New advances in understanding of use of coal bottom ash as an eco-pozzolan: Comparative study with fly ash, its traditional counterpart

The sharp decline in coal fly ash (CFA) production due to the ban on coal use in power stations threatens the continued manufacture of fly ash blended cements. This situation has prompted a search for scientifically, technically, economically and environmentally feasible alternatives. Against this backdrop, research is increasingly focusing on coal bottom ash (CBA), a material that has been disposed of in landfill for decades, creating significant technical, environmental, economic and health challenges. This study examines 2 coal ashes (fly ash and bottom ash) produced at the same power station. It conducts chemical, physical, mechanical and microstructural characterization to identify differences between the 2 materials and assess their performance when incorporated into cement matrices at 10–30% replacement volumes. Various standardized techniques and methods (XRF, XRD, laser grading) are employed to analyse the starting ashes, their pozzolanic activity (pozzolan/lime system) and the physical and mechanical behaviour of the resulting blended cements (rheology, calorimetry, mechanical strength and microporosity). Both ashes present similar chemical compositions and meet the requirements for cement and concrete production, according to EN 450–1. Their XRD spectra are also largely comparable and exhibit only minor crystalline differences, the most notable being the presence of anhydrite in the CFA. Between 7 and 28 days of reaction, the CBA demonstrates greater lime fixation capacity than the CFA. No appreciable differences are observed in normal consistency water demand, though in the 30% CBA paste the initial setting time is 60 min faster than in the CFA paste. The compressive strength and microporosity values over time show no significant differences between the two ashes, following the same trend as the reference Portland cement mortar. In summary, CBA emerges as a viable alternative to standardized fly ash since it fulfils the same chemical, physical and mechanical requirements as its CFA counterpart without any adverse effects, according to EN 450–1.