Innovative development of high-strength concrete with ultrafine metakaolin-heated POFA binders and fine coal bottom...

The production of high-strength concrete (HSC) remains heavily dependent on cement and natural sand, despite the urgent need to reduce the carbon footprint of construction materials. Meanwhile, substantial quantities of palm oil fuel ash (POFA) and coal bottom ash (CBA) remain underutilized, and most existing studies treat ultrafine metakaolin (UFM), heated POFA (HPOFA), or CBA in isolation rather than as an integrated eco-concrete system. This study develops and evaluates an innovative HSC in which UFM and HPOFA serve as blended binders and CBA partially replaces natural fine aggregate. The main objective is to quantify the influence of ternary system on strength development relating to time, permeability, sulfate resistance and microstructural characteristics. A full factorial experimental program was conducted on HSC mixes containing 10-15% UFM, 20-30% HPOFA, and 0-20% CBA as fine aggregate. The results showed that 15% UFM maximizes compressive strength for binary blend, while a combined 15% UFM and 20% HPOFA is optimal for ternary blend. Overall, optimization of the UFM and HPOFA with CBA as fine aggregate system identified 10% UFM, 20% HPOFA and 10% CBA as the best-performing combination. The incorporation of 10-15% UFM with 20-30% HPOFA reduces permeability to 1.80-1.97 × 10^-13 cm/s. The mixer containing 15% UFM and 20% HPOFA exhibits only 14.2, 8.6 and 11% reductions in compressive strength, tensile strength, and UPV, respectively, after 300 days of immersion in 5% Na₂SO₄ solution. Microstructural analysis confirms that UFM and HPOFA refine the pore structure, limit sulfate ion ingress, and diminish ettringite-induced microcracking. The life cycle assessment (LCA) indicates that mixes with 15% UFM and 30% HPOFA in the presence of 10-20% CBA achieve the lowest environmental impacts. The study offers mechanistic insights and practical mix-design guidance for regions with similar waste profiles and aggressive sulfate environments.