The effects of a hydrochloric acid pre-treatment on the physicochemical properties and pozzolanic performance of rice husk ash

This paper investigates the effects of acid normality (0.01–6 N HCl) and combustion retention time (0.25–16 hours) on the pozzolanic properties of pre-combustion acid-treated rice husk ash. The pozzolanic reactivity was quantified by adding ground ash to saturated Ca(OH)2 solutions and monitoring the time-dependent electrical conductivity and pH of the solutions. Also, the strength activity of ashes from different processes was measured by testing the compressive strength of mortars. It was observed that acid treatment results in ashes with higher SiO2 content, lower alkali and unburned carbon content, better grindability, and smaller particle size, in comparison with ash from non-acid treated husks. Acid leaching increased the lime reactivity of the ashes and decreased their sensitivity to prolonged combustion times. Further, acid treatment with 0.01 N HCl was found to be sufficient, as the use of stronger acids did not considerably improve the pozzolanic reactivity of rice husk ash.