Studies on the Effect of Retention Time of Rice Husk Combustion on the Ash's Chemo-Physical Properties and Performance in Cement Mixtures

This study investigates the effect of retention time of rice husk ash (RHA) combustion on its characteristics. The retention time was changed from 15 min up to 32 h. The chemo-physical and microstructural properties characterization methods included monitoring of electrical conductivity and pH variations of saturated lime-RHA solution, X-ray fluorescence, and diffraction. To assess the pozzolanic performance of RHA, a control mortar was produced as a reference with a water-to-cement ratio of 0.45 and a flow of 84%, and three cement replacement percentages by RHA of 5, 10, and 15% were applied for each retention time. The results show that with an increase in retention time, the EC and pH changes steadily decrease, suggesting lower RHA reactivity at longer retention times. However, no crystalline phases were found in RHA up to 32-h combustion, primarily because of the rapid cooling of the yield ash. In contrast, the compressive strength of RHA blended mortars increases up to 4 h combustion and then decreases. A possible explanation for the lower strength activity of short retention time RHA is offered in this study.