At the beginning of the course, we made a distinction between natural building and earthen building. Earthen building is a subset of natural building. Basically, it means building from clay and sand. Natural building includes things like wood, bamboo, stones, glass, etc. What follows focuses on fundamentals of earthen building.
There are three primary components of earthen building sand, clay and straw.
Have you ever looked really closely at sand? Sand is actually a bunch of tiny rocks. Because sand is a rock, it does not absorb or release water. Sand gives our earthen mixtures compressive strength, the ability to push down and hold weight. Sand is the aggregate component.
This is the glue that holds the mix together. Clay is the binder component. It is sticky, expandable and it readily reacts with water. Clay is the finest of all the components in our mix and as such, water has a tough time moving through clay. Clay demonstrates properties of plasticity; it expands when reacting with water and then contracts as it dries out.
Sand and clay are the essential components. This is the formula for adobe. Mixing different ratios of sand and clay together is how many ancient civilizations created monolithic structures that have been standing for hundreds of years. There’s still room for innovation though.
Straw or whatever fibrous material you have locally is the third and final ingredient in most earthen mixes. The fiber provides tensile strength; this is pulling strength. It is the ability of something to be stretched and pulled. Tensile strength helps increase flexibility of structures as well. Try pulling a piece of dry straw apart with your hands. It’s nearly impossible, you can cut it or tear it easily, but you won’t be able to pull it apart with sheer force. The key is using fibers that are dry, if its still green then it could rot inside the earthen wall and serve no purpose.
Coming into the course, I was hoping there might be a formula or something for mixes. I thought someone might teach me to follow x:y:z ratio and I could scale it infinitely around the world. This is not the case. There is no formula. Earth is variable. The clay in our backyard in Darien, IL is a different beast than the clay found on the Volcanic island of Ometepe or the colorful clays found near the Honduran/Nicaraguan border. Sand is also extremely variable, all kinds of different sizes, shapes, consistencies. Beach sand and sand from a mine is completely different. As such, a formula that works in the Philippines might not work so well for Nicaragua.
So what to do then? Experiment. Run small tests using different ratios, use different materials that are locally available and see what works best. Things to notice are cracking, hardness, how long it takes to dry, etc. It’s best to run a series of experiments working on optimizing the clay:sand ratio first and then adding in as much fiber that will fit. You can start by making blocks or balls using different clay:sand ratios such as 1:1, 1:2, 1:3, 1:4 and 1:5. Let the samples dry, and observe. Toss around the samples, step on them, try and chip them, drop them from shoulder height, see how many drops it takes to break, etc. Do more experiments. Do as many experiments as you can to feel confident about the ratio. It’s better to have a few samples cracking and looking funky than it is to have your entire home crack and fall apart.
Fail fast and cheap. Figure out what works and build from there.
This is the first in a series of posts about Natural Building.