Enhancing Performance of Self-Compacting Concrete Using Fly Ash and Waterproofing Admixture: A Study on Mechanical Strength and Water Resistance

Concrete in hydraulic and marine structures is highly susceptible to deterioration from water ingress, which accelerates degradation and reduces service life. While self-compacting concrete (SCC) improves placement efficiency, achieving both strength and impermeability remains challenging in durability-critical environments. This study investigates the combined use of Class F fly ash (FA) and SIKA WT-220 PMY waterproofing admixture in SCC. FA was used at 0–30% cement replacement, and SIKA at 2% of binder. Compressive strength was tested at 28 and 90 days, alongside water absorption and ISAT for permeability assessment. The 20% FA + SIKA mix achieved early high strength and reached 63.91 MPa at 90 days, while 30% FA + SIKA recorded the lowest water absorption (1.96%), a 37.9% improvement over control. SEM analysis confirmed matrix densification and reduced porosity due to the synergistic action of FA and SIKA. This study presents a novel and sustainable approach to producing high-strength, low-permeability SCC, addressing a gap in literature on combined FA–crystalline admixture systems. The results highlight its suitability for long-term use in aggressive, water-sensitive infrastructure.