Microstructure and Micromechanical Properties of Lunar Regolith Simulant–Based Geopolymers

This research investigates the microstructure and micromechanical properties of lunar-simulant-based geopolymers. Lunar simulant geopolymers were made by activating Off Planet Research H2N lunar highlands simulant with a sodium silicate activator under terrestrial conditions (1.0 atm and 1.0 g). A central composite experiment was performed to quantify the effects of solution-to-simulant ratio, curing temperature, and silica modulus on the microstructure, micromechanical properties, and macroscale compressive strength of lunar simulant geopolymers. The compressive strength ranged from 6 to 37 MPa and was influenced directly by silica modulus and indirectly by curing temperature and solution-to-simulant ratio. Lunar simulant geopolymer microstructures feature large (100 to 1,000 μm) unreacted simulant particles bound together by a matrix of sodium aluminosilicate geopolymer gel and small partly reacted simulant particles. Micromechanically, the matrix comprised a softer geopolymer gel phase and a harder partly activated simulant phase whose properties resemble those found in traditional geopolymers. The simulants included some extremely hard particles, but the primary constituents had micromechanical properties resembling terrestrial aggregates. The micromechanical properties and volume fractions of the matrix phases do not predict the bulk mechanical properties by themselves. Still, they could be used with numerical microstructures to simulate the structural scale performance of the lunar simulant geopolymer system.