Soil Construction: Where—And How—Building With Clay Thrives

As concrete’s prevalence continues to pose a danger to the long-term health of the environment, can clay- and soil-based materials serve as a viable option for builders?
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Video produced by Justin Hearn

In some ways, clay is the most basic material available to humankind. Because it is readily available in the natural landscape, humans have been building with clay for as long as we’ve been creating structures.  

There are countless examples of historical clay-based buildings, including the Pueblo adobe construction of native New Mexicans to the portable wood frame structures constructed by Australian aboriginal people, which were insulated with clay to protect the inhabitants from both cold and heat. Even on an individual level, a young person’s first attempt at building a structure usually takes the shape of a dirt-and-water amalgam, whether in the form of a backyard mud wall or a moated sandcastle built on a family trip to the beach.  

“If we look at construction throughout history, humans have found an infinite amount of interesting solutions that use the local resources,” said Emanuela Del Gado, a professor at Georgetown University who studies complex materials using computational modeling and statistical mechanics.  

Around the world for centuries, humans have been creating structures out of the most readily available materials: earth and water. But why do some of these structures—like the ancient fort at Hierakonpolis—survive, while others have long since turned to dust?  

According to Del Gado, there’s still a lot we don’t know about what makes soil construction work. Finding these answers might just be the key to discovering a new, modern approach to this incredibly affordable and eco-friendly material. 

How soil construction works 

How does soil construction actually work? To understand the basic principles of soil construction, think about the last time you built a sandcastle.  

“When you build a sandcastle,” Del Gado said, “you start with sand that is dry and completely disaggregated. We know if we wet it a little bit, we can get it to hold together. Grains of sand do not attract each other, but if you put in just enough water, it will form thin layers that attach to the individual grains and hold the building of the sandcastle together. But since that water is just liquid water, it will eventually evaporate out from between the grains. So, you left the castle there for the rest of the day under the sun, the water starts to evaporate and the castle, little-by-little, disaggregates.”  

As the water evaporates out from between the grains of sand, the resulting structure is unstable—and will eventually collapse.  

In contrast, lasting soil construction structures have water integrated more profoundly into the structure of the material, so that as the water evaporates the material hardens rather than becoming softer.  

“There are some clays, and therefore in principle some kinds of soils, which have a very particularly interesting reaction with water, similar to what happens with cement,” Del Gado said. “The powder reacts with the water, and it starts to produce a material that’s very sticky, which eventually hardens like a rock.”  

These materials are called geopolymers; understanding how and why they work is a topic currently under investigation by Del Gado and other leading researchers in the world of material sciences.  

Why is dirt important? 

As the construction industry moves to reduce the impact it makes on the environment, local materials like clay and soil can do a lot to reduce a project’s environmental harm.  

“In construction, there has always been the necessary constraint of wanting to build based on resources that are available everywhere and are relatively cheap,” Del Gado said. “That’s one of the reasons cement is such a ubiquitous building material—you can produce it from rocks anywhere on earth.”  

But the production of cement is especially damaging to the environment—so much that some scientists have called concrete, which requires cement in its production, “the most destructive material on earth.” Del Gado said that developing a better understanding of how soil and clay materials work could potentially allow the industry to reduce its usage of concrete on a vast scale.  

“You could theoretically mix a certain amount of ordinary cement powder with a complimentary geopolymer,” Del Gado said. “That would allow you to reduce the amount of cement that you use for each building.”  

The consequences of such a reduction could have a big impact on the environmental footprint of the construction industry—from lowering the amount of water needed to set cement structures to shrinking the emissions output associated with the manufacture of cement. 

Ancient technology, modern research 

Successfully creating geopolymers in ancient earthwork constructions seems largely to have been a result of trial and error, with builders experimenting with locally available materials to create stronger structures.  

“One great example of builders using local—and possibly even waste material—as a complementary component in construction is the Great Wall of China, where they found traces of rice proteins in the mortar,” Del Gado said. The sticky rice paste used to supplement the more expensive slake lime mortar bonded so strongly between the stones that, in some places, weeds still can’t grow in the cracks—creating a more stable mixture.  

Returning to these ancient technologies in the modern construction industry means taking a more deliberate, research-based approach to materials usage.  

“The question now,” Del Gado said, “is can we go back to that technology and, with our knowledge and capacity to understand how material works, can we find a modern-era version of those technologies that can actually help us solve the problems we have now?”  

Should builders embrace a return to building with stone?