Synergistic effects of fly ash as filler on the mechanical and durability properties of recycled concrete aggregates solid masonry blocks

The depletion of natural aggregates (NAs) and the escalating generation of construction and demolition (C&D) waste have emerged as major environmental challenges in the construction industry. This research focuses on developing eco-efficient solid masonry blocks (SMBs) utilising 100% recycled concrete aggregates (RCA) and incorporating fly ash (FA) as a filler material by replacing 30% of fine RCAs (FRCAs). The study investigates the influence of the incorporation of FA at different cement-to-aggregate (C:A) ratios by examining the physical, mechanical and durability properties of the SMB, supported by microstructural analysis. The results demonstrated that the inclusion of FA improved particle packing and reduced internal voids, leading to a 5.59–6.54% increase in hardened density, and compressive strength (CS) was enhanced by 57.18–140.70% compared with blocks produced solely with RCA. Water absorption decreased to 2.83–3.91%, while sorptivity was reduced up to 19.14%, demonstrating improved resistance to moisture ingress. Under acid, chloride and sulphate exposure, FA-modified blocks exhibited lower deterioration in terms of mass and strength. Microstructural observations revealed a denser matrix, enhanced interfacial transition zones, and additional hydration products. Overall, the findings confirm that FA-modified blocks meet structural masonry requirements and offer a durable, resource-efficient alternative for sustainable construction practices.