Durability of Fly Ash and Limestone Concrete Under Combined Sulfate and Chloride Attack

This study investigates the long-term durability of concrete incorporating fly ash (FA) and limestone powder (LP) when subjected to simultaneous sulfate and chloride ion attack. Three concrete mixtures—100% ordinary portland cement (PC), 25% FA-blended concrete, and 15% LP-blended concrete—were immersed in 5% sodium sulfate (S) and combined 5% sodium sulfate + 3.6% sodium chloride (SC) solutions at 30 °C for a period of two years. Ion ingress, phase evolution, and microstructural changes were assessed using ion chromatography, X-ray diffraction (XRD), and scanning electron microscopy (SEM/EDS). Results showed that the presence of chloride ions significantly accelerated sulfate ingress and compound formation. The LP mix exhibited the most severe deterioration, characterized by elevated formation of expansive phases such as ettringite and gypsum, leading to internal cracking and substantial compressive strength loss under SC exposure. In contrast, the FA mix demonstrated superior performance due to enhanced chloride-binding capacity, pozzolanic C–S–H formation, and reduced permeability, which limited deleterious phase development. The findings highlight that while LP improves early sulfate binding, its limited chloride-binding capacity increases vulnerability to sulfate–chloride deterioration. FA-modified concrete offers a more durable alternative for structures exposed to aggressive multi-ion environments by enhancing microstructural stability and long-term performance.