Low-carbon concrete solutions are showing high potential

Cement and concrete have built entire civilizations, according to the Portland Cement Association (PCA), the major trade group that represents U.S. cement manufacturers. They're not wrong. 

From Roman aqueducts to interstate highways, the Parthenon to Habitat 67, concrete defines so much of how we navigate and consider our built environment. And in recent history, it has also wrought terrible harm on our natural environment. 

A quick scroll down PCA's site reveals how the association has pivoted to making "a sustainable future" and "carbon neutrality" core parts of its mission. This is notable, given that portland cement manufacturing (made by heating limestone and clay minerals in a kiln up to 1450°C to produce clinker, which is then ground to a fine powder and mixed with gypsum) is responsible for around 8% of global CO2 emissions. Its carbon footprint is lower than steel or aluminum, but it is also consumed at much higher rates. So, what gives? Is this heavy-handed greenwashing or a sign of an increasingly climate-conscious industry?

In some respects, the cement industry has been shifting course for decades. The use of supplementary cementitious materials (SCMs) such as coal fly ash and steel slag to replace portions of traditional cement in concrete production – effectively upcycling industrial waste products with a lesser carbon impact – is nothing new. But within the last five years, give or take, the number of companies – both startups and established brands – producing some variation of low-carbon concrete has exploded.

The new normal 

“Low carbon concrete is becoming the lingua franca of the industry,” says Chris Bennett, owner of Bennett Build, a consultancy for sustainable concrete solutions. Indeed, what was once a niche business has rather quickly (and quietly) claimed outsized market share, to the point where major infrastructure developments, from mass transit projects to data centers to public buildings, are now being built with low-carbon concrete blends.  

The use of traditional portland cement in concrete mixes has become “the less common outcome,” according to Grant Quasha, CEO of Eco Material Technologies, a leading producer and distributor of SCMs, which the company harvests from coal ash landfills in Pennsylvania, Georgia, and other sites. (The company currently has seven active harvesting sites.) One such product is Eco Material’s proprietary PozzoSlag®, an engineered pozzolanic cement made using coal combustion waste, manufactured at room temperature. According to the company’s measurements, with a 50/50 mix of PozzoSlag and traditional cement, the blend demonstrates a 20 percent strength gain over portland cement after 28 days. It is also a nearly zero carbon product. 

To date, PozzoSlag has been used in several infrastructure projects in Texas, including in the runways at George Bush Intercontinental Airport in Houston, and portions of Interstates 45 and 2. Other fly ash-based cement blends produced by Eco Material have been used in the Hoover Dam Bypass bridge, the 2013 extension of the Bay Bridge, the recently completed Populus Hotel in Denver, and dozens of other large-scale developments. The company’s client list includes the likes of Google, Meta, Intel, and Amazon. And just last week, Eco Material opened a new rail terminal in the Long Island City section of Queens, which will enable the company to transport and distribute up to 50,000 tons of harvested fly ash annually to the New York City Metro area.

According to Quasha, public infrastructure is where low-carbon cement’s impacts are and will be felt the most. He also cites findings from the PCA that indicate how much more market share blended cements have claimed in recent years, all while meeting ASTM performance specifications. If nothing else, the scale of projects in Eco Material’s portfolio is evidence enough that lower-carbon concrete solutions are the new norm. The economics of it all make sense, he says. “This isn’t a science experiment. It’s just accelerating.” 

No single solution

There are numerous blend types and ingredients that go into low-carbon cements, from natural pozzolans to synthetic fly ash. One increasingly popular option is Type 1T ternary-blended cement, made by mixing varying amounts of portland clinker, gypsum, pozzolan additives, and limestone. 

Technological innovation in this industry has also increased exponentially. Massachusetts-based Sublime Systems uses an electrochemical reactor to synthesize calcium silicate-based Sublime Cement®, which can employ various rocks and minerals as well as industrial waste as feedstock, and releases zero carbon during production. This product is now embedded in the net-zero carbon office building 1 Boston Wharf. CarbonBuilt, a company based in Torrance, Calif., has developed a proprietary blend known as Reversa® Binder that replaces a percentage of portland cement. And the list goes on. 

“There is no magical solution for decarbonizing our industry,” insists Luis Baquerizo, director of the central research laboratory for building materials company CalPortland. Instead, he cites his company’s “multi-initiative approach,” comprising material innovation, process efficiency, and alternative technologies. One example where these paths converge is a reactive process called mineralization.

Baquerizo describes a closed-loop system in which demolished concrete is reclaimed for its aggregates. “It’s already a cement-rich material. When that hydrated cement is exposed to CO2, it becomes a new mineral, in this case calcite. It’s almost returning to its original state.” His company is currently building a reactor facility in Mojave, CA, which is powered by 24 megawatts of renewable wind energy. 

CalPortland isn’t the only industry player working to perfect mineralization. This suggests another positive shift for the industry towards circularity, where instead of seeking out the next best SCM and carbon-reduction metric, manufacturers are finding ways to reclaim and repurpose existing product. 

Prioritizing resources

According to Bennett, not every low-carbon solution is necessarily a good one, and not every company in this space is necessarily trading in low-impact products. He highlights the fact that replacing cement with a high concentration of SCMs, be it from rock, glass, or other minerals, “only increases the water demand” when mixing the blend. “It doesn’t behave the same; the slump isn’t the same, and you get crews adding water on-site just to keep it workable. A higher porosity equals weaker concrete,” he says. And in places with acute water shortages, added demand only further depletes the water table. 

He claims the biggest myth about low-carbon concrete is that you must add SCMs to make it low carbon. “There isn’t necessarily anything wrong with portland limestone cement. Its just  most people aren’t trained on how to treat it differently.” Bennett’s preferred method, in the simplest terms, is to use less cement. But such an approach also demands paying closer attention to how one is hydrating the cement. One viable solution involves introducing nanomaterials as a chemical admixture so that more of the cement is activated, and “we can observe hydration and the cement’s chemical properties at the nano scale.” This has proven to yield a product with superior tensile strength and lower global warming potential. 

All told, the increased deployment of so-called green concrete on behalf of tech giants, public works projects, and more is beyond encouraging. It clearly pencils out, and the environmental dividends look good on a corporate ESG report. It is transforming how we build our civilization.  

But to Bennett’s point, a healthy dose of skepticism is appropriate, especially when being told one solution or the other is a low-carbon panacea. 

Justin R. Wolf is a freelance writer covering the architecture, design and construction industries. He lives in Maine.