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Taking a Hard Look at Concrete’s Carbon Footprint

Oct. 7, 2021
Princeton, MIT and private industry are making inroads in sustainable materials

If there was any thread of hope for skeptics that global warming could be controlled by merely incremental changes to industrial processes and our way of life, the United Nation’s blue-ribbon Intergovernmental Panel on Climate Change dashed that permanently in August.

As U.N. Secretary-General António Guterres said in releasing the group's report, it was “a code red for humanity. The alarm bells are deafening, and the evidence is irrefutable.”

The report said the scientific consensus the brink of reaching the global increase of 1.5 degrees Celsius above pre-industrial levels was “… perilously close. We are at imminent risk of hitting 1.5 degrees in the near term. The only way to prevent exceeding this threshold, is by urgently stepping up our efforts, and pursuing the most ambitious path.”

Given that reality, it’s clear that every component of the current economy and our lifestyles can and must be rigorously examined for possible radical transformation. Given that challenge, and thinking strategically, why not target an important source of CO2 that stands out because it’s an industry that’s changed little in the past two hundred years and is thus likely to be rife with opportunity for profitable and environmentally sound change?

Specifically, I mean concrete (I’ll spare you bad puns involving the adjectival form of the word…).  

We’ve been using it since Roman times, and the basic method has changed little since the introduction of portland cement in 1824. Let’s be careful to differentiate between cement and concrete, often used interchangeably (I plead guilty) but having crucial differences.

Cement combines limestone and clay. Its creation is the major environmental culprit: the ingredients are heated (at present, largely by fossil fuels) to 2600 degrees F for the limestone to chemically decompose, resulting in calcium oxide that goes into “clinker,” the final cement product. As much as two-thirds of concrete’s C02 emissions result from heating the limestone and that chemical process.

Concrete is the final product of combining cement with aggregates such as sand and gravel plus water, which binds them through a process called hydration.

Cement and concrete’s contribution to global warming are staggering: A ton of CO2 is created for every ton of cement created, and an estimated 8% of all CO2 releases are attributed to concrete production, second only to iron and steel production among industrial sources.

At the same time, concrete is essential to the trend toward urbanization worldwide (especially in developing nations) which, paradoxically may be a positive for global warming because it reduces the pressure to cut rainforests—so eliminating concrete isn’t the answer: Improving it is.

The cement/concrete problem currently attracts the best and brightest. Start-ups and university researchers have created a wide range of promising innovations in the last decade, using both new technologies, such as the IoT, and differing chemical processes. Most address a single factor, such the cement’s composition, the fuel source, or how the CO2 is managed:

Similar to many other industries, IoT systems for concrete factories can cut raw materials  and energy use up to 20%, an added benefit to those realized by energy source and minerals substitutions.

Some experiment with alternative fuel sources such as old tires, waste lubricating oils, refuse-derived fuels or waste plastics to reduce fossil-fueled ones.

A UCLA team created CO2NCRETE, capturing and using the CO2 emissions from another major culprit—fossil-fueled power plants—to create a cement-like building material made of hydrated lime, which absorbs CO2.

The Sustainable Cements Group at Princeton University is one of several trying to reduce the amount of clinker, or eliminate it. They’d substitute byproducts such as  steel slag, clays, or coal-fired power plants’ fly ash.  This might reduce emissions 40-80% compared to portland cement.

For greatest benefits both environmentally and economically, it seems the best approach is a holistic on,  examining everything from the fundamental chemistry to how to make concrete structures last as long as possible (on the theory that the longer the building or pavement lasts, the more the environmental impacts will be amortized). Perhaps the influential MIT Concrete Sustainability Hub sums up this approach best:

“More concrete is produced than any other material on Earth. In the foreseeable future, there is no other material that can replace concrete to meet our societies’ needs for housing, shelter, schools, and infrastructure … but its attractive properties have lead to massive use that contributes approximately 5% of global CO2 production.

The statement adds that changing the concrete industry to preserve its benefits while minimizing environmental impacts, “requires a holistic approach in which progress in concrete science translates into innovative structural concrete engineering applications,,” from pavement to walls structures, “whose impact on sustainable development are evaluated with advanced environmental-econometric impact studies.”

Given that perspective, the company that stands out is a New Jersey startup, Solidia,  with the only approach simultaneously addressing not just the CO2 issue, but also three other major global warming-related issues as well: energy use, water use, and solid waste.

Solidia uses a new formula for cement. Its hydrothermal liquid phase densification (rHLPD) process uses less limestone (50% vs. 66%) and operates at significantly lower temperature (1200 degrees F) to bond and harden the particle mixture, cutting greenhouse gases by up to 30% while also saving energy. Another money saver is that it can be used in existing kilns.

In the concrete curing process, the Solidia Cement reacts with, and traps, CO2 instead of using water, consuming up to 240 kg of CO 2 , leaving 3-5% of the finished product as solid CO2 and allowing recovery of the water that’s used.  The company also claims that, “Unlike traditional ordinary portland cement (OPC), Solidia precast products can be recycled before curing, potentially eliminating tons of concrete landfill waste every year.” 

At this point, the Solidia process is only being used for precast pavers, although the company is beginning to also apply it to the larger, ready-mixed, market.

The company’s new CEO, Bryan Kalbfleisch told me in a phone interview that the company didn’t set out to create such a holistic solution, but was quick to expand its approach as new benefits emerged. In particular. once they realized that CO2 could be substituted for water, they made that a feature, especially for areas such as Belgium or Africa where there are water shortages.

Creating imaginative new concrete production materials and processes won’t automatically solve the industry’s CO2 problem.  This year’s Florida condo collapse disaster is a reminder that any method must go through rigorous testing to assure strength and durability. Technology may help there as well:  IoT sensors such as SmartRock take the guesswork out of field testing with usual methods such as field-cured cylinder break tests to make certain the concrete meets all tests.

Meanwhile, several states, led by New York and Colorado, are trying to boost the adoption of innovative concrete methods by instituting emissions standards for concrete used in public works, because the new methods are, at present, more expensive.

Efforts to reduce CO2 emissions from cement and concrete production are critical, particularly if they allow the economy and society to still enjoy concrete’s other positive contributions. At the same time, they may also provide inspiration to tackle long-standing practices in a variety of other industries that contribute to global warming that are no longer tolerable given the current “code red” threat to humanity and the planet.

W. David Stephenson is CEO of Stephenson Strategies (Millis, Mass.), a consultancy specializing in applying the Internet of Things (IoT) to sustainability and creative approaches to aging.  An IoT thought leader, he wrote The Future Is Smart (HarperCollins), one of the first guides to IoT strategy.

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