Valorisation of bottom ash in concrete: serviceability, microstructural and sustainability characterisation

The present study investigated the synergistic influence of bottom ash as a Portland cement (PC) and natural fine aggregate (NFA) replacement in concrete. Coal bottom ash (CBA) is a heavy ash that settles at the bottom of the combustion chamber of a thermal power plant. It was ground (GCBA) for two hours prior to replacing 10–30% PC, while CBA was used in raw form to replace 25% and 50% NFA. The mechanical properties along with durability properties (accelerated carbonation and chloride penetration) were studied after 28 days and 90 days of curing. Non-destructive tests were also performed to check the quality of CBA-based concrete. Microstructural characterisation was conducted using various techniques, namely X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. The concrete with 20% GCBA and 25% CBA (G20C25) reported the best performance in terms of parameters studied owing to the pozzolanic reactivity of GCBA and the filler effect of fine CBA. The microstructural investigations also validated the findings and trends observed in the experimental results. Well-fitted mathematical models were derived and optimisation was carried out using the desirability function approach. Multi-objective optimisation recommended 21.80% GCBA and 24.17% CBA as the optimum amount, resulting in a significant reduction of 19.19% and 18.19% in carbon footprint and eco-cost, respectively, compared to the control mix.