Materials: Decarbonising Cement

Kerry Hebden talks to the researchers and companies aiming for greener concrete processes

WHETHER it’s as eye-catching as the impressive Christ the Redeemer statue, the Hoover Dam, or Sydney Opera House, or whether it’s as ordinary as the local office block you work in, there is no escaping concrete and the impact it has had on our modern world.

Concrete is so ubiquitous, that according to researchers it accounts for around half of all human-made things, and if our love of the grey stuff continues, in just under 20 years there will be just as much, if not more concrete on the planet than living biomass

It’s not hard to see why then, that cement, the “glue” that binds concrete together is the second-most used material on Earth after water. A staggering 4.3bn t of cement was produced in 2021 alone, and with the world’s population set to grow to almost 10bn people by 2050, 68% of which are expected to live in cities, the demand for cement to build housing and infrastructure will also grow accordingly.

But, it is not just the quantity of cement that is eye-watering, so too is the effect that it has on the climate when made. As much as 600 kg of CO2 is released for every tonne of cement produced and that’s because the main ingredient in cement is clinker. Clinker is the basis of Portland cement, the most widely-used cement in the world, and it forms as a rotating kiln cooks a mixture of clay and limestone and a few other minor compounds at temperatures of around 1,400-1,500°C. The resulting marble-sized nodules are then ground down to make a fine powder and mixed with sand, gravel and water to make concrete. It’s an energy-intensive process that not only releases CO2 as the limestone undergoes a reaction during the precalciner stage (CaCO3 -> CaO + CO2), but the fuels used to heat the kiln are typically hydrocarbons such as coal or gas.

As such, around 7% of anthropogenic CO2 emissions every year come from cement production, a figure that needs to be scaled down if humanity is to achieve net zero emissions by 2050. For the cement industry, this means targeting a 3% annual reduction in CO2 for the next eight years, according to the International Energy Agency (IEA). However, its latest cement tracking report states that the direct CO2 intensity of cement production – which encompasses energy-related and process emissions – increased by around 1.5%/y during 2015-2021, a figure largely attributed to a higher global clinker-to-cement ratio.

Innovations in cement

To get back on track, more investment in innovative technologies, such as electric kilns and carbon capture and storage (CCS) is needed, says the IEA, which sees CCS as a critical component in cutting cement emissions. Carbon curing, an idea that has been around for at least 50 years, and is an approach used by Solidia, which cures concrete with CO2 rather than water, is a possible alternative; so too is injecting CO2 into fresh concrete during mixing to lock it away. Some firms such as Canadian startup CarbonCure, are already utilising this method to limit emissions.

Another way to reduce CO2 is to reduce demand for clinker, by substituting it with waste materials like blast furnace slag and coal ash, for example. Around 30-40% of clinker can be substituted in this way without compromising cement strength, according to researchers at Imperial College London.

Ironically however, as other industries such as iron and steelmaking look to decarbonise their processes, byproducts such as fly ash are likely to become less available; therefore companies and researchers need to look at other ways to make cement that helps reduce its effects on climate change. It’s a challenge that some groups have already taken up. Taking inspiration from ancient Rome, a group at the US’ Stanford University, led by Tiziana Vanorio, is working on a prototype to use volcanic rock as a base for cement, a technique employed by builders more than 2,000 years ago.