Estimation of fly ash reactivity for use in alkali-activated cements - A step towards sustainable building material and waste utilization

This paper addresses the detailed characterization of coal fly ashes with respect to their utilization in alkali-activated cement systems, thus maximizing the use of fly ashes in the construction industry. A technique was developed to estimate the reactivity of low calcium fly ashes in alkali-activated systems. The technique is based on a K-value which combines three characteristics of a fly ash. Two characteristics - amorphous phase percentage and specific surface area determined through Blaine measurement - are observed quantities. The third characteristic - degree of polymerization of silica in the amorphous phase of fly ash - is a calculated parameter. To take into account the water demand of fly ashes, which will influence the amount of water needed to get a workable mix, the shape factor was used to adjust the Blaine specific surface area. The relationship between the proposed K-value and compressive strength of alkali-activated fly ash pastes is approximated by a linear function. The correlation coefficient of the relationship varied from 0.961 to 0.833 for 1 and 91 days compressive strength respectively. The proposed K-value can firstly be used to rank fly ashes for their suitability to produce high strength alkali-activated materials and secondly when calibrated for a specific activator and curing conditions, to predict the compressive strength of alkali-activated fly ash binders.