Heat of hydration of Portland Cement-Metakaolin-Fly ash (PC-MK-PFA) 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 the rate of heat evolution (dQ/dt in J/gh) during hydration, and the heat of hydration, (Q(t) in J/g). For binary PC-PFA blends PC hydration is enhanced in the very early stages of hydration, but at extended periods (up to 120 h) an increase in PFA replacement level causes a systematic reduction in heat output. For binary PC-MK blends the results suggest that the MK initially diminishes PC hydration but the subsequent pozzolanic reaction of MK increasingly contributes to the heat output causing some blends to exceed the heat output of the PC control. For both systems a principal controlling factor in the PC hydration rate (and the heat evolution rate) is the water requirement of the pozzolan, but for PC-MK blends the pozzolanic reaction of the MK makes a significant contribution to the heat output. However this reaction is controlled both by the availability of water and the supply of Ca2+ ions from the hydrating PC which introduces an increasing level of complexity to the heat output versus time profiles. When combining MK and PFA in ternary PC-MK-PFA blends the MK has a dominant influence on the heat output versus time profiles.