Clays for Construction

No material goes farther back in construction history than clay. In recent years exciting new techniques and technologies centering on this age-old material are changing parts of the building industry.

Overview of Clay Properties

Clay is not like plaster. Plaster 'sets'. Clay dries. Clay shrinks when it dries (6% is typical) and it will turn back into mud if re-wetted. Clays are plastic and vary widely in the degree of plasticity (this property relates to others that you need in this application). Clays also vary widely in their water permeabilities (time to dry), dry strength, drying shrinkage and stickiness. So if you have a recipe from a book, stop and think. Be ready to test to see if that recipe needs adjustment for the type of clay you want to use.

Clays of higher dry strength generally dry slower, shrink and crack more (micro-cracking at bond-points will reduce strength). That means that a compromise can be reached where strength is adequate, drying time is shortest and drying shrinkage is least (typical clays shrink 4-6% on drying). Kaolins are the least plastic and fastest drying, bentonites are the hardest and slowest drying (in fact, bentonite particles are so small that it is possible to mix only small amounts in the water before a gel is formed). The difference in particle size and surface area between these two extremes is one-hundred-fold! In strength: ten-fold. You are likely going to need something in between these two extremes. "Ball clay" is a less pure (than kaolin) but dries much harder and shrinks more. However, ball clays also vary widely in dry strength. Natural fireclays and stoneware or earthenware pottery clays are less hard than ball clays but stronger than kaolins. You may like to try tests using a pure kaolin and a pure ball clay (these are available in 50 lb bags are are inexpensive). The performance difference between these two extremes will give you direction on how to mix them to get a compromise. If natural clays are available to you, consider mixing them into a plastic form and comparing their drying shrinkage and hardnesses. To increase the stickiness and strength of a natural clay, add small percentages of bentonite. Another factor to consider is the fineness of the clay powder. If you need to improve drying speed consider adding feldspar or calcium carbonate, these are inexpensive non-plastic mineral powders (the particles will create micro-channels within the clay to route water out quicker).

Earth plaster

Earth plasters have been used for thousands of years to cover both the interior and exterior of structures made using a variety of construction techniques. From the wide array of our own mined materials and those we import, we have selected a few light colored (and therefore easily pigmented) materials that span a wide range of fluid and dry properties. We describe these materials well in our documentation. You can blend these clays and condition with hardeners, plaster and mineral fillers to make them smooth, easy-to-apply and resistant to wear and weathering. For those with more specific needs, we stock a wide range of useful materials (e.g. naturally pigmented clays or even our standard ceramic mixes). Click here to learn more.

Straw Bale Housing

Light Straw-Clay: Straw is coated with a clay binding slip (slurry) and rammed into forms to produce blocks or walls of very low density (and good insulating properties). It has thus been dubbed Light Straw-Clay. Often little consideration is given to the type of clay used. However, this deserves attention (see overview section above). Bond points will have less strength if there are coarse particles between them (e.g. sand), finer particled clays will work the best to bond the straw.
Earth Blocks

Many companies around the world have developed pressing machines and polymer or other hardening systems that enable the making of blocks on the construction site, blocks that do not need to be burned in a kiln. Traditional techniques are also alive and well in many countries. The headache of adapting the pressing process to a new material at each construction site can be eliminated by employing consistent pressing clays from a supplier as either the full mix or to condition onsite materials.

Tile

The tile industry is by far the biggest ceramic industry user of clay and Plainsman Clays has a line of raw materials that are ideal to make many types of tile. We have the flexibility of making products solely from our own mined materials or from mixtures of them and imported raw materials and can supply powders of high water content. We also have fusible slip products that can be used to make engobes.
Bentonite for Stucco

Highly plastic clays (especially bentonite) are added to stucco mixes to make them stickier, apply better and dry slower. We import and mine bentonites, smectite and hectorite. These clay vary widely in their plasticity, you might find that our best bentonite will work in a much lower percentage than the one you have been using.
Architectural Commissions

If you are making a large wall panel, mural or free-standing architectural commission, we can help with a clay adapted to your needs. These projects present many challenges, you really need a clay that is intended for this use (not for pottery or small-scale sculpture). We can also assist you with the development of vitreous, textured, speckled and weather-resistant engobes and glazes and help you to deal with freeze-thaw (so your piece does not fracture) and the prevention of efflorescence (so your work does not turn white after a few rainfalls!)

By Tony Hansen
Follow me on

Got a Question?

Buy me a coffee and we can talk