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RESEARCH PRODUCT

Influence of aluminates phases hydration in presence of calcium sulfate on silicates phases hydration : consequences on cements optimum sulfate

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The mechanical properties of hydrated cements need to be optimized according to the nature of cement products. Among the factors of optimization, the addition of calcium sulphate intended to regulate the reactivity of tricalcium aluminate (C3A), in quantity and quality in cement is an essential parameter.The advancement of knowledge on the mechanisms of hydration of each cement phase allows a study devoted entirely to the concept of optimum of sulphate. We studied simple system that we complicated and we studied the evolution of hydration, porosity and mechanical properties of cement at different age. The first system studied was the mixture C3S/gypsum, the objective was to determine whether there was an optimal effect of calcium sulfate on hydration and mechanical strength of C3S as presented in the literature [1]. The results showed there was not optimum sulfate in the C3S/gypsum system but there was a specific effect of calcium sulfate on the hydration and the mechanical properties of C3S. The sulfate adsorption on the C-S-H surface is the cause of the change nucleation and growth process of C-S-H. This has resulted in increasing hydration degree of C3S and compressive strength. The second system studied was biphasic clinker C3S/C3A ground with hemi-hydrate and gypsum. An optimum sulfate has been observed, which move out with time to high rate of sulfate. The optimum sulfate was observed when the hydration of C3S, during the accelerated period, takes place simultaneously or slightly before the exothermic peak due to the high dissolution of C3A and precipitation of AFm. We showed the presence of AFm during the accelerated hydration of C3S, is the cause of the microstructure modification in the cement paste: the porosity increases with calcium sulfate addition but the hydrate assembly is more dense.