Material Characterization Studies on Low- and High-Carbon Rice Husk Ash and Their Performance in Portland Cement Mixtures

In this study, low- and high-carbon rice husk ash (RHA) in their as-received and ground forms were characterized by means of different methods in order to evaluate their performance in Portland cement mixtures. RHA-cement pastes and mortars, at three different RHA replacement levels of 0 %, 10 %, and 20 %, were prepared at a constant water/cement ratio of 0.485. Results from this investigation indicate that the material characteristics of low- and high-carbon RHA were significantly different in most of the tests conducted. In the as-received condition, the high-carbon RHA had a greater bulk density than low-carbon RHA, but their bulk densities were comparable after grinding. The low-carbon RHA was more effective in its pozzolanic reaction than high-carbon RHA in both ground and as-received conditions. The microstructures of both low-carbon and high-carbon RHA cement pastes were denser than those of control pastes. At a given dosage of RHA and superplasticizer, the ground and the as-received low-carbon RHA mixtures performed significantly better than the high-carbon RHA mixtures. Similarly, mixtures with low-carbon RHA showed significantly higher strength activity indices than those with high-carbon RHA at both dosage levels investigated. Grinding RHA was found to be beneficial in all the tests conducted, as the ground RHA mixtures depleted more calcium hydroxide, registered higher flow values, and possessed greater strength than unground RHA mixtures at all replacement levels. Thus, the grinding process significantly helps in utilizing both high- and low-carbon RHA in concrete.