One of the most conventional and energy efficient methods to improve the engineering properties of concrete is the utilization of waste materials and by-products. Among agricultural wastes, rice husk ash (RHA) has exhibited the greatest potential to enhance the characteristics of cement mixtures. However, the investigations on the properties and performance of RHA are slim compared to its potential advantages and few systematic studies can be found in the literature regarding parameters influencing the chemo-physical and microstructural characteristics of RHA along with its performance in cement mixtures. In this study, the effect of retention time of rice husk is systematically investigated. The chemo-physical and microstructural properties characterization methods include electrical conductivity (EC) measurement, X-ray fluorescence and diffraction. The husks were burned under absolutely controlled conditions in a muffle furnace under static air at 700 °C for 15 minutes, 1 hour, 4, 8 and 16 hours. Three cement replacement percents of 5%, 10% and 15% were applied for each retention time. A control mortar was produced as reference with a water to cement ratio of 0.45 and a flow of 250 mm. In total, 16 series of mix designs were made and tested. The results show that an increase in retention time the EC change is steadily decreased. However, the compressive strength of RHA blended mortars is increased up to 4 hours combustion and for higher retention times decreased. Nevertheless, no crystal phases were found up to 16 hours combustion. It is concluded that the pozzolanic performance of an additive is not only affected by the lime reactivity and several other parameters may control it.
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