Performance of low amorphous silica fly ashes in mortars

Fly ash based cements remain the most consumed pozzolanic cement in developing countries like India. However, despite such high demand, only 60% of the fly ash generated in India is consumed. One of the major reasons for this under-utilisation appears to be the lower quality of Indian fly ashes compared with fly ash generated elsewhere. Fly ashes produced in Indian power plants are generally low-calcium class F fly ash, with a low amorphous fraction (<50%). In the study reported in this paper, an attempt was made to try to understand the impact of low amorphous silica fly ashes on hydration and microstructure variations in blended cement systems using isothermal calorimetry, X-ray diffraction, thermal analysis, scanning electron microscopy and mercury intrusion porosimetry. Durability characteristics were studied by sorptivity measurements. The results showed that the low amorphous silica fly ashes improved both the durability and mechanical performance of the blended cements. These results will be useful in improving fly ash utilisation rates in India by the cement and ready-mix concrete industries.