Mars Colony Cement & Concrete

JoshNH4H · 2012-02-03 17:49:52

So, I think it's time to bring up the issue of cement and concrete for a Martian colony. Concrete is a great building material for foundations and is used often in construction on Earth. It seems that the primary uses for concrete are in foundations and in building very large structures. I would think that a small Mars colony would be more interested in the former than the latter, and the lower gravity may mitigate the need for this somewhat. Nevertheless, at the very least cement of some kind will be needed to be used as a mortar for bricks.

A quick note on vocabulary, which I mention because I didn't know: Cement and Concrete are two different things. Concrete is a mixture of Cement, aggregate (e.g., rocks or soil), and water. Cement is the binder.

On Earth, the production of cement involves heating limestone (Calcium Carbonate), to 1450 C to produce Calcium Oxide, which is then mixed with other things (On Earth, it can include, ash, sand, or gypsum). To make concrete, this is then mixed with water, along with aggregate (rocks/dirt, I think), and allowed to dry/harden.

There are lots of problems with using this mixture on Mars. Firstly, there is no limestone to be found on the Red Planet whatsoever, because limestone was made umpteen years ago from the shells of living creatures. Calcium is also relatively rare on Mars compared to Earth, with Calcium Oxide composing approximately 7% of the mass of the soil. Presumably it would be more concentrated in places. I don't know if we could expect a hundred percent or even fifty percent anywhere. Anyone know anything about this?

The other problem is getting the concrete or cement to set. The primary problem here is the low temperature. Because the setting of cement involves water, you can't really do it below the freezing point of water. The low pressure of Mars is also an issue here, in that the water will likely evaporate instead of being physically/chemically absorbed into the mixture (I believe that's what happens, of course feel free to correct me if I'm wrong).

4Frontiers has made a suggestion which helps with the first suggestion but not the second. In this powerpoint they talk about the possibility of using Sorel Cement instead of Portland Cement. Sorel Cement is made of magnesium oxide and chloride, and magnesium is apparently more concentrated and easier to get at. I can't comment on this. They claim that it would be able to set in martian conditions because it is a non-hydraulic cement, but I'm a bit skeptical. It is not wise to underestimate what 6 mb of pressure or -50 C can do; Liquid water does not fair well. It is still an interesting proposal, of course, but I would like someone more knowledgeable about this before I comment much in either direction.

So: We need a material that can be made from materials which are known to exist or could be expected to exist on Mars (gypsum does fall into this category, if anyone was wondering), and which will set properly at Martian temperatures (seeing as we have Sorel cement for the inside applications). While this cement should be able to set at martian temperatures, there is no guarantee that it will necessarily remain at those temperatures. For example, even with insulation it is not totally inconceivable that the foundation for a building could get above freezing.

GW has suggested Ice in a composite with Basalt Fiber. This has actually been proposed on Earth, where it is known as Pykrete. It's good down to about -15 C, seems like. Should work for most purposes, though I will admit that I would like to find something that would work at room temperature as well as at Martian ones.

I think this is a sufficiently long post to say that I don't know what the best solution would be. Does anyone have any ideas?

SpaceNut · 2012-02-03 21:20:39

I think you hit it right on the head but when we live on Mars at first we will need protection until we can get an atmosphere to help shield us. That said we will most likely be under ground partially or completel.. That means some tunneling which would lead to a creation of a grainulated material from the digging process of which the exposed tunnel needs a liner to seal the walls tightly.

http://www.lpi.usra.edu/meetings/isru97/PDF/LIN.PDF

David McKay seems to be a name that comes up often in searches for the subject...

Using Fabricators to Reduce Space Transportation Costs

GW Johnson · 2012-02-03 23:39:01

Here on earth the aggregate in concrete is small stones. Dirt as we know it here does not work. Too fine-grained, and the organics decompose, leaving behind voids. The key to concrete as a building material is the sand and stones: two widely disparate particle size distributions as reinforcement.

Here, concrete is cement plus sand (around 0.5 mm diameter) plus gravel/small stones (around 50 mm diameter). And, both the sand and the stones are washed very clean, and thoroughly dried. Not only that, they are carefully sieved for their size distributions. The chemistry that sets the cement is a hydration reaction of the calcium oxide with water. It is slightly exothermic, and requires the water to be in the liquid phase.

Any concrete variant on Mars will require a source of calcium oxide (unslaked lime) for the cement. Sand and stones should be no problem, but the lime is a problem. That means we need something widely available that is peculiar to the physical chemistry of Mars, to serve as the source material for cement.

I suggested ice because it is widely available there and here, and because the same sort of particle reinforcement would provide similar properties. Plus, on Mars, temperatures are well below the freezepoint of water.

There are fiber-reinforced versions of concrete in use here. So a basalt fiber reinforced material would make a lot of sense there. With ice there is no chemical reaction, only a phase change. But, the result is a strong composite material anyway.

Building materials (like concrete) are generally particle-reinforced composites, and they have to be very inexpensive, and widely available. We may not be able to identify viable candidates until men have actually landed on Mars and played with the local materials. It is difficult in the extreme to program a robot to look for stuff like this, precisely because you won't know it until you see it.

That's why I keep saying don't plan on successful ISRU on the first mission. Do the detailed exploration and the ISRU experiments on the first mission, and use that new knowledge to plan an ISRU scheme that actually will work, on the second and subsequent missions.

GW

JoshNH4H · 2012-02-03 23:54:16

Spacenut- I sure love those USRA links. Pure gold, every one of them.

It doesn't look like either of those sites give specific instructions as to how to make concrete, specifically with regards to the ability of concrete to set outside on Mars.

On a somewhat related note, I wonder how much basalt fiber it would be possible to suspend in water. If you can get a lot of it to stay in suspension, you can just pour in the slurry and let the water evaporate. On second thought, for anything larger than maybe half a meter to a side you'll probably end up with a layer of basalt fiber and then pykrete in the middle.

I suppose the other option is to pour red hot liquid basalt straight into the ground. That'll be messy, though, and dangerous.

Another possibility is to take the regolith, mix it with the Iron Oxide fines, then mix it with a strongly acidic water mixture, which will dissolve some of the salts. With time, it will set into a concrete-like mixture.

Looking at the options, I'm starting to think that pykrete is probably our best option. This is to say nothing of mortar for bricks, though. Silicone is possible for that, though it would be a very energy intensive mortar. I like the idea of using Sorel Cement for that, just maybe with a water-sulfuric acid or water-ammonia mix instead of just plain water, since that would have a much lower freezing point.

Last edited by JoshNH4H (2012-02-03 23:56:22)

Midoshi · 2012-02-04 11:18:58

As far as availability of carbonates goes, there's a good chance it could be mined from subsurface deposits. In 2010, Michalski & Niles spectroscopically identified carbonates in Leighton crater, which had excavated them from deep in the crust. It appeared to mostly be calcite (CaCO3) and siderite (FeCO3). A very similar observation was made about 1,000 km away in 2011 by Wray, which suggests there is a massive subsurface carbonate deposit spanning much of Mars. This is what was theoretically expected from our knowledge of the atmosphere and geology, but up until now there hadn't been any hard evidence for it. You see some details at the links below:

http://aram.ess.sunysb.edu/tglotch/TDG30.pdf
http://www.nasa.gov/mission_pages/MRO/n … 10308.html

JoshNH4H · 2012-02-04 14:18:33

So that's definitely good news. If there will be calcium, then, I suppose we can use plain old portland cement, which is good news. It probably won't be able to set under Martian Conditions, still, though, right? Might we want to add in some other compounds (like Sulfuric acid, maybe?) to lower the freezing point of the water? I suppose we could also add in a hydrate that releases a lot of energy upon hydration, but reacts fairly slowly.

Portland Cement will probably be the mortar of choice, though.

I've heard that concrete can be embrittled by high concentrations of CO2. Is this true? Will something need to be done about this?

louis · 2012-02-05 08:08:49

Wouldn't it just be easier to have a "heat tent" that you place over the construction site with a heat source inside to replicate Earth temperatures?

Midoshi · 2012-02-05 13:12:45

Sulfuric acid would neutralize the calcium alkalide, and you'd just wind up with gypsum (CaSO4). You can actually use this as a mortar, as the Egyptians did with some of their later structures, but it's not as durable as lime mortar. Mars does have large natural gypsum deposits, so that might be worth looking at some more. In construction gypsum = plaster of Paris.

I found a short paper on construction materials for Antarctica. Apparently they try to shy away from concrete as much as possible because of bad experiences with it under the extreme conditions. It mentions some practical problems one might not foresee from an armchair.
http://www.antarctica.gov.au/__data/ass … arctic.pdf

Glandu · 2012-02-06 04:36:30

As I said in the other thread, composite quality is heavily dependant on the link between the matrix & the "cloth". Pykrete is not a concrete, it's a composite. It works well because there is no bubbles between wood & ice. In less technical words, wood is "wet" with water. Will basalt be "wet" with water? Or will it need a yet-to-be-ionvented chemical add-on to be wet?

louis · 2012-02-06 07:03:46

Midoshi wrote:

As far as availability of carbonates goes, there's a good chance it could be mined from subsurface deposits. In 2010, Michalski & Niles spectroscopically identified carbonates in Leighton crater, which had excavated them from deep in the crust. It appeared to mostly be calcite (CaCO3) and siderite (FeCO3). A very similar observation was made about 1,000 km away in 2011 by Wray, which suggests there is a massive subsurface carbonate deposit spanning much of Mars. This is what was theoretically expected from our knowledge of the atmosphere and geology, but up until now there hadn't been any hard evidence for it. You see some details at the links below:
http://aram.ess.sunysb.edu/tglotch/TDG30.pdf
http://www.nasa.gov/mission_pages/MRO/n … 10308.html

"No problems where encountered, however, in achieving concrete strength or in any damage from freezing." That sounds like a rather hopeful conclusion to me. The problems they experienced in the notoriously strong winds of Antarctica shouldn't be replicated on Mars with its weak winds, and as suggested above, a tented covering with a heat source in side should help all round.

Midoshi · 2012-02-06 09:52:30

Glandu wrote:

Will basalt be "wet" with water? Or will it need a yet-to-be-ionvented chemical add-on to be wet?

I don't know about basalt specifically, but water is more attracted to hydrogen bond to the oxygen in silica (SiO2) than with other water molecules (this is why you get a meniscus in a glass of water). Basalt is mostly silicates (-SiO4), so I my educated guess is that it would "wet" pretty well. But if not, it wouldn't be difficult to "weather" the basalt fiber surface to make it "stickier" (hydroxyl groups or something, perhaps).

JoshNH4H · 2012-02-06 15:39:31

Louis- Because convective heat loss is not significant on Mars, it would actually be better to have a "heat sheet" that was coated in a reflective metal such as aluminium and minimized heat loss. Even with that, though, I suspect that the energy costs would be pretty significant. I don't have numbers to back this up because the calculus required to do so is annoying and I don't feel like doing it right now. (There is also the possibility that I am not yet mathematically equipped to do this calculus, though I don't think it likely because it doesn't seem to necessitate multivariable calculus). This would largely be dependent upon how long it took the concrete to set.

Midoshi- True, I hadn't thought about the fact that Calcium oxide is basic. That report (as well as GW saying in the other thread that he had trouble getting concrete to set below -2 C, and simple logic) generally seems to indicate that concrete as it's usually used on Earth will not work on Mars without heating, especially seeing that it's much colder than Antarctica. While the thinner atmosphere would help, conduction would still be a significant loss.

wrt Ice Basalt Fiber- As Midoshi said, the high affinity of water for glass and presumably similar materials would likely make the issues involved in removing bubbles smaller. This might be improved by heating the water-basalt fiber mix to get the basalt fiber to hydrate a bit. This is probably a good idea anyway because hot water (for reasons which I understand are not known at this time) freezes faster than cool water.

For mortar, I would suggest that Plaster of Paris might be sufficient, and if it weren't could conceivably be reinforced with Basalt Fiber. In other words, I'm suggesting that we use colligative properties to lower the freezing points of a water-sulfuric acid solution, and then expecting this to react with CaO to form CaSO4. I don't know if we can get it all the way to Martian temperatures but I suspect we could get it pretty low given that they're miscible in each other.

Last edited by JoshNH4H (2012-02-06 16:00:35)

louis · 2012-02-06 17:59:27

JoshNH4H wrote:

Louis- Because convective heat loss is not significant on Mars, it would actually be better to have a "heat sheet" that was coated in a reflective metal such as aluminium and minimized heat loss. Even with that, though, I suspect that the energy costs would be pretty significant. I don't have numbers to back this up because the calculus required to do so is annoying and I don't feel like doing it right now. (There is also the possibility that I am not yet mathematically equipped to do this calculus, though I don't think it likely because it doesn't seem to necessitate multivariable calculus). This would largely be dependent upon how long it took the concrete to set.
Midoshi- True, I hadn't thought about the fact that Calcium oxide is basic. That report (as well as GW saying in the other thread that he had trouble getting concrete to set below -2 C, and simple logic) generally seems to indicate that concrete as it's usually used on Earth will not work on Mars without heating, especially seeing that it's much colder than Antarctica. While the thinner atmosphere would help, conduction would still be a significant loss.
wrt Ice Basalt Fiber- As Midoshi said, the high affinity of water for glass and presumably similar materials would likely make the issues involved in removing bubbles smaller. This might be improved by heating the water-basalt fiber mix to get the basalt fiber to hydrate a bit. This is probably a good idea anyway because hot water (for reasons which I understand are not known at this time) freezes faster than cool water.
For mortar, I would suggest that Plaster of Paris might be sufficient, and if it weren't could conceivably be reinforced with Basalt Fiber. In other words, I'm suggesting that we use colligative properties to lower the freezing points of a water-sulfuric acid solution, and then expecting this to react with CaO to form CaSO4. I don't know if we can get it all the way to Martian temperatures but I suspect we could get it pretty low given that they're miscible in each other.

Josh,

Yes, I was assuming a heat tent with a reflective internal lining. I doubt that during the day it will be that significant an amount of power required - remember Inuits used to warm the interior of an igloo with a tiny candle. But in any case energy is the one thing we will have plenty of on Mars.

JoshNH4H · 2012-02-07 00:50:37

Josh,
Yes, I was assuming a heat tent with a reflective internal lining. I doubt that during the day it will be that significant an amount of power required - remember Inuits used to warm the interior of an igloo with a tiny candle. But in any case energy is the one thing we will have plenty of on Mars.

I actually don't agree with that in the slightest. I did try to do the math and, though it was beyond my ability, it is fortunately a generic enough problem that the internet provided a solution; namely that:

Where:

qr is the total heat flux through the surface
k is the thermal conductivity of the material
r is radius from the centerpoint
T is temperature

Note that in this circumstance the 4 should be a 2 because we are looking at a hemisphere instead of a full sphere.

If you take the limit as r2 goes to infinity (which is to say, the fact that the temperature at infinite distance from the centerpoint approaches the mean for Mars), you will see that qr approaches zero. This indicates that once the system reaches thermal equilibrium, where the temperature gradient does not change as time progresses, there will be no heat flux. However, seeing as this involves warming the entirety of the planet Mars (modelled in this case as an infinite flat surface with infinite depth, which would be a close enough model if there were an actual heat flux involved), this would actually require a tremendous amount of energy. in reality, there would be some energy consumed in warming the regolith immediately surrounding the structure and then after that the primary energy loss factor would come from radiative losses from the warmed soil. This is calculable but it would be annoying to do so, and it is subject to variation based on fine changes in the external circumstances such as what structures are near the place where the concrete is being set.

Radiative heat loss is actually a pretty big loss factor. For example, an object with mars-average albedo (.2) radiating at 300 K will radiate at 370 W/m^2. Keeping in mind the fact that the radiating area will be significantly bigger than 1 m^2 (even if it's not at 300 K, it may be fairly close to it), there will be a significant loss of heat from the concrete.

Now, whether we have plenty of energy on Mars is a question of whether or not we decide to build a large excess of concentrated solar power plants or not. But regardless of what the capacity for energy generation relative to need is, there is a floor on how much power will cost to the colony as a whole based on the cost of producing it. Note that I said "the colony as a whole." Regardless of what subsidies may exist, someone is paying for the energy and it won't be cheap. Even on Earth solar of any kind isn't competitive with fossil fuels without subsidies, and on Mars the sunlight is only less intense. It will be the method of choice to generate power but only because there are no fossil fuels available (remember, there are ~zero pollution risks on the planet Mars), and an early colony won't be doing nuclear yet. I will reiterate that the statement that there will be boatloads of energy to waste is false. Resources spent creating unneeded generating capacity could be better spent doing other things, such as expanding the colony or raising the quality of life of the colonists.

SpaceNut · 2012-02-07 21:08:30

There was something that I recalled about using ground up Granite in a mixture that made very strong to weather conditions surfaces.....

http://www.crushers.com.ng/news/use-of- … ction.html

http://lejpt.academicdirect.org/A17/085_096.pdf

GW Johnson · 2012-02-07 22:06:48

In the other conversation under "3D Printers", I just today described an ice-matrix Mars concrete, including how to form the basalt fiber rebar, and how to pre-stress beams for bending resistance, the same way we do here. That conversation had veered into concrete, too.

GW

JoshNH4H · 2012-02-08 15:58:48

GW-

I took a look at your post in the other thread. Seeing as this is the dedicated thread, I'll post my response here (not that there's anything unreasonable about following the track of the thread, I just try to keep it centralized). Anyway, your basic proposal as I understand it was to use water, which in cold conditions would form itself into ice, as a cement, mix in crushed up rocks as an aggregate (what is the purpose of an aggregate, by the way?), and Basalt fiber ropes for extra strength if such is needed or desired.

It seemed to me that you implied in your post that water ice has an inherent "stickiness" to it. It was my understanding that the "stickiness" of water ice is due to melting and then refreezing when introduced to a warmer object. Were you suggesting that water ice, even without melting, is more adhesive than concrete, or was I misconstruing your post?

On the topic of your proposed formulation: Sounds good to me. I would imagine that strengths might even be superior to pykrete, seeing as basalt fiber is significantly stronger than the wood fibers that they used to produce pykrete.

On the topic of energy: I am going to make a post on this in the Carbon and Carbon Monoxide thread, but I think you might be going a little bit far in attempts to conserve it.

Midoshi · 2012-02-08 21:59:52

Water ice can indeed be as hard as rock at Martian (or even Antarctic) temperatures. At -20°C it is comparable with limestone, and by 210 K (average global Martian temperature) you are approaching typical igneous rock hardness. See Fig. 2 in the link below:
http://books.google.com/books?id=fyrrPtBKKgUC&pg=PA198

JoshNH4H · 2012-02-08 23:45:29

I guess that's no particular surprise. I would imagine that the dangers when it comes to structural failure for icecrete foundations would be failure by melting. It will definitely important to make sure that the floors are properly insulated to prevent a weakening or evaporation of the icecrete.

I would also imagine that adding in aggregate and basalt fibers would increase the strength significantly, or at least a little bit.

Glandu · 2012-02-09 04:23:02

IIRC, the chinese did build a railroad line in marshes between central China & Tibet. The important thing is, they are climatizing some fo their fundations, to keep them frozen forever. Melting would mean destruction of the line.

It means, keeping those things frozen is doable - at least on earth. I have no clue on the energy costs of this architecture.

GW Johnson · 2012-02-09 17:18:56

The way I had in mind to make the suggested martian "ice" concrete would be to mix liquid water, local sand, and local rounded rocks (yep, rounded, no sharp edges or corners), and pour that into a form the same we do ordinary concrete here. Then just let it freeze solid. The liquid water would freeze to the aggregate and stick there, just like the little boy's tongue on the subzero light pole.

In such a formulation, the water ice takes the place of the Portland cement in concrete here. The other two components (sand and rocks) are the same. Here, concrete has both the sand and the rocks, and tests in compression as much stronger than mortar mix, which is just Portland cement and sand. Both are way stronger than Portland cement alone without the sand. There's something about a particulate embedded in a matrix that is far stronger than the matrix alone, even without any fibers.

As for reinforced martian "ice" concrete, you rig the basalt fiber braided ropes, or the steel rebar, inside the empty forms, then pour the concrete in to fill the forms. This embeds the reinforcement inside the structure.

To do pre-stressed beams, rig the forms and reinforcement, plus the empty pipes or tubes from one end to the other. Then pour your concrete mixture. Once fully hard, slip the pre-stress rods through the tubes, add thrust washers on each end, add the nuts, and tighten savagely, but to an engineered torque (so that the tensions in the rods are what was intended).

I see very little difference between doing this with Portland cement here and water ice there. Except that it takes about 30 days for Portland cement to reach full cure strength here. I doubt it takes that long to freeze the water and lower the ice temperature to below -20 or -30 C there. Once that cold, the stuff should be very stout.

Plus, ice composites are harder to melt than straight ice. The dirty snowbanks of plowed snow beside the road are the last to melt.

Rounding sharp edged rocks merely requires tumbling them in a drum with some steel hammer shot.

GW

Last edited by GW Johnson (2012-02-09 17:19:21)

JoshNH4H · 2012-02-09 18:01:36

GW- Sounds good to me. The only place I can think of where that won't be a workable idea is in the eventual spaceport. There, they'll probably go for cast basalt blocks cemented together using heated mortar (Or I guess regular mortar with a Water-H2SO4/Some other compound if necessary mix designed for minimal melting point).

GW Johnson · 2012-02-10 14:36:19

About the only three things I can think of RE: "ice concrete" are:

1. Boiloff of water into the near-vacuum that is the Martian atmosphere. It'll be in a stout form. I guess lay some relatively impermeable tarps over the freshly-poured material's free surface, and quickly bury that with local dirt several inches to a couple of feet deep (that's just a wild guess). The overburden pressure should stop most of the boiloff until the thing can freeze solid.

Fixing the wild guess: Somebody with a steam table handy could find the vapor pressure of water at 32 F (0 C), and convert that to dirt depth at a typical loose dirt effective specific gravity of 1.68 and 0.38 gee on Mars. I don't have one here.

2. What do we use for forms? No lumber on Mars. Old spacecraft shell plating?

3. Exposed ice concrete structures will be subject to slow sublimation into the near vacuum, reduced somewhat by the aggregate. Dirt cover should stop that. Based on the probe and rover results to date, maybe 3 inches (7-8 cm) of dirt will do. Maybe even less.

I'd use this stuff for in-ground foundations and building structures that get buried or built underground and buried.

For roadway-trackway uses, it'll need a few inches of the dirt cover on top of exposed surfaces. But it should be the same hell-for-stout heavy structural material that concrete is here at home.

Makes me wonder if there is a coating we could apply that would stop the sublimation? Ideas? That would make exposed bridge beams possible. Sort of important.

For rocket landing pads, in some locations just grade off the dirt to expose bedrock. Land directly on that.

Other places where the bedrock is irregular or too deep, that's where the mortared basalt blocks come in real handy. If you find an old lava flow, just saw them out quarry-style. Why waste the energy to melt basalt you already have to cut? It doesn't even need to be basalt. Actually, almost any rock will do.

GW

JoshNH4H · 2012-02-11 01:04:28

The vapor pressure of water at 0 C is 6 mb, which is actually just about the same as Martian atmospheric pressure. I would say just pour the water and let it freeze, and once it's frozen enough put some tiles and maybe a sheet over it to prevent further evaporation once that's happened. Of course the vapor pressure is going to be even lower below 0 C.

Forms- Would Iron work? I don't see why not. Sheet metal. It'll be lighter in the Martian gravity. It'll also conduct away the heat nicely. The other thing that I was thinking: Why are forms needed, especially for, say, foundations? Is there anything unfeasible about digging a hole and just pouring the icecrete mix in? That would even make it make a nice bond with the surrounding regolith, which I would imagine would be pretty desirable for a foundation (As an aside, will foundations actually be necessary for Martian buildings? I suppose they probably will, considering the amount of regolith that will be piled on top to provide pressurization.).

Preventing sublimation- Well, I would imagine that with time an encrusting regolith layer would form. But by the same token we clearly don't want "with time," we would want it immediately. For pre-formed objects I think that we would actually seek to avoid icecrete seeing as pre-formed cast basalt could be used. In cases where whatever it was both absolutely had to be poured and had to be exposed directly to the atmosphere, I would look at a sheath of cast basalt bricks, or maybe a film of PVAc polymer (Polyvinyl Acetate- Made by radical polymerization of the vinyl acetate monomer, made by reacting ethylene and acetic acid with a bit of oxygen over a palladium catalyst). Hydrocarbons tend to be relatively energy intensive to make but a thin film shouldn't be such a problem, especially not when it finds definite use as it does here.

I'm definitely in agreement with you on the rocket launch thing, though that is of course somewhat far in the future.

GW Johnson · 2012-02-11 10:08:42

That 6 mbar vapor pressure at 0C is a very nice result. I calculate about 3.8 inches or 10 cm dirt cover will stop all sublimation once the water freezes, and that's in total vacuum. If we use liquid water very near the freezepoint, the vapor pressure is not much higher, so a maybe a foot or 30 cm of dirt cover over a plastic tarp would do nicely while the ice concrete "sets up". That's figured at 0.38 gee, too. It's only 1.5 inches of dirt at 6 mbar here on Earth, inside a hard vacuum chamber.

That means casting ice concrete and getting it solidified without evaporation loss will be relatively easy. Simple backhoe work. Nice. We're going to need an electric backhoe.

My understanding of Mars's atmosphere is that the surface pressure is 2-4 mbar over most of the planet. The 6 mbar is down in the bottom of a very deep hole: Valles Marineris. That's pretty close to vacuum conditions in any practical sense for most of the settlement locations.

You're quite right, some foundations need no forms, only a shaped hole. It depends upon whether you want the top of slab above grade or not. For the rest, sheet metal and maybe rods or tubes as stakes will likely do. Be nice if it was scrap needing re-use. Any sort of building is going to need a foundation to be stable. Unsupported walls sink into and tip over if just set upon dirt. Slow, but fatal to the structure. Takes less than a year here to tear up a loose cement block wall in the garden.

Still thinking about preventing sublimation from exposed ice. Sure would be nice to build bridge beams from water, dirt, rocks, cheap plastic pipe, a few steel rods, and some nuts and washers. Your idea of some sort of plastic paint is very interesting.

GW

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