Synergistic effects of microwave curing regimes on early, mid, and long-term strengths and microstructural performance of fly ash-slag based 3D-printed geopolymers

• Microwave heating enhances mechanical strength of 3D-printed geopolymers. • Increasing heating time improves C-A-S-H gel formation and phase transitions. • Excessive microwave heating can lead to higher porosity with larger pores. • Microwave curing optimizes both ultra-early and long-term material performance. This study investigates the synergistic effects of microwave heating on the early, mid, and long-term flexural and compressive strengths of fly ash-slag-based 3D-printed geopolymers. Results show that flexural strength reaches its peak with 30–45 s of microwave heating at 1260 W, while compressive strength is maximized with 15–30 s of heating at 1800 W. Thermal analysis indicates gel dehydration and carbonate decomposition, with greater mass loss at longer microwave heating times. XRD, FTIR, and SEM collectively confirm the formation of C-A-S-H gels, calcite, and enhanced geopolymerization and densification over time, which contributes to improved mechanical properties. MIP analysis reveals that extended microwave curing increases overall porosity and coarsens the pore structure, which presents a trade-off between achieving high ultra-early strength and maintaining optimal long-term microstructural density. Microwave curing accelerates early strength development while maintaining durability, offering a rapid, low-carbon, and energy-efficient alternative for sustainable 3D-printed construction. These findings underline the potential of microwave-cured geopolymers to replace OPC in additive manufacturing, optimizing performance while significantly reducing the environmental impact of construction. This approach supports the principles of cleaner production by valorizing industrial waste (fly ash and slag) and reducing the energy footprint of the curing process.