Analysis of the effects of rice husk ash on the hydration of cementitious materials

Rice husk ash (RHA) is a highly reactive pozzolanic material produced by the controlled burning of rice husk, and it is widely used as a mineral admixture to produce high-performance concrete. The addition of the rice husk ash has complex effects (such as the cement dilution effect, the pozzolanic reaction, and the absorption and the release of mixing water) on cement hydration. Current models do not explain all these complex effects. This paper fills this gap by presenting an analytical model to simulate the hydration of the cement-RHA blends by considering both the cement hydration and the RHA reaction. The proposed model considers the influence of factors including the water to binder ratio, the RHA replacement ratio, the absorbed water in the RHA internal pores, the fineness of RHA (that is the mean particle size of the RHA), and the amorphous SiO2 contents, on the hydration of the cement-RHA blends. We find that compared to the plain Portland cement paste, the hydration degree of the cement in the cement-RHA blends is improved due to the dilution effect. The calcium hydroxide contents in the cement-RHA blends decrease with the increase in the RHA replacement ratio. The proposed hydration model is verified by using experimental data on the RHA blended concrete with different water-to-binder ratios and different RHA substitution ratios.