Use of synthetic calcium hydrosilicate in Portland cement compositions

Abstract

We investigated the patterns of the synthesis of calcium hydrosilicate of tobermorite composition in the "amorphous silica–calcium hydroxide–water" system. The gradual interaction of calcium hydroxide and amorphous silicon(IV) oxide leading to the formation of synthetic tobermorite was confirmed by X-ray diffraction analysis. It was determined that the optimal duration of the synthesis of calcium hydrosilicate was 5 days, which, according to electron microscopic studies, ensured the production of grains with a size of 0.4–4 m. The influence of the synthesized tobermorite on the change in strength characteristics of cement stone during setting was studied using cement-sand mortar mixtures with sands of different particle size distribution. A comparative analysis of the effect of storage conditions of the synthesized product on the structure-forming processes of cement stone was carried out. It was established that the modifying effect of synthetic tobermorite was observed when it was dried after synthesis and stored in a dry state. Conversely, prolonged storage of calcium hydrosilicate in a wet state led to a loss of its activity and a decrease in the strength characteristics of cement stone. The mechanism of the influence of calcium hydrosilicate additive was studied by measuring the change in the rate of heat release of cement dough at the initial stage of hydration. It was shown that the addition of dry calcium hydrosilicate accelerated the physicochemical processes of hydration of clinker minerals, while the addition stored in wet conditions slowed down hydration. Electron microscopic studies of cement stone after 28 days of setting confirmed that the introduction of dry calcium hydrosilicate contributed to the formation of a structure characterized by a higher degree of crystallization and an increase in the strength of the samples.