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Amazingly, I could not find a topic on these basic building materials in the life support section. But they are extremely relevant to construction on Mars. So I thought it would be worthwhile creating one.
Part 1: Cob
Cob is a building technique that has been used in England for many centuries. Clay and sand are mixed with straw and are used to build up walls by hand in layers. Some cottages made from cob are centuries old. In England, damp is a constant problem. Cob walls are built upon a damp proof layer of heavy stones. The structure is usually covered by an overhanging thatched roof. From a modern perspective, the value of cob is questionable. The main costs associated with building a house are labour. Why use a material that is weaker in compression and vulnerable to damp, when much stronger materials like baked brick and concrete are available and won't cost much more? This is why cob isn't used much anymore. Apart from the occasional garden shed.
On Mars, a number of things look different. Firstly, we are unlikely to have available large quantities of portland cement. We may be able to access liquid water and Martian regolith is essentially dessicated clay. The Martian surface is also very dry, which removes a significant problem for cob. Compressive strength of dry cob is about 1.2MPa. That is sufficient for quite sizable structures, especially if walls are tapered outward towards the base.
https://www.icevirtuallibrary.com/doi/a … n.22.00022
The straw provides tensile reinforcement and also prevents crack propagation. On Mars, we would need to either grow natural fibres to reinforce the cob or instead use a synthetic fibre. This could be something simple like shredded polyethlene.
For dome and tunnel structures that are relatively small, I wonder if we could do something even simpler and less labour intensive. Maybe we can cast structures out of wet mud? For a dome, we would create a formwork by inflating an inner polymer dome, within a second outer polymer dome. A wet clay produced by mixing fine regolith with water, would be poured between the layers. The outer dome will be permeable. The cast mud will rapidly dry in the thin, dry Martian atmosphere. Once dry, we deflate the inner dome and remove the outer. We now have a free standing dome made from dry mud. Next, we cover the entire dome with several metres of raw regolith. We paint the interior of the dome with lime, to provide a hard, impermeable inner surface. Next, we pressurise.
The strength of the dry clay material will be no more than a few atmospheres. This limits the size of the domes that we can produce in this way. But this method allows us to build habitable volume very rapidly. We simply inflate a double skin polymer dome, fill the gap between the layers with mud and let it dry. The polymer can be used over and over again. The only materials needed are regolith and water. It would be neat if we could find a way to recapture the water. Liquid water will not be an abundant resource unless we can find an aquifer. One way of doing this would be to make the outer polymer dome impermeable and the inner dome permeable. As the mud dries, the inner volume will fill with water vapour. We can run a compressor, which will compress the vapour back into a liquid for reuse. In this way, we coukd recover almost all of the water needed. So the only material we need in abundance is regolith fines, which are everywhere.
Last edited by Calliban (2023-11-27 16:56:28)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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Adobe is as ancient as cob and remains in use as a building material in dry climates.
https://en.m.wikipedia.org/wiki/Adobe
Adobe is unfired bricks, made from a mixture of clay, sand and water. Straw is sometimes added. The bricks are cast in moulds and are left to dry in sunlight. The resulting bricks are then bonded into walls with mortar made from the same material. Adobe bricks vary in compressive strength, but most US building codes specify a minimal compressive strength of 2MPa. On Mars, without the threat of damp or rain, walls should remain strong and free from erosion for centuries. Following the production of a cast mud dome, adobe bricks could be used to construct a central pillar, which would provide additional design factor against the weight of the overburden.
Internal walls could also be made from adobe. All water used in the manufacture of adobe can be recovered. On Mars, we will not have wood or abundant metals to make furniture for early settlements. Seats, tables, beds, cupboards and sideboards, could be made from adobe bricks and painted with lime or polished with wax. Stairways can be made from adobe, with stone paving. Floors will be polished stone, set in soil based mortar. Even sinks, baths and showers can be adobe, if sealed with an oil or wax. This suggests that we could built entire cities on Mars from little more than dirt, using endlessly recycled polymer formwork and water that is used again and again.
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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Same mixture as brick which we do have but as far as building the structure after making them might not seem a possible first round building but if we can layer the mixture into a dome and build out from that we can build brick use into the continued expansion of the building. It is possible to build arches as well to create the central area as well.
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