As I discussed a few posts back, the most salient feature of a truly closed ecosystem is hermeticity of the encosure. For a CELSS with stability, that means not only a very low permeability, but also a high reliability over a long time (multiple human generations at least). So last summer, I started researching materials with an eye skewed toward Martian resources and conditions and eventually stumbled into magnesium based cements. As I researched this further, I gradually convinced myself that a magnesium phosphate cement was THE ideal material for constructing Martian habitats.
- Nearly as impermeable as glass
- Even lower coefficient of thermal expansion, hence better reliability in environments with extreme temperature swings (like Mars)
- Sets at close to freezing (0°C) temperatures, addition of antifreeze could potentially even lower that to -60°C
- Two part aqueous slurry chemistry requires only modest energy, but could be used in autonomous deposition (i.e., remote 3D printing)
- Two part aqueous chemistry also has some repercussions for delivery that invoke biomimicry
In late September, I found out about the New Worlds 2016 Symposium, and submitted an abstract, which was accepted, then put together a paper and ulimately presented in early November in Austin. I also raced to file a provisional patent before submitting these publications, and planted the seeds of a new venture. Those venture seeds are germinating at my Cerambotics website. As of the date of this post, New Worlds still has not published the papers, or videos of the presentations. My slides can be found at http://cerambotics.com/NW2016presentation.html.
The mostly unasked question I infer from others versed in this topic of ISRU hab materials is how do I rationalize using wet chemistry when most other space-hab construction efforts are currently focused on sintering methods? I should say that I am a proponent of developing sintering methods in general. I think sintering is a grand idea on the moon and asteroids, where water is likely to be limited, but because of the low gravity in these scenarios it will necessarily be high temperature sintering to make up for low pressure. High temperature means high power. On Luna, and asteroids inside Mars' orbit, there is abundant solar energy to tap into. At Mars, however, insolation is only half that at Earth orbit, and on the surface, it is further attenuated. Without tethering to vast surface solar arrays or orbital power beaming, it is unlikely that a large structure could feasibly be sintered on Mars. At the same time, water is expected to be abundant on the surface, or rather just under the surface. While temperature and pressure conditions on Mars lead to sublimation of surface water, indications point to an abundance of permafrost at several inches below. The water content here will need to be addressed even for sintering this material, as it will create voiding in the sintered material if not first extracted. Better to use it, especially if there is a way to build habitat walls with less energy. Now, with my proposal, there must be a way to keep the aqueous slurries from freezing at Arctic temperatures. One solution is to just add heaters to the plumbing. That adds energy requirements, albeit nowhere near to what sintering needs. Another solution is antifreeze. Fortunately, one of my slurry constituents already acts as an antifreeze: phosphoric acid. I have some other candidates I am investigating for the magnesium slurry. One of those candidates is a material already identified in Martian regolith: perchlorates, which are known to lower (in the right amounts) the freezing point of water to nearly -70C.
So to summarize my rationale, there are several reasons driving me away from sintering and some good indications that wet chemistry is actually very feasible on Mars.