The rice husk used throughout this work was brought from the rice milling plant in Ayuthaya province which is located in the central region of Thailand. The dried rice husk was fired at temperatures of 400, 500, 550, 600, 650, 700 and 800 °C for 1 h in the muffle furnace. The rice husk ash (RHA) was removed from the furnace and rapidly cooled down to ambient temperature between 30 °C and 32 °C by spreading on a tray. The RHA was then ground to particle size less than 45 μm using Los Angeles abrasion machine. X-ray fluorescence (XRF) was used to determine the percentages of SiO2 present in the RHA. The 28-day strength activity index with Portland cement was tested. The properties of RHA firing at various temperatures are shown in Table 2. The optimum temperature of 650 °C was employed to prepare the rRHA which was used throughout the experiment. This temperature was selected from the strength activity index results and the SiO2 content as shown