a foaming agent was prepared from anticoagulated ox blood . the anticoagulant was fibrisol ™ formed as an aqueous solution of 10 parts fibrisol ™ to 100 parts water . the anticoagulant was added to the ox blood with stirring . the anticoagulated ox blood was placed in a 40 litre pressure vessel with a 2 . 5 % by volume of the mixture of a 50 % aqueous sodium hydroxide solution . the blood and sodium hydroxide solution was then boiled under a pressure of 103 kpa for 3 hours . the pressure was released and the solution allowed to cool naturally to room temperature . the blood and sodium hydroxide solution was then neutralised using 2 . 5 % by volume of the mixture of a 40 % aqueous hydrochloric acid solution , the blood , sodium hydroxide and hydrochloric acid were boiled under pressure for a further 3 hours . the pressure was released and the resulting solution allowed to cool naturally to room temperature . the solution was stabilised using a 2 . 5 % by volume aqueous solution of ferrous sulfate ( iron ( ii ) sulfate ), the stabilised solution was heated under pressure for 30 minutes and then allowed to cool naturally to room temperature . finally , the foaming agent ( i . e . the product of the previous processing steps ) was concentrated by boiling in an open vessel to remove water until the volume had reduced to 50 % of the volume prior to the commencement of the concentration step . this required boiling for 4 - 5 hours . the foaming agent of example 1 was used to prepare a concrete replacement using the apparatus of the specific embodiment of gb 2 330 086 . a 4 . 04 : 1 water to foaming agent solution was foamed and mixed with sharp sand ( 30 wt % water ) and 100 % portland cement ( opc ). the apparatus was operated at a speed of 10 . 04 kg / minute of sand and cement . the product density was optimised as shown in example 3 , a concrete replacement product of density 1810 kg / m 3 was obtained by feeding the water and foaming agent to the foaming inlet using a water flow rate of 3 . 49 l / minute and a foaming agent flow rate of 0 . 00318 l / minute ( a foaming agent solution flow rate of 0 . 00967 l / minute and a percentage foaming agent of 24 . 72 wt %). the flow rate of the foam into the mixing chamber was optimised to 0 . 2 l / minute in order to obtain a concrete replacement of density 1810 kg / m 3 . the density of the concrete replacements produced using the process of example 1 was analysed . altering the flow rates for the water and foaming agent used to form the foaming agent solution , and of the resulting foam ( the foamed foaming agent solution ) was found to provide products of different densities , as shown in table 1 below . water flow rate was adjusted to ensure that the total water level in the concrete replacement product includes the water present in the sharp sand . the water content of the concrete replacement product of this example is 35 wt %, the flow rate of water was adjusted for the amount of water present in the sand , from 3 . 5 l / minute ( where the sand is dry ) to the flow rates shown below . fig1 shows the relationship between the flow rates of the foaming agent which form the foaming agent solution . fig2 shows the relationship between the flow rate of the foaming solution once foamed and fig3 shows the relationship between the density of the concrete replacement product and the water flow . the strength - to - weight ratio was determined for the concrete replacement product of example 3 at four densities . this was compared to strength - to - weight ratios derived from literature densities and strengths of commercially available concretes . the values are provided below : the average strength - to - weight ratio of the concrete of example 3 was 1 . 40 %, the average for the known concretes was 1 . 15 %. as can be seen the performance of the inventive concrete replacements is generally superior to that of known concretes . fig4 to 7 show the fine structure of the cement material as including macropores and micropores in combination .