“Water is very, very valuable on the lunar surface,” says Marlies Arnhof, a member of the Advanced Concepts Team at the European Space Agency and a coauthor of the research. “So one of our main goals with this study was to reduce the amount of water necessary to produce a geopolymer.”
Superplasticizers are materials that are used to reduce the water content of concrete and geopolymers while maintaining their flowability. On Earth, superplasticizers are typically hard-to-pronounce substances like naphthalene and polycarboxylate. But as Arnhof and her colleagues discovered, urea works just as well and could easily be sourced on the moon. Rather than filtering out contaminants in astronaut urine and recycling the waste water, the pee could be stored in a tank and harvested for urea.
To test the idea, the researchers mixed synthetic urea powder with lunar regolith simulant to make cylindrical structures the size of a fist and let them dry under a weight. They then simulated using the material in a 3D printer by extruding it in layers through a syringe. They compared the results with conventional geopolymers. “It performed quite well,” says Anna-Lena Kjøniksen, a materials scientist at Østfold University College and coauthor of the study. “It seemed to give the best overall results, especially when it came to avoiding crack formation.”