HEXAVALENT CHROMIUM LEACHING FROM CEMENTS AND CEMENT COMPOSITES
Chromium is one of the 25 most widespread elements in the Earth’s crust and occurs as an indelible non-volatile trace element in raw materials used in cement clinker production (clay, limestone and iron additives in particular). Chromium in raw materials mentioned above is mainly in form of trivalent chromium. Naturally occurring chromium (III) is not initially harmful, since it is chemically stable. Only at high temperatures found in cement rotary kilns, inert trivalent chromium oxidizes to form more dangerous hexavalent chromium. The paper presents the results of the experimental investigation of the hexavalant chromium releasing from cement samples and cement composites to the water environment under various conditions. The samples of Portland cement as well as the cement composites with different composition were used for the experiments. The chemical composition of cement samples and cement composites were determined by using X-ray fluorescence analysis (XRF); the soluble hexavalent chromium concentrations in cement leachates and cement composites leachates were analysed by colorimetric analysis. The determination of chromium (VI) by spectrometry was based on the reaction of hexavalent chromium with diphenylcarbazide by forming the purple coloured compound. The content of water soluble chromium was measured to be lower in cement composites leachates than in cement leachates. The soluble hexavalent chromium concentrations in cement composites were measured from 0.13 to 0.22 mg/kg and 0.07 to 0.15 mg/kg, when studied powdered and compact cement composites leachates, respectively. The concentration of Cr (VI) in cement leachates reached the average value of 2.85 mg/kg.
Determination of hexavalent chromium in South African cements and cement-related materials with electrothermal atomic absorption spectrometry
This paper seeks to develop a method to determine the total chromium (VI) in cement and cement-related materials, with the particular aim of ensuring that no changes take place in the original state of chromium owing to the sample preparation procedure. The selective extraction of chromium (VI) in a cement matrix, based on treatment with 0.1 M sodium carbonate (Na2CO3) solution and subsequent determination of chromium by electrothermal atomic absorption spectrometry has proposed and applied to South African cement clinkers, natural gypsum, limestone and certified reference materials. Between 30% and 80% of the total chromium in the cement clinkers are Cr(VI) compounds and 8-26% of the total amount of Cr(VI) species is water soluble. This disproves the general perception that all Cr(VI) in cement and cement-related materials is water soluble. Results from an analysis of a sulphate-resisting Portland cement compared with the certified values were used to verify the analytical performance of the proposed method.
Degradable polymers : principles and applications
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