Experimental and statistical optimisation of pervious concrete modified with fly ash and silica fume

This study investigates the influence of supplementary cementitious materials (SCMs), namely fly ash (FA) and silica fume (SF), on the fresh, mechanical, durability and microstructural properties of pervious concrete. A range of mix designs incorporating varying water-to-cement (w/c) ratios (0.36 and 0.38) and SCM contents (10%, 20%, 30% FA and 5%, 10% SF) were evaluated, followed by optimisation using response surface methodology (RSM). Results indicated that increasing w/c ratio enhanced workability across all mixes, with FA significantly improving flow and SF reducing it owing to its ultra-fine texture. However, a combination of 30% FA and 10% SF led to a 110% improvement in workability. Porosity and permeability displayed a linear relationship with SCM content and w/c ratio. Maximum values were observed at 5% SF, but higher SF levels reduced these properties. Compressive strength, ravelling resistance and residual strength were positively influenced by SF but reduced with FA alone. Microstructural analysis revealed that both FA and SF promoted secondary calcium silicate hydrate formation, leading to matrix densification and improved durability. Optimisation through RSM identified an ideal mix design balancing strength and permeability, affirming the synergistic benefit of FA and SF in sustainable pervious concrete.