Carbicrete makes cheaper, stronger concrete blocks carbon negative
By Larry StewartOctober 19, 2022
There’s a lot of research into reducing cement’s carbon footprint because cement kilns generate 8% of the world’s greenhouse gas emissions, and the big publicly traded concrete producers and users have carbon-reduction targets to meet. With cement normally responsible for half of concrete’s carbon footprint, a 2016 startup called Carbicrete is commercializing a process to make precast concrete blocks without it.
Carbicrete replaces cement in the concrete used to produce precast concrete with slag from the steel-making process and cures it with carbon dioxide. Carbicrete’s concrete process eliminates the GHG emissions associated with cement production, and permanently sequesters CO2 in curing.
More carbon is captured during curing than is emitted during manufacture of Carbicrete, making the resulting concrete carbon-negative. For every 2,200 pounds (1 tonne) of concrete produced using this process, 330 pounds (150 kg) of CO2 are abated/removed.
Making traditional cement emits carbon two ways. Extracting calcium oxide by heating calcium carbonate, typically as limestone, to about 2,700 degrees Fahrenheit (1,480 Celsius) 1) releases carbon dioxide, which accounts for 40% of limestone’s mass, into the atmosphere; and 2) fossil fuels that normally fire cement kilns create additional carbon emissions.
Cement in traditional concrete is cured with water that causes the calcium to react with the CO2 in the surrounding air, turning it back into strengthened calcium carbonate.
Slag provides the calcium oxide in Carbicrete. It is cured by injecting the concrete and calcium oxide with CO2 captured from industrial sites. Curing creates calcium carbonate, which fortifies the concrete.
“It’s effectively doing the same job as hydration in a regular concrete product,” Carbicrete CEO Chris Stern told dezeen.com. “The chemical composition of a concrete block that we make is exactly the same as a regular concrete block but we take a different road to get there.
This curing process takes 24 hours and stores away three kilograms of CO2 per concrete masonry unit (CMU) by converting it into a stable mineral, alongside an additional kilogram of CO2 that is pulled from the surrounding air during curing.
The steps in Carbicrete’s process are much like those of conventional concrete production, with a few key differences:
- With cement-based concrete, the first step involves mixing cement with aggregate and water. With CarbiCrete, cement is replaced with steel slag, which is mixed with the other materials using standard equipment.
- The mix is then poured into a conventional block-making machine where the CMUs are formed.
- In order to cure the concrete, it must be placed into a specialized absorption chamber into which CO2 is injected. Within 24 hours, the concrete has reached full strength.
Carbicrete says its concrete blocks can be produced at 20% lower materials cost and reach up to 30% better compressive strengths. The company also says the process can be implemented easily in any precast plant. Absorption chambers capable of curing 2,400 units per day are being installed at the facility of Quebec manufacturer Patio Drummond to complete the commercial pilot phase of the rollout.
While a traditional CMU releases 2 kg or CO2 emissions, each Carbicrete block produced sequesters 3 kg of CO2. So the process not only helps take carbon out of the building process, but also will generate carbon offset credits that will be valuable to companies looking to meet their carbon reduction targets.