Assessment of microstructural, physico-mechanical and durability of residue fly ash as a sustainable cementitious material after cenosphere extraction

Fly ash (FA) is a heterogeneous industrial byproduct composed of several valuable, lightweight microspherical components, including cenospheres. The continued extraction of these components, particularly cenospheres for tailored applications, has raised concerns about the potential depletion in quality of the residue fly ash (RFA). This may affect the microstructural, physico-mechanical and durability properties of the resulting concrete, thereby increasing the likelihood of the RFA being entirely abandoned as waste, increasing environmental pollution and industrial waste management cost. To evaluate the potential variations, this study investigated the pre- and post-cenosphere extraction performances of four fly ash samples from Malaysian power plants, with focus on microstructure, physical, mechanical and durability performance. A range of cement pastes and concrete mixes was examined, each case incorporating FA and RFA in proportions ranging from 0 to 40% with cement. The durability of the concrete in a chloride-exposed environment was assessed using a simulation-based approach. The findings indicated a moderate enhancement in early compressive strength development in the RFA concrete at 7 and 14 days, which is attributed to the enhanced surface area and reduced setting time. Overall, an increase of about 8-37% in surface area was observed, with a corresponding 10-33% reduction in workability, a 4% -12% decrease in compressive strength at 28days, and a 13-15% reduction in flexural strength. Furthermore, only slight variations were observed in the initial corrosion ingress, concrete deterioration propagation and overall durability performance. These findings demonstrate that despite the reduced workability, strength, and setting time, the RFA concrete retains its durability in chloride environments, suggesting its viability as a sustainable cementitious material in concrete production with minor modification.