The comparative study of using wood ash in lieu of fly ash in South African blended cement paste and mortar

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A comparative analysis of the properties of fly ash (FA) and wood ash (WA) blended cement, produced in the controlled environment of a cement factory was examined. Since FA has been adopted in South Africa as a material for blended cement, the possibility of using wood ash in lieu of FA to produce blended cement was investigated. The blended cements were prepared and tested at AfriSam (South Africa) Ltd. Roodepoort, with 10 % and 25 % FA and WA as partial substitution for Ordinary Portland Cement (OPC) clinker (CEM1). The chemical composition of FA and WA, the setting times of the blended cement pastes and the compressive strengths of the blended cement mortars were analyzed and discussed. The results indicated that FA and WA have combined SiO2, Al2O3 and Fe2O3 constituents of 89.29% and 72. 58%, respectively. Also, lower initial and final setting times were observed with 25% WA samples as compared to the control sample and FA samples. WA blended cement showed faster rate of hydration than FA blended cement at early curing age resulting in mortar compressive strength of 26.2 MPa and 25.4 MPa as well as 22.6 MPa and 19.2 MPa for 10% and 25% WA and FA substitution respectively. However, FA blended cement mortar exhibited higher late strength than WA blended cement mortar with 10% FA blended cement mortar resulted in higher compressive strength than control sample at 90 days. The combined synergy of WA and FA in blended cement is envisaged to produce mortar of desirable early and late strengths.