After years of slow headway, building design and industry professionals say sharp reductions in the climate impact of concrete are possible now. That is significant because cement, the critical glue that holds concrete together, is so carbon-intensive that if it were a country, it would rank fourth in the world as a climate polluter. 

The Global Cement and Concrete Association this year committed to zero emissions concrete by 2050. No single solution has surfaced to reach this goal. But an expanding set of data tools and departures from tradition are starting to add up. 

Take LinkedIn’s new headquarters in Mountain View, California, which eliminated 4.8 million pounds of carbon dioxide that would have been embedded in the new building, much of it by cutting back on cement. Jenny Mitchell, the company’s senior manager of design and build, works under the gun — parent company Microsoft has committed to removing all its historic carbon from the atmosphere. 

Mitchell believes concrete will actually get to net zero. “I think it is a tall task, but I think we can,” she told 200 people at the virtual Global Concrete Summit this month.

To help get there, Mitchell’s team uses a tool that’s swiftly gaining traction called EC3, for Embodied Carbon in Construction Calculator. EC3 launched last year under the auspices of the Carbon Leadership Forum in Seattle.

The free calculator compares the embodied carbon of similar products. Rock aggregate that travels by barge could have a much smaller carbon footprint than aggregate that travels by truck, for example, even if it comes from farther away.

The EC3 software works by comparing Environmental Product Declarations (EPDs) that are fed into it by suppliers. Picture a nutrition label, but instead of calories and carbohydrates, it lists carbon quantities. 

“The number of EPDs for concrete is exploding," rising from 800 to 23,000 over the past year or so, said Don Davies, president of Magnusson Klemencic Associates, a structural and civil engineering firm in Seattle. “Embodied carbon is starting to be a differentiator as to [which firm] gets the work.”

The building blocks of concrete’s carbon footprint

Every batch of concrete is a specific mix called for by an engineer who takes into account what is required for a building element. Mixes can also depend on whether the concrete needs to be pumped upward hundreds of feet or to flow into tight spaces. Concrete varies naturally, too, because it’s made of limestone, clay, sand, gravel and water.

Engineers and suppliers can make use of this variability to reduce the energy and carbon emissions embedded in concrete from its key ingredient: cement, the powder that fuses rock and sand together into concrete when mixed with water. 

Cement is usually made of roasted limestone and silica. It accounts for 80 percent of concrete’s carbon punch. 

There are two reasons for this. First, the kilns that roast the limestone need to approach temperatures as high as 2,600º F, which requires immense amounts of fuel. Second, the chemical reaction at the heart of the process creates calcium oxide and drives carbon off into the atmosphere.

The World Cement Association keeps its own database of cements. The best performing company so far is Dalmia Cement, based in India, at 528 kilograms of carbon dioxide released per ton of cement, according to Ian Riley, the WCA's chief executive officer. Developing countries actually tend to have newer, more efficient cement plants than those in countries that led the Industrial Revolution.

Certain additives can also lower concrete’s carbon footprint. Clay can add strength and lower emissions in cement. But ironically, some additives useful for strength are themselves byproducts of carbon-polluting industries.

For example, fly ash, the solid waste product left over from the coal-fired power plants that cause health and safety problems for nearby communities, has long been added to concrete to lower its carbon footprint and make it stronger. As coal plants are phased out, some worry that fly ash supplies are becoming depleted.

Iron slag waste from blast furnaces is used in a similar manner and can also be scarce. One startup company, Sioneer, is using recycled glass as an alternative to fly ash and slag. 

Then there are a host of companies pursuing innovations to capture carbon emissions in ways that can be fed back into cement production. Blue Planet has come up with a method to make artificial limestone by rolling sand grains in the stream of pollution flowing from fossil fuel power plants. It is building its first factory in Pittsburg, Calif., right next to a natural-gas-fired power plant. 

Another startup in the space, CarbonCure Technologies, embeds CO2 into fresh concrete, strengthening it while trapping the carbon through a mineralization process.

Zeroing in on the lowest-impact concrete

One significant remedy for concrete is just giving it more time to cure. For generations, suppliers have been forced to deliver batches that cure in 28 days. The faster you want your concrete to dry, the more cement you have to add. Some jobs may need to move fast. But with conscientiousness about carbon growing, many jobs may choose to chart a different course.

“Over a 19-month project, what is 10 days?” Mitchell asked. “If we have these conversations early enough, we can work through it.”

“We are now specifying at 56 or 90 days and many of our colleagues are also,” said Christopher Drew of Adrian Smith + Gordon Gill Architecture. 

AS + GG uses a tool to project the lifetime carbon emissions from their buildings, both from embodied carbon and overall energy use. The tool, called One Click LCA, can even automatically update to reflect the changes on a local grid that make a material cleaner.

Engineers and architects tend to be too prescriptive in the concrete they demand for particular projects, said Lionel Lemay, executive vice president of the National Ready Mixed Concrete Association. Many experts agree this is actually adding millions of tons of unnecessary carbon emissions by demanding mixes that can’t be modified to reduce their impact. 

One cutting-edge technology being explored by the industry is carbon absorption from the air. Left exposed, concrete actually sucks up small but meaningful amounts of carbon dioxide after the structure is finished, perhaps 3 percent of what went into making the concrete structure.

Simply asking for two bids— one that has the lowest overall cost and one that offers the best price for the lowest amount of carbon — could yield significant benefits, Magnusson Klemencic’s Davies suggested. “If it only costs 5 percent more and could cut the carbon footprint in half, I have buyers who would go for that.”

There is no technological magic wand that removes carbon from the concrete-making process, but there is a fast-changing palette of options, AS + GG’s Drew said. “Fly ash, Blue Planet, CarbonCure, high-strength rebar,” he said. “We should be looking to combine all the different solutions.”