Enhanced Utilization of Coal Ash in Construction: Physicochemical Properties and Compressive Strength of Blended Cement and Geopolymer Mortar

The properties of fly and bottom ashes produced by Morupule power stations in Botswana are analysed for their pozzolanicity. Furthermore, the influence of physical, chemical and microstructural properties of the ashes on the performance of compressive strength of fly ash–based blended cement and geopolymer mortar was evaluated. Physical characterization including particle size distribution, specific gravity and morphology was carried out with Malvern Mastersizer-2000, Le Chatelier flask apparatus and Carl Zeiss FEGSEM Gemini 500. The chemical composition and mineral characterization were determined by S8 Tiger XRF and Empyrean PANalytical 4 KW XRD systems. Blended cement mortar was produced by intergrinding CEM I Portland cement with fly ash in different proportions from 0% to 50% at 10% increments. The geopolymer mortar was synthesized by activating oxides in the ash with varying quantities of sodium hydroxide, ranging from 8 to 14 moles. The findings indicate that the two fly ashes are categorized as Class F, while bottom ashes from Plants A and B are classified as Classes F and C, respectively, due to differing oxide composition percentages. An increase in the proportion of fly ash in blended cement results in a decrease in compressive strength, with the maximum compressive strength of 52 MPa attained at 10% fly ash. The increase in sodium hydroxide concentrations affects compressive strength at various curing ages, achieving a peak compressive strength of 39 MPa at 28 days with a sodium hydroxide concentration of 14 moles. The findings were further corroborated statistically by the analysis of variance (ANOVA). The presence of heavy elements in coal ash underscores the imperative for safe reutilization of the material rather than its disposal in an ash pond. This study suggests that perceived waste can be converted into wealth, while simultaneously alleviating potential environmental and ecological hazards.