Investigation of the reaction mechanism of blended fly ash and rice husk ash alkali-activated binders

This study investigates the influence of the chemical and physical properties of two abundantly available waste by-products in Sri Lanka, fly ash and rice husk ash (RHA) as precursor materials for the synthesis of alkali-activated binders. The suitability of the two types of fly ash and the replacement of fly ash by RHA (10% and 20% by weight of the binder content) were assessed. The study reports the development of compressive strength together with an in-depth analysis of the reaction mechanism of the blended RHA alkali-activated binders. The 100% fly ash mortar achieved the optimum compressive strength of 38.9 MPa at 28 days. Replacement of the fly ash with 10% and 20% RHA reduced the compressive strength by approximately 14% and 43%, respectively. The higher specific surface area of RHA and relatively higher unburnt carbon content in RHA were identified as the major factors influencing the low compressive strength obtained. Furthermore, the addition of RHA increases the reactive silica in the gel matrix and leads to an increase in the Si/Al ratio (3.70–3.89), which has a negative effect on the compressive strength. The difference in solubility rate of precursor fly ash and RHA negatively affect the formation of the gel matrix which is hypothesized as a further reason for the lower compressive strength observed in the RHA mixes.