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How To: Make Cement

Posted by on 2012/04/03

Sorry about the slight delay between posts, I can’t promise it won’t happen again, but I’m back for the moment so let’s dive directly into a pool of concrete.

Mortar has been around since the dawn of civilization, and the art of masonry is one of the first skilled trades.  Now, I’m no mason, and until I get a lot more research done, I’m not going to presume to instruct anyone else on the art; but knowing how to mix your own mortar lays the foundation for not just surviving, but thriving.

I don’t think I need to get into the benefits of stone structures, so lets just get down to business here, which is how to mix our own cement out of commonly available stuff.  Thankfully, we’ve got a lot of options in this field.

The Babylonians primarily used clay bricks, mortared with pitch.  We already know how to do that, essentially gluing clay bricks together. The Egyptian pyramids were mortared by mixtures of mud/clay/sand, gypsum, or lime.  The Greeks essentially ‘invented’ the first hydraulic cement (hardens under water) by mixing lime and volcanic ash.  Romans used a similar lime based mortar, rigorously and carefully measured and skillfully worked.  Modern concrete is cement mixed with sand and gravel at a 1:3/1:4 ratio, activated with water.

The basic tenets of cement:

1. Cement is a powder that works like a glue, activated by water. Different cement recipes will yield different properties in the cement.

2. The more water you use, the easier the cement will flow, but  the weaker the final structure will be and the longer it will take to set.  For quality purposes, aim for a low water to cement ratio.
How to make lime-based cement:

Used for milennia by nearly every civilization in the world, lime-cement is good enough for you, old man. In order to make it, we need lime.

Limestone, Chalk, and Seashells are essentially calcium.  Lime is derived by heating calcium at high temperatures (in that earth kiln we built) which through the magic of chemistry, turns it into quicklime.  Once you add water to quicklime, it creates an exothermic reaction called “Slaking”.  Once you have slaked lime you will either have a powder or a putty depending on how much water you added.  That’s the good stuff.   Superior concrete/cement starts with superior lime,the whiter the better,  impurities will appear as dark spots or mottling.

Most recipes involve 2 or 3 parts sand to 1 part slaked lime, but some go as high as 5:1.  The addition of other elements like  heat treated clay powder, or other crushed particulates, will give a variety of different results.  The addition of gravel will create the familiar concrete; and if you happen to have some volcanic ash lying around, then use that instead of sand and you’ll have a concrete worthy of the parthenon.

The best sand, according to Charles James’s Military Dictionary is round and sharp, evenly grained sand that is washed in clean water prior to use.

The chinese used terra cotta instead of sand, we can use fired clay; which requires lime of the utmost purity – such a combination will produce ‘hydrolic lime’ that can set under water.

Cement dries and hardens upon exposure to air.  The burning of the lime forces the CO2 out of the lime, and as the reactive slaked lime interacts with the air, the process of ‘carbonation’ causes it to ‘cement’.

We’ll get deeper into the chemistry of cement in later entries, but for now; suffice to say that you’ll need to learn how to find and identify limestone or other sources of calcium in order to add this tool to your survival toolbox.

So here’s a quick and dirty lime-cement recipe we can rely on:

What you’ll need –

Limestone – although you can substitute oyster shells if no limestone can be found.  Then again, limestone should be pretty easy to find considering it makes up about 10% of the sedimentary rock on the planet.  Here’s a technical definition:

Limestone is a chemical sedimentary rock that is at least 50 percent calcium carbonate in the form of the mineral calcite. Limestone may also contain small particles of other materials, such as quartz, feldspar, clay minerals, pyrite and siderite, and it may contain larger nodules of chert, pyrite or siderite. Different types of limestone are classified based on the texture, mineral content, origin and geological age, but all limestone is part or wholly organic and may contain fossilized shells and plants. – some other website

How to identify limestone:

It’s white or beige in color, impurities will cause further variations in the color.  Fossils may or may not be present in the rock.  It has a hardness of 3 on the Moh’s hardness scale, which means that it’s easily scratched and broken.  Calcium carbonate is a base, so it will react with acid, say the stomach acid of that deer you just killed, or some vinegar you’ve been brewing in your hut. Limestone won’t crumble easily, but it can be crushed.  The grain size of limestone will vary, the finest grain is the best for our purposes.  Limestone has a low cleavage, it will fracture rather than cleave.  If it appears to be composed of layers, it’s probably sandstone not limestone.  Once its crushed into powder, it should be predominantly white.

Okay. So you’ve found some limestone, now it’s time to burn it.

We’ll need a kiln.  You can either construct an above ground kiln out of firebricks (clay) or use an earth or pit kiln.  You need to get the fire up to a temperature of around 900 degrees. Hardwoods will do the job easily. Oak works.

One method called a lime clamp consists of multiple heaps of alternating layers of limestone and coal above a fire hole, covered with clay and/or sod to keep the heat in.  The process, while easy enough to do in the wild, is notoriously wasteful. You’re better off taking the time to construct a kiln or a furnace.

You don’t need to powder the stone, or even crumble it, but the larger the rocks the higher the heat and longer the time required; so breaking it down will speed up the process. After the lime is burned, it will crumble easily; it will actually crumble as it burns and the carbon dioxide is released. It will be pretty evident when it’s done.  In the case of over-burning, it will create hard portions of dense lime that need to be screened out of the quicklime and crushed.  They’ll be more difficult to slake than evenly burnt lime, but will still work for our purposes, just not for high quality durable construction.  Under-burnt lime will have portions that just will not slake, so those chunks need to be removed. Again, i’ll stress, don’t handle the product bare handed because it could take your flesh off, and you need your flesh. Also, if you haven’t already got a dust mask, you should make one before working with quicklime because you don’t want to breathe the shit in either.

Now, the result – quicklime, is insanely useful; and we’ll go into it’s various applications at some later point.  It’s extremely caustic, and can burn flesh. Quicklime is used in a variety of campsite chemistry crafts, and assuming we’re doing this all in a linear fashion; you should always make more quicklime than you need and store it in a sealed container. On exposure to air, it will begin to carbonate.  For our purposes we want slaked lime, so add water, it will hiss and crumble.

So here’s a bit about slaking lime.

When slaking quicklime, there’s an immediate exothermic reaction, which means that when combined with lime, the water will hiss and boil, which can be dangerous.  If you want to speed up your production time, you can mix your sand, water, and quicklime all at the same time; once the chemical reaction is complete, then you can use it immediately as a mortar. Adding gravel or crushed brick will speed up the setting time.  The proportions of these ingredients are debatable, I’d suggest trial and error.

The amount of water that you use will change the properties of your cement; more water will result in a putty, less water will boil off and leave you with a powder.  For best results use relatively pure water at room temperature.  A low water:lime ratio is 2:1, which will almost always yield a powder, and a high ratio is 6:1 which will almost always result in a slurry that will dry to a putty.   During the slaking process, if it’s feasable, you want to stir the solution as it cools to make sure that you’re fully slaking the quicklime.

Once your slaked lime is exposed to air for extended periods of time, it will re-carbonate, in other words, it will cure back into limestone. It can take a long time to cure, but once it does, it’s exactly what you need it to be; tough, resistant, sturdy, and binding.  So use your newly made lime cement as a mortar for stonework or apply it directly to the surface like plaster (because it’s plaster).  Pure lime cement is almost useless as a building material as it will shrink and crack if used exclusively, which is why we add sand and other particulates.

Modern hydraulic cement, or ‘portland’ cement is made by mixing lime and clay-stone instead of sand, or in addition to sand.  Like any other endeavor, the more sophisticated our tools become, the greater the ability to manufacture this material becomes.  In order to reliably create portland cement we’ll need to fire our limestone and clay together to temperatures far in excess of what we’re capable of doing at our current tech level, so this is something we’ll have to revisit later.  In the meantime, simply mixing fired clay (or volcanic ash if you have some) will help to make a water resistant cement that depending on the qualities of the materials used, may or may not be able to set under water.

Really, we have to think of cement as a glue, as that’s really all it is; but once you add your particulates like sand and gravel, then you have concrete, which is a reliable mortar, building material, and generally useful stuff to know how to make.  Concrete made from sand/gravel/lime is not particularly water resistant and will not set in wet conditions.

Now, actually using the cement to build structures is going to require a basic lesson in masonry and some tools which we’re going to need to learn how to make.  Both of these things will have to be addressed later.

The tools we’ll need:
As these tutorials go up, I’ll try to remember to come back here and add links.

Trowel – Best made of metal, but wood will work – we’ll consider this a simple tool and get to it in one of the next tool making sections.
Level – Precision is important here, making a level is almost as difficult as trying to do stonework without one; we’ll try to come up with some low-tech solutions.
Chisel – A hunk of shaped, tempered, strong metal – one of the most important tools we’re going to try to figure out how to make
Hammer – A rock won’t cut it, we’ll need to make ourselves a metal and wood hammer.

The skills we’ll need: Hopefully, we’ll get around to these tutorials as well.


Wow I did not think there was going to be this much information to parse, and we haven’t even gotten into the basics of masonry yet.  We’ll also be revisiting this section too as we’re going to need quicklime and lime cement for a lot of other projects down the road.


4 Responses to How To: Make Cement

  1. Cleon E

    This stuff is extraordinary thanks for sharing keep it up.

  2. David Nash

    I just found your site, love the how-to section. I bookmarked this. Great work…

  3. Paul Fairfield

    Are the listed ratios by volume or by mass? Or am I splitting hairs?

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