Engineers are cultivating a more sustainable future of building materials.
By Thomas K. Grose
As the planet’s population increases, so does the number of buildings needed to contain it. About five billion square meters rise from the ground annually, calculates the Global Alliance for Buildings and Construction—or each week the equivalent of a city the size of Paris.
But as the world races to check greenhouse gases and slow climate change, the building industry has come under scrutiny. It’s responsible for a whopping third of energy-related CO₂ emissions, according to the United Nations Environment Programme, so the urgent need to address global warming has begun to force change. The concrete industry, for instance, has vowed to cut emissions 25 percent by 2030 and reach net-zero by 2050.
Sustainable building materials will play an important role in achieving such ambitious goals. The good news: construction materials that once seemed impossible are now hitting the market at an unprecedented rate.
It’s Alive!
Historically, the construction and materials manufacturing industries have been reluctant to adopt new products. The most popular building materials, concrete and steel, are cheap and easy to use. However, they each individually contribute about 8 percent of global greenhouse emissions.
Concrete ranks second only to water as the most-used substance on Earth. Wil V. Srubar III, associate professor of civil, environmental, and architectural engineering at the University of Colorado Boulder, has developed a different approach to the material. Since the production process of Portland cement—the key binder in concrete—has a huge carbon footprint, his team has replaced it with a living alternative.
Using biomineralizing bacteria—organisms that produce the calcium carbonate in natural structures like mollusk shells and corals—they manufactured a new building material that replaces the Portland cement binder with a bacteria-friendly one Srubar invented. It proved a solution to the challenge researchers had previously encountered in attempting to add biomineralizing bacteria to concrete: the high pH levels in the mixture killed the creatures. “We engineered an environment . . . that’s more conducive to living organisms,” Srubar says.
Compared to traditional Portland-cement concrete, Srubar notes, the material “can achieve up to 90 percent embodied carbon reduction.” And research has demonstrated that it is just as strong and inexpensive as traditional concrete, he continues.
Architects, engineers, and contractors are serious when they pledge to clean up the built environment, Srubar asserts. “These are very, very ambitious goals, so there is tremendous demand for new technologies.” His company Prometheus Materials aims to tap into that need by commercializing the living concrete. Srubar sees potential for the material to replace the traditional version in both the developing and developed world.
Growth Market
Cross-laminated timber (CLT) is one of several engineered woods considered green, due to trees’ ability to sequester carbon from the atmosphere. The material is made by gluing together several sheets of wood with each layer’s grain perpendicular to the one below. The pattern affords CLT the strength of concrete and allows it to replace that high-emission substance in walls, floors, and ceilings.
In addition, nearly all cross-laminated timber structures are prefabricated. They fit together piece by piece on-site, “like an Ikea kit,” explains Lech Muszynski, Oregon State University professor of wood science and engineering. That speeds the pace of construction and reduces needed heavy equipment to just one crane and one delivery truck, says the CLT manufacturing expert, further shrinking the materials’ carbon footprint.
Though it has been around since the early 1990s, CLT has only recently gained traction, with a spate of high-rise buildings using it. The twenty-story Sara Cultural Centre in Skellefteå, Sweden, opened last fall, and, later this year, the twenty-five-story Ascent apartment tower will welcome Milwaukee residents. The fact that these buildings garner headlines show how rare the material still is, Muszynski points out.
Why has CLT been slow to take off? “First and foremost is a lack of familiarity,” he says. Also, its prefabricated nature means its use is considered custom work. With CLT, manufacturers must be involved in the building’s design and construction from the beginning, because the material has to be precisely manufactured to fit the design and carefully assembled with input from the manufacturer. As Muszynski explains, that’s a new approach that’s beyond the comfort level of many architects and engineers.
However, he thinks the CLT industry is ripe for commodification, which would allow for modular construction of houses and apartment buildings that follow a similar design. “They don’t have to be identical, but based on the same model.”
One caveat: CLT’s carbon storage is temporary, the professor stresses, because the sheets eventually end up in a landfill where their carbon is released as they decompose. Still, he adds, that process “is delayed for at least a generation or two, and a lot can happen in that time,” such as the development of new technologies to mitigate CO2 release.
Muszynski is optimistic about CLT’s future. He predicts that, within five to ten years, CLT will become “a more significant part of the solution” to cut construction industry emissions.
Moral Fibers
A new entry into the mass timber market comes from the lab of Liangbing Hu, materials science and engineering professor at the University of Maryland. He has invented a “superwood” engineered at the nano level. The smallest wood fibers, Hu explains, are naturally aligned in the same direction—just like carbon nanotubes, one of the world’s strongest materials. He found a way to chemically treat wood to remove the lignin, a natural polymer that glues the fibers together. The wood is then compressed so that the fibers touch, which strengthens the material.
Hu sees his wood as a replacement for steel and aluminum, both of which require immense amounts of energy and heat to produce. Little energy is needed to make the superwood, and it can be manufactured at room temperature. Unlike steel production, Hu says, the manufacturing “will be carbon negative, if you take photosynthesis into consideration.”
He uses wood from fast-growth trees that are both inexpensive and lightweight. That makes the superwood—equal in strength to steel, he says—five to six times lighter. The goal, Hu says, is to also make his material less expensive.
Hu is interested in CLT as well, which he calls “an emerging and attractive option for replacing concrete.” There’s potential, he says, to make CLT from superwood, further increasing its strength while making it thinner. “There are a lot of design opportunities there.” Hu’s spinout company, InventWood, is investigating superwood’s commercial potential, working with companies in the traditional wood industry to scale up the process and integrate it into the manufacturing chain.
Budding Strength
Concrete structures rely on steel rebar to handle tensile load. But salt used on icy roads or from coastal air can eventually permeate concrete, causing it to flake and exposing the rebar to corrosion. That can shorten a structure’s lifespan. One solution is epoxy-coated rebar, which doesn’t corrode, “but it’s ungodly expensive,” says Daniel Walczyk, a mechanical engineering professor at New York’s Rensselaer Polytechnic Institute. Another solution is rebar made from fiber-reinforced polymer (FRP). It also works, but unlike steel it cannot bend, which limits its design potential.
Walczyk, along with Alexandros Tsamis, an assistant professor of architecture at RPI, are looking to hemp—yes, the plant marijuana comes from—to produce flexible and resilient rebar. “Hemp is a very strong natural fiber,” Walczyk says, adding that it’s twice as efficient at sequestering carbon as trees. Use of hemp in construction isn’t new. “Hempcrete,” a material that can in some cases replace concrete, has been used in France since the 1990s and is starting to take off globally as legal prohibitions on the use of industrial hemp have eased.
Using hemp for rebar is a new twist, however. Walczyk and Tsamis say their plant-based rebar is at least as strong as steel, can bend, and won’t corrode—tripling the life of a concrete structure. It’s a mix of hemp and flax within a matrix of a meltable polymer, or thermoplastic—in this case a bioresin that’s derived from plants. The manufacturing process also takes little energy, especially compared to FRP. Moreover, builders incorporating the material would use less concrete, as they would not need extra to protect the rebar.
The dynamic duo is also inventing a machine that could be used onsite or in a factory to manufacture the rebar. Rope-like material would feed into the machine, Walczyk says, and then it would “basically spit [the rebar] out like a 3D printer.” Onsite rebar fabrication would further cut emissions because the only delivery would be a coil of rope. It should also be cheaper than other options because the feedstocks are all commodity items.
The US legalized the use of industrial hemp in 2018, but funding agencies remain hesitant to back hemp research because of marijuana’s stigma. Walczyk and Tsamis have launched a company, FiberWerks, to commercialize their inventions, but they still mainly work in the lab. If they were “flush with money,” Walczyk says, they would likely be market-ready within two years. But they’re still reliant on internal funding, “so right now we’re just plodding along.” Venture capitalists have expressed interest in the technology. That’s encouraging, he says, “but it’s not yet ready for primetime.”
Rubble Brick Road
Kiln-fired bricks are another common building material requiring considerable energy and intense heat—temperatures between 1,300ºC and 1,700ºC—to make. While bricks can be recycled for reuse, the process is expensive and labor-intensive. Gabriela M. Medero, an associate professor of civil engineering at Scotland’s Heriot-Watt University, has spent twenty years researching uses for construction waste, much of which ends up in landfills. About ten years ago, she produced her first prototype of a brick made from rubble, including bricks, glass, gravel, sand, and gypsum. Dubbed the K-Briq, it requires no firing or cement.
The K-Briq has less than a tenth of the carbon footprint of fire-baked bricks, Medero says, but is just as strong and durable—and competitively priced. How did she accomplish making bricks without a kiln? “That’s part of the secret,” she says.
In 2019, she cofounded a company, Kenoteq, to commercialize the K-Briq technology. A major component of the business model is the ability to manufacture bricks near the construction instead of the clay extraction site. Eliminating the need for long-distance shipping further minimizes K-Briq’s carbon footprint. Kenoteq has already fired up production to produce 3 million bricks a year. That’s a small percentage of the 2.5 billion bricks Britain used last year, but still impressive for a small startup, which now has industry partners to license the technology in Europe, the US, and Canada.
Medero believes there is a growing community within the construction industry ready and willing to adopt greener materials. “The sector needs to change,” she says. “More and more, there is a push for more sustainable solutions.” And who knows? The road to greener infrastructure may be partially paved with K-Briqs.
Thomas K. Grose is Prism’s chief correspondent.
Design by Toni Rigolosi