Physicochemical Analysis of Recovered Fly Ash and Their Suitability in Mortar and Concrete Applications

Despite the shift toward cleaner energy, coal-fired power plants remain a significant source of global energy, generating excess fly ash that accumulates in large stockpiles—often persisting for decades even after the plants are decommissioned. The study investigated the physicochemical properties and suitability of fly ash recovered from seven ash dams for use in cement-based applications. The recovered ash was beneficiated by drying, breaking agglomerates, and sieving to meet specifications for use as a cementitious material. Subsequently, analyses were conducted for particle size, pH, density, loss on ignition, scanning electron microscopy, oxide composition, X-ray diffraction, thermogravimetry, Fourier transform infrared spectroscopy, and the compressive strength of mortar samples. The results indicate that the samples are Class F fly ashes, containing amorphous aluminosilicates, with a comparable physical, chemical, and mineralogical composition, and that they meet specifications for use in cement-based materials. This remains true despite a slight increase in sulphur-bearing phases in the Kusile ash associated with the plant’s desulfurization technology. Furthermore, the compressive strength results show that, compared with the reference Portland cement mortar, fly-ash-blended mortars exhibit higher strength gain at later ages, indicating good pozzolanic reactivity, though the degree of strength gain depends on each ash’s fineness, amorphous content, and mineralogy.