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.