Construction techniques for the early days of a Mars colony.
A great deal of discussion has taken place regarding how to get colonists to Mars, and the various competing plans. The same is true, to a lessor degree, about the initial settlement and what it would look like in its first few days. However, not much has been said regarding how the colony can begin bootstrapping itself.
A few days after landing - what then?
The colonists will need to begin using Martian resources (otherwise, every pound of anything they use is going to be upmass from Earth.) For example, on of their first needs will be to increase their living and working space, because the initial landers will be small and cramped. They'll further need space for growing crops and livestock.
So, you can get this expansion space one of three ways; ship it up in section from Earth (hard, and nightmarishly expensive) create it from local resources, or some combination of the two methods.
One option for ISRU (In Situ Resource Utilization) is to make brick from Martian soil. The brick structures would then need to be made airtight to roughly the same degree a space station would need. This is not easy. The brock would also need to be protected from cracking, because a crack would mean explosive decompression.
So, that brings us to water. The colonists will need a lot of it (Brick-making uses a lot of water, as does masonry). Therefor, the colonists need a water source. On some areas on Mars, there are thought to be accessible underground aquifers, but we simply do not yet know where they are. Even on Earth, drilling a water well is often a hit or miss proposition.
However, what is certain is that there is accessible water on Mars. We know it exists in vast quantities in, and in areas near, the northern polar ice cap. Therefor, even if it turns out it can't be found in the mid latitudes (though it may be there too, and if so, no problem), we know where to find some, and the colony will surely go where there is water - it would make no sense to put it where there isn't any.
What this means in practice is that, no matter where on Mars it is, the colony will have an ample water supply.
That also means that, if water can be used to create the needed living space, it would be a great benefit.
The good news is that water can do exactly that. Instead of using brick, use ice. One of many advantages; ice is airtight. A drawbacks are that ice is also prone to cracking, though no as much as masonry construction.
Fortunately, there's an answer, one that harks back to World War II. It's called Pykrete.It was developed during WWII for Project Habakkuk, with the goal of building massive, virtually unsinkable aircraft carriers out of ice, to protect North Atlantic shipping.
Pykrete is roughly 13% by weight wood pulp, sawdust, or other absorbent fiber. The rest is water. Ratios vary a bit depending upon the fiber used. The resulting materiel is frozen, and once frozen it is much stronger than ice, and less prone to break. It has a crush strength estimated by some as 1100 PSI, others (especially when using pulp rather than the original sawdust) claim it is higher, 2000 to over 4500 PSI. For comparison, concrete is about 5000.
For an early colony shortly after landing, wood fiber or pulp would be an issue. I think a substitute could be used; clay. And clay has been found on Mars. Any sedimentary silt would do in a pinch. It won't have as much strength, but that can be overcome by simply making the walls thicker.
How does one build in pykrete on Mars? Use tractors (which the colonists will have to have anyway) to excavate large holes in the regolith (or find a handy small crater). About 40 feet in diameter and twenty feet deep would do.
The water would need to be degassed, which is easily accomplished via exposing it to the near vacuum of Mars. This would also aid in prechilling the water; they'd want it to be close to freezing.
To prepare the site, a segmented form could be used; anything that would define the interior dimensions of the construction. The regolith will serve for the exterior. The water and clay (or fiber) are then mixed and poured in continual thin layers, freezing as they go (to avoid giving it a chance to seperate) in the frigid temperatures of Mars. The pykrete will also freeze on contact with the regolith, creating its own outer layer form.
Pouring from the bottom up, the finally reach the roof. A domed form, plastic over iron rods, could be used, and the pykrete applied to the top. At the end of it, you have a solid construct, with walls, ceiling, and floors of yard-thick pykrete. It's already airtight. (An airlock or connector would be put in place pre-pour).
The structure would then be covered by a yard or two of regolith. The interior would need to be insulated and covered for comfortable living, but local materials would work well for that - perhaps fiberglass from local silica. Once insulated, the interior could be heated to normal room temperatures without harming the protected pykrete.
Above, I used a round shape for an example. Other methods may prove superior, such as creating large pykrete, "bricks" and putting them in place with pykrete "mortar". Or a rectangular shape might be preferred.
