Formation and thermal stability of calcium silicate hydrate substituted with Al3+ ions in the mixtures with CaO/SiO2 = 1.5

Abstract

The influence of aluminum additive on the formation mechanism and thermal stability of calcium silicate hydrates was determined. Calcium silicates hydrates were synthesized in the primary mixtures with the molar ratios of CaO/(SiO2 + Al2O3) = 1.5 and Al2O3/(SiO2 + Al2O3) = 0.025 or 0.05. The hydrothermal synthesis has been carried out in unstirred suspensions under saturated steam pressure in argon atmosphere at 200 °C temperature for 4, 8, 16, 24, 48 and 72 h by applying extra argon gas (10 bar). It was determined that Al2O3 additives have a significant influence on the formation mechanism of synthesis products as well as their stability during the isothermal curing. At the beginning of the reaction, this additive retarded the formation of calcium silicate hydrates but stimulated the crystallization of CASH and C2S which remained stable under all experimental conditions. The thermodynamic calculations, in the mixtures with Al2O3/(SiO2 + Al2O3) = 0.025 molar ratio, showed that the greatest possibility that C-S-H(I) and C2S with incorporated Al3+ ions in the structure will crystallize, because the obtained Gibbs free energy value is the lowest (\(\Delta_{\rm r} G_{\rm T}^{0}\) = −80.3 kJ and \(\Delta_{\rm r} G_{\rm T}^{0}\) = −104.5 kJ). Meanwhile, in the mixtures with larger amount of aluminum, there is the greatest possibility that 4CaO·Al2O3·9.5H2O and 4CaO·Al2O3·6.5H2O will be formed. The obtained results were confirmed by the thermodynamic calculations and instrumental analysis data.