Monday, December 5, 2011

homemade concrete

this summer, some friends ( i'm married to one of them now! ) and I got together around a bonfire for the usual: to share good stories, good drinks, and good roasted marshmallows. while we were at it, I thew some limestone in the fire (only cook limestone in an extremely well ventilated area as this process releases carbon monoxide).
i did this because cooked limestone ( quicklime ) is one of the key ingredients to concrete and i wanted to find out if i could make some in my own back yard. it's another one of those basic human skills that many people used to know how to do with what was around but few now know about as a result of the complicated technological manufacturing processes which have re-mystified this ancient art for us.
for those who don't know, "the land" (the name of the property this blog is about) is also a place where we experiment with ways to continually reduce the area from which we draw resources for our way of life. the eventual goal of this hobby (perhaps i'll do this full time someday) is to create a walkable glocal culture. - this means a community of people who can participate in knowledge sharing with the world of the global economy while having everything required in the forms of relational capital, food, water, shelter, and beauty within a walking distance from where they sleep. the walking distance part is huge because this helps to prevent the culture from participating in ecological and economic overshoot.
anyhow, concrete is the foundation and structure of most new buildings on planet earth and many of the oldest and best ones. making it can be a complicated process to be sure, but that's my job: to know how the process works and to clearly disseminate that information to others. since i'm so into local economies, i've documented the process below for anyone else who might be interested in utilizing this knowledge as part of their own local economy. i'm only making a very small batch in this case for testing purposes so you'll see hand tools and buckets in the photos but hopefully it communicates the basic steps and a few of the basic principles clearly enough for someone to take away a few ideas about possibilities to upscale this process within their particular situation. with a decent sized earth kiln, a commercial mixer and some dry storage space, one could produce copious amounts of local concrete without ever having to visit lowes or home depot again... if they get the mix right. 
so with no more delays, here is a summary on how we did, and you can, make home made concrete:

1) gather the sandiest looking soil you can find. don't use beach sand. beach sand makes low grade concrete (the granules are too worn and smooth to be an effective aggregate - the recipe calls for sharp sand). i'm just using a shovel and wheel barrel for the gathering job.

2) sift the soil through a fine steel mesh screen into a bucket or some vessel that will hold water and in which the soil and water can be stirred in a large enough quantity. this step removes much of the organic debris, clay (which is often clumped with other clay) and small stones.

3) once you have sifted enough dirt into the bucket or trough (enough being as much as you can fit and still do what i'm about to describe), fill the vessel up with as much water as you can and begin to stir. i filled the vessel just under 1/4 full with dirt before adding about 1/2 bucket of water so that i could stir it all around and not make too much of a mess. the idea here is to get the clay to particulate into the water. the second you stop stirring, the sand begins to settle to the bottom. most of the sand content in the soil will settle within the first five seconds because it is heavier than the clay. i then begin to pour the muddy water (full of the clay particles) into another bucket where the clay will settle for the next several hours. once the clay is settled, i can reuse a portion of that water for mixing or i can water the landscape and set the clay aside for other purposes (clay can be useful.. think clay tiles on your roof or floor).
repeat the above process several times and you will begin to notice that the sand in the bottom of the bucket becoming purer and purer until the bucket is ready to be emptied of water for the last time.

4) scrape the sand out onto a smooth dry surface from which you can gather it up when it is completely dry ( i didn't let the sand dry completely on my first batch which hurt the quality ).

5) now you have the sand, one of your four main ingredients (sand, lime, aggregate, water), it's time to cook the limestone.
by the way, try not to use salt water (ocean, sea water) for any kind of conventional concrete. salt water is only used with special cements using elements like MgO. the salinity can potentially weaken the long term strength of the concrete and corrode steel reinforcing if you're doing a structural pour.
limestone is fairly easy to identify once you know how. limestone comes in a number of varieties, some of which are better for making construction grade lime but that's a science in and of itself that i'm not even going to touch here. cooking lime is a part of the process where you have to be: a) safe, first, foremost and b) inventive. i'm not going to get into either of these very much, i'll just show you what we did. 
the first objective is to find a place where you can light a really big fire that won't catch on the surrounding landscape. we chose a pit which is in the side of slightly inclining terrain, sort of like digging in to the side of a hill. we then covered the space with sheet metal supported by angular steel members and weighed down with stones. we placed the stones on a steel grill so that we could get a really hot fire going and before pulling the grill out from under the payload of stones that drop down in to the hot coals. there we leave them lying until they are visibly red hot. the major mistake i made at this first batch came from only letting them cook for several hours. depending on the kind of limestone and the size of the pieces, it can take over 24 hours for a stone to cook all the way through. i'll describe this phenomenon in further detail below.  you want the stones to stay red hot for around 18 hours if they're about the size of a fist or a little larger, so that they can cook right through to the core.

6) don't get too close to the fumes from the fire (carbon monoxide) and keep a close watch on the heat, you want to keep the stones red hot.

7) as mentioned, i only cooked these stones for several hours so only the outer 1/4" of stone became quicklime. you'll know it's quicklime because it will flake/ break right off and turn into a fine white powder if you squeeze the rocks with your (gloved) hands or strike with an object like a hammer. traditionally, once the cooked stones had cooled, they would be thrown at a tilted metal or wood grate with 1/2" - 1" gaps and the stones that were pulverized by the impact and fell through the grate were used for construction grade projects while the stones that didn't make it through the grate were kept for agricultural use (altering the ph of the soil upward in highly acid soils).
always wear gloves when handling this stuff, it will severely dry out any skin it touches. it will also crackle, smoke, hiss, and reach temperatures of around 130 degrees F if it gets the slightest bit wet. putting moisture on the limestone is called slaking the lime. you want to slake the lime later on. for now you want to keep it dry and break it up as much as you can to get the good lime for your concrete/ cement mortar mix.

8) i'm jumping a few steps ahead now to mention that before you do what you're about to do which is mix homemade concrete, you want the form or mold you plan to pour in to afterward ready. here we have a simple form as i try to make some experimental Roman dimensioned concrete pavers.

9) now we're ready to set completely dry sand onto the smooth, clean, dry surface where we will do the mixing.

10) 1 part lime (which is your cement), 2 parts sharp sand, and 3 parts aggregate ( which was traditionally a small gravel, often a metamorphic rock, that could be sifted for locally but which is not used here ) are your solid ingredients. add water to this mixture and stir until you have the desired consistency for your pour.
here we're using a method which was often used to make a lime cement mortar before the industrial revolution. you hollow out a small area in the center of the sand with a shovel. pour the parts of lime into the hollow area, and then pour 1 part water onto the 2 part lime in the middle of the 3 parts sharp sand around the outside.

11) then to the best of your ability, smooth the sand up over the lime making a smooth sand dome. here you see the sand begin to crack. it will actually billow up and crack open because of the reheating taking place within the freshly slaked lime. try to keep the dome smoothed down to maintain pressure. 

or, you can just slake the lime and begin to mix the two together.

12) i don't really show step 12 but after about 20 minutes of step 6, begin chopping and mixing the sand and quicklime with your shovel or trowel and adding just the right amount of water to keep the consistency about where you want it to be leading up to the pour. you want to be sure to really mix the two together well. during this stage, you can add the 1 part aggregate as well if you want to make a structural concrete. here, we're really only creating a rustic cement mortar mix and solidifying it in the shape of a block, but add some aggregate, quality quick lime (which this first experiment failed to produce in quantity), and some steel reinforcing, and you're looking at some very tough home made concrete. 
pour the mixture into the form and let sit for the recommended time for the size pour you're undertaking. i let this paver sit for several days before I pulled it out of the form. most concrete will cure to within a few percentage points of it's final strength within a month of pouring.

click here for a brief intro into how the Romans did it. also, I'm really interested in others' experiences with making concrete and mortar mixes from the basic ingredients, so feel free to comment below and share with the rest of us what has and hasn't worked in your situation. thanks goes out to the Elliots and to Emily for all their help and for bringing tacos.


  1. I live in the North of the hard red clay...thinking I'd better think more about making bricks...

  2. good point material girl. limestone isn't readily available to everyone although
    it is the third most common "rock" (16.5% of continental land surface) on the surface of the earth behind sandstone and shale ( which are also useful building materials ).

    slide 39 of this uconn lecture...
    ... explains that 75% of the continental surface (land surface) on earth is comprised of sedimentary stone ( also here: ).
    of that 75%, 22% is limestone (hence the percentage listed in the first paragraph) which is why limestone was, as a building unit and as a source of the main ingredient for cement, the backbone of the Greek, Roman, Egyptian, and Mayan building trade empires, among others.

    so you're not too far away from granite country up there which is also a great
    building material and clay will probably be the center of my next new experimental bonfire party complete with stories, beverages, and s'mores.. once I've done a few more concrete making parties. wood fired clay tiles are waterproof if done properly. Barefoot Architect, by Johan Van Lengen provides a great preliminary description for wood fired clay tile making and has a lot of really
    helpful cartoon drawings to explain steps without all the super technical jargon, and it's only about 10$ on amazon with shipping ( though it's about 2" think with decent information ).

    i've also been exploring hand quarrying methods for various stone types and hope to share summaries of what I've learned and tried in upcoming entries.

  3. i doubt if i can identify lime stone. can i use shells or coral?

  4. Because limestone is available pretty much everywhere in the world, you have to consider the climate you live in when making concrete. That’s why you have to constantly assess the weight and the strength of your end product. Limestone is a fairly brittle material (you’ve made that clear) and will break easily even under slight pressure.

  5. As sea shells are comprised of calcium carbonate, they have also been used to create various concrete and mortar mixes:

  6. Mixing concrete is something that I enjoy doing, although I must say that it is indeed very tiring! Anyway, what I like most about this is that I can freely create my own design, limited only by my creativity. I’ll surely consider trying out limestone next time. Thanks for the idea!

    -Alphonse Daigle