- The lack of the availability of concrete ready mix on Luna has been cited as a problem for lunar colonies. Indeed, the scarcity of water on Luna leads some to predict that this old standby of structural engineering will be left behind on Earth. What can replace it?
- What is needed is a cement that is liquid at reasonable temperatures for construction and cures at reasonable temperatures, filler material in fine grain size for mortar and filler in grades of coarser chunks for concrete. All materials must be reasonably available on Luna.
- One possibility for cement is an analog to the dental filling amalgams used on Earth. Amalgams are alloys of Mercury which are mixed from liquid mercury and a powder containing the balance of the ingredients, such as silver or gold. Minor amounts of other elements have been shown by experience to produce good results. Amalgams can be mixed and cure in a few minutes at room temperature. The mixed alloy is pressed into a cavity and fills it. The filling is typically shaped by carving as it hardens within minutes.
- Sodium, potassium and aluminum are more plentiful on Luna than mercury, gold and silver. So one should try forming an alloy from sodium potassium eutectic (NaK) which is liquid down to -12 degrees C and powdered aluminum. Minor additions to the alloy could be added to the molten aluminum before it is formed into a powder. Magnesium, calcium, silicon, titanium and boron might be considered. Mixing the powder and liquid portions of such an alloy could be done at near standard temperature and vacuum on Luna. Experiments on Earth could be done in a glove box with an argon atmosphere. Experiments with sodium or potassium on Earth are inherently dangerous. Sodium and potassium are fire hazards and explosion hazards. Mishandling them can result in caustic chemical burns. Anyone undertaking such experiments should acquaint themselves fully with proper procedures first.
- The merit of a particular composition of lunar cement will include that the powder and liquid portions mix quickly to form a paste that can adhere to whatever pieces one wants to cement together, that there is sufficient time before setting to position the pieces to be cemented, and that the cement reaches a high strength bond in a reasonable amount of time. It would be nice to have a cement that can be used for structures containing oxygen and water, but that will need to be a totally different material. The NaK liquid used could contain minor amounts of other ingredients such as silicon or calcium. The aluminum powder could contain minor amounts of sodium or potassium. The actual formulation will depend upon cost and performance evaluation.
- Sodium and potassium can sublime away in a vacuum and the eutectic evaporates so these things would be stored in sealed containers on Luna and the eutectic would be exposed to vacuum only for a limited time when mixing the cement. Vapor deposition occurs on Luna whenever materials with a substantial equilibrium vapor pressure are exposed to vacuum. So vapor deposition must be taken into account. A sodium, potassium, aluminum cement could be used in a mortar to hold bricks together on Luna wherever the structure will not house water or an oxygen atmosphere. Although there are some concerns with using an aluminum powder, NaK based cement in a vacuum; the fact that it could be used in a vacuum at all is a particular advantage. Where people and thus an oxygen atmosphere will be contained something like a silicone polymer is called for. Carbon needed to produce silicone polymers will likely need to be imported.
- A lunar concrete made with dry-mix/steam-injection method has been put forward by T. D. Lin et al. Since hydrogen is the only portion of the concrete that may need to be imported from Earth and hydrogen is only a small portion of the weight of the concrete, this may be the material of choice for some applications. The amount and availability of hydrogen on Luna is still not well known. Polar sources of hydrogen are still promising.