John Stahl, our hemp stone expert says the following: “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, 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.
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).
We use about 10% Portland 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.
Read the full article at HempBuildingMaterials.com
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