Carbon sequestration is an emerging technique that permanently removes a greenhouse gas from the atmosphere.
Preventing its release lessens the potential for human-caused climate change, especially when applied to carbon-intensive processes like cement production and usage. If this technique catches on in construction, it could revolutionise how you approach cement pouring.
What is concrete carbon sequestration?
Concrete carbon sequestration is a reaction that chemically converts carbon dioxide (CO2) into solid lime or calcium carbonate, perpetually trapping it within a cement-based product. Cement and water are critical components of this process.
When CO2 dissolves in water, it forms carbonic acid. Carbonic acid generates carbonate, which combines with calcium ions to form solid crystals. This chemical reaction naturally sequesters carbon from the atmosphere, permanently trapping it within the hardening material.
How this technique is used in construction
Unless you’re working on cutting-edge projects, you probably haven’t encountered this method before. While humans have known about carbonation for ages, applying it in construction is a relatively new approach. However, it has already been done successfully several times. The 725 Ponce in Atlanta, Georgia, is a great example.
While this 3,400-square-metre mixed-use building’s design called for exposed concrete, it needed to be sustainable. Thomas Concrete — the concrete supplier — used 36,000 cubic metres of a carbonation mix, removing 750 tonnes of CO2 from the atmosphere. This reduction is equivalent to 888 acres of forest absorbing CO2 for an entire year.
Suppliers and builders are increasingly using this technique because they want to reduce their climate change contributions. Since concrete accounts for 8 per cent of CO2 emissions worldwide, it is one of the most potentially impactful targets.
How often is concrete carbon sequestration used?
While sequestration isn’t an industry staple, it is becoming more common. Technology-based solutions already exist. For instance, CarbonCure Technologies Inc. — the company that helped with 725 Ponce — manufactures greenhouse gas removal equipment.
You could use CarbonCure equipment to inject captured CO2, which you can get by placing a specialised machine over a flue into fresh concrete while you mix it. This prevents it from contributing to embodied carbon over its operational and end-of-life phases.
The benefits of concrete carbon sequestration
As you know, concrete and variable temperatures don’t mix. Unless you pour it between four and 30 degrees Celsius, the temperature will adversely affect the water, cement and aggregate ratio. A lower compressive strength or internal cracks are possible outcomes.
With carbon capture, you could make your mixture more resilient. Research shows temperature significantly impacts its compressive strength and carbonation rate. Samples cured at ambient and high temperatures saw significant strength increases. Moreover, they reached carbonation depth two times faster than those cured at five degrees Celsius.
In addition to permanently trapping a greenhouse gas within a hardened cement-based product, this method makes your mixture more durable. Hessam AzariJafari — the MIT Concrete Sustainability Hub’s deputy director — says this approach can increase the compressive strength of cement-based products by densifying the paste. If done correctly, you increase structural resilience while shrinking your carbon footprint.
How this impacts construction’s sustainability
You can use sequestration to neutralise greenhouse gas emissions during the life cycle of your built structures. It creates a natural carbon sink, offsetting the emissions associated with production and eliminating embodied emissions.
Improving your industry’s sustainability is vital because the built environment is a major source of greenhouse gas emissions. Concrete contributes 22 per cent of the built environment’s embodied emissions, which is second only to steel. Addressing this issue at scale could significantly improve the sector’s sustainability.
Will this technique catch on in construction?
Since commercial construction relies heavily on cement, sequestering CO2 may catch on relatively quickly. Even if you aren’t concerned with the environmental impacts, increasing your pour’s compressive strength is undeniably valuable.
