Reducing emissions is front of mind in the concrete industry. It is and it will be for decades to come. There has been progress. Industry emissions are down an estimated 21% in seven years. Portland limestone cement (PLC) and other blended formulations are part of the success story.

The sobering reality is that increased demand offsets emissions success. Demand has tripled since 1990. Global cement production now stands at 4.4 billion tons. Ten years ago, researchers at the Columbia University Climate School projected that level for 2050. And we're already there.

The IEA’s 2018 Technology Roadmap seeks a 2050 global clinker ratio of 0.60. Achieving this goal requires a 40% increase in the use of clinker substitutes. However, supplies of common SCMs like fly ash and blast furnace slag will decrease 16% by then. Alternative SCMs must address the shortfall. That's another 1.2 billion tons of alternative SCMs by 2050. An expanded use of SCMs is key to meeting emissions targets.

Increasing demand raises the stakes in the quest for carbon neutrality. Supplementary cementitious materials (SCMs) are a key part of a necessary revolution.

History of SCMs

More than 2500 years ago, the ancient Greeks used volcanic rock as an SCM. It came from the island of Santorini in the Aegean Sea. The material lined a water storage tank on the nearby Isle of Rhodes.

The word “pozzolan” is derived from the name of an ancient Roman town, Pozzuoli, now a part of Naples. The area supplied volcanic pumice and other volcanic materials for some of the earliest concrete materials. Pozzolans tend to increase carbonation, a reaction that absorbs CO2 from the air and permanently sequesters it. Unfortunately, this process increases the potential for corrosion in traditional black steel reinforcement. Modern rebars like fiber-reinforced polymer (FRP) are a corrosion-resistant alternative.

Common SCMs are byproducts of various industrial processes. Examples include fly ash, blast furnace slag, silica fume, and rice husk. There are natural pozzolans like metakaolin, calcined clay, and calcined shale. Others are of volcanic origin. One is volcanic ash; the other is a glass-like substance called obsidian.

Iowa State’s National Concrete Pavement Technology Center publishes tables noting SCM characteristics. Researchers there cite the positive impact on both fresh and hardened concrete.

The Center notes how SCMs commonly replace Portland cement. They are now used in approximately 60% of all concrete mixes in the United States.

Consider a PEC Consulting graph published in the March 2022 edition of the International Cement Review. It illustrates global emissions disparities in the industry. Consider that Chinese CO2 emissions have skyrocketed during the three decades of soaring cement production.

Using SCMs to reduce OPC consumption could reduce the industry’s GHG emissions by 20%. At the same time, they often make concrete stronger while increasing lifespans. Popular mixes resist thermal stress, alkali-silica reactions, and sulfate attacks.

New Take on Flexible SCMs

Traditional SCMs face a future complicated by dwindling and/or regionalized supplies. Consider a report by ChemAnalyst following Q1 2022. It said, “Pricing offers for Si02 grade GGBFS have increased due to escalating downstream demand from the cement industry amid inadequate supplies from regional players.” Fly ash supplies dwindle as coal-fired power plants shut down. Terra notes that certain “technologies in our industry rely on scarce or depleting feedstocks that often have to be transported great distances.”

Scalability is another concern. Terra asserts that, “many sustainable and green technologies never reach scale because the underlying technology is too expensive to use in the real world.”

However, Terra is one company tackling the challenge. Terra’s OPUS SCM replaces 35% of Portland cement. At that level, it reduces CO2 emissions by 70% and NOx emissions by 90%. Its proprietary process is very flexible. It can incorporate varying supplies of local feedstocks and waste products. D.J. Lake, Terra’s founder and VP of R&D, says, “Terra’s technology is one important piece of the transition to safe, long-lasting, and carbon-neutral cement."

In 2023, Terra intends to begin construction on its first commercial-scale production facility. Its capacity is an estimated 250,000 tons per year.

Future Possibilities

Essential elements of concrete's carbon neutral future will likely include:

• Certain kilns powered by electricity from renewable sources, others by green hydrogen

• Full capture of CO2 released during calcination

• Injection of that CO2 into locally produced concrete

Reversible carbonation and calcination reactions show promise in delivering cost-effective carbon capture. However, material availability is a challenge. It also requires CO2 uptake per gram of sorbent that is still theoretical. The efficiency of carbon capture success is currently limited. There are too few carbonation/calcination cycles, which leads to sintering and pore blockage.

Case Study: UCLA Carbon Built

In 2021, UCLA Carbon Built won a prestigious XPrize for its carbon capture technology. The composition of Carbon Built’s blocks are inspired by nature’s “original cementation agent” found in seashells. CarbonBuilt substitutes hydrated lime for OPC to reduce carbon emissions by half. The benefit is amplified by the pace at which hydrated lime absorbs carbon dioxide.

As the concrete hardens, the process sequesters CO2 from flue gas. Flue gas impurities like H2O and SO2 inhibit the carbon capture capabilities of CaO. Going forward, new synthetic CaO-based CO2 sorbents may address key challenges.

About PACA

The Pennsylvania Aggregates and Concrete Association (PACA) keeps its members and the public abreast of industry innovation. Our team welcomes any questions you may have about your upcoming concrete project. Please contact us at your convenience.