Toward Sustainable Lunar Construction: Optimization of Composite Alkali Activators for Nano-SiO2-Modified Simulated Lunar Soil-Based Geopolymer with Macro- and Microstructural Characterization

Lunar soil-based geopolymers are promising in situ construction materials, but their reliance on Earth-transported alkali activators limits sustainability. This study developed a nano-SiO2-modified simulated lunar soil-based geopolymer (NS-SLSG) with reduced activator demand by optimizing sodium hydroxide (SH), calcium hydroxide (CH), and sodium silicate (SS). Inductively coupled plasma atomic emission spectrometry (ICP-AES), orthogonal testing, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Thermogravimetry–differential scanning calorimetry–derivative thermogravimetry (TG-DSC-DTG), Energy-dispersive X-ray spectroscopy (EDS), and Mercury intrusion porosimetry (MIP) were used to evaluate apparent elemental-release characteristics, flowability, mechanical properties, and microstructure. The experimental results indicated that the measured Al and Si concentrations were particularly sensitive to the SH dosage; SH and SS exerted comparatively greater effects on flowability, whereas SH and CH had greater effects on mechanical performance. Adding 0.75 wt.% NS reduced the total admixture mass in the experimental formulation by 23.9% while increasing compressive and flexural strengths by 26.0% and 19.5% to 36.15 and 8.26 MPa, respectively. Microstructural results suggested that SH promoted depolymerization, while CH may have enhanced Ca2+-assisted polycondensation, with corresponding variations in gel composition and pore structure. These findings indicate a viable strategy for improving the resource efficiency of lunar construction materials.