Effect of Rice Husk Ash Blending of Portland Cement on Compressive Strength of Gap-Graded Concrete

Rice husk ash (RHA) is a very reactive pozzolanic material. It has been successfully used as a mineral admixture in concrete. Rice husks are readily and in large quantities available in Vietnam. Hence, it would be of economic interest to study the use of RHA for the production of high-strength and durable concretes based on indigenous raw materials. This concept should also encompass the aggregates, with coarse-grained crushed rock (granite) and very fine river sand as natural candidates. The particle size distribution of this sand violates the building code, however. Since blending with coarse (imported) sand would be too expensive, this study focused on gap-graded mixtures. The application of the RHA in conjunction with a naphthalene-based superplasticizer in the gap-graded concretes with very fine sand made it possible to produce high consistency and cohesive mixtures with a relatively low sand content. Twenty-eight days compressive strengths values of 70-90 MPa were obtained for mixtures with slumps of 140-225 mm and water to binder ratios of 0.40-0.33. For stiff mixtures, with water to binder ratios of 0.35-0.27, the 28-days compressive strength values were in the range of 60-100 MPa.