Influence of the availability of calcium hydroxide, sulphates, and alkalis on hydration of supplementary cementitious materials

Abstract

The hydration of siliceous fly ash (FA), slag (S) and metakaolin (MK) with calcium hydroxide (CH) was studied at 27 °C to isolate the roles of CH, alkali and sulphate. The influence on rate of hydration was studied along with phase assemblage and strength development. While the FA was the most slowly reacting SCM, it was also the least influenced by the addition of alkalis and sulphates. For MK and S, not only the rate of hydration is influenced, but the phases formed are also affected by the amount of alkalis and sulphates. The addition of sulphates delays the hydration of alumina in S and MK. In more reactive SCMs such as MK, the availability of CH can be a limiting factor in the reaction of the SCM. The results show that to achieve maximum possible hydration and contribution to strength by SCMs, an optimisation of the alkali and sulphate content is required.