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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.

In this study, efforts were undertaken to produce ultra-high-strength mortars (UHSM) from conventionally available materials. Selection of mixtures and optimization of mixtures were achieved by adopting the basic principles of UHSM and through trial studies. A preliminary investigation was carried out to assess the effects of different curing regimes on the strength development of UHSM. Effects of fiber addition, heat treatment, and preset pressure on the rate of strength development of UHSM mixtures were individually assessed to better understand their benefits in the production of UHSM. Mechanical properties of the heat-treated UHSM were then investigated to determine the performance of the UHSM developed from conventional materials. Results from this study suggested that for the production of UHSM, a multiple curing regime was best suited and was then used for further studies. Data on strength development indicated that heat treatment increased the strengths by 57% to 75%. With application of preset pressure, the compressive strengths of UHSM were increased significantly by 15% to 18%. The optimum quantity of fibers to produce UHSM was found to be 2% to 3%. Effectiveness of fibers in increasing the strength of UHSM was found to be higher for heat-cured specimens than for normal water-cured specimens. The maximum target strengths obtained from the UHSM were 194 MPa, 24 MPa, and 31 MPa for compression, split tension, and flexure, respectively. The UHSM also showed improved energy absorption and toughness characteristics, especially at higher-fiber dosages.

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