Chemical and physical surface modification of waste rubber for cementitious composites: Mechanisms of interfacial bonding and performance recovery

Billions of tires every year reach the end of their useful life, which results in significant global waste. Tire waste around the globe leads to socio-economic challenges such as pollution and high management costs, etc. One of the solutions is the use of tire waste in construction, which is one of the fastest-growing industries. Incorporating tire waste in the form of fine rubber particles in mortar or concrete mixes will not only lessen the site dumping burden but will also help in the overuse of natural resources, as well as boost the circular economy cycle. Tire waste can be used in concrete both as fine and coarse aggregate. However, the hydrophobic nature of rubber particles weakens the interfacial transition zone (ITZ) with the cement matrix, leading to substantial losses in mechanical properties, typically a 40-60% reduction in compressive strength at higher replacement levels. Surface treatment of rubber particles is therefore essential to improve hydrophilicity, enhance ITZ bonding, and recover mechanical performance. This review provides a comprehensive and critical evaluation of physical, chemical, and emerging treatment methods for crumb rubber used in cement-based materials, with particular emphasis on microstructural modifications and their correlation with mechanical properties. Physical treatment includes water washing, cement coating, and chemical treatments include NaOH, KMnO 4 , acid solutions (H 2 SO 4 , HCl, CH 3 COOH), and silane coupling agents while emerging and advanced methods include partial oxidation, UV/thermal treatment, MICP bio-treatment, and microwave treatment. For each method, the review synthesizes findings on surface morphology changes (SEM), chemical transformations (FTIR), wettability improvements (contact angle, sedimentation), and resulting mechanical performance. Cement coating offers the most cost-effective improvement (37-64% strength gain at 22% lower cost) but adds significant weight. KMnO 4 treatment delivers superior hydrophilicity with 21-49% compressive and 37% flexural gains, at a higher cost and time-sensitive optimization (12h). H 2 SO 4 treatment achieves the highest strength improvement (56%) through carbonization-induced porosity, though over-exposure causes severe degradation. Emerging methods like microwave and MICP treatments show promise but lack validation in cementitious systems. This synthesis provides evidence-based guidance for selecting treatment methods based on performance requirements, economic constraints, and sustainability considerations.