Re-Imagining Waste: CBA-Modified High-Strength Mortar as a Blueprint for Greener Construction

The search for viable alternative resources is essential for advancing sustainable development in the construction industry. A significant global concern is the substantial generation of industrial waste, particularly coal ash byproducts such as fly ash (FA) and coal bottom ash (CBA) from thermal power plants (TPPs). India ranks as the third-largest producer of coal ash globally and the second-largest in Asia, generating approximately 105 million metric tonnes annually. While TPP-derived wastes have been extensively studied in masonry mortars, the potential of CBA as a partial or complete replacement for natural fine aggregates (NFA) in high-strength mortar (HSM) remains significantly underexplored. This study investigates the fresh, mechanical, and microstructural properties of mortar incorporating CBA as a substitute for NFA, specifically up to a 100% replacement level Flow table tests revealed improved workability with increasing CBA content, which is attributed to its porous microstructure; however, significant bleeding was observed at higher replacement levels (≥75%). The dry density consistently decreased with the addition of CBA with a reduction of up to 19.27% at full replacement. Ultrasonic pulse velocity (UPV) values declined with higher levels of CBA but improved with curing age. The mortar incorporating up to 100% CBA retains appreciable mechanical properties despite a progressive reduction in compressive strength (C_S) with increasing CBA content. The observed compressive strengths for the different mixes were as follows: control mix (CM) at 36.72 MPa, mix with 25% CBA (CBA25) at 25.56 MPa, mix with 50% CBA (CBA50) at 19.69 MPa, mix with 75% CBA (CBA75) at 16 MPa, and mix with 100% CBA (CBA100) at 9.93 MPa. All mixes exceeded the minimum strength criteria, confirming their classification as HSMs at all replacement levels. These results highlight the potential of CBA as a sustainable alternative in construction materials, supporting efforts toward resource efficiency and environmental sustainability in the industry.