Hydrothermal synthesis and characterization of Na+ and [Al3+ + Na+]-substituted tobermorite in CaO-SiO2·nH2O-H2O System

Abstract

The influence of Na+ and [Na+ + Al3+] additives on the formation of 1.13 nm tobermorite in the environment of saturated steam at a temperature of 175 °C was determined in this paper. The composition of initial mixtures corresponded to the molar ratios of CaO/SiO2 = 0.83 (C/S = 0.83) or CaO/(SiO2 + Al2O3) = 0.83 (C/(S + A) = 0.83) and Al2O3/(SiO2 + Al2O3) = = 0.025 (A/(S + A) = 0.025) when amorphous silica was used as an SiO2 component. The amount of NaOH additive corresponding to 1, 3, 5, 7 or 10 % Na2O from the mass of dry materials was added. Isothermal curing durations were as follows: 2, 4, 8, 16, 24 hours. According to the obtained results the NaOH additive stabilises semi-amorphous calcium silicate hydrate C-S-H(I) and retards its crystallisation into tobermorite. A bigger impact on hydrothermal reactions is made by an additive of γ-Al2O3, which considerably speeds up the formation of tobermorite, whereas a complex additive of [Al3++Na+] positively impacts the formation and crystallinity of 1.13 nm tobermorite when the durations of synthesis are short. Due to Al3+ insertion into a crystal lattice, 1.13 nm tobermorite of a mixed thermal type forms in stirred suspensions, and [Al3+ + Na+] insertion creates conditions for the formation of anomalous tobermorite.