The effects of calcium hydroxide and activator chemistry on alkali-activated metakaolin pastes

The impact of calcium hydroxide as an additive in sodium hydroxide- and silicate-activated metakaolin pastes is presented with a primary aim to ascertain the specific effects of calcium on the reaction kinetics and phase formation. FTIR and XRD are employed to understand the interaction of calcium hydroxide (10 wt% replacement of metakaolin) in 1, 5 and 10 M NaOH-activated pastes (and equivalent molarity Na₂SiO₃-activated pastes), with a focus on its impact on the nanoscale properties of the sodium-alumino-silicate-hydrate (N-A-S-(H)) gel. In situ FTIR and ICC are used to assess the impact of calcium hydroxide on the reaction kinetics, including the rate of metakaolin dissolution and associated rate of N-A-S-(H) gel growth. It is found that calcium hydroxide is effective at accelerating the reaction kinetics of 5 M Na₂SiO₃-activated metakaolin due to the formation of an initial calcium-rich gel that promotes additional metakaolin dissolution and enhances N-A-S-(H) gel formation.