Fresh and strength properties of high volume ultra-fine fly ash cement mortar with calcinated limestone powder

The growing demand for sustainable building materials has led to an increased quest for substitute cementitious systems that can mitigate environmental impact while maintaining the characteristics of traditional cementitious systems. Ordinary Portland cement (OPC) production contributes significantly to CO₂ emissions, necessitating the development of sustainable alternatives. Fly ash, being an industrial by-product, is used with cement to make high-volume fly ash blended (HVFA) cement, which offers substantial environmental and economic benefits by limiting the use of ordinary Portland cement. Yet, its extensive usage is hindered by performance downsides such as slower early-age strength and deferred setting time. This study aims to develop a sustainable mortar system by integrating multiple industrial by-products, including ultra-fine fly ash (UFFA), calcinated limestone powder (CLP), and bauxite powder (BP), activated with alkaline activators (AA). The primary objective is to replace 40–60% of ordinary Portland cement, thereby improving the mechanical and durability properties of the mortar compared to conventional mortar systems. In the production of sustainable mortar, incorporating 40–60% of ultra-fine fly ash, 10–20% of calcinated limestone powder, 0–10% of bauxite powder, and 3–9 g of alkaline activator. Mortar specimens were prepared with varying proportions of industrial by-products: 40–60% ultra-fine fly ash, 10–20% calcinated limestone powder, 0–10% bauxite powder, and 3–9 g of alkaline activator, maintaining a 1:3 binder-to-fine aggregate ratio. Comprehensive testing was implemented, including workability assessment, setting time determination, compressive strength evaluation, flexural strength testing, fracture toughness analysis, and detailed microstructural characterisation. The optimum mixture composition, consisting of 40% ordinary Portland cement, 40% ultra-fine fly ash, 15% low thermal calcinated lime powder, and 5% bauxite powder with alkaline activator made from 6 g of magnesium carbonate powder, demonstrated superior performance characteristics. This optimal blend achieved enhanced mechanical properties compared to conventional mortar while successfully replacing 60% of ordinary Portland cement with industrial by-products. The improved performance of the optimum mixture can be attributed to synergistic effects between ultra-fine fly ash, calcinated limestone powder, and bauxite powder under alkaline activation. Future research should investigate the long-term durability performance of the developed sustainable mortar under various environmental exposure conditions.