Effect of temperature on the hydration of Portland cement blended with siliceous fly ash

• The reaction of quartz powder at 80 °C strongly enhances the compressive strength.
• Almost no strength increase of fly ash blended OPC at 80 °C was found after 2 days.
• Siliceous hydrogarnet is formed upon the reaction of fly ash at high temperatures.
• Temperature dependent change of the system was simulated by thermodynamic modelling.
• Destabilisation of ettringite above 50 °C correlates with sulphate content of C–S–H.

The effect of temperature on the hydration of Portland cement pastes blended with 50 wt.% of siliceous fly ash is investigated within a temperature range of 7 to 80 °C.The elevation of temperature accelerates both the hydration of OPC and fly ash. Due to the enhanced pozzolanic reaction of the fly ash, the change of the composition of the C–S–H and the pore solution towards lower Ca and higher Al and Si concentrations is shifted towards earlier hydration times. Above 50 °C, the reaction of fly ash also contributes to the formation of siliceous hydrogarnet. At 80 °C, ettringite and AFm are destabilised and the released sulphate is partially incorporated into the C–S–H. The observed changes of the phase assemblage in dependence of the temperature are confirmed by thermodynamic modelling.The increasingly heterogeneous microstructure at elevated temperatures shows an increased density of the C–S–H and a higher coarse porosity.