Comparison of lunar and Martian regolith simulant-based geopolymer cements formed by alkali-activation for in-situ resource utilization

Building infrastructure for space exploration and habitation pose tremendous engineering challenges. Characterized by extreme temperatures in the lunar environmental cycles at the poles (the proposed landing for Project Artemis), the vacuum of space, and very limited access to water, most conventional construction materials, such as ordinary Portland cement (OPC, are not suitable as construction materials under space conditions. Not only does OPC require a significant amount of water, it is prohibitively costly to transport from Earth to the Moon or Mars. These limitations present a significant challenge for traditional approaches to manufacturing construction materials, suggesting fundamentally new, innovative approaches are necessary

Space exploration, therefore requires in-situ resource utilization (ISRU) developing construction materials using Martian/Lunar regolith instead of terrestrial resources. Lunar regolith is the topsoil layer on the lunar surface and it consists of mainly anorthosite and basaltic rocks. Consequently, in addition to the lunar regolith samples returned by the Apollo astronauts, significant research by NASA and other nations study regolith properties using rover missions as well as instrumented satellites, resulting in simulant soils consisting of terrestrial volcanic ash and other natural minerals that mimic the geochemistry and morphology of a variety of lunar and Martian regoliths for earth-based research.

Several strategies have been proposed for extra-terrestrial construction, including solar-sintered regolith and sorel cement. Extra-terrestrial cement is mainly required for developing vertical takeoff/landing pads for Martian and lunar surfaces to protect against secondary ejecta consisting of debris and dust kicked up by landings and take-offs, as well as habitats to protect humans and equipment against the harsh environmental conditions, including radiation. Compared with other strategies, the geopolymer binder has several advantages. It can serve as a bulk binder because it is rich in aluminosilicate minerals, and it requires less energy and water to prepare. Interestingly, the feasibility of producing geopolymer binders using Martian and lunar regolith simulants has been demonstrated. However, more studies are still needed to understand geopolymer binders fabricated from different regolith simulants and methods.

In their research efforts to provide cutting-edge technology that will support such long-term exploration and inhabitation of space, Dr. Jennifer Mills, Dr. Maria Katzarova and Professor Norman Wagner from the University of Delaware created a framework for comparing lunar and Martian regolith simulant-based geopolymer binders fabricated via alkali activation for ISRU. The authors also studied the environmental exposure effects (sub-freezing temperatures, vacuum and high temperatures) on the strength of the binder cement. They commenced by formulating geopolymer binders using lunar and Martian regolith simulants. Finally, they measured the material properties and characterized the microstructural evolution of the geopolymer binders under different environmental conditions. Their work is currently published in the journal, Advances in Space Research.

The researchers reported the successful conversion of one Martian regolith simulant and three lunar regolith simulants to geopolymer binders via alkali activation with sodium silicate. They recommended decreasing the solid content and increasing amorphous aluminosilicate content for effective activation of Martian simulant because small particles and aluminosilicate enhance the comprehensive strength of the binders under ambient curing. The comprehensive strength of all the three lunar simulant binders was decreased in vacuum and low-temperature conditions but increased in high temperatures (600 °C). A reduction in the comprehensive strength of Martian binders after heating was attributed to the high content of magnesium and iron.

Overall, variability in minerology, morphology and chemical composition of the regolith simulants exhibited different impacts on the mechanical properties of the final binders. The comparison of the strength properties of the binders with those in literature also emphasized the effects of these parameters on the comprehensive strength despite the differences in formulation and processing procedures. Furthermore, it was noted that extreme conditions could enhance the effects of chemical differences on reaction time and binder conversion due to the close link between reaction kinetics and chemical compositions.

In summary, the authors explored the application of lunar and Martian regoliths to fabricate high-performance construction materials suitable for use as landing pad structures for rockets. The findings support the need to further develop geopolymer binders for ISRU construction. It also provided a framework for understanding the possible differences between regolith simulants across literature research and under different environmental exposures. In a statement to Advances in Engineering, the team explained that the findings of their study will enable the development of a landing pad within the current payload limitations by sending activators to the lunar surface. Prof. Wagner’s research group includes significant efforts to develop geopolymers for earth-based sustainable construction materials, showing the dual use of this advanced space technology development to improve the human condition here on Earth.