Global production of ordinary Portland cement (OPC) remains a major contributor to anthropogenic greenhouse gas emissions, accounting for roughly 7% to 9% of total global carbon dioxide output. In response to urgent environmental mandates, geopolymer concrete (GPC) has emerged as an innovative, sustainable construction material that completely eliminates or substantially reduces cement by utilizing aluminosilicate industrial by-products such as Class F fly ash and ground granulated blast furnace slag (GGBFS) activated through alkali solutions. This extensive research evaluates the mechanical properties, long-term durability, and microstructural evolution of GPC formulated with varying proportions of fly ash, GGBFS, and 100% replacement of natural coarse aggregates with recycled concrete aggregates (RCA). Rigorous experimental testing protocols analyzed compressive strength, split tensile strength, flexural behavior, water absorption, rapid chloride migration, sulfate attack resistance, and scanning electron microscopy (SEM) over curing periods extending up to 90 days. The findings demonstrate that an optimal 70:30 fly ash-to-GGBFS binder ratio combined with tailored sodium silicate-to-hydroxide proportions yields exceptional early-age strength, superior microstructural densification, and robust durability, presenting an advanced framework for sustainable infrastructure development.