Making Carbon Negative Concrete: Efforts to Decarbonize the World

Concrete, which is closely related to our lives, emits more than 4% of the world's total CO2 emissions. In today's world, attempts are being made to decarbonize concrete through various processes, such as materials, manufacturing methods, and hardening methods.

TEXT BY OLIVER FRANKLIN-WALLISTRANSLATION BY MITSUKO SAEKI

WIRED(UK)

Rock-like composites, concrete. We have built churches, houses, roads, bridges, skyscrapers and even factories on this material. Humans consume more than 4.1 billion tons of concrete each year. This is more than any other material except water, and most people are probably standing or sitting on concrete at this very moment.

This is a problem. This is because concrete, and especially its main ingredient, cement, is doing great harm to the environment.

The cement industry emits 2.8 billion tons of carbon dioxide (CO2) each year. This is more than the annual CO2 emissions of any country other than China and the United States, and accounts for 4-8% of global human-caused CO2 emissions.

To meet the Paris Agreement goal of keeping global temperature rise below 1.5°C and never exceeding 2°C, we need to reduce the amount of CO2 emitted from cement production to at least 16% by 2030. need to be reduced. But for now, CO2 emissions continue to rise, largely due to the number of large-scale construction projects underway in China. The concrete industry is now in a race against time to solve a very difficult and unpredictable problem.

Creating a Carbon Negative Company with Concrete

The way concrete is made has changed very little since the 19th century. All you have to do is mix coarse aggregate such as pebbles and gravel, fine aggregate such as sand, cement that acts as an adhesive, and water.

"The biggest problem with concrete is the method of making cement. To make cement, you first have to make a clinker," said a structural engineer at the University of Bradford in the UK. Professor Ashraf Ashauer says: Clinker, the raw material for cement, is generally made by mixing lime, clay, gypsum and other materials and firing it in a kiln.

“Clinker needs to be fired at extremely high temperatures, around 1500 degrees Celsius, which releases a lot of CO2,” explains Ashauer. In addition, as the clinker burns in the kiln, the lime breaks down into calcium oxide, releasing even more CO2.

One method of deoxidizing concrete is to replace cement with fly ash (fly ash produced when burning coal), bottom ash (ash that falls to the bottom of the furnace), or blast furnace slag (produced during steelmaking). It is to replace it with materials such as slag). Cement makers have been using these scraps for years, but a wave of coal-fired power plant shutdowns is sapping supplies, leaving many looking for alternatives.

For example, Canadian concrete maker Carbicrete says it uses steelmaking slag, a by-product of steel production, as a substitute for cement. “We produce 250 million tons of slag each year during steelmaking,” explains Carbicrete CEO Chris Stern. “Steel slag has been used for road paving for many years. Besides road construction, the finer slag is used in land reclamation and sometimes as fertilizer. It will never be used."

After the materials are mixed, a process called "hardening" is required to harden the concrete. Have you ever seen a worker pouring water over a freshly poured cement base? That is hardening work. The typical water-based method requires about 28 days to harden.

Carbicrete, on the other hand, uses carbon dioxide to harden the concrete. CO2 emitted from industrial sites is collected and injected into concrete, where a chemical reaction produces calcium carbide, or lime, which hardens. "We are now a 'carbon negative company' where we absorb more CO2 than we emit," says Stern. “By selling our products, we are able to effectively eliminate the marginal cost of CO2 capture.