Hydration peculiarities of high basicity calcium silicate hydrate samples

Abstract

The influence of aluminum additive on the thermal stability and hydration peculiarities of calcium silicate hydrates were determined. Calcium silicate hydrates were synthesized in the primary mixtures with the molar ratios of CaO/(SiO2 + Al2O3) = 1.5 and Al2O3/(SiO2 + Al2O3) = 0 or 0.05. The hydrothermal synthesis has been carried out in unstirred suspensions under saturated steam pressure in argon atmosphere at 175 °C temperature for 24 h by applying extra argon gas (10 bar). It was estimated that after 24 h of synthesis, the intensities of diffraction peaks and the quantity of α-C2SH were close, suggesting that Al2O3 additive have no influence on the quantity of formed α-C2SH. It was determined that Al2O3 additive changes not only the sequence of the formed compounds, but also their stability during the calcination in the temperature range 100–1000 °C. It was estimated that pure sample and sample with Al2O3 additive are stable till 400 °C. Meanwhile, in the higher temperature of calcination (450 °C), both samples fully recrystallized to anhydrous C2S phases: x-C2S and calcium olivine; at the same time, the crystallinity of sample with Al2O3 additive was 1.37 times higher. The microcalorimetry and the crystallinity studies show strong direct dependence of α-C2SH samples reactivity on the level of its amorphization. The best results were obtained after calcination at 450 °C, intensive exothermic effect with the maximum (~4.25 mW g–1), appeared after ~4.5 h, and cumulative heat after 70 h–188.92 J g–1. The addition of Al2O3 during synthesis did not influence significantly the initial hydration stage: the first exothermic peak was found to be analogous to that of pure system. However, in further heat flow evolution, the two-step hydration process was observed: The first weak peak (~1.0 W g–1) appeared after ~4 h of hydration and was followed by second acceleration of reaction rate after ~5.5 h. The obtained results were confirmed by the simultaneous thermal analysis, microcalorimetry, and X-ray diffraction analysis.