Concrete is a material widely used in construction, but its manufacture is one of the main causes of carbon dioxide (CO2) emissions in the world. CarbiCrete has developed a solution called Carbon Negative Concrete which uses steel slag instead of cement and can absorb more CO2 than it emits during production., providing a way to build sustainable structures without harming the environment. Carbon negative concrete is a sustainable and environmentally friendly option for the construction of buildings, roads and other structures.
It is said that concrete is the most widely used material on Earth after water. You can find it everywhere, but you never think about it. Modern society, as we know it, was built on this.
The manufacture of cement, a key ingredient in concrete, accounts for 8% of global emissions. What could replace concrete reliably and affordably?
A start-up called CarbiCrete has developed a promising solution: carbon-negative concrete.
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cement
The key ingredient in concrete is cement, a complex compound made up of calcium, silicon, aluminum, iron and other ingredients. These elements are heated to extremely high temperatures, causing a chemical reaction in which some elements burn and the rest turn to dust. First, coal or natural gas is burned to generate the electricity and heat needed to reach such high temperatures, and second, the chemical reaction of cement compounds emits CO2.
Cement powder is mixed with aggregates such as sand and gravel, and when water is added another chemical reaction takes place which causes the whole mixture to harden, reaching full strength in just under ‘a month.
Carbon Negative Concrete
CarbiCrete has completely eliminated the cement and replaced it with steel slag. Once the iron is extracted from the ore to make steel, the slag is what remains. It is not uncommon to use slag as aggregate in construction, especially for paving roads.
They mix the slag with aggregates and water, then pour the mixture into forms to make CMUs (concrete masonry units, the concrete blocks used for construction).
The last step is to harden the blocks so that they harden and reach their full strength. This happens in an absorption chamber where CO2 is injected, causing another chemical reaction: “During the carbonation process, CO2 is continuously captured and converted into stable calcium carbonates, filling voids in the matrix to form a dense structure and giving concrete its strength.“. Full strength is reached in 24 hours.
What makes CarbiCrete carbon negative rather than carbon neutral is that the company uses CO2 gas from industrial vents in its absorption chambers.. This way CO2 is not generated and some of what has been removed from the atmosphere is absorbed.
The company claims that its CMUs have equal or better mechanical and durability properties than cementitious CMUs, including up to 30% higher compressive strength and better freeze/thaw resistance.
How to scale?
However, a possible downside is that since CO2 absorption is a critical part of the process and must be carried out in a special chamber, CarbiCrete can only be used in pre-made form; it cannot be placed in a mixer truck and poured in place at a construction site. Rather than selling CMU, CarbiCrete licenses its technology to concrete manufacturers, who can apply the company’s technology in precast facilities. Depending on the size of the absorption chamber, the technology could be used to make blocks, panels, beams, or any other prefabricated product.
CarbiCrete claims that if a typical CMU production facility adopts its technology, the environmental impact can be significant, with 20,000 tonnes of CO2 reduced and eliminated, 4,400 m3 of water saved and 33,000 tonnes of landfill avoided each year.
We are still a long way from becoming a true sustainable building technology, but carbon footprint concrete, even on a small scale, is a step in the right direction.
More information: carbicrete.com
