Pozzolanic Properties of White Cowpea Husk Ash

This research attempts to empirically investigate the pozzolanic properties of White Cowpea Husk Ash (WCHA), an agricultural biomass waste, at different percentages of its use as partial replacement of cement in concrete. WCHA was obtained after the calcination of white cowpea husk for 3 hours at 5500C. X-ray Florescence (XRF) analysis performed revealed that the sample of WCHA is a Class C pozzolana, which contains 65.4% of the combination of SiO2, Al2O3, and Fe2O3. The WCHA shows increase in consistency with increase in the WCHA content. This was attributed to the high Loss of Ignition (LOI) of WCHA compared to that of the cement. In addition, the results indicated that the initial and final setting time of WCHA – Cement blended concrete increase with increase in the WCHA content. The delay in setting times of WCPA-Cement paste could be attributable to the slower pozzolanic reaction. The density of the concrete decreased as the WCHA content increases. Generally the compressive strength of the WCHA concrete increased with increase in curing age and decreases as the WCHA content increased from a strength of 28.6 to 20.0 N/mm² giving a percentage reduction of 30.1 %. The strength reduction is also attributed to the modification of the bonding properties of the binders’ hydrates.  However, the 28 days compressive strength of concrete with up to 10 % WCHA content (26.4 N/mm²) satisfied the design characteristic strength of 25 N/mm². Beyond this limit, the compressive strength of the concrete fell below the design strength. Hence, 10 % WCHA could be regarded as the optimum dose for grade 25 concrete.