Our interest in hemp and other alternative fibers for papermaking has led us to considerable research in the use of these same fibers for building materials. There are a number of approaches towards the use of these fibers in building. Some of them are based upon the model of the wooden board, and various strategies of pressed fiber with heat, pressure, and various binders have been tried with a view to creating a substitute for wooden boards and/or panels.

     We are pursuing an alternative approach, which seems to us to be a more ecologically efficient solution, since it is closer to the earth, and seems to offer greater advantages and less cost. The basic idea is to use a preparation of hemp hurds and lime to create a kind of cement ("Agstone") which may be used to construct floors and walls which are stronger and more durable than cement or concrete, yet less brittle and much lighter. It also exhibits superior properties of insulation, waterproofing, and fireproofing, and can effectively replace not only the structural elements of a wall, but it can stand alone without any additional exterior or interior wall covering. In addition to offering all of these benefits, the material is easy to work with, and can end up saving considerable expense when compared with traditional building methods.

     Our research with this approach began with the use of hemp hurds and lime which we learned about on our visits to the makers of Isochanvre, in France, but since we don't have enough hemp hurds here to work with, we have adapted the idea to the use of other materials: rice straw (a good choice because of the high silica content like hemp hurds) or just about any available chipped up weeds, brush, straw, or other agricultural waste. In the last couple of years we have been implementing these ideas through experimental construction projects.

     Over the last couple of years, we have experimented widely with different recipes and procedures for building with Agstone, so it is time to update this article with current ideas about the process and give some general recipes which you can use as a jumping off point for your own investigations.

     I express it this way because there are so many possible variations depending upon your intentions, your available materials, your budget, your tools, your time, and your labor that it is impossible to give definitive recipes or procedures. For instance, if you are trying to make the strongest, most durable, and most aesthetically pleasing structure possible, you might elect to use, for example, imported Italian marble. If your intention is more along the lines of building a solid, functional structure using appropriate materials and technology and at the lowest possible cost, you may want to use some of the recipes that follow. But even here there are choices: you can purchase fine clay, sand, fly ash, and rice straw, or you could use your own available sub-soil and whatever weeds and brush you clear from your own property. I follow the latter strategy, intending to use our own available materials whenever possible, even though we realize that better materials can be used. The first principle of appropriate building technology is to use materials that you already have locally, and this is the guiding principle that we attempt to follow.

     Before I give actual recipes, let me briefly discuss some of the different approaches to low cost building technologies. Since experimenting with our own methods, I have learned a lot about what others are doing: adobe, cob, straw bale, etc. Earth structures such as adobe and cob represent a very ancient technology of building, and it has many advantages, chiefly low cost, but one disadvantage: it is not really waterproof. While many structures have survived intact for many years, they are usually annually maintained, and/or sealed with a top coat of some sort of lime plaster. Agstone is somewhat similar, except that the addition of lime makes a stronger, more durable, and much more waterproof structure. It has a much harder finish and can be used much like concrete. Straw bale is a very interesting concept, except that all that straw takes up a lot of space, and it needs to be covered anyway. When you consider the interior and exterior covering (for which you might well use a material such as our Agstone), you will probably end up using as much or more of this outer material anyway, so there seems to be little advantage other than the extended insulation. To summarize this brief overview, I have looked at a number of alternatives, and I still favor the approach we have worked out.

     While there is a wide variation of possible recipes that will all work, it is useful to have a basic understanding of the chemistry involved with the use of lime. When lime (hydrated lime; calcium hydroxide) is used for an exterior coat, it hardens up by simply combining with the carbon dioxide in the air to change back to the limestone from which it was derived (by burning). However, interior lime hardens up by a different chemical reaction: it combines with materials high in silica to harden up into stone. The classic way to do this is to combine lime with clay. This is "Roman cement," and it is really the origin of the whole idea.

     Other sources of silica are also used, but there seems to be no agreement about the chemistry involved. Sand is very high in silica, but because of the particle size, it is less significant than clay. However, I have experimented with mixes higher and lower in sand, and my personal experience is that sand is an essential ingredient in the mix. Mixes with little or no sand are just not as hard or durable as mixes made with plenty of sand. Then there is the question of the silica component in the aggregate, the agricultural waste, weeds, chips, or straw. Hemp hurds are high in silica, and that is probably one of the reasons why the hemp hurds plus lime formula has been such a success in France. My personal experience confirms that hemp hurds perform better than just about any other similar aggregate material. Rice straw is also very high in silica, and, while I have not had personal experience using rice straw, I am confident that it would turn out to be an excellent material for this purpose.

     Then there is the sand and clay. This ideally should come altogether if you can find a good subsoil that has a high percentage of clay mixed with sand and silt (no organic matter).

     Finally, there is Portland cement. Many purists recommend eschewing the use of Portland cement altogether for technical and religious reasons. I will avoid this controversy here except to mention that the technical arguments suggest that too much Portland cement creates problems, but small amounts of cement mixed with lime avoid those problems. We use about 10% cement in our mix because it works: it helps it to set up quickly, and it assists the chemical reaction with the lime. This small a concentration avoids all of the objections to its use other than its supposedly high environmental cost of manufacture, in terms of fossil fuel use, and the release of CO₂ into the atmosphere. Anyway, I have experimented with and without cement, and I have found that using only lime for a binder will usually set up hard eventually, but it may take quite a long time to do so. We haven't yet found a really good local source of clay; perhaps if we did we could effectively eliminate cement from our mix altogether.

     My latest addition to the recipe is wood ash. The main reason I want to use it is because there is only so much wood ash I can add to my compost, and I want to use it up. Also, lime is highly alkaline and this high pH is required for the chemical reaction to take place, so it can't hurt to use it.

     I draw the line, however, at incorporating old bottles, broken or whole. This may be purely an aesthetic consideration, since the glass may be at least as useful in the mix as the wood ash I want to use up, and it is certainly nice to use your refuse for a building material. Perhaps I may get over this prejudice and start using it. When you have a lot of Agstone to pour, whatever you can throw in to extend the mix saves time and money.

     In addition to all of the above, we also use rocks and gravel, especially in the lower courses of our structures. It just makes good sense to use up these rocks where they can do no harm. The more rocks you use up, the less Agstone you will need to use.

     Our process allows the use of a much greater quantity of agricultural waste (chips, weeds, or straw) than other methods. We keep changing our recipe, experimenting with new ideas, but the following should give you a very sturdy mix:

     For purposes of working out formulas, I always start with one bucket of chips/weeds/straw and call that 10 parts, by volume (we find that formulas measured by volume are much easier to reproduce than formulas by weight). Our mix is as much as 50% chips, weeds, or straw. We use this material because it is freely available and because all those pieces strengthen the mix. We use a random mix, striving for some variation in the size of the pieces. If broken glass is used, that would be part of the 50%.

     We used to begin by adding the lime to the chips or weeds, but we have found that it is better to mix up the lime, clay, sand and cement with water and then add the chips or weeds. We recommend three parts of lime (15%), a total sand/clay portion of about five parts (25%), and about two parts Portland cement (10%).

     There you have it -- this mix works very well for us, but you can modify it in many directions. You can add chipped up paper and cardboard to extend the mix, but it makes it softer and weaker. You could add more sand and clay, more lime, less cement, broken bottles, floor sweepings, old clothes (shredded), broken toys . . . We pour on site in wooden forms, which are removed the following morning, but you could also press bricks and mortar them together with the above mix, leaving out the chips, weeds, and/or straw.

     You can apply a top coat of lime plaster, which is just hydrated lime and water with fine sand, to smooth out the surface. For a really smooth inner surface, you could add gypsum to the plaster, along with any natural pigments.

May 7, 2002 --

     As I look over what I have written earlier, I see that we still use a very similar mix, although we often use even more chips -- up to 2/3 of the total volume.

     But I would like to add that, while pouring in forms works just great for foundations, for building walls I would recommend making bricks first. I want to construct a form in three pieces -- a piece of plywood with two permanent 2" x 6" sides attached, and two sets of parallel 2x6" dividers, with interlocking notches, to make up a mold for 64 bricks. Fill the mold with agstone, and the next morning you can gently move the fresh bricks to a spot to finish drying/curing. In this way, you can keep up a production of 64 big bricks per day for a building project (if you need more, use multiple molds).