Study on the mechanical properties of fly ash cement mortar reinforced with alkali-treated sisal fiber and sodium sulfate

In this study, the feasibility of replacing 50 % of cement with fly ash to reduce cement consumption was investigated. Sodium sulfate was used as an activator to enhance early strength and mitigate the risk of reduced early strength due to the high replacement ratio, leading to a cost-effective and eco-friendly strategy. Additionally, sisal fibers were incorporated to improve toughness, with the fibers treated using sodium hydroxide to enhance their mechanical performance within the matrix. The mechanical tests reveal that adding 4 wt% sodium sulfate optimizes early strength, while excessive sodium sulfate and high alkali treatment of fibers harm the long-term integrity of the material. Microscopic analyses indicate sodium sulfate accelerates early hydration, increases alkalinity, dissolves fly ash more efficiently, and promotes hydration product formation. The early formation of ettringite provides nucleation sites for gel growth. Alkali treatment removes low-molecular-weight components from sisal fibers, increasing cellulose crystallinity and reducing chemical reactivity. SEM imaging shows that alkali-treated fibers develop wrinkles, improving their interaction with the matrix and enhancing mechanical response. Utilizing fly ash and alkali-treated sisal fibers is a promising approach, offering a sustainable and economical solution for improving composite material performance. • Sodium sulfate activation boosts early strength in 50 % fly ash cement mortar. • Alkali-treated sisal fibers enhance the toughness of the composite. • Alkali treatment improves sisal fiber crystallinity and fiber-matrix bonding. • Sodium sulfate and treated fibers together enhance strength and toughness. • Performance enhancement mechanism was studied by XRD, FT-IR, TG, & SEM-EDS.