<p>The utilisation of Recycled Coarse Aggregate (RCA) from construction and demolition waste in geopolymer concrete offers a viable waste valorization pathway but is limited by the presence of porous adhered mortar that degrades performance. This study evaluates the combined effects of RCA replacement level and surface treatment on the fresh properties, mechanical behaviour, durability and microstructure of low-alkali, hydroxide-free geopolymer concrete cured under controlled ambient conditions. A hybrid binder comprising Ground Granulated Blast Furnace Slag and Fly Ash in a 40:60 ratio was activated using a Na₂CO₃–Na₂SiO₃ low-alkali activator system, eliminating the need for sodium hydroxide and thermal curing. RCA was incorporated at 0, 25, 50, 75 and 100% under five surface conditions. Untreated RCA significantly reduced workability, strength and durability, particularly beyond 50% replacement. In contrast, chemical and mineralogical treatments effectively improved RCA quality. At 50% RCA replacement, sodium silicate-treated mixes achieved a 28-day compressive strength of 44.5&#xa0;MPa and a static modulus of 29.5 GPa, approaching control values. SEM–EDS analysis indicated interfacial densification and pore refinement. Overall, 50% RCA replacement combined with sodium silicate or CO₂ treatment is identified as a practical and scalable waste valorization strategy for durable low-alkali geopolymer concrete.</p>