This paper presents a research work on the effect of rice husk ash (RHA)/lime (CaO) as a partial replacement of cement in concrete production. A total number of ninety (90) cubes were cast, thirty (30) cubes for trial mix to obtain optimum water cement ratio (W/C) for the actual mix and another sixty (60) cubes were cast for actual mix on the percentage replacement group of 0%, 2.5%, 5.0%, 7.5% and 10% of both rice husk ash and lime respectively. The setting times of RHA/Lime and ordinary Portland cement (OPC) paste, workability, saturated surface density, chemical composition of RHA and compressive strength of the concrete were determined. Results showed that RHA/lime has influence on the setting times because both initial and final setting times of the cement paste with RHA/lime have higher setting times. The initial setting time ranges from 140 mins to 159 mins at 2.5% to 10% of RHA/ Lime respectively, as compared with the control specimen, which has 95 mins. The steady increase in the initial setting time is also reflected in the final setting times, and ranges between 216 mins – 261 mins. Slump decreased from 18 mm to 0 mm.Compacting factor test ranges between 0.89 – 0.81, the specific gravity of rice husk ash was found to be 2.13, and the bulk density 530 kg/m3. Lime (CaO) content of RHA 1.36%, silica (SiO2) content 67.30% and loss on ignition (L.O.I) 17.78%.The average compressive strength at 2.5% replacement level of RHA/lime was 25.3 N/mm2 at 21days which decreased to 18.7 N/mm2 at 28days with other replacement level of RHA/CaO, respectively. This is initially closer to the strength of the control concrete specimen which has average compressive strength of 25.6 N/mm2 at 21days but behaves differently later by increasing to 27.8N/mm2 at 28 days. The decrease in the RHA/CaO replacement situation could be attributed to the variation of the chemical composition of RHA/CaO.
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