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Modern cities are built from concrete with steel rebar because it is abundantly available and provides enough tensile strength. Concrete requires cement. Cement is made from limestone. Limestone is made from coral and skeletal remains. On lifeless planets there are no organic materials such as limestone because there was never any life. There's basically just a lot of basalt and other igneous rocks.

If you were going to build a city on another planet, what kind of materials would you use to provide the kind of tensile strength that you need?

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    $\begingroup$ Possible duplicate of What useful materials exist on a lifeless planet? $\endgroup$ – Werrf Feb 16 '17 at 18:00
  • $\begingroup$ Related: How to cut corners while rebuilding after apocalypse? $\endgroup$ – Theraot Feb 18 '17 at 8:40
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Feb 19 '17 at 4:59
  • $\begingroup$ Concrete has terrible tensile strength but excellent compressive. This is why reinforced concrete has the metal bars running through it, the metal provides tensile strength and the concrete compressive. $\endgroup$ – Tim B Feb 20 '17 at 12:07
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    $\begingroup$ People have been building cities without cement for thousands of years, using stone or wood or fired bricks or mudbricks. $\endgroup$ – AlexP Feb 21 '17 at 9:18

19 Answers 19

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I would actually use... cement, because your premise is wrong:

On lifeless planets there is no organic materials such as limestone because there was never any life.

Limestone is not an organic material (I guess the right word would be an "organic compound"). Limestone is just calcium carbonate. And there's no reason why there couldn't be sedimentary deposits of calcium carbonate without biological elements.

If calcium (and carbon, and oxygen, and hydrogen) present in the planet's crust, you will be able to synthesize cement out of it, albeit at a much higher cost than using calcium-carbonate-rich limestone.

You mention:

There's basically just allot of basalt and other igneous rocks.

Feldspar is an igneous rock containing calcium. Melt it down to separate the silica and aluminium, and you'll be able to synthesize cement.

Granted, the cost would be very expensive. So to answer this question one would need to know the geological composition of the planet's crusts, to see which useful compounds are readily available. Methane-rich planet? Synthesize plastics. Aluminium-silica rich? That means clay, which means ceramics. The best material to build cities with will depend on the availability of local compounds (which varies wildly from planet to planet), and the industrial tools that the colonists can drop on the planet (e.g. nuclear furnaces).


Edit: User @Luaan pointed out that limestone is a biogenic substance, one produced by alive organisms, and that made me remember about oil-eating bacteria.

So if the lack of biogenic materials is a problem, but the planet's crust has the chemical elements needed for those materials, a solution would be to use engineered bacteria (or other microorganisms) to produce those materials (e.g. turn feldspar into limestone). This would fall into the umbrella of terraforming.

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    $\begingroup$ A better word would be "biogenic" - formed through life. Most of the limestone we use is biogenic in origin, but not all of it - and on a lifeless world, you could expect that the dissolved calcium (captured by marine life on Earth) would precipitate from the seas directly (e.g. in similar processes that created the salt and gypsum deposits of the mediterranean). It probably wouldn't be anywhere as cheap as on Earth, though - life has a nice way of concentrating materials, which makes them much cheaper to exploit. $\endgroup$ – Luaan Feb 16 '17 at 13:46
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    $\begingroup$ Locate planet → send a probe with engineered bacteria → wait for limestone deposits → profit. :-D $\endgroup$ – IvanSanchez Feb 16 '17 at 14:04
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    $\begingroup$ See also: Gypsum which, at least from that article appears to form naturally absent any biological process. Any culture capable of pursuing a building project on a lifeless planet should be able to handle the toxic by products of liberating the necessary components. $\endgroup$ – Joshua Drake Feb 16 '17 at 14:50
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During the roman empire they didn't know modern cement, but they built "small thingy" like the Pantheon by using pozzolan, which is basically volcanic ash.

That would be a good material to use.

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    $\begingroup$ They didn't know modern cement but they had their own Roman Concrete $\endgroup$ – Maurycy Feb 16 '17 at 11:05
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    $\begingroup$ @Maurycy: Which IMHO is a heck of a lot better than modern concrete, at least for durability. Roman concrete structures are still standing (and some are being used) after 2000 years. My modern concrete driveway is crumbling after no more than 50 years. $\endgroup$ – jamesqf Feb 16 '17 at 19:27
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    $\begingroup$ +1. A lot of people forget that "cement" is a generic term, and that the stuff we most commonly use these days is more correctly known as "ordinary Portland cement". There are other kinds of cement. Volcanic pozzolan is one, and fly ash is another alternative. $\endgroup$ – Jules Feb 16 '17 at 20:14
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    $\begingroup$ @jamesqf I don't know where you live, but it might also be related to the fact that your driveway is regularly massacred by a wheeled machine weighing half a tonne. Not to mention possible freezing and unfreezing water inside it during winter. It might just be that in warmer roman empire the latter was not a big issue. But I've also read somewhere that roman concrete was less prone to cracking than the modern one so maybe that's part of it. I wouldn't be so quick to call the roman absolutely better though without more information :P $\endgroup$ – Maurycy Feb 16 '17 at 22:12
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    $\begingroup$ @Maurycy: Not all the Roman Empire was warm. For instance Aventicum en.wikipedia.org/wiki/Aventicum#Destruction_and_rediscovery or the foundations of the (mostly wooden) town of Lousonna (now Lausanne), both in Switzerland. I've lived there, and it does snow in the winter. And sometimes in the spring, too :-) $\endgroup$ – jamesqf Feb 17 '17 at 5:13
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Carving.

Take the ancient Jordanian city of Petra, for example. They put giant steps into a mountain so that they could test the quality of the rock.

The benefit to building steps was so that nobody would have to hang by ropes or dangle off the mountain - during the build process.

It was this technique that enabled the constructors to stand on secure ledges while they literally carved around the mountain rock.

That being said, I'll assume your builders don't have hundreds of years to commit with a hammer and chisel. So you might want to pack some impressive technology.

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    $\begingroup$ There are some truly impressive underground cities in Turkey and Iran where people dug down and out into sandstone.. $\endgroup$ – Joe Bloggs Feb 16 '17 at 11:24
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    $\begingroup$ In Ethiopia they carved churches out of rock as well. $\endgroup$ – Matthieu M. Feb 16 '17 at 19:23
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Use any kind of mortar that isn't Portland cement.

The substance that, today, we usually just call "cement" is more properly called Portland cement, and it was only invented in 1794, by a chap named Joseph Aspdin. There were cities in the world long before 1794, so it's clear that cement is not required for cities. The stuff that's between the bricks in a wall is mortar and, while cement is the most common form of mortar today, many other mortars have been and still are used.

There is plenty of information about mortar. Mortar can be made from all kinds of substances, including gypsum, lime, clay, asphalt and good old mud. Of these, only asphalt requires life, though lime is made from limestone and the largest deposits of that on earth are life-related.

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High precision cut stone.

When stones match together, you do not need mortar to seal them together. Given a high enough precision, you could cut hexagonal blocks and use them to build your walls. These blocks would give more stability than the standard rectangular "brick" when subjected to earthquakes and other vertical and side to side horizontal motion. Front to back horizontal motion could possibly "jenga" a block to slide out of place. Actually a slightly concave surface on the upper 3 sides and a slightly convex surface on the lower 3 sides will sync is all up and make the walls immovable.

The stone constructions in the Andes used massive custom cut stones with no overall pattern. These have proved remarkably resistant to strong earthquakes that sometimes happen in the area. There are stories of the stones "dancing" during a strong earthquake, only to settle back into place when the earthquake ended.

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  • $\begingroup$ Cutting stones into lego like bricks would work as well. $\endgroup$ – Ezra Feb 19 '17 at 18:10
  • $\begingroup$ @Ezra I had a thought that all you need to do is make the lower 3 sides slightly convex and the upper 3 sides slightly concave, and everything becomes immovable. $\endgroup$ – Michael Richardson Feb 21 '17 at 6:12
  • $\begingroup$ @Ezra - Lego's mechanism actually becomes impractical beyond a certain size and weight; the blocks simply snap off the "nubs" and start to shift. Blocks used for things like retaining walls use a tongue and groove system, and can withstand tons of lateral load for decades (but not indefinitely). Beyond that you're talking about the concave/convex shaping for stacked stones from the above answer. $\endgroup$ – KeithS Apr 6 '17 at 18:08
  • $\begingroup$ @KeithS 'lego' like retaining walls are actually very common. greensrecycling.co.uk/images/slides/lego-concrete4.jpg $\endgroup$ – Ezra Apr 6 '17 at 18:24
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Pyramids. Big blocks of stone (basalt / granite / sandstone) and good masons.

Obtain carbon from carbonaceous chondrite meteorites. Find source of calcium (Gypsum minerals in fossil seabeds). Make cement. Also if carbon / hydrogen available then plastics.

Use clays and fire interlocking blocks and or create glasses.

If the technology available refine iron/steel, aluminium, magnesium.

Cities could also be below ground.

Also note that on a planet without life you will also probably be dealing with a reducing atmosphere.

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    $\begingroup$ why would there be "fossil seabeds" on a lifeless planet? $\endgroup$ – Lorry Laurence mcLarry Feb 16 '17 at 10:21
  • $\begingroup$ Seas are the result of large bodies of water. Life is not required. The water dissolves inorganic minerals from rocks. Sodium, chlorine $\endgroup$ – pHred Feb 16 '17 at 10:24
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    $\begingroup$ ... calcium & etc. Continents move, weather patterns change, seas dry out, voila, fossil sea beds. No life required. Also note that most of the water on earth comes from crustal rocks. $\endgroup$ – pHred Feb 16 '17 at 10:59
  • $\begingroup$ You probably don't need carbon from meteorites - it's nothing rare even on Earth, where most of it is stolen by life. On a non-living planet, you'd have a basically inexhaustible supply in the atmosphere (assuming it wasn't lost over time as on Mars, of course) and in various geothermal deposits. Hydrogen is plentiful if you have water deposits - on a water/ice-less world, it might be a bit more of a problem. And the reducing (and possibly dry) atmosphere might make it trivial to build long-lasting buildings from materials that aren't usually considered particularly desirable on Earth. $\endgroup$ – Luaan Feb 16 '17 at 11:32
  • $\begingroup$ @Luuan Yep. Cracking CO2 also a useful source of oxygen. $\endgroup$ – pHred Feb 16 '17 at 11:39
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Steel frames, glass or ceramic cladding and a prefabricated composite sandwich for the floors replacing the current prefab concrete.

Not a vast difference from how most city towers are built currently, but quite heavy on industrial base, so not for initial colonisation but rather for a mature colony. Tower foundations are currently built with poured concrete, but if you stick to low rise until suitable levels of biological matter have built up, guano for example, then deep foundations are less of a critical factor. You won't need high rise until you've built up a large population anyway.

Initial colonisation will probably be done with corrugated iron huts and other prefab facilities.

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  • $\begingroup$ Steel is made of iron, and iron (as pointed out in one comment here worldbuilding.stackexchange.com/questions/71387/… ) needs life to be deposited in ores $\endgroup$ – L.Dutch Feb 16 '17 at 10:12
  • $\begingroup$ @L.Dutch, that just means you'll need to process a different ore, tougher but far from impossible. $\endgroup$ – Separatrix Feb 16 '17 at 10:17
  • $\begingroup$ @L.Dutch, it's less important than the fact that a lifeless world probably won't have much oxygen in the atmosphere. $\endgroup$ – Separatrix Feb 16 '17 at 10:19
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    $\begingroup$ @L.Dutch Most of the ores we mine and use on Earth are, yes. But it certainly isn't the only source - you also have iron sulfides, and without oxygen, iron dissolved in the oceans. We can already "mine" some underwater sources directly, it just isn't economical thanks to competition from those oxygen-driven deposits. In a future where we can colonise other planets (possibly in other star systems), it's a good bet they'd be even easier to exploit. Instead of iron ore, you'd have iron - if you're really lucky, you wouldn't even need much refining. $\endgroup$ – Luaan Feb 16 '17 at 11:19
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There's a whole history of NASA planning buildings on the moon, but if you want something semi-conventional:

Steel and glass

Very future-present. Entirely inorganic. Requires only iron (very common planetary mineral) and silica, which need to be purified. This is usually done by reacting with carbon, but if carbon is scarce other reducing agents exist and can be recycled.

Plastic

If you can get carbon from somewhere, like the atmosphere, then you can chain it together to produce light modular structures made from polymers.

Other Ceramics

Clay is inorganic, although produced by running-water ancient processes. There's a whole range of other materials that can be made through "firing" processes, and tend to be very hard.

Carved Basalt

Inca-style. Especially with modern cutting technology. It's not as easy to work with as concrete and tends to produce low, massive buildings, but they don't have to be that way with modern techniques and the use of some structural steel.

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  • $\begingroup$ Plastics are basically frozen petroleum made of long organic hydrocarbon chains and tweaked a bit. You would be hard pressed to make it on a lifeless planet. $\endgroup$ – ohwilleke Feb 17 '17 at 15:09
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    $\begingroup$ If you can get carbon and hydrogen? Then you just need to run it through the Fischer-Troph process. Requires energy input, but then so does everything. Carbon may be available as carbonate rocks: planetary.org/blogs/emily-lakdawalla/2010/2540.html $\endgroup$ – pjc50 Feb 17 '17 at 16:16
  • $\begingroup$ But they...what? $\endgroup$ – Brian Rogers Feb 17 '17 at 17:25
  • $\begingroup$ @BrianRogers oops, fixed! $\endgroup$ – pjc50 Feb 17 '17 at 18:58
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One of the oldest builing materials is Loam. It has good strength and can be used for walls and even bigger builings by using it in combination with some pillars. (You must protect it from water aswell)

It even helps with good room humidity :)

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    $\begingroup$ Sorry C.Fe. Loam is a biologically produced material. $\endgroup$ – pHred Feb 16 '17 at 9:51
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    $\begingroup$ @LorryLaurencemcLarry You shouldn't be this negative towards someone trying to help. "Stupid answer" is inappropriate. $\endgroup$ – Secespitus Feb 16 '17 at 9:57
  • $\begingroup$ @pHred While my short research I could not find any part of Loam that needed some sort of biological procedure. Please tell me what is formed through such a process so I can expand my knowledge and add a note to my answer. $\endgroup$ – C.Fe. Feb 16 '17 at 10:28
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    $\begingroup$ @LorryLaurencemcLarry The question never mentions skyscrapers, It only mentions "modern cities". There are big cities wth almost no skyscrapers, so giving an answer on a material that can be used to buid a few floors and is available in large quantities is a feasable answer. (Even though maybe more for the outer rim of a city if you want skyscrapers) Using some form of (coated) rebar can increase the strength of loam in almost the same way (not same strength) as it does with steel, so I added Loam for its similarities to concrete. $\endgroup$ – C.Fe. Feb 16 '17 at 10:34
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    $\begingroup$ @pHred In general finding a soil on the earth without some organic matter in it is hard, so we probably cant quite tell. Researching this topic is also not really done, as we have other materials and organic matter is so common in our soil, that it would take work to extract it. By the geologists definition loam is a material qualifying for the asked question, so I think we should stop our debate (akthough it was fun) and agree on: Loam is a feasable material for the question, but its debatable how good loam would be without biological components. Have a nice day. $\endgroup$ – C.Fe. Feb 16 '17 at 11:22
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If you build with arches (or vaulting which is sort of the 3-d equivalent if you consider a wall as 2-d) there's no tensile strength needed... all the load is compressive. You can use precision-cut blocks or just about any material that will stick together to fill the gaps between blocks that are not precision cut. So you can build using whatever rocks are available... and anything that is vaguely cement-like for gap-filling. The building material doesn't have to need any tensile strength and nor does the filling material, it just needs enough strength to hold itself in place. You could even use something like lead hammered in.

Look at all the mediaeval cathedrals in Europe, Roman aqueducts, castles... all of them are just about 'heaps' of stone in compressive loading with nothing tensile at all. You only need reinforced concrete or steel or timber to make lintels and flat spans... without it, you're limited to arches and somewhat more massive floors - you built a barrel-vault 'ceiling' and then you probably fill the top of the barrel vault up with loose fill to get a 'level' surface for the next floor up. Okay, no wood for the roof, you just make it vaulted and waterproof... and fit drains.

No skyscrapers, but you can build several floors up before the walls at the bottom get so massive they use all of the room space. But it's a lifeless planet... so you can build wide and low, there are no neighbours to complain.

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Even if cement isn't available you really just need something to hold your aggregate together. Two ideas that spring to mind are:

  • Bags
    With bags you just fill them up with any old sand, dirt, rocks or whatever you happen to have lying around. You then just stack them up. You can even design them to reinforcing bars through them and/or tie them together with straps.
  • Glue
    With glue well you just, well glue stuff together. The glue could be almost any kind of plastic or resin you desire. Mixed with the right aggregate it could be as strong as steal.
  • Bags + Glue
    Just put the two ideas together, aggregate, mixed with a resin and maybe some kevlar or carbon nano-tube fibers, poured into kevlar or tyvex bags that are already the correct shape and size. Stack and tie them together.
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Look at the work being done to use Lunar Regolith as a building material.

The three ways that I've seen are to use heat to fuse it into a glass, use heat to sinter it together or import a polymer to use as the fixing agent. The third method is not too expensive and the volume of polymer to the volume of fixed material is relatively small.

See this article.

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Cities don't necessarily need to be tall. They can be wide. While concrete reinforced steel is commonly used in skyscrapers, its worth remembering that there's other materials strong in compression

Assuming its a lifeless rock, you have lots of space to build. Quarry whatever ignesious rocks you have and build out rather than build up..

The incans had a city that was built entirely out of precision cut, interlocking stones. With reasonably high tech cutting tools you could do similar work on harder materials. While I'm not a civil engineer, I do also wonder if you could get a similar effect to rebar (the tensile strength of steel adding compressive force to the stone here) by drilling through and threading/clamping steel rods or wire .

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You can build it out of anything depending on conditions. In very dry places house of mud work. If no rain or bugs, consider sugar. If cold you can make them from Ice. You can make them from trees, salt, metals, foam, plastics.... get the picture. BTW on the moon, melting the sand will create a form of glass cement that be used for construction.

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    $\begingroup$ Where do you get sugar and trees on a lifeless planet? $\endgroup$ – ohwilleke Feb 17 '17 at 15:11
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    $\begingroup$ @ohwilleke maybe it's a giant galactic jaw breaker $\endgroup$ – Justin Ohms Feb 17 '17 at 23:33
  • $\begingroup$ Sugar is made from some of the most common elements in the universe. Through photosynthesis, plants produce glyceraldehyde-3-phosphate (G3P), (C6H12O6) or (as in cane and beet) sucrose (C12H22O11). So with a little sunlight, plants can be used to make a structure similar to honeycombs that can be used to build structures. Or you can use giant honeybees. The point of my answer is that variety of construction materials is only limited by your imagination. $\endgroup$ – tony lester Feb 18 '17 at 16:18
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Cities solve a problem for its residents. Could be safety, community, commerce, weather protection etc... There really is no one reason to live in a city. So the design of a city has a lot to do with the problems of people that live in it. In a environment with perfect weather, no hostile neighbors or crime then a society could flourish in tents and simple structures.

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Robert Zubrin, who has thought about colonizing Mars extensively, suggests that the best building material for that planet may be brick. It can be made easily out of the clay abundant in the Martian regolith, and although water (which is scarce) is required for the manufacturing process, nearly all of it can be recycled. He also suggests that most structures would be built underground, both to protect their inhabitants from radiation (which Mars' atmosphere, lacking an ozone layer, does not block, but which a few feet of soil would protect against), and to compensate for the fact that the air pressure in man-made structures would be much higher than the Martian atmosphere.

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You say that concrete is necessary for construction, that concrete requires cement, and cement is made from limestone that contains organic materials. Hence no limestone or concrete for building materials when building cities on a lifeless planet.

Any simple chemical compound can be chemically synthesized from elements or from simpler compounds.

Limestone may be formed from organic remains, but

Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3)

https://en.wikipedia.org/wiki/Limestone1

Calcium carbonate contains calcium, carbon, and oxygen.

Portland cement is made by heating limestone (calcium carbonate) with other materials.

On a lifeless planet with no limestone calcium carbonate and the other materials can be obtained by heating up rocks to melting or vaporizing temperatures and centrifuging the liquid or vapor to separate the elements and storing the elements separately until they are needed.

Compounds may be synthesized by machines like 3-D printers that form chemical compounds element by element, perhaps. They would take elements from their storage bins as needed.

Melting or vaporizing mountains of rock to reduce to elements would take immense amounts of energy. But the space colony will probably have fusion reactors to supply almost unlimited energy, and/or gigantic flimsy space mirrors to focus vast amounts of sunlight on small areas to generate energy.

Thus if a lot of concrete for construction is needed on a lifeless planet the cement will be chemically synthesized.

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You can also carved rock, the same way the guys have been doing it before the (re)invention of modern-day cement. The cities of Petra would be a good example.

You can also resort to adobe houses, made from mud bricks. Just make sure to kiln-fire them.

For the decorative purposes, look for marble in the planet you've landed on.

You can also synthesize calcium carbonate for use in cement.

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Any structure built on Mars would either be underground or be buried over. Now here is a novel idea. Build it out of Pycrete. Mars is so cold at night that the few hours of daylights it would stay frozen. It would provide a great barrier to radiation, and would actually help keep structure warmer at night. It is also easy to work with. Water in Pycrete could also serve as the reserve for the colony. Building would be made by using inflatable bags which are filled with water and dirt to make the structure.

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