This investigation aimed at developing BSA hydrogels as a controlled release system to study the release behavior of spin-labeled coumarin-3-carboxylic acid (SL-CCS) and warfarin (SL-WFR). The release profiles of these spin-labeled (SL-) pharmaceuticals from BSA hydrogels prepared with different procedures are compared in detail. The mechanical properties of the gels during formation and release were studied via rheology, while a nanoscopic view on the release behavior was achieved by analyzing SL-drugs–BSA interaction using continuous wave electron paramagnetic resonance (CW EPR) spectroscopy. The influence of type of drug, drug concentration, duration of gel formation, and gelation methods on release behavior were characterized by CW EPR spectroscopy, EPR imaging (EPRI), and dynamic light scattering (DLS), which provide information on the interaction of BSA with SL-drugs, the percentage of drug inside the hydrogel and the nature and size of the released structures, respectively. We found that the release rate of SL-CCS and SL-WFR from BSA hydrogels is tunable through drug ratios, hydrogel incubation time and gelation procedures. All of the results indicate that BSA hydrogels can be potentially exploited in controlled drug delivery applications. The admproducts ,5, nanocproducts ,7, smartproducts ,9, hydroproducts , hydrogeproducts ,12,13, aHydrogels are entangled networks made from proteins, hydrophilic polymers, or small molecules with the capability to retain water and biological fluids in large amounts without dissolving, due to the presence of physical and chemical bonds between the polymeric chains ,15,16. TIn order to develop a novel drug delivery vehicle, we selected bovine serum albumin see A as our The mechanism of hydrogel formation from albumin by heating and chemical crosslinking has been previously established . ThermalSeveral reports are available on the use of albumin for drug delivery purposes, but they have been largely based on nanocarrier systems. For instance, Pápay et al. studied the pulmonary delivery of apigenin, a natural polyphenol with antioxidant activity from BSA nanoparticles . GharbavElectron paramagnetic resonance (EPR) spectroscopy is a non-destructive and highly specific method used to monitor paramagnetic compounds containing unpaired electrons. The majority of materials, except for paramagnetic transition metal ions that possess intrinsic free radicals, are not detectable by EPR due to the absence of paramagnetic centers. Thus, by spin labeling or spin probing techniques, which are based on the covalent or non-covalent corporation of paramagnetic substances such as organic nitroxide radicals into the drug, the protein, or both, it is possible to study drug–protein interactions in drug delivery systems. Furthermore, through this method, information about the local environment, motional parameters, and ligand binding of the protein can be obtained, which allow for deep insights into the release properties ,25,27,33Coumarin (CCS) and warfarin (WFR) are widely prescribed as anticoagulant drugs for the treatment of thromboembolic disorders, which are among the most common and often fatal cardiovascular diseases. Moving toward a more controlled delivery of these types of drugs can avoid problems related to their oral administration including variability in dose response due to their narrow therapeutic window or considerable interaction with other medications ,37,38. Pi, to describe the gelation technique (i), which can be either thermally (T) or pH (P) induced methods, temperature (θ) and pH (p) of hydrogel preparation, incubation time , and time-release  indicating the release of drug over a specific time, respectively. For instance, BSAT  refers to the thermally prepared gel at 65 °C for 5 min and pH 7.4, and the release behavior was studied 48 h after gelation.Bovine serum albumin  was purchased from Sigma-Aldrich  and used as received without any purification. 4-Hydroxy TEMPO-labeled coumarin-3-carboxylic acid (SL-CCS) and 4-carboxy-TEMPO-labeled warfarin (SL-WFR), synthesized previously , were usThe 3 mM precursor solution of BSA was prepared by dissolving BSA powder in deionized water. After 1 h of stirring at 100 rpm at room temperature, the final solution was sterilized by a 0.45 µm filter. SL-CCS and SL-WFR powders were dissolved separately in DMSO to obtain 26 mM stock solutions and added into the BSA precursor solution with three different drug:BSA molar ratios into glass vials. Hydrogels were prepared by adopting the procedures described in our paper . ThermalOur release experiment was based on the addition of 1 mL 10× PBS on top of the hydrogels and incubating vials in a thermomixer at 37 °C. In order to maintain sink condition, after removal of 12 µL release medium at various time intervals for deeper analysis, this amount was replaced with fresh PBS. The mechanical properties and viscoelastic behavior during the formation of the hydrogels were investigated using rheological characterization. Evaluation of storage and loss moduli  provides information on gelation point . According to the shape and magnitude of storage and loss moduli, it is possible to obtain an insight into the gel stability and define mechanically robust and weak hydrogels.−1 frequency.Rheological measurements of hydrogels were performed on a Physica MCR 301 rheometer  using a CP50-2/TG plate as a measurement system. After placing the 2 mL of precursor solutions, namely BSA and different ratios of CCS/WFR:BSA, on the surface of the rheometer plate, the measurement gap was covered by silicon oil to avoid water evaporation. Time dependent curves of storage and loss moduli during gel formation were obtained using a 0.5% oscillatory strain and 1 rad sCW EPR spectroscopy provides information on the release mechanisms by plotting the double integral of the released spin-labeled drugs in the EPR spectra versus release time intervals, which is a measure of the released molecules. Furthermore, a deeper understanding of the drug–protein interaction in the hydrogel and the release medium can be obtained by analyzing the rotational motion of the spin probe. For instance, immobilized radicals show slow rotational dynamics that are coupled to the much slower rotational motion of the protein, while freely tumbling spin probes show no or little attachment to proteins.The CW EPR measurements were conducted on a Miniscope MS400  benchtop spectrometer with a microwave frequency of 9.4 GHz recorded with a frequency counter . The temperature of the samples was set to 37 °C using temperature controller H03 (Magnettech). All experiments were performed at a microwave power of 15 mW, a sweep width of 15 mT, and modulation amplitude of 0.2 or 0.03 mT . Simulations of recorded EPR spectra were evaluated in MATLAB using the EasySpin program package. This program was based on applying the Schneider–Freed approach to solve the Schrödinger equation for slowly rotating nitroxides [Sample preparation was as follows: about 12 µL of desired solution were filled into 50 µL glass capillary  and capped with tube sealant .Spectral-spatial electron paramagnetic resonance imaging (EPRI) is a well-suited technique to reveal a spatial tomogram as well as an EPR spectrum. The former derives information on the macromolecular structure of the sample while the latter indicates the micromolecular structure arises from mobility of the paramagnetic center .In this work, EPRI was used to monitor the release procedure by analyzing hydrogels, while the released substances in the medium were investigated by applying CW EPR.The EPRI spectra were obtained using an L-band spectrometer  equipped with a re-entrant resonator, operating at a microwave frequency of about 1.1–1.3 GHz.The measurement parameters used for 2D spatial imaging were set as follows: center field 48.8 mT, scan width 8 mT, scan time 60 s, and modulation amplitude 0.02 mT.The samples were prepared by placing 600 µL of precursor solution into cylindrical Teflon sample holders (9.3 mm diameters and 8.76 mm height) and keeping them in an oven  with different incubation times. After gel formation, sample holders were placed into 100 mL flasks containing 65 mL 10× PBS and kept in an incubation shaker at 37 °C. For EPRI measurements, at different time intervals, the sample holders were taken out of the buffer, externally dried with a tissue and subsequently transferred into the spectrometer.Dynamic light scattering (DLS) measures fluctuations of the scattering intensity arising from Brownian motion of particles in solution. By analyzing these fluctuations, information about the diffusion correlation and particle size can be obtained. DLS is one of the most popular techniques used as a non-destructive method to characterize complex liquids, proteins, and polymer structures ,42,43.A Litesizer 500  equipped with a 40 nm semiconductor laser with a wavelength of 658 nm was used for these experiments. The instrument measures with six runs and the duration of 30 s for each run and the temperature was set to 37 °C (with 4 min of equilibration time).To obtain the hydrodynamic radius and intensity correlation function, the release medium was analyzed by transferring 100 µL of release sample into a Quartz cuvette  with no further filtration. Thereafter, the samples were measured to gain insights into the nature of the released molecules and gel fragments.Mechanical and viscoelastic properties of hydrogels can be considered essential design parameters for pharmaceutical and biomedical applications. In the field of drug delivery, the integrity of the hydrogel plays a crucial role in protecting a therapeutic agent in the case of a non-biodegradable system until it is released out of the system, while the flow properties of some gels gain importance since they are used as injectable carriers for drug delivery purposes ,45.Physical protein gels are formed by non-covalent interchain associations involving hydrophobic, van der Waals, solvation, and electrostatic interactions determining the rheological behavior of proteins. The rigidity of the protein network arises from the strength and number as well as pattern of these interactions ,46. FiguT and BSAT increased up to 29,000 and 20,000 Pa after 60 min, respectively. Therefore, robust hydrogels can be formed by the heat induced technique even at 59 °C, which is slightly below the denaturation temperature of BSA. However, the G′ value of BSAT was, as expected, considerably higher than that of the BSAT. In our previous work [For thermally induced hydrogels, the precursor solutions of BSA were heated and kept at 65 °C and 59 °C . In both samples, the storage modulus dominated the loss modulus instantly, reflecting the gelation point. We defined a hydrogel to be mechanically tough when the G′ value exceeded 10,000 Pa after 1 h. The value of G′ for BSAIn To probe the impact of CCS and WFR on albumin gelation kinetics, different ratios of these drugs were added to the precursor solutions of BSA and rheological measurements were repeated at different temperatures (see Time dependence of storage and loss moduli for the system including BSA and CCS with three different molar ratios at 65 °C are depicted in We have previously investigated the effect of fatty acids on gel formation by adding different concentrations of stearic acid (SA) to a BSA precursor solution (SA and 16-DSA are substitutable in their binding to BSA). Unlike CCS and WFR, fatty acids delay gelation onset time and considerably affect the mechanical properties. Generally, BSA consists of seven long chain fatty acid binding sites that obstruct protein denaturation and final gelation and increase the thermal stability of protein .Drug binding to HSA in solution has been well studied by a spin labeling and CW-EPR spectroscopy approach, which provides information on the maximum number of binding sites per protein as well as the dynamics of SL-pharmaceuticals .c,τ which is calculated from diffusion tensor D  and BSAP at 2:1 SL-CCS:BSA ratios, respectively. Due to the delayed gel formation of samples prepared by the pH-induced method, much longer incubation times were required. Results from spectral simulation revealed that in a hydrogel prepared by the thermally induced method at 65 °C, almost 26% of SL-CCS showed freely tumbling rotation, 29% were strongly bound to BSA, and 44% had intermediate rotational motion. However, with the addition of acid, the percentage of freely rotating and intermediately immobilized SL-CCS increased to 33% and 51% while that of the strongly bound ligand to BSA decreased to 14%. In general, the strongly immobilized state in BSA could stem from ligand binding sites, which are located at the interfaces of α-helices. The intermediately immobilized ligand with faster rotation is typically situated in water swollen regions of BSA with a β-sheet structure [tructure . Acidic tructure . Figure In It can be seen that all 0.5:1 and 1:1 ratios in the SL-CCS loaded BSA hydrogels prepared by different methods showed an initial fast release before reaching a plateau after 24 h. The initial release of large amounts of drug corresponds with the diffusion of release medium into the protein network, which dissolves the entrapped SL-CCS close to or at the surface of the hydrogel. However, the second and slower release is due to the depletion of drug in the inner part of gel matrix, which leads to the increase in the diffusion process length. In contrast, the kinetics of the SL-CCS:BSA 2:1 ratios almost corresponded to a zero-order release with an initial burst effect. It is possible that by increasing the ratio to 2:1, the binding sites from which the later release occurred are fully occupied and instead, the amount of drug between the hydrogel networks increases, which results in a drug release at a constant rate.The effect of initial drug loading and preparation methods on the release profile is also discussed in EPR spectra of the released components from BSA hydrogels with SL-CCS prepared with lower incubation times  are shown in We applied electron paramagnetic resonance imaging (EPRI) to study the spatial localization of a spin probe in a nondestructive way and followed the position of the probes inside the slowly dissolving gels not in the release medium. This technique combines spectral information with the spatial distribution of paramagnetic species, which permits monitoring of the changes in the property in different parts of the investigated sample ,53.y-axes, respectively; the spatial dimension is shown along the x-axis. By applying spatial cuts, it is possible to obtain information on intensity distributions at certain positions. T and release of SL-CCS from the BSA hydrogel during the first 4 h. It can be seen that the signal amplitude decreased over time, which illustrates the release of the SL-pharmaceutical from the hydrogel. Moreover, the EPR signal intensities slightly shifted to the right (bottom of the hydrogel) with release time, indicating that release occurs from the gel surface.The amount of SL-pharmaceuticals inside the BSA hydrogel was calculated by double integration of the central spatial cuts. To better understand the components released over time, we determined the hydrodynamic radii and correlation functions using dynamic light scattering (DLS). In the present work, BSA hydrogels with good mechanical properties were prepared as delivery hosts for pharmaceutical applications. The whole gelation procedures, namely thermally and pH induced methods, have previously been explained in detail . SL-CCS Additionally, EPR spectroscopy and DLS were applied to shed more light onto the interaction of SL-pharmaceuticals with BSA hydrogels as well as the nature and size of the released structures. We have shown that differences in chemical substitutions of SL-CCS and SL-WFR and binding capacities of BSA for SL-pharmaceuticals can deeply affect drug–protein interactions. Since a higher percentage of SL-WFR is attached strongly and intermediately to BSA hydrogels, more of these components are released compared to the SL-CCS incorporated BSA hydrogel, in which a higher percentage of freely tumbling SL-drug was found in the release medium. Moreover, a higher percentage of SL-CCS/SL-WFR intermediately  and strongly  bound to BSA was released from less rigid hydrogels due to faster penetration of water into the gel network. This led to the expedited release of albumin dimers or individual proteins when compared to the mechanically robust hydrogels. We hope that this study aids in paving the way to employing the full potential of BSA hydrogels as a suitable controlled release system and their implementation in biomedical applications.


Then, the compressive strength was tested, and the microstructure analysis of the ready-made lightweight concrete (LWC) was performed. The degree of penetration of aggressive solutions into the cracks of the samples was assessed by means of applying 1% phenolphthalein solution. Changes in the weight of lightweight concrete samples after the test period were estimated. The obtained test results indicate that the decrease in the durability of lightweight concrete can be classified as a long-term process. Concrete with GEGA and GAA showed high resistance to aggressive environments. Moreover, the environment containing chlorides turned out to be the most aggressive, while the environment containing sulfates proved to be the least aggressive. The higher the concentration of the destructive factor was, the faster the corrosion process went. This has been proven by measuring the pH using phenolphthalein and carrying out microscopic examination. Concretes containing aggregates made of foamed glass and sintered fly ash are suitable for use both in traditional construction and in facilities exposed to an aggressive environment .In light of the scientific research, the corrosion of concrete structures is one of the main problems that may reduce their durability due to the negative impact of the natural environment. The paper analyzes the influence of the type of component on the selected properties of lightweight concrete subjected to the influence of aggressive liquids. Four concrete mixes were prepared with a granular aggregate made of foamed glass (GEGA) and aggregate made of sintered fly ash (GAA) with the use of a mineral additive: silica fly ash. The prepared lightweight concrete after one year was exposed for 60 days to the following environments: strong acid—HCl, 1% and 2% concentration, weak acid—CH Concrete is the most popular construction material all over the world, and its production and consumption are constantly increasing . It has 4, Na2SO4, K2SO4, MgSO4, (NH4) 2SO4, and acid—H2SO4. The defense mechanism in the case of sulfate salt penetration into the concrete is mainly based on the buffering effect of the binder components that maintain the alkaline environment of the concrete. The migration of sulfate ions with significant concentration into the concrete structure poses a risk of the formation of corrosive chemicals, in particular: ettringite (C3A·3CaSO4·32H2O) and gypsum (CaSO4·2H2O), as indicated by the authors [Concrete corrosion can be caused by external physical or chemical factors . Sometim authors ,26,27.42−) and the decrease in the pH of the solution in the pores of the concrete result in the lack of durability of the hydrates. In the grout, in the outer layer, exposed to the environment, a solution containing gypsum appears [The increase in the concentration of sulfate ions , and sodium sulfate (VI) (Na2SO4).The aim of the work was to present the problem concerning the durability of lightweight concrete (LWC) exposed to the influence of harmful factors of the external environment. The main task was to link the impact of destructive factors on lightweight concrete in general, and specifically on its durability. For this purpose, four different concrete mixes were prepared containing granular foamed glass aggregate (GEGA) and sintered fly ash aggregate (GAA). The lightweight concrete samples were exposed to aggressive aqueous chemical solutions of 1% and 2% concentrations of hydrochloric acid (HCl), acetic acid  were designed. The concrete composition was designed assuming a variable water/cement ratio and the type of lightweight aggregate. The main parameter considered in the corrosion resistance analysis was the water/binder ratio, which changed along with the content of cement and ash in the recipes and constituted 0.5 (R1), 0.45 (R2), 0.4 (R3), and 0.35 (R4). For all recipes, only artificial lightweight aggregates were utilized. In recipes for R1 and R2, GEGA2 mm and GEGA4 mm were used in the amount of 25% of the total volume of lightweight aggregate and GAA 8 mm in the amount of 50% of the total volume of the aggregates. In the R3 and R4 recipes, only GEGA 2 mm and GEGA 4 mm were used in the amount of 50% of the total aggregate volume. The formulas R1 and R4 contained fly ash as an additive fly ash. A chemical admixture, superplasticizer FK 63.30 , was applied in the lightweight concrete mixtures. The amount of admixture and the amount of mixing water in the four prepared concrete mixes was maintained at 0.8% of the cement weight. The composition of the mixtures is presented in 3 and then the remainder of the water was added. To test the compressive strength and the resistance of lightweight concrete to the influence of aggressive liquids, cubes with the dimensions of 100 mm × 100 mm × 100 mm were formed in accordance with the requirements [The proportions of the particular amounts of concrete components, given in irements . After 2The penetration depth (mm) of aggressive liquids into the structure of the samples was carried out after 30 and 60 days of exposure to aggressive corrosive solutions as required . The pen3COOH and 1% and 2% Na2SO4), were assessed after 30, 45, and 60 days from the moment of immersion in corrosive solutions. Due to the shortened test time—60 days, the concentration of 1% and 2% of aggressive liquids was selected. The choice of corrosive solutions  is conditioned by an attempt to assess the corrosion risk in the case of lightweight concrete including the mechanism of the influence of ions of strong and weak acids and salts, ions occurring in the natural environment .The results of changes in the average absorption of LWC samples, having been previously stored in water and used as a comparative material, subjected to three types of corrosive environments of different concentrations  to the compressive strength of samples stored in water of a constant temperature of 20 ± 2 °C. At the time of commencement of the corrosion resistance tests, the samples were one year old. Earlier throughout the 1-year period, all samples had been stored in a room with a constant temperature of 20 ± 2 °C and air humidity of 95%. The compressive strength test of each LWC type after 30, 45, and 60 days was done by means of an Advantest 9 Controls machine  with a maximum pressure force of 3000 kN, according to [The corrosion resistance of concrete, containing various types of lightweight aggregate, was determined as the ratio of the compressive strength of the samples stored in corrosive solutions . The research was carried out to determine the degree of the damage to the internal structure of the samples including the observation of the precipitated corrosion products. Before assessing the microstructure (SEM) of the selected concrete surfaces with the use of lightweight aggregate (GEGA and GAA), cubic samples with dimensions of 20 mm × 20 mm × 20 mm were cut out. The above-mentioned samples were ground and cleaned of dust.An example of a general view of the fractures of the surface of LWC samples (R1–R4) moisturized with a 1% solution of phenolphthalein, is shown in The penetration depth of corrosive solutions (mm), depending on their concentration in the structure of the test series (R1 ÷ R4), is presented in 3COOH and Na2SO4) with the selected levels of concentration (1% and 2%) and the moisture present in the pores of the concrete, completely immersed in an liquid, the subsurface layer of concrete undergoes a gradual process of penetration by aggressive corrosive liquids. The front of the penetration of aggressive solutions gradually moves into concrete, and the main reaction taking place in this process is the reaction of corrosive solutions with calcium hydroxide dissolved in the concrete pore liquid. As a result of this reaction, Ca(OH)2 leaches out, which reduces the alkalinity of the concrete , which in turn leads to an increase in the degree of the penetration of aggressive liquids into the structure of LWC, depending on its composition.On the basis of the examples , it can 2SO4) to 7 mm (2% HCl). As the w/b index decreased, a decrease in the depth of liquid penetration was also visible. Therefore, for R2 (w/b = 0.45)   the pene = 0.45) . In the  = 0.45) , the pen = 0.45) . The dec = 0.45) a. The hin the R4  mixture It was stated that the penetration depth of corrosive solutions, regardless of their type and concentration, depends on the composition of the designed LWCs . The wat3COOH, and 1% and 2% Na2SO4), were assessed after 30, 45, and 60 days from the moment of immersion in corrosive solutions. The weight of the corrosion-treated samples was compared with the weight of samples immersed in water. In both cases, samples had been dried in the temperature of 105 °C before weighing. The test results for LWC absorption after exposure to corrosive environments are presented in The results of changes in the average absorption of LWC samples, having been previously stored in water and used as a comparative material and samples, subjected to three types of corrosive environments of different concentration , from 8.1% to 9.6% (CH3COOH) and from 9.4%, respectively, up to 10.1% (Na2SO4). In the case of LWCs R3 (0.4) and R4 (0.35) made of GEGA 2 mm and GEGA 4 mm lightweight aggregates, each constituting 50% of the aggregate volume, the LWC R4 composition additionally uses fly ash in the amount of 20% of the cement mass. It was observed that because of the additional amount of fly ash, the water absorption of the R4 samples decreased by about 10%. When analyzing the test results, it can be observed that even though the GEGA aggregates were characterized by a much higher open porosity than the 8 mm GAA aggregate, the absorbability of LWC R3, R4 compared to LWC R1, R2 samples, stored in water and aggressive environments for 60 days, did not differ by more than 1% in any case. The observed differences in changes in the water absorption of the tested concrete samples (R1–R4) with lightweight aggregate depend on the exposure time and concentration of the selected corrosive environments, and on the physical properties of GEGA and GAA aggregates (2 phase). Ca (OH)2 shows the greatest susceptibility to aggressive corrosive environments in the cement matrix. Its dissolution enables the migration of aggressive chemical liquids into the LWC structure. The result of this phenomenon is a relatively high water absorption, which must be compensated by the increased cement content or the addition of fly ash.The obtained results of the tests and the analysis of the average values for changes in LWC absorption indicate an increase in aggressive liquid absorption in all environments compared to the water absorption of samples stored in water. In general, the LWC water absorption remained between 4.1 and 11.6%. The lower water/binder (w/b) ratio of LWC in the tested samples was assumed, and a lower water absorption was observed. In the case of w/b = 0.35, it did not exceed 9.6% in any aggressive environment after 60 days. The positive effect of the fly ash content was also visible, which limits the penetration of both water and aggressive liquids. For example, for LWC samples R1 (0.5) with a mixture of GEGA aggregate  and GAA  and with the addition of fly ash, liquid absorption in most cases was equal to or slightly lower than for LWC R2 (0.45) containing no fly ash and almost the same as for LWC R3 (0.4). This fact draws attention regarding its economic and environmental importance. Due to the limitation of the penetration of aggressive liquids, it is possible to use fly ash as a cement substitute in the amount of min. 20% of cement weight. Regarding the composition of LWC R2 with the mixture of GEGA and GAA aggregates in the same proportions as in the case of LWC R1, the lack of fly ash and an increase in the amount of CEM I 42.5 R by 50 kg/mgregates . GEGA aggregates , GEGA grThe density of LWC, depending on the exposure time to corrosive solutions compared to the density of the samples stored in the air-conditioned chamber, is presented in 3COOH, the weight loss of the samples was noted in the range of 4–11%. On the other hand, the weight loss of the LWC samples treated with 2% Na2SO4 ranged from 6% to 12%.The greatest changes in the mass and, consequently, in the density of the samples were noted when the LWC samples were exposed to a 2% HCl solution for 60 days. In this case, greater weight loss occurred in the LWC R1 and R2 group, and in the LWC R2 group, where the weight loss constituted 15% while in the case of LWC R1, it was only 13%. Lower weight loss in the case of LWC R2 resulted from the use of CEM I with the addition of fly ash. In this case, the ash was a cement substitute in the amount of 20%. Although in the case of LWC R1, the w/b ratio = 0.50 was higher than in the LWC R2 w/b = 0.45, and the weight loss in the HCL environment was 2% lower. In the case of LWC R3 and R4, whose w/b were 0.4 and 0.35, respectively, lower weight loss was recorded for the LWC R4. The results of the tests were influenced by the low w/b ratio = 0.35. In the case of LWC R4, fly ash was an addition of 20% to the cement mass. For the LWC R3, the weight loss constituted 11%, while for the LWC R4, the weight loss for the samples stored in 2% HCl reached 8%. When LWC samples were stored in the environment of 2% CHcm, cube) of the three single test results  defined in the adopted research terms.The compressive strength of LWC (R1÷R4), depending on the exposure time to corrosive solutions compared to the compressive strength of the samples stored in the air-conditioned chamber, is shown in 3COOH for 60 days, the weight loss of the samples was in the range of 28–38%. On the other hand, the decrease in compressive strength of LWC samples in 2% Na2SO4 ranged from 15% to 34%. The obtained test results of the average compressive strength in a function of time and the impact of selected corrosive environments, despite the lower ratio (w/c and w/b), suggest dissolution of the cement paste covering and filling the pores of light aggregate grains, and as a result, enable crystallization (including penetration) of corrosive compounds (The greatest decrease in LWC strength (R1 ÷ R4) was noted when the LWC samples were exposed to a 2% HCl solution for 60 days. In this case, greater decrease in weight concerned the LWC R1 and R2 groups, and, in particular, the LWC R2, where the decrease in compressive strength constituted 43%, while in the case of LWC R1, the compressive strength decreased by 41%. Lower weight loss in the case of LWC R2 resulted from the use of CEM I with the addition of fly ash. The sealing of the structure limited the influence of the aggressive liquid and the damage to the cement matrix. In the case of LWC R1, the ratio w/b = 0.50 was higher than LWC R2 w/b = 0.45, and the decrease in compressive strength in the HCl environment was 2% lower. In the case of LWC R3 and R4, whose w/b were 0.4 and 0.35, respectively, the decrease in compressive strength of the samples stored in 2% HCl for 60 days was the same in both cases and amounted to 38%. In the case of LWC R4, with an addition of 20% fly ash to the cement mass, the compressive strength increase was not observed. In the case of the LWC, the main factor influencing the strength of the LWC was the crush strength of the LWA. When the LWC samples were exposed to 2% CHompounds  in the vGraphical interpretation of the corrosion resistance coefficient of LWC (R1 ÷ R4) in the given corrosive solutions based on weight loss and compressive strength in environments of 2% concentration after exposure for 60 days is presented in 3COOH) with a concentration of 2%, the decrease in compressive strength for LWC R1 and R2 was from 28% to 38%. Exposure of LWC to a salt solution—sodium sulfate (VI), after 60 days, caused a decrease in the strength of LWC R1 and R2, and constituted respectively 31% and 34%, while in the case of R3 and R4, the decrease in compressive strength was observed at the level of 15% and 20%, respectively. The analysis of the test results proved the significant influence of the components used on the mechanical properties of LWC.The greatest decrease in the compressive strength of the LWC was visible after 60 days of exposure to 2% solutions of aggressive liquids described in this study. A summary of the test results is presented in Examples of microscopic images (SEM) of concrete samples (R1 ÷ R4) with LWA (GEGA and GAA) after 60 days of exposure to the corrosive environment are shown in 4) were visible. Their amount depends on the composition of the LWC. For example, after 60 days of exposure to a 2% HCl solution, changes in the microstructure in terms of the amount of precipitated ettringite in LWC R1 with the participation of the GEGA aggregate  and GAA  were visible, both in the case of the utilization of fly ash for LWC R2 with GEGA 2 mm, 25%, GEGA 4 mm, 25%, and GAA 8 mm, 50% and without the addition of fly ash  with the participation of different amounts of LWA (GEGA and GAA) suggest that in all cases, corrosion products  with cement and mineral additive such as fly ash allows a higher compressive strength of LWC over a long-term period (365 days) to be obtained compared to concrete with cement alone.The level of penetration of aggressive corrosive solutions depends on the open porosity of lightweight aggregates and the type of mineral additives.The presented test results for resistance to aggressive environments for LWC (R1 ÷ R4) with an aggregate (GEGA and GAA), modified with the addition of fly ash , indicate the influence of the LWC composition on the average compressive strength, even though the samples had been previously cured for a year (365 days) under standard conditions (temperature 20 ± 2 °C and humidity ≤95%). After a one year period, the samples were exposed to aggressive environments.The use of GAA aggregate in the amount of 50% of the total aggregate volume in LWC increased the strength of concrete by approx. 50%.LWC containing 100% of the total aggregate volume in the concrete is characterized by a higher resistance to corrosive environments by 8%.In general, the corrosion resistance decreases as the water/binder ratio increases.The introduction of silica fly ash or a mixture of fly ash as a partial replacement for cement causes significant changes in the chemical composition of the pore solution phase in the concrete structure with the use of lightweight aggregates.Microstructure tests (SEM) show that the type of mineral additive affects the degree of corrosion resistance of LWC. The highest corrosion resistance, considering the change in the average compressive strength after 60 days of exposure to a corrosive environment, is demonstrated by the concrete with a mixture of lightweight aggregate (GEGA and GAA) and fly ash.Concrete with lightweight aggregate (GEGA) and fly ash has a higher effect of corrosion resistance. The observed phenomenon may be the result of a slow pozzolana reaction as well as a slow process of filling the voids in the concrete structure with the products of the binding reaction.The appropriate durability of LWC subjected to a corrosive environment depends on the composition of the concrete: the type and amount of cement, the value of water/cement ratio, the type of aggregate, the type and number of concrete additives, and water/binder ratio. The selection and the right proportion should ensure a tight matrix in the LWC. Shaping the tightness of concrete in a particular LWC  should primarily result in limiting the capillary porosity of the cement grout. Based on the tests carried out for LWC and presented in this work, the following conclusions can be drawn:In the initial stage of concrete maturation, it results in a decrease in compressive strength, but in later stages, compressive strength gradually increases.10.The use of lightweight aggregates (GEGA and GAA), regardless of their mixing ratio and grain size, enables the migration of aggressive corrosive solutions, resulting in the precipitation of corrosive products such as ettringite. Its amount probably depends on the degree of sealing of the interfacial transition zone and the matrix as well as the lightweight aggregate absorption mechanism.


It is unclear based on published literature whether shorter courses of antibiotic treatment may be appropriate for urinary tract infections (UTI) in patients with SCI/D given their complex baseline clinical status. C. difficile infection or death within 30 or 90 days, respectively, after treatment completion. Statistical tests included Chi-square, Mann-Whitney U, and logistic regression. This retrospective cohort study was conducted at the VA San Diego Healthcare System (VASDHS), which has a dedicated SCI/D unit. Adults with SCI/D were identified for inclusion if they had received antibiotics for a positive urine culture in conjunction with UTI symptoms from 1/2018-12/2020. Individual UTI events were excluded if associated with potential sources of harbored infection, anatomic abnormalities increasing risk of bacteriuria, non-bacterial pathogens, concurrent infections prolonging antibiotic treatment, or antibiotic courses managed outside of VASDHS. Treatment groups comprised UTI events treated with no more than 7 days of antibiotics (group 1) versus more than 7 days (group 2). Study endpoints were recurrence or new incidence of UTI within 30 and 90 days after completion of antibiotic treatment and onset of C. difficile infection occurred in 1 patient in group 2 after 3 days of antibiotic therapy. Duration of antibiotic therapy was not predictive of treatment failure within 30 days of antibiotic completion. Factors predictive of treatment with longer courses of antibiotic therapy included hospital admission and more severe ASIA impairment scale score. One-hundred and seven patients with 241 unique UTI events were included in this study, with 79 events in group 1 and 162 events in group 2. Baseline characteristics were similar across both groups, aside from a higher incidence of hospital admission and more severe SCI/D based on the American Spinal Cord Injury Association (ASIA) impairment scale in group 2. Efficacy outcomes are described in Table 1. No deaths occurred within 90 days of treatment completion, and Table 1. Incidence of UTI after antibiotic completionThe findings of this study suggest that for some patients with SCI/D, UTI treatment lasting 7 days or fewer may be effective compared to longer courses of antibiotics and could be beneficial in reducing collateral damage from antibiotic use. All Authors: No reported disclosures


The mechanical properties, coarse quality of ATCA, and NCA were studied and compared. The compressive strength, splitting tensile strength, and compressive elastic modulus of ATWR are 221.51 MPa, 5.93 MPa, and 3.33 × 104 MPa, which are 1.31, 2.22, 1.40 times of that of NR, respectively. All of the quality control indices of ATCA meet the requirements of the current industry standards of China. Finally, the toxicity risks of ATCA concrete were investigated. It is illustrated that the leaching of main heavy metals including Sb, As, Hg, Pb, Cd, and Zn in the ATCA concrete under different pH conditions are below the regulatory limits. The utilization of antimony tailing has significant environmental and economic benefits.In this research, the sustainable applications for utilizing antimony tailing coarse aggregate (ATCA) in concrete is investigated. Comprehensive verifications were performed by a series of experiments on the characteristic of ATCA and the toxicity risks of concrete. Firstly, a real case study of utilization of ATCA as a complete substitute for natural coarse aggregate (NCA) in high strength concrete was conducted. Then, chemical composition of ATCA was tested. It is demonstrated that the essential mineral is SiO Antimony (Sb) is a metalloid element belonging to Group 15 of the periodic table, naturally occurring in the Earth’s crust ,3. Due tHowever, Sb and its compounds are considered to be hazardous to human health, causing degradation in liver and low blood sugar ,10. In tIn recent years, the green and sustainable production of concrete receives much attention ,20,21. ABesides, the toxicity risks of heavy metals must be taken into consideration when the wastes are used as building materials ,35. The Science coarse aggregate (CA) is the largest component of high strength concrete, significantly affecting the performance, it is necessary to investigate the characteristic of ATCA in tailing concrete. Furthermore, the toxicity risks of ATCA concrete should be evaluated to ensure the concrete is harmless to the environment. The aim of this study is to investigate the characteristic of ATCA and the leaching performance of ATCA concrete based on the previous work. A series of experiments was performed, such as chemical composition and mechanical properties of ATWRs, coarse quality of ATCA, and the toxicity risks of ATCA concrete, which were tested and discussed in the present work.The rest of this paper is organized as follows. 3 of NCA. However, the NCA of the nearby quarry is not suitable for C50 concrete, and the qualified supplier is 52 km to the site. After our investigation, we found that the compressive strength of the ATWRs from XKS met the requirements. Moreover, the distance from XKS to the project is only 20 km. Considering the above issues, the entire C50 concrete is intended to prepare with ATCA as a complete replacement of NCA. Using ATCA can not only provide a new method for the treatment of antimony tailing, but also solve the problem of shortage of raw materials for the project and create huge economic benefits.The construction of the Long-lang highway project, which is nearby XKS and located in South China’s Loudi City, Hunan Province, is chosen as a real case in the present work. As illustrated in In this part, the investment and environmental benefits are discussed by using ATCA concrete. The cost for ATCA administration, reclaiming, and transportation are respectively saving 90% , 85% (¥27 yuan), and 61% (¥25 yuan) per cubic meter compared with NCA. The cost saving of direct investment resulting from ATCA can be obtained with the following equation,2 land occupation for stacking ATWRs. It is very conducive to control the solid waste pollution on the environment and protect the ecology. Therefore, the utilization of ATCA as a complete substitute for NCA in concrete shows a great potential for economic and environmental benefits.In addition, the use of ATCA reduces about 15,000 mThe test sample was obtained by the five-point sampling method in the antimony waste ore stacking area in XKS. The stacking area is approximately a rectangle, divided into four areas by diagonals, and the sampling points are distributed at the intersection and quarter points of the diagonals. A total of 400 kg ATWRs were collected from each sampling point, 1 m from the upper surface. The collected ATWRs were mixed together and brought back to the laboratory. Some were processed into specimens for rock mechanical performance testing. The rest were crushed into ATCA, of which the particle size was 5–20 mm, used for chemical composition analysis, aggregate quality testing, and preparing for ATCA concrete performance experiment.X-ray fluorescence spectrometer  was used to analyze the chemical composition ATCA. Polarizing microscope  Co., Ltd., Beijing, China) was used to analyze the mineral composition of the test sample.2), and the subordinate mineral contained kaolinite, pyrophyllite, muscovite, and montmorillonite  is closely inlaid to form a dense aggregate with clear boundaries. Most of the grains are concavo-convex and suture-shaped. The crystal size is relatively uniform, generally varying from 20 μm to 150 μm. A small amount of chalcedony is distributed between grains. The late stage of quartz (Q2) is composed of fine vein-like aggregates, filling along the rock fissures (red dotted area). The vein wall of the late stage of quartz (Q2) is relatively straight, and the width of the vein varies from 100 to 300 μm, and the particle size of late stage quartz varies from 30 to 100 μm. The lithology of ATWR is quartzite. According to rock characteristics, it can be inferred that ATWR has good mechanical properties and can be used as CA for high strength concrete.The mechanical properties, such as uniaxial compressive strength, splitting tensile strength, and elastic modulus are the most important geotechnical parameters for intact rocks ,42. SincTo better investigate the rock mechanical properties of ATWR, NR form the qualified supplier for NCA, mentioned in The compressive strength of ATWR was 221.51 MPa, while the measured value of NR was 169.52 MPa. Since the lithology of ATWR is quartzite, its compressive strength is much higher than that of NR. There was a clear cracking sound as the ATWR sample was crushed, while the cracking sound of NR was relatively low.β is the angle between normal of crushing surface and loading axis; ϕ is the friction angle of testing samples, as shown in The loading process of both groups of specimens exhibits three stages, i.e., elastic stage, crack developing stage, and failure stage. While the failure modes of the two kinds of rock samples are obviously different, as illustrated in There are two methods to determine the tensile strength of rock: direct tension test method and indirect tension test method. For the direct tension method, the specimen production is more difficult and the experimental technology is more complicated. Therefore, the indirect tension method is generally used for testing. The failure load of the rock is tested based on the splitting method, and the splitting tensile strength is obtained by conversion.As shown in 4 MPa, while the measured value of NR is 2.38 × 104 MPa. The elastic modulus of NR is 71.5% of ATWR’s. Since compressive strength is an adequate index for mechanical properties, a close relationship exists between compressive strength and elastic modulus of the two kinds of samples.As illustrated in In this section, the mechanical behaviors such as cubic compressive strength, splitting tensile strength, and compressive elastic modulus of the two kinds of samples were tested simultaneously. The results show that the cubic compressive strength, splitting tensile strength, and compressive elastic modulus of ATWR are 1.31, 2.22, 1.40 times of that of NR’s. Therefore, the mechanical behavior of ATWR is superior to that of NR. This further shows that the ATWR possesses excellent mechanical property for the lithology of is quartzite and that it is very suitable for high strength concrete. It can be inferred that the ATCA as a substitution for NCA possesses perfect physical and mechanical properties. In order to further study the feasibility of ATCA for concrete, the quality of ATCA is investigated in the next section.To ensure the mechanical property  and duraThe sieving method was used to determine the graduation of ATCA and NCA. The gradation curve is illustrated in The density and water absorption of CA are key indices of material quality, which indirectly reflects the open porosity of the aggregate. The lower water absorption of the aggregate means lower open porosity, higher density, and better working performance and durability behavior for the concrete.The apparent density, gross bulk density, loose bulk density, void ratio, and water absorption are listed in In order to evaluate the ASR of ATCA, the alkali activity experiment was conducted. ATCA and NCA specimens with dimension of 25 × 25 × 280 mm were prepared for the experiment. For 14 days, all samples were placed in the NaOH solution, of which the concentration was 1 mol/L, and the temperature was (80 ± 2) °C. The length of the sample was measured, by which the expansion rate of length was calculated and the alkali activity of aggregate was evaluated. The expansion rates of ATCA and NCA specimens are 0.71‰ and 0.80‰, respectively, which are both less than 1‰ of the standard required.Other indices of material quality, including needle-flake content, firmness, crushing value, abrasion resistance, dust content, and clay lump content were test in this subsection. As per the code, all of the indices are classified into three grades, of which the ranges are shown in The content of needle-flake particles was tested by a needle-flake gauge, divided into three grades. As illustrated in The sample was soaked in a saturated sodium sulfate solution. After five cycles of immersion and drying, the mass loss percentage was measured and the firmness was determined. The results of the mass loss percentage of ATCA and NCA are respectively 3.0% and 3.6%, which are both classified as grade Ⅰ according to The crush value refers to the ability of CA to resist crushing and indirectly reflects the strength performance of CA. Pressure was applied to the sample by a press, and the mass percentage of the sieved fine material after pressure was used to determine the crushing value. The results of mass percentage of the sieved fine material of ATCA and NCA is 9.2% and 14.8%, which are classified as grade Ⅰ and grade Ⅱ, respectively.The abrasion resistance of CA was tested by the Los Angeles method. The mass loss of ATCA and NCA is 18.0% and 24.5%, respectively. As per the code, the abrasion resistance of ATCA and NCA are both classified as grade Ⅰ.The dust content was determined by testing the weight of particles with a particle size of less than 0.075 mm in the CA. The results of the dust content of ATCA and NCA is 0.4% and 0.72%, which are classified as grade Ⅰ and grade Ⅱ, respectively.Moreover, the clay lump content was determined by testing the weight of clay lump with a particle size of more than 2.36 mm in the CA. The results of the clay lump content of ATCA and NCA is 0.1% and 0.18%, which are all classified as grade Ⅱ according to The experiment results are summarized and showed in The results of water absorption and firmness of ATCA are superior to NCA, therefore, it can be inferred that ATCA concrete has better frost resistance. The alkali activity of samples seems increased for ATCA specimens. However, the difference is not significant and can be neglected. The tested results of firmness, crushing value, and abrasion resistance of ATCA are superior to that of NCA. ATCA possesses excellent abrasion resistance performance, thus it can be inferred that ATCA may be applied in concrete pavement. It is demonstrated that the compressive performance of ATCA is significantly better than that of NCA, which is completely consistent with the strength test results of the material. The dust content and clay lump content of ATCA are lower than that of NCA, and it is beneficial to enhance the bonding performance between ATCA and mortar. However, the needle-flake content of ATCA is higher than that of NCA. The factor for higher needle-flake content of ATCA could be attributed to higher compressive strength of ATCA, which tends to produce needle-like particles when crush into CA.The toxicity characteristic leaching experiment was conducted to evaluate the concentration of heavy metals from ATCA concrete and ensure that the concrete was harmless to the environment. The static soaking method was used to analyze the dissolution and release of the main heavy metals including Sb, As, Hg, Pb, Cd, and Zn in the ATCA concrete under different pH conditions according to GB 30770-2014 .A total of seven batches of leaching tests were carried out to simulate the leaching of heavy metal from ATCA concrete soaked in different pH solutions. Sulfuric acid or sodium hydroxide solution were used to adjust the pH values of the soaking solution at 4, 5, 6, 7, 8, 9, and 10.The concentration of the heavy metal was measured every two days and the test lasted for 14 days. The results are shown in As can be seen from the figure, pH has a great influence on the leaking of Sb, As, and Hg in the concrete. As for As, the changing trend of the tested result is consistent with Sb, illustrated in While the test results of Hg shows a different vibrational tendency, illustrated in Under varying conditions of pH range 4–10, the concentrations of Sb, AS, and Hg reach a steady state on the eighth day. Moreover, on the 14th day, the maximum concentrations of Sb, AS, and Hg are 0.266 mg/L, 0.067 mg/L, and 0.029 mg/L, respectively. While the regulatory limits of Sb, AS, and Hg are respectively 0.3 mg/L, 0.1 mg/L, and 0.05 mg/L. It is illustrated that all the test results are less than the regulatory limits. Therefore, the ATCA concrete can be considered as a non-hazardous material.2) and the lithology of ATWR was quartzite. The mechanical behavior such as cubic compressive strength, splitting tensile strength, compressive elastic modulus of ATWR and natural rock (NR) were studied simultaneously. The results show that the cubic compressive strength, splitting tensile strength, and prism compressive elastic modulus of ATWR are 1.31, 2.22, and 1.40 times of that of NR’s, respectively. Therefore, the mechanical behavior of ATWR is superior to that of NR. It is demonstrated that all of the quality control indices of ATCA meet the requirements of the current industry standards of China. Moreover, ATCA possesses excellent abrasion resistance performance, thus it can be inferred that ATCA may be applied in concrete pavement. The leaching of main heavy metals including Sb, As, Hg, Pb, Cd, and Zn in ATCA concrete under different pH conditions are below the regulatory limits. The ATCA concrete can be considered as a non-hazardous material.The chemical composition and mechanical properties of antimony tailing waste rock (ATWR), quality of antimony tailing coarse aggregate (ATCA), and leaching performance of ATCA concrete are studied in this work. Polarizing microscope and X-ray diffraction (XRD) analysis illustrated that the essential mineral of ATWR was quartz  and reduce about 15,000 m


As a repair layer mortar, the positive influence of silica fume in modified 5% silica PCM specimens was also found at early ages and with different moisture conditions at the interface compared to the normal PCM. In conclusion, the addition of silica fume to the PCM caused chemical connection at the concrete–PCM interface and improved the interfacial performance.The polymer cement mortar (PCM) overlay method is a promising solution for strengthening deteriorated concrete structures in which the occurrence of premature debonding at the interfaces prevents the strengthened structures from achieving full serviceability. The purpose of this study is to improve the concrete–PCM interfacial bond to prevent premature debonding. There are two main focuses of this study: (i) investigation of the effectiveness of adding 5% silica fume to PCM in forming a chemical connection between concrete and PCM, based on a direct single-surface shear test using two roughness levels of concrete (smooth and rough) and microstructure analysis and (ii) performance evaluation of the bond between substrate concrete and a PCM overlay with/without silica fume at early ages and with different moisture conditions at the interface, based on a bi-surface shear test using rough substrate concrete surface. The inclusion of 5% silica fume with PCM caused an improvement in the interfacial strength , with a smooth concrete substrate surface where mechanical bonding had less influence. In addition, lower Ca/Si values in the interface of modified 5% silica PCM specimens compared to the normal PCM specimens quantified by energy-dispersive X-ray spectroscopy (EDS) indicate the formation of a chemical connection at the concrete–PCM interface by transforming harmful Ca(OH) Currently, existing reinforced concrete (RC) structures are at risk of deterioration due to exposure to severe environmental conditions, unseen mechanical loadings, natural disasters, etc. Repairing and strengthening these structures is necessary to extend their lifetime rather than replacing the structural elements. Several strengthening techniques based on practical experience and scientific research have been proposed over recent decades, such as continuous fibre sheet bonding, steel plate bonding, and fibre-reinforced polymer jacketing ,2,3. The2 (approximately 85%) of silica fume make the repair material highly reactive [2 and ettringite. In the presence of water, the silica compound of silica fume reacts with Ca(OH)2 to form more C-S-H [2 gathered at the interface and increases the interfacial performance by forming more C-S-H at the interface.The PCM overlay method often upgrades the flexural capacity and stiffness of strengthened RC structures ,22,23,24reactive . Despitereactive ,31 have reactive  stated tre C-S-H . This reWith this aim, in our previous study , 5% sili2/g and density of 3.14 g/cm3, crushed stone with a maximum size of 19 mm and true density of 2750 kg/cm3 as the coarse aggregate, and sand with 2.68 fineness modulus and true density of 2680 kg/cm3 as the fine aggregate were used. Poly carboxylic acid liquid with a density of 1.05 g/cm3 was used as a water reducer. The high-early-strength cement was used instead of normal Portland cement to facilitate the surface roughness preparation (smooth surface) used in this study, to reduce the influence of hydration reactant in preparing the specimen to study the moistness state of the interface described in Substrate concrete was fabricated by mixing water, cement, sand, crushed stone, and water reducer. A commercially manufactured high-early-strength cement having specific surface area of 4550 cm2 (28.54%), Al2O3 (4.52%), SO3 (2.51%), CaO (62.64%), and Fe2O3 (1.78%). This indicates that the PCM used in this study has a higher silica content than conventional cement .Polymers, in the form of polymer concrete, polymer-modified concrete, or polymer impregnated concrete, are used in the repair of structures because of their superior properties to those of ordinary Portland cement . PCM is As supplementary material, silica fume is largely used to increase mechanical properties and durability. In this study, commercially available silica fume with technical characteristic as presented in Primers have been widely used as adhesives to increase the bonding performance of interfaces in repair systems. In this study, a commercially available primer with technical characteristics as shown in The surface roughness of the concrete substrate is considered a key factor in the performance of bonding between substrate concrete and a new overlay layer. Considering ease and suitability, two roughness levels of concrete substrate (smooth and rough) were used in this study, as shown in The composite specimens were prepared by casting concrete and PCM on different dates with a 14-day interval between pouring two materials. The time difference was chosen to allow the prepared concrete using high-early-strength cement to attain its strength before being repaired, to resemble existing concrete in real infrastructure. Half of the concrete specimens were cast first, followed by pouring PCM onto the last half to prepare composite specimens of sizes 100 × 75 × 75 mm and 100 × 100 × 75 mm for direct single-surface shear testing and bi-surface shear testing, respectively. After 14 days curing of half of the concrete specimens, the treated surfaces were kept face up in the mould for the PCM application. The treated surfaces were cleaned with high-pressure air to remove dust before casting the PCM. To study the influence of the state of moistness of the interface, the composite specimens were fabricated with the moisture state of the substrate concrete interface as air dry (AD), saturated surface dry (SSD), or wet after 14 days curing of the substrate concrete, so that the concrete reached a hardened state and the moisture content within the concrete exerted very little influence on the hydration reactants.The AD state was achieved by placing the specimen in a natural dry environment in the basement of the laboratory for one week before pouring a new mortar layer. The SSD condition was achieved by immersing the specimens in water for 1 day, removing the samples from water, waiting for about 2 h, and wiping off the excess moisture (if any) on the surface with a towel before placing the PCM. The wet state was made by putting the substrate concrete in water for 3 days, removing the specimen from the water immediately before casting the PCM, and the overlay mortar was placed without wiping away surface water. The states of moistness of the interface used in this study are shown in Once the specimens were cast, the moulds were wrapped with a polythene sheet to avoid the evaporation of moisture. Twenty-four hours after casting, all the formworks were demoulded, and the specimens were cured in water for 7 days to help the hydration process followed by dry curing for 21 days. Curing under dry conditions benefits the formation of polymer films that can fill the voids and strengthen the PCM . The totA = the area of the connected interface (mm2).Many test methods to evaluate interfacial shear strength have been proposed in the past, as summarized by Santos . Saldanh2 at the interface. In XRD analysis, a continuous 2θ scan mode from 10° to 75° was applied with a step size of 0.02 and scan speed of 1.5°/min. In addition, TG-DTA of the powdery samples collected from the interface was also performed up to 1000 °C, using a platinum open-topped crucible in N2 gas dynamic atmosphere (60 mL/min rate), and a heating rate (20 °C/min) to quantitatively identify the amount of Ca(OH)2 and CaCO3 at the interface.SEM-EDS was employed to identify the interfacial layer and to precisely understand the chemical bonding of the interface. A rectangular parallelepiped of size 15 × 15 × 5 mm including substrate concrete, PCM overlay, and substrate concrete–PCM interface was cut to fit the size of the SEM chamber. The prepared samples were vacuumed for 7 days in a vacuum chamber to keep them dry. The specimen surface was coated with a platinum coating to enhance electrical conductivity and to get a high-quality interface image. XRD analysis of the samples (in the form of powder) collected from the interface after the loading tests was performed at 40 kV and 40 mA with a Cu-K alpha radiation source to identify the intensity of Ca(OH)The laboratory test data of interfacial strength were further analysed statistically using one-way analysis of variance (ANOVA) to evaluate the influence of silica fume as a repair mortar and the impact of different influencing factors. The relevant calculation principles and formulae are presented in the literature . The sigSEM-EDS (JSM-6510LA) was employed to explore the microstructure of the interfacial layer of the composite specimens. The SEM images were further analysed using EDS attached in the SEM to precisely identify the interface and for quantitative elemental analysis. The EDS uses an X-ray source for the identification and quantification of the elements present at detectable concentrations. The prepared sample for this microstructure analysis contains both concrete and PCM parts with different elemental compositions. Therefore, EDS results of particular elements’ distribution over a selected area help to identify the interface precisely as the EDS technique is capable of producing relative elemental distribution maps using a different-colour index, as presented in Once the three phases of the composite specimen were confirmed, the interface was further analysed to record the elemental information using EDS, which can detect elements with concentrations up to 0.1 wt%. In this study, EDS was performed to determine the elemental composition of the interface layer by selecting points along the bond line (interface layer). Ten different points were selected along the interface for the elemental analysis, as presented in The Ca to Si (C/S) ratio is an indirect measure of C-S-H and calcium hydroxide (CH). A high C/S ratio implies that the specimen has high CH and low C-S-H, and vice versa . Interfa2 phase. The XRD results of the modified 5% silica PCM and normal PCM specimen presented in 2 [2 was found in the modified 5% silica specimen compared to the normal PCM specimen  to identify Ca(OH)ted in 2 . Lower pspecimen . Conclus3 (third step) [rd step) ,47. The 2 and CaCO3 in the samples were calculated by multiplying the molar mass ratios of Ca(OH)2/H2O and CaCO3/CO2 by the corresponding mass loss obtained in the second and third steps of the TG curves 2 (modified 5% silica PCM cases) compared to the bond formation in without silica fume  cases.As mentioned in the literature , the masG curves , and areThe average strength of the three specimens used in each group was recorded as the interfacial shear strength under specific conditions, and is presented with the standard deviation (SD) in p-value of 0.023 and 0.039 for the smooth and rough surface, and degree of influence “b”), as presented in 2 with pozzolans. An increase in the interfacial strength even with the smooth surface, where mechanical bonding had less influence, indicates a higher possibility of causing a chemical reaction at the concrete–PCM interface with silica fume addition that subsequently results in higher interfacial strength. On the other hand, in the case where primer was used at the interface, only a slight variation in the average interfacial strength for the normal PCM and modified 5% silica PCM (in both cases of surface roughness) was observed, as presented in p-value of 0.686 and 0.776 for the smooth and rough surface, and degree of influence “d”) also confirm that in the case where primer was used at the interface, silica fume inclusion had no effect. The use of primer at the interface results in a protective layer that prevents the small silica fume particles from filling the gaps among the cement particles of the substrate and hinders the flow of the water required for hydration of the cured concrete. It indicates that silica fume cannot perform in the expected way to strengthen the interface by the micro-filling effect or by chemical connection. Considering the negative impact of primers on the environment and the positive influence of silica fume (no primer cases) for enhancing interface bonding, in practical PCM strengthening applications, the inclusion of silica fume in the overlay material is highly suggested.The addition of 5% silica fume to the PCM in the overlay material greatly increased the interfacial strength compared to the normal PCM in both surface roughness levels when no primer was used at the interface, as shown in Many available reports ,51,52 haIn the repair system, the composite specimens are considered to have three phases/layers, including a concrete substrate cohesive layer, a repair material cohesive layer and an interface between them, termed the adhesive layer. The fracture modes are named according to the location of the fracture on the surface of the composite specimens. A fracture mode was named concrete cohesion (C) or PCM cohesion (P) when a fracture occurred either in the concrete substrate or in the PCM part, while an interface fracture mode (I) was named so when a fracture occurred purely through the interface, with or without tiny cracks in the concrete or PCM. Some fracture modes were categorized as composite fracture modes that include partial concrete and partial adhesive (I-C) fracture modes, in which some amount of concrete substrate or coarse aggregate was attached on the PCM side; partial PCM and partial adhesive (I-P) fracture modes, when some PCM was attached to the concrete substrate; and mixed concrete and PCM (C-P) fracture modes, where the fracture occurs in both the concrete and the PCM layer. The possible fracture modes of the composite specimens are shown in Visual observations were made carefully during and after the tests of the composite specimens to determine whether the specimens failed along with the shear plane (interface) or if the fracture was due to significant cracking in the overlay material or concrete substrate. Examples of adhesive and composite fracture modes of the composite specimens observed in this phase of the study are shown in The observed fracture modes of all the composite specimens in the direct single surface shear test, along with the corresponding interfacial shear strengths are shown in 2 in the PCM and Ca(OH)2 in the concrete to form secondary C-S-H. Because of the secondary chemical reactions of Ca(OH)2 and pozzolans, the microstructure of the interface can improve with time, leading to the creation of a denser interfacial zone with better durability. Consequently, mixing silica fume with PCM enhances the interfacial performance and shifts the fracture mode from pure interfacial to composite or mixed-mode fracture.The compressive strengths of normal PCM and modified 5% silica PCM were evaluated using cylindrical specimens according to the standard test method ASTM C39 . The inf2  in PCM, the silica compounds can react with Ca(OH)2 produced in the early-age hydration stage and increase its strength, causing a marginal strength difference between normal PCM and modified 5% silica PCM at the initial stage. This gain in strength became pronounced for the modified 5% silica PCM cases beyond seven days. A large gain in strength was achieved between the ages of 7 and 28 days: about 64% in modified 5% silica PCM mortar and about 40% in the normal PCM overlay mortar. The results of the statistical analysis show ‘no or very little’ significance between the ages of 1 and 3 days, which shifted to a significant and very significant level at ages of 7 days and 28 days, respectively. These results are presented with p-values and degrees of influence in 2 during the early hydration stage seems to contribute to the higher strength of modified 5% silica PCM mortar. In conclusion, the addition of 5% silica fume significantly increased the compressive strength of the monolithic specimens with time, though it had only a marginal effect at early ages.The strength gain in the modified 5% silica PCM cases was slightly higher compared to normal PCM cases, though it was considered insignificant up to 3 days of curing. There is a possibility that due to presence of higher SiOThe average interfacial tensile strengths of the three specimens, after excluding data when the difference between any measured value and the mean value exceeded 20% of the mean value, are shown in 2 of the substrate concrete and SiO2 of silica PCM of the overlay mortar to form secondary C-S-H. Additionally, silica particles can fit between cement particles and inside pores on the surface of the concrete substrate, resulting in a denser microstructure with higher intermolecular forces and mechanical interlocking. Consequently, the bond strength increased significantly with the inclusion of silica fume from the very first day of pouring the overlay mortar. The addition of silica fume to PCM is strongly suggested for practical PCM strengthening applications to withstand the induced stress by the continuous traffic movement during and after the construction work from the early ages.The outcomes of statistical analysis showed a positive influence of silica fume on interface performances from young ages of casting. The bonding improvement can be attributed to the chemical reactions between Ca(OH)After the mechanical testing, visual observations were made to determine the failure modes of the composite specimens: whether they failed due to cracks along the interface (shear plane) or due to significant cracking in the overlay material or substrate concrete. The failure modes were named according to the classification of the fracture surface as discussed in 2 to form secondary C-S-H that leads to a good bond between substrate concrete and the overlay mortar. The AD state of the substrate may accelerate water migration from repair mortar, leading to incomplete hydration near the interface and less bonding between substrate concrete and overlay mortar. The very small amount of water present in the AD state helps in the formation of fewer C-H-S at the interface, which does not satisfy the interface strengthening requirement, resulting in less bonding strength than SSD condition. The wet condition of the interface resulted in the lowest interfacial strength in all cases. The presence of excessive moisture at the interface in the wet moisture state cases may have clogged the pores of the substrate concrete and prevented absorption of new material, resulting in weaker bond strength. The bond strength in the SSD state of normal PCM was 24% and 4% higher than that of wet state at 14 and 28 days, respectively. The outcome of statistical analysis showed there is no or very little influence of the moistness of the interface on the bonding strength. The same phenomenon was also observed in a previous study [us study  in the cx) obtained using F distribution at 1% (F0.01), 5% (F0.05), and 10% (F0.1) significance levels to determine the level of significance of the factor in question. Following the significant criteria shown in A typical example of calculation and analysis of the relativity of the impact factors of the bond interface between substrate concrete and overlay layer using one way ANOVA is presented in The influence of silica fume on the interfacial shear strength of the composite specimen under different moisture states of the substrate concrete interface is shown in In the absence of primer at the substrate concrete interface: (i) The effect of the constituent of the overlay material attached to the concrete substrate on the bond strength is significant. The PCM modified with 5% silica composite specimen showed higher interfacial shear strength than the normal PCM composite specimens. The inclusion of silica fume significantly increased the interfacial shear strength compared to the normal PCM for smooth and rough concrete surfaces. (ii) Surface preparation (texture) significantly affects the bond strength and a better bond performance under shear stress conditions was acquired with a rough interfacial surface. The interfacial shear strength increased with increasing surface roughness for both the normal PCM and PCM modified with 5% silica cases. (iii) The mixing of silica fume in the PCM tended to shift the pure interfacial fracture mode (I) to a composite fracture (I-P) or (I-C) or mixed fracture (C-P) mode due to higher adhesion of modified PCM overlay with substrate concrete.With a primed substrate concrete surface, the interfacial bond performance does not depend on the inclusion of silica fume with PCM as the primer forms a protective layer and hinders the functionality of silica fume.2 during the early hydration stage.There is a high possibility that the bond strength increases significantly with the inclusion of silica fume from the very first day of pouring of overlay mortar due to the predominant reaction of silica compounds with Ca(OH)The interfacial strength is predominantly influenced by the moistness of the substrate concrete surface, the saturated surface dry interface state of substrate concrete facilitates bond strength development. Interface moisture state exerted a positive influence on the modified 5% silica PCM-concrete bonding performance, while it had no/insignificant impact on the normal PCM-concrete interface.2 into a large amount of C-S-H, which indicates the possibility of formation of chemical connections at the modified 5% silica PCM-concrete interface.The lower C/S ratio observed in the microscopic SEM-EDS test at the modified 5% silica PCM-concrete interface implies high C-S-H content, resulting in high strength. This is mainly due to the transformation of harmful Ca(OH)2 content observed qualitatively through XRD analysis and quantitatively through TG-DTA at the modified 5% silica PCM-concrete interface compared to the normal PCM-concrete interface. This suggests an increase in the extend of bond formation between silica compounds and free Ca(OH)2 (modified 5% silica PCM cases) compared to the bond formation in the without-silica fume  cases, thus contributing to the improvement of the interfacial performance in former cases.A decrease in the Ca(OH)The following conclusions can be drawn from this experimental investigation:Based on the results of the experimental program, statistical analysis and microscopic test, it can be concluded that the use of silica fume can achieve adequate bond strength with concretes substrate and can cause a chemical reaction at the concrete–PCM interface to enhance the chemical bonding between the concrete substrate and the overlay material. Considering the easy applicability of silica fume with PCM in practice, the environmentally friendly nature, and the ability to achieve adequate bond strength with concrete substrates (without primer), this study provides an evidence for the engineering application of silica fume in polymer cement-based repair materials.


D/IG ratio obtained from Raman spectra of the CNTs decreased to the lowest value of 0.84 after microwave treatment at 1400 °C, implying a high degree of carbon order. The presence of Fe and trace amounts of other elements were confirmed by the energy-dispersive X-ray spectrometer (EDS) and were postulated to have catalyzed the growth of CNTs. The mechanism of the SL-CNTs growth under microwave treatment was proposed and discussed.This study reports a novel method for synthesizing super-long carbon nanotubes (SL-CNTs) from cellulose via a microwave treatment process without an external catalyst. CNTs with a length of 0.7–2 mm were obtained via microwave treatment of cellulose biochar temperatures of 1200–1400 °C. Scanning electron microscope (SEM), together with high-resolution transmission electron microscope (HRTEM) results, were used to investigate the changes in the length and morphology of CNTs with respect to treatment temperature. The morphology of CNTs changed from twisted, curved, and threadlike to straight structures. The average length of CNTs after microwave pyrolysis at 600 °C was approximately 600–1800 nm, which after microwave treatment at 1300 °C and 1400 °C increased to about 1–2 mm. X-ray diffractometer (XRD) results confirmed the crystalline structure of CNTs with two prominent peaks at 2θ = 26.3° and 2θ = 43.2° correlating with the graphite (002) and (100) reflections. The I After their discovery in 1991 . The useCarbon nanotubes have been successfully synthesized using various methods, such as laser ablation , chemicaSuper-long carbon nanotubes (SL-CNTs) have gained increasing attention for numerous plausible applications, including sensors ,20, nano2 wafer, Fe catalyst, at a pressure of 1 atm, with helium/H2 (2.5:1 v/v) as the carrier gas. The carbon source was high-purity (99.99%) ethylene. SL-CNTs were synthesized at 700 °C for about 10 h. The CNTs were generated due to the super-saturation of a carbon-metal solution, which induced carbon segregation leading to CNT formation from the catalyst surface. Loiseau and Pascard [2O3 catalyst. They proposed the root growth mechanism based on the carbon source and real-time photography analysis and observed that the length of CNTs was time and temperature dependent. Ghemes et al. [The synthesis of SL-CNTs remains a challenge. There have been some attempts by other researchers to produce SL-CNTs. Chakrabati et al. ,28 synth Pascard  used the Pascard  used theDespite the recent achievements in the synthesis of SL-CNTs, the cost remains high due to the complexity of the synthesis methods. Moreover, methods of synthesis of SL-CNTs are not sustainable due to the use of carbon source gases derived from fossil fuels. Further, there is no report on the production of super-long carbon nanotubes (SL-CNTs) from biomass. Biomass is a natural carbon energy source and has gained increasing attention as a precursor  to synthCellulose derived from palm kernel shell (PKS) biomass was used as the feedstock in this study. PKS, purchased from Malaysia, was ground into a size fraction of 150–280 µm and subjected to the cellulose isolation method. The palm kernel shell sample was composed of 33.04 wt.% cellulose, 45.59 wt.% lignin, 23.82 wt.% hemicellulose, and extractives 9.89 wt.% . The che2 atmosphere. Microwave pyrolysis runs were carried out for 30 min. After the pyrolysis, the cellulose char, which contained CNTs, was collected after separation from the AC by sieving.In the first step, CNTs were synthesized from the cellulose sample under fast microwave pyrolysis. Cellulose pyrolysis was conducted in a temperature-programmed microwave oven , operating at a frequency of 2.45 GHz and maximum power output of 2000 W. The cellulose sample was mixed with a microwave receptor (activated carbon) at the ratio of 10:2 to reach the desired pyrolysis temperatures of 600 °C. The mixture of biomass and AC was loaded into a quartz reactor and placed in the microwave oven. A thermocouple was placed inside the reactor to monitor the sample temperature at 2 s intervals. High purity nitrogen gas (99.999%) at 400 mL/min flow rate was purged through the system for at least 30 min to eliminate air before experiments in a N2 atmosphere. Microwave treatment runs were conducted for 30 min at the desired temperatures of 1200 °C, 1300 °C, and 1400 °C.In the second synthesis step, high-temperature microwave treatment was used to obtain SL-CNTs. About 5 g of cellulose char was loaded in a ceramic crucible and placed into a larger custom-made quartz reactor (high-temperature resistant). A high-temperature thermocouple was placed inside the reactor to monitor the sample temperature. At 400 mL/min, nitrogen gas (99.999%) was purged through the system for at least 20 min to eliminate air before experiments in the Niw and sw represent refined weight portions of phase I and the internal standard, respectively, i,cw symbolize the original weight portions of phase i, and s,ww depicts the initial weight of the internal standard. The composition of the inorganic matter in cellulose was analyzed using X-ray fluorescence (XRF) , and the results are shown in −1 with an acquisition time of 3 min.The surface morphology of CNTs was studied using scanning electron microscopy . The image measurement was obtained using an in-lens detector at an energy of 2 kV to achieve a clear image . The CNTs were also analyzed with a high-resolution transmission electron microscope (HRTEM) (Fei Tecnai F20) equipped with energy-dispersive X-ray spectroscopy (EDS). The system was set at an accelerating voltage of 120 kV, and images were acquired digitally by a high-resolution camera. The obtained images were processed with digital micrograph ,37. The The morphology of the SL-CNTs after microwave treatment is presented in The EDS results of the SL-CNTs at 1400 °C d revealeThe HRTEM analysis of the SL-CNTs after microwave treatment at 1200 °C and 1300 °C is shown in d-spacing value of 0.32 nm was calculated for CNTs, slightly lower than that of natural graphite (0.335 nm). The typical peaks for CNTs in XRD spectra are generally detected at 2θ = 26° and 2θ = 43°, which illustrate the graphite (002) and (100) reflections, respectively . Figure To quantify the yield of SL-CNTs, the crystallinity of biochar samples was analyzed using Rietveld refinement quantitative analysis. The approach reported in  was used−1 corresponds to the stretching mode in graphite planes associated with the sp2 bond. The D band at about 1360 cm−1 correlates with disorder and defects in highly ordered carbonaceous materials or graphite crystals [D/IGI) is a significant parameter for studying graphite-like or crystalline carbon structures. It indicates the degree of carbon order; a high ratio indicates a high defect in the carbon structure, while a lower ratio signifies a high carbon order and lower defect in the carbon structure [D/IGI ratio is commonly used to evaluate the degree of graphitization in carbon materials like CNTs. The D/IGI ratio of CNTs decreased from 0.97 at 1200 °C to 0.87 at 1300 °C, finally to 0.84 at 1400 °C. These results show that the carbon order and graphitization degree of SL-CNTs increased at elevated temperatures.Raman spectra of the CNTs before and after high-temperature treatment is shown in crystals ,45. As scrystals . The D btructure . The ID/−1 (R1), 1635 (R2) cm−1, and 1515 cm−1 (R3) were considered. Aromatic carbon, aromatic ethers, and alkyl carbon bonds are linked to the R1 band [To better understand the alteration in the microstructure of CNTs after microwave treatment, further analysis was carried out by deconvoluting the Raman spectra of the SL-CNTs into five bands by adopting the Gaussian function method. The additional bands at 1196 cm R1 band . The R2  R1 band . The areR2+IR3I/DI ratio studied the formation of aromatic clusters during the growth of SL-CNTs. The amorphous carbon clusters containing 3–5 aromatic rings are represented by R2+IR3I, while DI reflects aromatic cluster development with more than six rings [R2+IR3/IDI ratio of SL-CNTs decreased from 0.89 at 1200 °C to 0.70 at 1400 °C, indicating the transformation of small aromatic systems to larger aromatic clusters, which in turn increased the carbon order of SL-CNTs. The monotonous increase in the G/IAllI ratio with temperature suggested that the graphitic structure of SL-CNTs was enhanced at higher temperatures. The R1+IR3/IGI ratio of SL-CNTs decreased from 1.60 at 1200 °C to 0.42 at 1400 °C. These results imply the removal of functional groups and the development of more ordered structures in SL-CNTs.The integrated band area ratios were defined and used to better understand the changes in the carbon structure of SL-CNTs, and the results are shown in ix rings . Figure −1 increased with temperature. This band is known as the overtone of the D band in the second-order Raman spectra. The shape, intensity, and frequency of the 2D band are affected by the chemical structure of CNTs and the number of carbon layers in the CNTs. The 2D/IGI values of less than one in the SL-CNTs . The volatiles left in the char were high molecular weight compounds that failed to generate enough vapor pressure to be released from the biomass . These vThe change in the morphology of CNTs from curvy to straight may be attributed to the structural modification at high temperatures. Mobility of carbon atoms leads to re-orientating the carbon structure, and changes in its physical properties can occur at elevated temperatures ,57. In aAlthough carbon nanotubes are being used widely in various technological fields, they still have some significant drawbacks that hinder their full potential due to their small dimensions and the lifespan of added catalysts. Previously, SL-CNT formation was dependent on the catalyst’s lifetime. The electrical and mechanical properties of CNTs can only be harnessed if the CNTs are long and continuous for some applications. This novel method of producing SL-CNTs is simple, safe, and environmentally friendly and an ethical approach towards synthesizing cleaner, high purity, and low-cost SL-CNTs from biomass without the need for an additional catalyst or carbon source under microwave treatment. Therefore, the method can provide a convenient means of producing selective CNT morphology, leading to the mass production of SL-CNTs on an economic basis. SL-CNTs pave the way for novel opportunities for capitalizing on CNT properties over large length scales. The SL-CNTs formed are more durable and possess strong electrical and mechanical properties, making them valuable for an increasingly large number of applications in the field of nanotechnology, material science, and even modern-day living. The advantages of SL-CNTs over their shorter counterparts include enhanced suitability for electrical conduction, multifunctional composites, sensors, and nanoelectronics. SL-CNTs offer a combination of flexibility, electrochemical reactivity, strength, electrical conductivity, and porosity, which will pave the way for a new era of technological advancement.D/IG ratio of 0.84 after microwave treatment at 1400 °C was obtained, indicating an increase in the carbon order in the SL-CNTs. The yield of SL-CNTs reached 26.89 wt.% at 1400 °C. The inherent inorganic matter in biomass was postulated to have functioned as a catalyst to facilitate the growth of the SL-CNTs. The carbon supply source for the SL-CNTs under microwave treatment was the aromatic hydrocarbons generated from the biochar. The change in the morphology and structure of CNTs after microwave treatment can be used to control the quality and structure of CNTs from cellulosic biomass. The method of SL-CNT synthesis presented here is a simple but sustainable method for the synthesis of nanotubes from renewable biomass without the use of any catalyst or external carbon source. Hence, this is a promising approach for large scale production of superlong carbon nanotubes that should find applications in various industries.Super-long carbon nanotubes (SL-CNTs) with lengths of up to 2 mm were successfully synthesized from cellulose biomass via microwave treatment without any catalyst or external carbon source. A low Raman I