Facing compositional heterogeneity and thermal cycling challenges in lunar regolith based geopolymer for sustainable extraterrestrial construction

Abstract

This study developed a high-fidelity lunar regolith simulant (GCD-2) that matched the physicochemical characteristics of Chang'e−6 real lunar samples. Then, a dry mixture of GCD-2 (acting as a precursor) and solid sodium silicate (acting as an activator) was used to synthesize the Lunar regolith based geopolymer (LRG) via the one-part method with the addition of water. An environmental-compositional framework was established to elucidate the coupling effects of extreme lunar temperature cycling (−178 °C–113 °C, 700 h/cycle) and elemental ratios (Ca/Si and Al/Si) in precursor on the mechanical properties of LRG. Multiscale characterization techniques, including Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy, X-ray Diffraction, Thermogravimetric Analysis, and Nitrogen Adsorption-Desorption measurements, were integrated with analysis methodologies (Brunauer-Emmett-Teller, Barrett-Joyner-Halenda, Density Functional Theory) to examine microstructural evolution. Results demonstrated that increasing Ca/Si ratio (from 0.26 to 0.35) enhanced the compressive strength of the LRG but exacerbated cryogenic susceptibility compared to the baseline group (Ca/Si = 0.26 and Al/Si = 0.34). Conversely, increasing Al/Si ratio (from 0.34 to 0.58) initially improved the compressive strength and reduced cryogenic susceptibility, followed by a decline in mechanical properties and an increase in cryogenic susceptibility. The LRG with Ca/Si of 0.35 and Al/Si of 0.58 achieved the maximum compressive strength (52.25 MPa), along with superior post-cryogenic strength retention. An optimized operational period for mixing and casting of the LRG at lunar surface was suggested to be within 100 h before high-temperature phase of the lunar day. Furthermore, cryogenic degradation in the LRG with high-Ca system was determined by the coupled internal and external factors, whereas the degradation in the low-Ca systems was predominantly affected by the external factor. The internal effect involved the collapse of the gel network, propagating the degradation of the pore structure, while the external effect included pore water freezing-induced cryo-suction and gel phase incompatibility, which governed detrimental pore evolution. The results of the current study can provide theoretical foundation for optimizing lunar construction method, advancing cost-effective and sustainable extraterrestrial habitat engineering.