Behavior of a high-volume fly ash fiber-reinforced cement composite toward magnesium sulfate: a long-term study

Sulfate attack is one of the severe concerns for concrete's durability in sulfate-rich soil, groundwater, and the marine environment. The ingress of dissolved sodium and (or) magnesium sulfate in concrete leads to the formation of expansive products such as gypsum, ettringite, brucite, and magnesium-silicate-hydrate (M–S–H), causing extensive cracking and disintegration of concrete based on the severity of the attack. The consequence of ingress of magnesium sulfate is more severe than sodium sulfate. The present article aims to assess the long-term behavior of a novel cement composite incorporating 80% class-F fly ash (F-FA) and 20% ordinary Portland cement with varying volume fractions of polypropylene fibers exposed to 5% magnesium sulfate solution for up to two years. The compressive strength, weight, and volume changes of the specimens measure these effects. The mixes with higher volume fractions of PP fibers undergo a 40% reduction in compressive strength, 6.7% weight gain, and 3.5% volume change at two years. The morphological features revealed through SEM images and EDX analysis find the formation of M–S–H, brucite, gypsum, ettringite traces, and unreacted F-FA. The outcomes of this study encourage the utilization of F-FA to a much higher volume to help reduce the carbon footprint and promote sustainability.