<p>Cement production accounts for nearly 8% of global anthropogenic CO₂ emissions, highlighting the urgent need for low-carbon, circular alternatives in construction. This study explores the use of Waste Rubber Powder (WRP), a non-biodegradable by-product from end-of-life tires, as a partial cement replacement in M20 and M30 grade concretes. WRP was incorporated at 0%, 5%, 10%, 15%, and 20% by weight. The impact of WRP was evaluated through compressive and splitting tensile strength; water permeability, Rapid Chloride Penetration Test (RCPT), and microstructural analysis via Scanning Electron Microscopy (SEM).The results indicate that 5% WRP replacement achieves the optimal balance between mechanical performance and environmental benefits. Compressive strength reductions were minimal 2.4% for M30 and 4.1% for M20 and chloride ion penetration increased slightly (6.3% and 7.8%, respectively). However, WRP content exceeding 10% significantly compromised concrete performance, with strength losses up to 18.7%. SEM analysis revealed increased porosity and disrupted interfacial bonding. M30-grade concrete exhibited superior resistance to WRP-induced deterioration, attributed to its denser matrix and enhanced hydration. This study establishes 5% WRP as a viable threshold for sustainable cement substitution without sacrificing structural integrity. Future research should focus on integrating WRP with supplementary cementitious materials and assessing long-term durability under real-world conditions. </p>