Cement has a surprisingly large climate impact, contributing around 8% of global CO2 emissions. To align the sector with the Paris climate goals, big changes will be needed. The sheer scale of these changes can seem tricky, but there are a number of ways emissions could be cut.
Cement and climate change
Cement is a binder which is mixed with sand, gravel and water to produce concrete, the most widely used construction material in the world. “Basically, you name some form of infrastructure or building and it’s got some kind of concrete in it,” says Johanne Lehne, co-author of a Chatham House report on low-carbon cement in 2018.
Cement is an essential part of concrete because it acts as the glue that holds it together. Roughly 50% of CO2 emissions from Portland cement, the industry standard, come from calcination, the chemical reaction used to make clinker, a key ingredient. Here, limestone is heated to high temperatures to produce calcium oxide in a reaction that also generates CO2. “This is what makes cement decarbonisation so tricky, but also so fascinating,” says Lehne. “It’s not just about shifting to a different kind of fuel use. You’re really talking about completely transforming the material itself if you’re trying to actually get to completely net zero.”
Of course, the high temperatures needed to heat cement kilns also result in huge amounts of emissions. Around 40% of CO2 from the cement industry is produced in this way, while the remaining 10% comes from mining and transport of the raw material ingredients.
There are several ways these different emissions can be reduced, and a number of policy levers which could be pushed to help this.
Use it less
There are a number of ways cement use could be reduced without simply stopping building. Cement is also often ‘overused’ in buildings compared to what is needed to ensure their structural soundness, says Lehne, for example by building thicker walls or using a higher proportion of cement in concrete than needed. Structures can also be optimised to use less cement, for example by using a ‘capillary web’ system, while longer-lasting building will also result in less cement use over the long term.
In some situations it may make sense to replace concrete with sustainably-sourced wood, both avoiding the emissions from cement and potentially turning buildings into carbon sinks.
Reduce fossil energy use
Newer cement kilns are more energy efficient than older ones, so replacing them can cut emissions. Chinese and Indian plants are already much lower in carbon emissions than in Europe, because they use newer technology, says Ian Riley, chief executive of the World Cement Association, an industry body.
Alternative lower carbon fuels can also replace fossil fuels for heating the cement kilns. In Europe, there has already been a strong shift to using waste and biomass instead of coal, which has brought down emissions considerably, says Lehne, although this has caveats when considering the broader shift to a more circular economy. “We definitely don’t want to be locked into a system where we’re just burning our waste.” Meanwhile, US start-up Heliogen is trying to develop a system using concentrated solar energy to generate the high temperatures needed for industrial processes such as cement production.
Finally, electrifying cement production is both technically possible and no more pricey than other options to reduce cement emissions, according to a 2019 feasibility study from the CemZero partnership in Sweden. Electrification is an area where a breakthrough technology would be “incredible” says Lehne, although the technology readiness is currently far away, she says.
Cut the clinker
The global ‘clinker-to-cement ratio’ sat at 0.70 in 2018, a slight increase on 2014 levels. Reducing this ratio can shave off emissions. It needs to fall to 0.66 by 2030 to be in line with the International Energy Agency (IEA)’s sustainable development pathway, for example.
Other similar cementitious materials, such as industrial by-products blast furnace slag and fly ash, can be used as a substitute for clinker to some extent. Quantities of these are limited, but companies are working on other ways to increase clinker substitution with alternative cementitious materials. LC3, a blended Portland cement developed at the University of Lausanne which recently went into large-scale production in Colombia, cuts the clinker ratio to 0.50 by adding limestone and a low-grade kaolinite clay, both materials which are widely available.
However, clinker substitution can only ever go so far in cutting emissions by itself: residual emissions will remain so long as conventional Portland clinker is used.
More than 98% of the world’s cement is currently made using Portland clinker, but cements can be made using alternative binders. Some are already in small-scale use, but they largely remain in the realm of breakthrough technologies that “really aren’t necessarily there yet,” says Lehne.
US firm Solidia produces a cement which absorbs CO2, rather than water, as it hardens. A very clean CO2 source is needed and it can only be used for precast moulds, rather than ready-mix concrete which currently makes up the majority of the industry. Scaling up such carbon-cured cements could potentially absorb 1.4 billion tonnes of CO2 per year by 2050, fixing it into cement, according to a paper in Nature last year.
There are a host of other low or zero carbon cements being developed. But building designers need to gain confidence that they are safe, robust and long-lasting, while many are currently far more costly than Portland cement, says Riley
John Provis, professor of cement materials science at the University of Sheffield, studies alkali-activated cements, sometimes known as geopolymers, another group of cements often seen as promising for cutting carbon. They are made by combining aluminosilicate precursors – a wide range of materials with differing availability, reactivity and cost worldwide – with a small amount of a reactive chemical. “Alkali-activated cements don’t need the high-temperature kiln processing step [of Portland cement] and don’t use limestone, so can potentially have much lower CO2 emissions,” says Provis, noting CO2 reduction is on the scale of 40-80%. DB Group’s Cemfree and Wagners’ Earth Friendly Concrete are two cements of this type already being made in the UK.
The biggest drawback of basically all the alternatives is that none is as robust and versatile as Portland cement, says Provis, who argues for a ‘toolkit’ approach to novel cements. “I guess my big pitch is that we need to move away from a one-size-fits-all solution,” he says. For example, different cements could be used for high-rise buildings than for roof tiles.
Capture the carbon
In order to get the cement industry to net zero, carbon coming out of cement kilns will likely need to be captured and either stored or utilised in some way, such as in ‘carbon-cured’ cements. “We don’t think that the industry will be able to decarbonise completely without also having carbon capture and usage,” says Riley.
The industry is holding out hope for carbon capture, even if it hasn’t been scalable so far, says Lehne. “I will admit that I don’t really see an alternative for the cement sector,” she says. “If it’s going to be carbon neutral, there has to be some form of carbon capture and storage as well.” However, she notes, the logistics of storing carbon means capture and storage cannot be implemented in all places there are currently cement plants. “Although CCS is taking a long time to scale, that is not a technology readiness problem,” she says. “It is a business model and policy challenge.”
Policy could play a huge role in pushing cement incumbents and start-ups to explore ways to cut CO2 from cement, and construction firms to start to use these new technologies. Globally, there has been relatively little policymaking on reducing the climate impact of cement. “It’s no surprise that these industries have been quite slow to move; they haven’t been pushed to move,” says Lehne. In Europe, for example, the cement industry is included in the European Trading System (ETS) but up until now has received free allocation of credits. This could change under an ETS revision next year, however, while the EU Commission is considering the sector for a proposed border carbon adjustment pilot.
Lehne argues a package of policies is needed to push cement to decarbonise. This would include ensuring money is going to research and development and implementing effective carbon pricing, but the biggest lever to get cement to zero emissions may actually be demand side policies, she says. For example, construction firms could be required to use less concrete in buildings or switch to other materials. Governments could also create clean cement purchasing mandates as a way to really stimulate action from the cement industry. “A massive missing component is simply creating the market for these cleaner industrial products,” says Lehne.
Setting standards which allow novel cements to compete with Portland cement is another crucial step to changing things, adds Provis. “Our current standards give a list of recipes that we’re allowed to follow. We need to be able to specify materials based on performance, not based on recipes.”
The industry may well end up needing all these levers if a climate-compatible cement sector is ever to become a reality. “For the alternatives to work, the economics have to add up and that’s really difficult,” says Provis. “But it can’t be allowed to be impossible.”
By Jocelyn Timperley