The influence of γ-Al₂O₃ and Na₂O on the formation of gyrolite in the stirring suspension

Abstract

Influence of γ-Al₂O₃, Na₂O and hydrothermal synthesis parameters on the gyrolite formation process was established at 200 °C. The molar ratios of primary mixtures were CaO/(SiO₂ + Al₂O₃) = 0.66 and Al₂O₃/(SiO₂ + Al₂O₃) = 0 or 0.025. The amount of NaOH, corresponding to 5% of Na₂O from the mass of dry materials, added in the form of solution. It was determined that in the stirred suspensions under favourable synthesis conditions (active SiO₂ component, dispersive starting materials) gyrolite already forms after 16 h of isothermal curing at 200°C. The stirring affects the sequence of intermediate compounds: gyrolite crystallizes together with Z-phase. γ-Al₂O₃, as the additive in gyrolite synthesis is not usable, because it stimulates other calcium silicate hydrates formation. Na₂O additive positively affects gyrolite synthesis because the duration of gyrolite synthesis shortens almost 2–3 times (6 h, 200°C) to compare with pure mixtures. However, gyrolite is not stable in the mixture with Na₂O and begins to turn into pectolite already after 16 h of isothermal curing. Moreover, the presence of (Na₂O + γ-Al₂O₃) additive stabilizes gyrolite and prevents its transformation to pectolite. Obtained results were confirmed by XRD, DSC and SEM/EDS analysis methods.