• Optimized multi-solid waste geopolymer achieved 61.34 MPa compressive strength using RSM, demonstrating high-performance sustainable construction material.
• Class-C FA is the most influential factor on compressive strength, followed by BCC, while GP enhances structural density.
• Ca from Class-C FA participates in geopolymerization, forming hybrid N–(C)–A–S–H gel, improving mechanical properties.
• RSM-based optimization enables synergistic utilization of multiple solid wastes, promoting circular economy and environmental sustainability.