Making Use of Urea

To establish a sustained presence on the Moon, humans need habitats capable of withstanding unfriendly outer space conditions such as vacuum, cosmic radiation and extreme temperatures. However, constructing said habitats would require huge volumes of material that are too heavy to feasibly bring from Earth. An alternative approach is to make use of materials already present on the lunar surface, a key idea known as in-situ resource utilisation, or ISRU.

Previous studies have shown that geopolymer might be a promising material for ISRU construction on the Moon, since lunar regolith – the loose mineral dust on the Moon’s surface - can be used as the major ingredient for the geopolymer recipe¹. The other necessary ingredients - water and alkaline activator – have the potential to be sourced on the lunar surface as well, meaning that no material needs to be brought from Earth. However, water on the lunar surface is rare and highly valued². In addition, geopolymer mixtures with increased solid content have been shown to promote greater compressive and tensile strength³. The challenge therefore is to alter the geopolymer ‘recipe’ to use as little water as possible, while keeping within the confines of what is available to use in-situ.

Rising to this challenge are a group of researchers from the ACT and Østfold University College, who, as part of an ongoing joint Ariadna study, are investigating the potential of urea as a superplasticiser for geopolymer mixtures. Superplasticisers are chemical additives that reduce the volume of water required in the geopolymer recipe, while at the same time ensuring good workability is retained in the mixture – this is vital to ensure the material can be moulded or printed into a desirable shape. But why urea? In a crewed mission to the Moon or Mars, urea becomes a readily available in-situ resource: after water, it is the most abundant component of human urine. Although this small organic molecule is typically treated as a waste product, urea has the ability to reduce viscosities of aqueous mixtures via the breaking of hydrogen bonds⁴. Urea is therefore proposed as an ISRU geopolymer superplasticiser, meaning it could soon go from being a humble waste product to a valued by-product.

In the collaborative study, geopolymer mixtures with urea have been compared to more commonly used superplasticisers (namely, polycarboxylate- and naphthalene-based), as well as geopolymers with no superplasticiser additions. To simulate the extreme temperature conditions of the lunar surface, all samples were also subjected to vacuum and freeze-thaw cycles during curing. So far, the following promising results were found in samples with urea superplasticiser additive:

• Water and alkaline solution demand was reduced
• Material workability and flow properties for additive manufacture were improved
• Favourable compressive strength results were shown compared to other materials that have been proposed for lunar in-situ construction

Overall, geopolymers with urea demonstrated highly efficient use of in-situ resources, with enhanced mechanical properties. Furthermore, these results can be valuable for Earth applications as well: urea is an easily accessible, cheap plasticiser, and its use can reduce water requirements and improve workability of geopolymer.

The results of this study, which also benefitted from collaboration with Technical University of Cartagena, Spain and University of Padua, Italy, have been published in the Journal of Cleaner Production. Additional investigations of this joint study are now underway, focusing on reinforcing geopolymers with in-situ basalt fibres to further enhance their strength.