Casting Concrete in a New Light

New technologies can reduce concrete's large carbon footprint.

Concrete has been around for a long time.

The earliest concrete-like structures were built by the Nabatean people circa 6,500 BC, in what is now Syria and Jordan. They were concerned with the “dryness” of the mixture — what we now refer to as “slump” — and tamped the concrete in place using specialized tools, similar to the modern practice of vibrating. By 200 BC the Romans were building an empire of concrete, using volcanic ash, lime and seawater. 2,000 years later, many of these structures not only still stand today but are still in use.

There have been refinements over the succeeding millennia — Portland cement, air-entrainment, etc. — but Ankhhaef, who oversaw construction of the Great Pyramid of Giza for the Pharaoh Khufu around 2,560 BC, would immediately recognize the grey material discharging from a modern concrete mixer.

Today, we use approximately four billion tons of cement a year globally and, barring the advent of some better building material or an unforeseen drop in population growth, that number will continue to rise over time. The International Energy Agency forecasts an increase in cement production over the next 30 years to five billion tons per year. Bill Gates quotes an estimate that the world will add two trillion square feet of buildings by 2060, the “equivalent of building another New York City every month for the next 40 years.”

Usually absent from the prevailing narrative on global warming and the need for sustainable technologies is the fact that the chemical and thermal combustion processes involved in cement production leave a large carbon footprint, accounting for approximately eight per cent of global CO2 emissions annually. By comparison, total greenhouse gas emissions from the transportation sector — the bête noire of the green movement — are only four per cent higher. If the concrete industry were a country, it would rank third in CO2 emissions, behind only China and the United States.

Photo by Ricardo Gomez Angel on Unsplash

According to a 2018 study, carbon emissions from the production of cement and concrete will have to decrease by at least 16 per cent by 2030 in order to meet emissions targets established by the Paris Agreement on climate change. The report cautions, “Steeper reductions will be required if assumptions about the contribution from carbon capture and storage (CCS) technologies prove to be optimistic. The trends all point to regulatory, financial and societal pressures on the horizon, especially for cement companies without a detailed plan for a Paris-compliant pathway.”

The challenge for cement producers arises from the convergence of rising demand and pressure to reduce emissions. Current Intergovernmental Panel on Climate Change models are largely dependent upon the deployment of significant amounts of carbon capture and sequestration, and here new concrete technologies may play a role.

California-based CarbonBuilt employs Reversa technology, developed by UCLA researchers, to utilize captured industrial CO2 for concrete production. During mixing, Portlandite (also known as calcium hydroxide or hydrated lime) is introduced to cut the Portland cement content with low-cost and low-carbon cementitious and filler materials. Concrete is then cured with CO₂. CarbonBuilt claims, “The Reversa process reduces emissions by at least 60 per cent… through a combination of utilization (permanently embedding CO₂ into the concrete) and avoidance (reducing CO₂ emissions associated with the raw materials).”

Nova Scotia-based CarbonCure offers a technology that introduces recycled CO₂ into fresh concrete during mixing. The CO₂ — purchased from industrial gas suppliers — then undergoes a mineralization process and becomes permanently embedded as calcium carbonate. This increases compressive strength and uses less cement. Every cubic yard of concrete produced using CarbonCure technology saves an average of 17 kilograms (25 lbs) of CO₂ emissions. The company states, “An average high-rise building using CarbonCure would save approximately 1.5 million lbs of CO₂ emissions, the equivalent to the carbon absorbed by 888 acres of forest in a year.”

Another approach reduces the carbon footprint by cutting cement content with less carbon-intensive materials. LC3 (limestone calcinated clay cement) is a product developed at the University of the Swiss Ecole Polytechnique Federale, in conjunction with Cementos Argos of Columbia. The clay contains very little carbon, releasing virtually no CO₂ when it’s heated, and it can be processed at a lower temperature than limestone. A double win for green. Argos says the technology cuts energy consumption by 30 per cent and reduces carbon output by almost half.

The concrete carbon footprint remains a large and growing global problem. Major organizations now working on relevant strategies includes the UN Environment Programme (UNEP), the International Energy Agency (IEA) — working with the industry-led Cement Sustainability Initiative (CSI) — and the Energy Transitions Commission. And while the ancient Nabateans couldn’t have anticipated their ingenious discovery would one day threaten humanity, the truth is that the industry must change in order to survive.

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