Setting times of Portland cement-metakaolin-fly ash blends

In the present study two pozzolanic materials are used, Metakaolin (MK) and Fly Ash (PFA), as binary and ternary partial replacement binders with Portland cement (PC) to investigate their effect on standard consistence and setting times of paste. To maintain standard consistency for MK-PC pastes increasing w/b ratios are required with increasing MK levels, whereas PFA has little influence on consistency. For binary MK-PC pastes there are substantial increases in initial and final setting times at 5% MK followed by decreases at 10 and 15% MK before further increases at 20% MK. For binary PFA-PC pastes however setting times increase at approximately the same rate up to 30% PFA and then increase more sharply between 30% PFA and 40% PFA. In general the effects of both PFA and MK on consistency and setting times in binary pastes are reflected in the behaviour of the ternary pastes although the two pozzolans do not behave completely independently of each other. The different ways in which the two pozzolans influence setting times is explained in terms of differences in their pozzolanic activities and in the manner in which they influence the availability of water for hydration.