Sustainable enhancement of chemical durability and microstructural stability in cement mortar incorporating sodium silicate–silica fume treated recycled fine aggregate

Sustainable construction techniques increasingly promote the use of recycled fine aggregate (RFA) from demolition waste; however, its elevated porosity and poorly adhering mortar frequently compromise durability, especially under harsh chemical conditions. This study presents a novel pre-soaking treatment utilising a combination of sodium silicate (SS) and silica fume (SF) solutions to enhance mortar adhesion and reduce its chemical reactivity. Mortar mixtures were made using natural fine aggregate (NFA), untreated recycled fine aggregate (RFA), and RFA subjected to various degrees of sodium silicate-silica fume (SS-SF) treatment. The durability performance was assessed by submerging specimens in 5% H₂SO₄ and 5% MgSO₄ solutions for 90& 180 days, and subsequently measuring weight loss, compressive strength, and ultrasonic pulse velocity (UPV). The 20% SS/2% SF (T4) mixture exhibited the greatest resistance, resulting in approximately 40% reduced mass loss, a 30% increase in residual strength, and a 20% increase in UPV compared to the untreated RFA mortar. Microstructural analyses verified less gypsum and ettringite production, enhanced C–S–H stability, and a denser matrix. The results indicate that SS–SF pre-soaking is a practical modification method for significantly improving the chemical durability of RFA-based mortar.