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The world which I am working on is set in the far future on a colonized/terraformed planet that has become reduced to a medieval-like level of technology. It was not previously inhabited by any life, and thus has no fossil record and no limestone. Lime-based mortar has been used for thousands of years, and thus is very important for the construction of large stone buildings. Does this mean having large stone architecture in a world without limestone is impossible? If the calcium carbonate is present on the planet but not stored in limestone, would it be accessible and usable in other ways? Are there alternatives to lime that could be used as mortar?

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    $\begingroup$ Lime based mortar has been used for thousands of years, it's also not been used for thousands of years and far longer than thousands of years, people have built dry stone structures with no mortar, including some very large structures, like entire cities . so what was your actual problem again? because I just don't see one here. $\endgroup$
    – Pelinore
    Aug 7 at 5:59
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    $\begingroup$ Building buildings without mortar has also been done for thousands of years, and for more thousands of years than using mortar. Wood, stone and sun-dried brick buildings. Mortar is used more usually for fired brick buildings rather than stone buildings. On the other hand, a world whixh had no life has no soil, and growing crops with no soil would be a much tougher problem. $\endgroup$
    – AlexP
    Aug 7 at 6:08
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    $\begingroup$ Gents, this planet has been terraformed by an advanced civilisation that has devolved relatively recently. Why wouldn't the soil still be ok-ish with some bare minimum of organic content, bacteria, and essential minerals? $\endgroup$ Aug 7 at 11:42
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    $\begingroup$ The planet was terraformed over thousands of years by a beyond-advanced sci-fi civilization. There's a little bit of handwaving, but the assumption is that the planet has been terraformed for long enough for most of Earth's natural processes to take hold, just not enough time for limestone to form yet. $\endgroup$
    – Nitro
    Aug 8 at 0:20
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    $\begingroup$ Additionally, after posting this question, I solved it myself. Assuming calcium carbonate is still present on the planet, then it can be sequestered by mollusks, just as was done before limestone formed in the first place on Earth. Thus, these mollusks can be burned to produce lime rather than burning limestone. Additionally, gypsum mortars and clay mortars were used historically, they were just generally replaced by lime mortars over time because lime is simply better (because of the water proofing). It's by no means impossible to use worse mortars, buildings just won't last as long. $\endgroup$
    – Nitro
    Aug 8 at 0:23

8 Answers 8

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Tufa and travertine

Tufa and travertine are inorganically precipitated calcium carbonate.

Heat them. They're a perfect limestone replacement, and exactly what you need. If you have pozzolanic soil, there's no reason you can't do better than mortar and make cement, exactly as the Romans did.

Note that travertine can be produced directly from ultramafic rock without any biogenic limestone playing a part.

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    $\begingroup$ both of these occur in large minable deposits, so although rarer they will work just fine. $\endgroup$
    – John
    Aug 7 at 12:09
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Clay mortar.

clay mortar building

Mortars made out of clay were widely used in places where clay was abundant and lime or limestone hard to come by. The linked article has a lot of stuff on clay mortars as used in Scotland.

Clay Mortars for Masonry Buildings

Lime is generally considered to be the most common mortar material for traditional structures, yet in many parts of Scotland, especially areas where clay-rich soils are common, clay was commonly used as a mortar in masonry building. Sometimes seen as an archaic and purely vernacular tradition, clay mortars were in fact used up until the end of the 19th century and possibly later in some locations. A suitable mortar should contain heavy clay from a silicate-rich soil that can bind the matrix of the mortar together and support the compression loads of the masonry. Such material, normally found below the topsoil layer, was dug out of the ground and sometimes used directly for bedding the masonry and filling the wall core, or mixed with aggregates and straw to form a mortar.

Clay is a product of weathered stone. There should be clay on a planet with a hydrologic cycle. Maybe even more clay because there will be more erosion and weathering without surface plant life to limit it. And maybe better clay omn your world because the components of topsoil that make it good for life (e.g. organic matter) make it worse as mortar. On this lifeless world, weathered surface soils might be suitable for use as clay mortar and so people would not have to dig for subsoil.

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Sea shells are one of many other ways to make quicklime.

you can roast sea shells in the same way you cook limestone and get the same results. Tabby concrete is made this way and it is what early Spanish settlers used in the new world, since they were in coastal areas with no limestone. Human settlements can generate a LOT of sea shells so supply is not a huge issue. Plus of course they can always farm shellfish for the shells with a bonus of food.

As Sean OConner have pointed out you can do the same thing with travertine a non-biological mineral found around hot springs. There are several other minerals mentioned in other posts that will work as well. You can even use eggshells, anything made of calcium carbonate will work.

enter image description here

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    $\begingroup$ Sea shells freshly harvested from the newly introduced non indigenous life? that works, may limit your supply somewhat depending on how plentiful the introduced sea life has become so far .. additionally, any shell will do, how about escargot farms 😁 $\endgroup$
    – Pelinore
    Aug 7 at 12:29
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    $\begingroup$ @Pelinore snail shells also work fine. and of course you can farm snails and shellfish. $\endgroup$
    – John
    Aug 7 at 13:15
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    $\begingroup$ """It was not previously inhabited by any life, and thus has no fossil record and no limestone.""" ... prob also no sea shells $\endgroup$ Aug 8 at 19:31
  • $\begingroup$ @CaffeineAddiction but if the are terraforming the planet they ae going to bring snails and shellfish. $\endgroup$
    – John
    Aug 8 at 23:19
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Frame challenge:

Mortar is not required; It just makes things easier. Interlocking bricks can be used and the construction method is formally known as "dry stone" and is older than mortar and makes for more durable walls. There is debate about the construction methods used such as cutting, poured/molded, or re-formed.

One example are the Incas at Saksaywaman in Peru:

enter image description here enter image description here enter image description here https://ashtronort.wordpress.com/2016/03/28/precision-fitting-of-massive-incan-blocks/

I seem to remember there also being walls somewhere that literally look like your typical puzzle piece blocks, complete with interlocking circular tabs and slots. But I can't remember the location. I thought it was Gobele Tepe but apparently it's not. If anyone remembers, please tell me so I can post it.

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    $\begingroup$ most of those walls still use mortar, only the outermost surface edge fits perfectly, there is a big gaps behind that filled with simple mortars. The walls without mortar are small few and in places of extreme wealth. researchgate.net/figure/… $\endgroup$
    – John
    Aug 7 at 16:31
  • $\begingroup$ @John It would not surprise me if walls or lesser importance did use mortar just to make things easier. Nonetheless, it is possible without mortar given sufficient worker skill (and time). $\endgroup$
    – DKNguyen
    Aug 7 at 18:41
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    $\begingroup$ @John: The Cyclopean walls of Mycenae are definitely not "small". (And they are 3500 years old.) $\endgroup$
    – AlexP
    Aug 7 at 19:46
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    $\begingroup$ @DKNguyen literally every wall in the images above are mortared walls, dry stone walls can be made and they have limitations, but the Incan walls above are not examples of mortarless walls. the vast majority of Incan walls use mortar. $\endgroup$
    – John
    Aug 8 at 3:50
  • $\begingroup$ @AlexP Mycenae is also not Incan, Or do you think I mean dry fit stone walls can't exist? but just to be clear Cyclopean walls also often used mortar. $\endgroup$
    – John
    Aug 8 at 3:54
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Alkali based mortars and cements

If we ignore CaCO3 altogether:

Yes,using aluminosilicate geopolymers/alkali activated cements.

You can make cements using sodium or potassium hydroxide, and an amorphous aluminosilicate mix, such as certain pozzolanic soils, flyash, dehydrated clays, geothermal silica, etc. Obviously, it'll be much more costly than Portland cement or lime mortar as alkali hydroxides are comparatively scarce. Nonetheless, it's possible.

If you had adjacent deposits of natron and kaolinite, you'd be well on your way. It could be discovered as a side product of a porcelain making operation.

Another alternative for indoors only is bonding things with sodium silicate (waterglass), which you can make from sodium hydroxide and geothermal silica. You just mix up any powder in it and it sets. It is insanely, incredibly hard and strong...BUT it is attacked by water, so can only be used in a completely dry environment, which is why it's not used everywhere.

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It is possible to make cement without limestone. If it is just that limestone is missing then aragonite or calcite could be used. If something a little different is needed try dolomite CaMg(CO3)2 or even Magnesium carbonate.

With sufficient heat they will all decompose to reactive oxides that could be used in cement.

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    $\begingroup$ Aragonite and calcite will work fine but chalk is biogenic in origin, so you should fix that. $\endgroup$
    – John
    Aug 7 at 12:11
  • $\begingroup$ Yes I see now ok my bad I will fix it. $\endgroup$
    – Slarty
    Aug 7 at 16:53
  • $\begingroup$ thanks for that $\endgroup$
    – Slarty
    Aug 7 at 21:30
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Sulfur can be used as binder to make concrete and mortar. The recipe is 20%-32% molten sulfur; 10%-20% fine silica, mica and carbon filler; and rest rock aggregate.

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Volcanic ash

was used as a binding material pretty much before lime.

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  • $\begingroup$ I think (the right type of) volcanic ash forms alkali activated cement. $\endgroup$ Aug 8 at 10:44
  • $\begingroup$ Do you have any information about this? I know the romans used volcanic ash in combination with lime to make strong concrete. And I know about plenty of talks of using it as a replacement in modern concrete. $\endgroup$
    – John
    Aug 8 at 15:05
  • $\begingroup$ @John Google "Glukhovsky cement" or similar. Basically, if you have a reactive aluminosilicate, it can react with lime to form 'normal' cement, or alkali to form geopolymer cement. Either Ca or alkali mixes tolerate a little of the other but there's a big incompatible composition region. Volcanic ash can be (among other things) alumonosilicate, or alkali aluminosilicate mixes, and thus, like flyash, are useful in both systems. $\endgroup$ Aug 8 at 21:29
  • $\begingroup$ @John If you're interested in Roman concrete formulations, search strings involving pozzolan are useful. $\endgroup$ Aug 9 at 0:12

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