Raw, fired and stabilised earth
Clay has a poor carbon footprint (close to reinforced concrete1) because it is necessary to fire the clay at high temperatures to obtain bricks or tiles for example. This heat treatment modifies the characteristics of the clay material and makes it resistant to immersion in water.
By avoiding the firing stage, raw clay has a very low carbon footprint if used locally. The absence of thermal modification of its properties makes raw earth easily reusable.
Stabilised raw earth is unfired earth to which a quantity of cement or lime is added in order to make it more resistant, especially to erosion. The addition of cement has a negative impact on the carbon footprint and hinders the recyclability of the soil when the building is demolished.
A technique for every kind of earth
Soil used in construction is called "mineral" soil; "vegetable" soil is reserved for agriculture because it is made fertile by the presence of decaying plants and living organisms.
Mineral soil is a material composed of gravel, sand, silt and clay, in different proportions depending on the location. A diversity of soils therefore corresponds to a diversity of construction techniques, whether load-bearing or not. A balanced earth, for example, will be ideal in the form of rammed earth walls or compressed earth bricks (CEB), while a more clayey earth will be used to make cob or plaster.
When we talk about concrete, we are really talking about the material we use most in the construction of our bridges and buildings: cement concrete. Soil is also concrete, but clay concrete.
So what is concrete? It is a mixture of aggregates of different sizes bound together by a binder - in the case of earth, clay. As with cement concrete, the proportion between these different constituents influences its strength, workability and processing.
By adding water to the soil, we liquefy it and can shape, mould and compress it. The removal of the water by evaporation then solidifies the clay and makes it hold the given shape. Unlike the irreversible chemical setting of cement concrete, clay concrete can be rehydrated and reformed ad infinitum. This is a major advantage in making it a material of the circular economy.
It is also important to know that earth works in compression, but has poor resistance to traction. This is why earth will not take the form of a beam as wood would, but rather an arch or a vault, like stone. For good resistance, certain design and dimensioning rules must therefore be respected: ties, solid/void ratio, maximum slenderness, presence of lintels, etc.
Clay, unlike cement, sets in the air. It is therefore necessary to remove the formwork or demould the clay quickly and to ensure good ventilation and a good drying time. However, experiments are currently underway on "poured earth", i.e. earth that is used like cement concrete. This requires adding cement to the soil or other types of chemical binders, or working on the formwork so that it allows air to circulate (still in the experimental stage).
Adding cement to soil is also a common technique to increase its load-bearing capacity. However, what it gains in strength, it obviously loses in ecological footprint and 'recyclability', so think twice!
It is also common to add additives to earth to change its characteristics and this practice is not new: we can cite as an example the addition of cow dung or blood (remember that the vegan label is very recent 😉 ) to plaster preparations to avoid cracks.
Earth can be eroded by rain or lose strength through capillary rise. This is why you will usually hear earth builders say that this material requires "a good hat and boots", i.e. an overhanging roof and a base.
Earth is relatively accessible to non-professionals, making it a material of choice for self-builders. However, training and professional advice are strongly recommended for load-bearing applications.
Although earth is readily available and very cheap, it is labour intensive and time consuming. For this reason, it is rarely found on our 'classic' European building sites, where labour and site time are drastically reduced in order to minimise construction costs and maximise profits. However, there are original forms of building sites, such as participative building sites, which allow this material to be reintroduced into our current constructions and restore its lost social dimension.
Earth is sensitive to rain erosion, but it is also very resistant to moisture if the moisture is allowed to escape. This is because soil is a 'perspirant' material, meaning that it is permeable to water vapour. This gives it the ability to absorb excess moisture from the air and release it later (e.g. when the room becomes drier) or vent it to the outside. For this principle to work, however, it is imperative that the other materials making up the façade (interior and exterior cladding and insulation) are also breathable to allow for this transition of moisture and to avoid condensation in the walls which would have a disastrous effect on the earth structure. The earth, by regulating the humidity, contributes to the quality of the indoor air (see also our article "Mens sana in domus sana").
Due to its high density, earth has a high thermal capacity, i.e. it is able to store heat and thus dampen temperature peaks: this phenomenon is called thermal phase shift. In winter, this characteristic allows the heat from the sun to be stored and gradually released into the building, even after the sun has gone down. In summer, it slows down the entry of heat, keeping the interior of the building cool. However, thermal capacity should not be confused with insulating capacity. In our latitudes, earth must be supplemented with insulation to protect against the cold in winter, either as an extra layer or by mixing it with a natural insulating material such as straw, hay or other material (also known as light earth).
« Recyclability » and « reparability »
If the earth is used raw, not stabilised with cement and not mixed with other materials (such as straw for example), it can be recycled endlessly. A good blow of pickaxe, a little water, and there it is ready to be re-used for a next construction! It is also non-polluting and can be stored on the ground until its next life.
An earth building is also a resilient building. This is because the material is tolerant and can be easily maintained. You will know, for example, that clay plasters can simply be repaired with a wet sponge after an impact, which is not the case with cement or synthetic plaster!