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Fundamental Investigations into Performance of Carbon-Neutral Rice Husk Ash as Supplementary Cementitious Material

Although the benefits of rice husk ash (RHA) as a pozzolan have been known for a long time, its use in concrete has been limited. Past efforts to produce RHA have yielded ashes, some with high carbon content and others with some crystalline silica, neither of which is desirable for use of RHA as a pozzolan. Within the past decade, the production of off-white-colored RHA containing high levels of amorphous silica and very little carbon has become possible through innovations in incineration techniques. Results are presented from a study conducted to ascertain the properties of a low-carbon-content RHA and evaluate it as a supplementary cementing material. In addition, the as-obtained RHA was ground to a smaller particle size to study the effect of finer ash on properties of cementitious mixtures. Specific tests conducted include pozzolanic reactivity and microstructure of RHA and other properties of cementitious pastes and mortars. These include flow, initial and final setting time, compressive strength, flexural strength, and split tensile strength. In addition, other tests such as water absorption, effective porosity, and rapid chloride ion permeability were conducted to ascertain the improvements offered by RHA in enhancing durability of cementitious mixes. Comparative studies were conducted with silica fume mixes. Results from this study are very encouraging and suggest that RHA produced with a consistent quality (i.e., high amorphous silica content and low carbon content) can serve as an environmentally friendly and sustainable supplement for cementitious material needs in future; it can reduce the carbon footprint of concrete.