Concrete ranks as the most popular construction material in the world. But its key ingredient, cement, is responsible for 8% of global carbon dioxide emissions each year. Scientists want to replace concrete with a more environmentally friendly material, and one candidate is soil. In one of the most recent iterations of these efforts, the Banerjee Research Laboratory at Texas A&M University has created a tool kit for using local soil to make construction materials.
Concrete production, especially production of its binding agent, cement, releases massive amounts of carbon dioxide (CO2). “If [cement production] were a country, it would be the third-largest emitter in the world,” said Gaurav Sant, a professor of civil and environmental engineering and materials science and engineering at the University of California, Los Angeles.
“We need to go carbon neutral by 2050 and carbon negative thereafter,” Sant said. To do that, the construction industry needs to drastically change or replace concrete. “We’re talking about disrupting and transforming our entire basis of society as a whole in the next 30 years.”
The modern form of concrete, a mixture of sand and gravel bonded by cement and water, has been used for only the past 150 years or so. The development of modern concrete reinforced with steel allowed builders to erect massive structures, giving us city skylines dominated by skyscrapers. With the growth of additive manufacturing, a process in which layers of concrete are 3D printed one on top of the other in a predetermined design, more complex building parts can be created more efficiently.
“A lot of emerging economies are going through a massive construction boom, and if we do this all in concrete, the consequences for the environment are going to be catastrophic,” said Sarbajit Banerjee, a chemistry professor at Texas A&M University, at the 2020 meeting of the American Chemical Society (ACS).
Building with Backyard Soil
Banerjee and Aayushi Bajpayee, a Ph.D. candidate in Banerjee’s lab, wanted to develop a sustainable material that could work with existing building codes and concrete-based construction methods. For their source material, the team settled on soil.
That idea came from both nature and history. In nature, termites make impressive use of soil, building intricate and durable mounds. “That was one of our motivations, and the second one was ancient times,” said Bajpayee. “In ancient Rome and India, there are a lot of places [where people used] soil.”
The difference between ancient earthen structures and a soil-based concrete alternative is that in ancient times, the main instinct for such construction was survival, said Bajpayee. “Now, our main instinct is sustainability.”
Banerjee and Bajpayee used clay soil from a colleague’s backyard in College Station, Texas. They combined the clay, a water-repelling additive derived from beets, and sodium silicate to bind everything together. A 3D printer then extruded this material according to the desired design, forming a model a few inches tall.
They hope that their method, presented at the 2020 ACS meeting and published in Frontiers in Materials, can serve as a chemical tool kit to be used at any construction site, particularly in remote or hostile environments. Once a construction team analyzes the local soil, they can tweak the ingredients in the tool kit, mix their material, and start printing.
By cutting out the energy-intensive production steps, using local resources, and eliminating transport concerns, Banerjee predicts their material will have a much lower emissions profile than concrete, but they’re still running simulations to nail down the figures. “I think the numbers are going to be significant,” he said.
But soil-based replacements have limited applications. Although they could prove valuable for building housing in remote areas, on the basis of the current work, Sant doesn’t think these materials are viable for larger structures.
Gnanli Landrou, cofounder of Oxara, a sustainable construction material start-up, said his group uses a process similar to Banerjee’s to make a soil-based building material. “The goal is not to replace concrete, but to efficiently use concrete where needed,” he wrote in an email. His group wants to use their product, Cleancrete, for housing or nonstructural pieces of larger buildings. “Overall, we want to enable access to sustainable and affordable building materials and homes.”
More robust replacements for concrete are in the works. Sant, for example, has developed CO2Concrete, a cementless concrete in which CO2 from industrial waste gas reacts with calcium hydroxide to bind everything together. “You get structural cementation with limestone in a seashell,” said Sant, which is exactly how CO2Concrete forms. It’s similar in cost and function to concrete, he said, but with half the carbon footprint.
“Concrete is still the gorilla in the room because it’s been the norm for many years,” said Banerjee. “There are costs involved that are not cheap, rising CO2 emissions that are tremendously large. We can do better than that.”
—Jackie Rocheleau (@JackieRocheleau), Science Writer