Valorization of partially combusted wood fly ash with nano-silica as low-impact alternative to coal fly ash in cementitious composites

The declining availability of coal fly ash (CFA) due to the phasing out of coal-fired power plants has created an urgent demand for alternative supplementary cementitious materials (SCMs) in sustainable concrete construction. This study aims to effectively recycle wood fly ash (WFA), a by-product from wood biomass combustion, as an SCM for greener construction practices. While WFA has demonstrated promising potential, its partially burnt nature negatively impacts mechanical properties due to the presence of porous biochars. This study explores the valorization of WFA as a low-carbon SCM through the addition of a small dosage of nano-silica (NS) to mitigate these drawbacks. Ternary-blended cementitious composites were prepared by replacing cement with 15 % and 30 % WFA, combined with 3 % NS, to evaluate fresh, mechanical, durability, and microstructural properties. The results were compared with composites made of commercially available classes C and F CFA. It was observed that although NS addition negatively affected the fresh properties, it greatly enhanced the mechanical properties, chloride resistance, water absorption, and freeze-thaw durability of the WFA-modified composites. The improved freeze-thaw resistance can potentially extend the service life, thereby enhancing environmental compatibility. The mix having 15 % WFA and 3 % NS outperformed the control one in both mechanical and durability aspects, indicating that a low-level NS replacement can effectively mitigate the drawbacks of partially combusted WFA. Overall, CFAs demonstrated better performance than WFA at the 30 % level.