Influence of temperature on early hydration of Portland cement–metakaolin–slag system

The hydration of ternary blended cements comprising Portland cement, metakaolin and blast-furnace slag was studied at different temperatures of 30, 40, 50 and 60 °C by isothermal calorimetry. Phase composition of hydration products was determined by thermal analysis and X-ray diffraction for sample hydrated at 60 °C. Hydration involves different sequential and successive reactions in which mechanism, kinetics and products are strongly influenced by temperature. Activation energy for each exothermic reaction was determined. The linear trend of heat flow maxima for 2nd, 3rd and 4th exotherm versus temperature was revealed. The first peak is related to formation of mostly amorphous aluminate hydrates with no evidence of crystalline ettringite. Formation of first crystalline ettringite detected by X-ray diffraction was correlated with the distinct 3rd peak that occurs after 2nd “silicate” peak that corresponds to formation of calcium silicate hydrates and calcium hydroxide. The 4th peak recognizable at temperatures above 40 °C is related to the conversion of ettringite to hemicarbonate.