Effects of Fly Ash Content and Curing Age on High Temperature Residual Compressive Strength of Strain-Hardening Cementitious Composites

Class F fly ash is an integral component of most Strain Hardening Cementitious Composites (SHCCs). It reacts with the calcium hydroxide produced in the primary hydration of cement. As calcium hydroxide disintegrates at lower temperatures than other cement hydration products, reducing it through the pozzolanic reaction with fly ash has been shown to improve the thermal stability of SHCC. However, the degree (or extent) of the pozzolanic reaction increases with curing age. Therefore, the effects of fly ash content on the residual compressive strength of SHCC after exposure to elevated temperatures must be investigated at different curing ages, which is the motivation behind this study. Three different SHCCs with fly/ash to cement (FA/c) weight ratios of 1.2, 2.4, and 3.6 were tested at 28 days and 150 days under residual conditions after being subjected to high temperatures of up to 600 °C. The results show that the beneficial effects of high FA/c ratios on the residual compressive strength, associated with the accelerated pozzolanic reaction of fly ash at high temperatures, are lost at the long curing age.