Fixing concrete's carbon footprint

It is the most widely used substance on the planet after water, an essential ingredient in modern cities and — depending on where you live — supports everything from the roof over your head to the dams and bridges that make up essential infrastructure.

But concrete, a wonder material that has revolutionized construction and raised living standards across thaw me world, is one of the most powerful drivers of global warming.

The cement industry is responsible for about 8% of global carbon dioxide emissions, more than double those from flying or shipping. If it were a country its yearly pollution would only be topped by the US and China.

Yet more than 4 billion tons of cement are produced each year to build houses, motorways, flood defenses and more. Its use is expected to keep rising as people in poorer countries move to cities and demand standards of housing and infrastructure that have long been enjoyed in richer parts of the world. China, which makes over half the world's cement, poured more in just the few years from 2011 to 2013 than the US did in the entire 20th century.

"The fundamental challenge is that [concrete] is incredibly carbon intensive, it is a definite problem, yet we probably will continue to use more of it," said Johanna Lehne, a specialist in decarbonizing industrial processes at climate think tank E3G in Brussels.

Why climate-friendly concrete doesn't exist

Concrete has a simple recipe. It is made by mixing cheap rocks known as aggregates — typically fine sand and chunky gravel — with cement and water. When combined, these last two react to form a binder that glues the components tightly together.

Producing the cement is what makes concrete so dirty. Manufacturers burn fossil fuels to heat rotating kilns to temperatures above 1400 degrees Celsius as part of a process that turns limestones and clays into what's known as clinker — the main component in cement. The chemical reaction to break down the limestone is what releases the bulk of the CO2.

But because that process is an inherent part of cement production, there is no obvious technology to eliminate the emissions from concrete. Unlike the power or transport sectors, for instance, cement presents "a fundamental technical challenge," said Lehne. The concrete industry lacks the equivalent of wind turbines or electric cars.

How to clean the cement industry

In October 2021 the Global Cement and Concrete Association, a lobby group whose members represent 80% of cement production outside of China and include several Chinese manufacturers, released a roadmap to fully decarbonize the industry by 2050.

About 40% of the planned savings broadly involve changes to produce cement and concrete more efficiently. That includes heating kilns without using fossil fuels — perhaps from burning rubbish in waste incineration plants — or replacing some of the clinker with waste from steel and coal plants.

Almost one-quarter of the emissions cuts come from designing more efficient buildings and extending their lifetimes — processes over which the industry has little control. This could mean architects and engineers retrofitting old buildings instead of knocking them down and designing new ones to last longer. 

The final — and most speculative — third of the savings come from capturing carbon dioxide after it has been released. 

Can carbon capture technology turn grey concrete green?

Although the technology to capture carbon dioxide exists, it is expensive and untested at scale, and its development in the cement industry is still in the early stages. That means a cornerstone of the industry's plans rests on a technology that isn't yet ready to fix the problem.

"In the next 10 years we have to make that technology mature — and prove the industrial scalability and the commercial scalability of the technology," said Thomas Guillot, CEO of GCCA, which has urged policymakers and investors to coordinate with the industry to develop the necessary infrastructure. "It's not something that would be easy."

By 2030 the GCCA — which has called for help from local governments and other actors in the supply change — wants carbon capture technology applied at industrial scale in 10 cement plants. The first of these is being built by German concrete producer Heidelberg Cement in Norway, where it hopes to suck in half the CO2 emitted at the plant and permanently store it. The GCCA roadmap lists 29 carbon capture projects under various stages of development at cement plants across the world.

While analysts have praised the roadmap for its realistic 2050 targets, they have also criticized its vague short-term commitments to cut emissions. The GCCA members have not yet made detailed commitments explaining how they will cut pollution this decade. That will come later in the year, said Guillot. "What we want to do is to really walk the talk and to transform commitments into actions, global visions into local requirements." 

Future solutions

There are smaller-scale solutions that show early signs of promise.

In Sweden, a pilot study by energy company Vattenfall has shown cement can technically be made from electricity without using fossil fuels. Other researchers are exploring how CO2 could be injected into crushed concrete and reused as an aggregate. In France, one company has successfully converted cement bypass dust into lightweight aggregates using CO2 captured on-site. 

The cost of capturing carbon is still a large barrier for cement manufacturers, said Maarten van Roon, chief commercial officer at Carbon8 Systems, the company behind the technology. By turning waste into something usable instead of paying for landfill, "we help remove a cost from the supply chain and that, in turn, helps us justify an expense for putting the innovation on site."

Concrete's carbon footprint could also be lowered by using sustainably sourced timber in construction. But replacing concrete with timber at large scale would put enormous pressure on the planet's beleaguered forests.

"Most people think concrete has a huge impact on the environment, and they are right," said Jorge de Brito, a civil engineering professor at the University of Lisbon in Portugal, who has published a study assessing green concrete alternatives. "But concrete has that impact because it's the most used material."

Edited by: Tamsin Walker