Recycling cement, a green revolution for the construction industry
17.09.2024
Cement is an irreplaceable material. It is practical, easy to make and cheap. Cyrille Dunant, a materials scientist at Cambridge University, describes it as ‘a handful of dirt’. But this ‘dirt’ is one of the components of concrete, the basis of the vast majority of buildings. According to the expert, some four billion tonnes of concrete are used worldwide every year, making it the second most widely used substance after water. According to the scientist, concrete is a mixture of sand, rocks, water and, of course, cement, which functions as ‘the bond that unites the components’. So far so good, but cement has a problem: it is the cause of 90 per cent of the carbon emissions from concrete use.
That is why Dunant’s team of Cambridge researchers is working on a way to recycle cement and reduce the environmental impact of the construction industry, which is responsible for 7.5 per cent of the world’s carbon emissions, according to The Guardian in a report. Dunant is a senior research associate at the prestigious British think tank and also has a PhD in the study of cement, so he understands like few others why this material pollutes so much.
‘Cement manufacturing emits CO₂ for two reasons. One is that to make it you need to burn coal to produce heat, or maybe gas or some other kind of fuel. And the other reason is the limestone that has to be burned in the process. This limestone is lime and CO₂ together. The heat separates the CO₂, so you end up emitting it″, notes the Swiss-British scientist.
The reason for the high emissions from cement is not that the material itself is particularly polluting, but the sheer quantity in which it is produced worldwide. ‘Many people think that cement emits a lot of CO₂, but this is not true. It emits little CO₂ and consumes little energy, but we use a lot of it,’ the expert explains.
According to the scientific journal Nature, cement production has tripled in the last forty years. ‘Cement is a very good and very efficient material, but it has a huge impact,’ adds Dunant.
The construction industry is far from meeting emission cut targets, according to the employers themselves. A survey by the RICS network of infrastructure professionals at the end of 2023, in which 4,600 experts took part, shows that buildings are still responsible for 40% of global emissions. As much of this is due to cement, initiatives have emerged such as that of Sacyr, which developed a concrete from the revalorisation of glass waste that replaces between 10% and 20% of the cement needed in concrete. However, traditional Portland concrete remains the industry standard.
Steel is the key
The method being developed by the Cambridge University team involves taking advantage of steel recycling to also recycle cement. No new machinery is needed, because it uses the same electric kiln that is used for steel.
The difference between steel and cement is that the former is electrically conductive and the latter is not. However, melting the steel is only the first part of the process, after which the remains have to be ‘cleaned’, something that is usually done with lime: ‘What lime does is to collect all the impurities. And the mixture of lime and impurities is called slag. Instead of lime, in this case we add cement, which is largely composed of lime, so it is really good for cleaning steel,’ explains Dunant.
To add cement, it must first be separated from the other components of concrete. This process requires a lot of energy and is one of the reasons why recycling cement has never been attempted before, according to the scientist. ‘It’s a process where you put a lot of effort into it for little gain, but here we have a researcher called Rohit Prajapati who did his PhD on separating cement and other aggregates.’ The Cambridge team managed to make this separation a more efficient process and thus remove one of the big barriers to cement recycling.
The process as such consists of charging the electric kiln with steel, cement and some additional lime, as the separation of cement is not always perfect and may need to be ‘compensated’. Dunant explains didactically: ‘You melt the steel and, on the one hand, you have new steel to make ingots, bars and beams and, on the other hand, you have the slag that you can cool down quickly and then grind to powder. You add a little gypsum and you have new cement. With that and the new steel you can build a new building.
An irreplaceable substance
Ever since Portland cement was invented in England in the 19th century (there have been precursors to modern cement since the days of Ancient Rome), it has remained an immovable standard. ‘There is no one who could conceive of building certain infrastructures without concrete and cement,’ says Dunant. There is still no viable alternative today, but even if there were, the Cambridge scientist points out that recycling would still be needed for the millions of tonnes that already exist on the planet.
In the European Union, the Landfill Directive establishes penalties for those who place concrete waste in landfills, so countries must look for alternative ways to dispose of it. In the United States, the world’s second largest producer after China, regulations are much laxer and much of the unused concrete ends up in landfills.
The Cambridge research team is receiving EU funding to collaborate with companies across the concrete supply chain. On the steel side, the main partner is Spain’s Celsa, which in addition to its headquarters in Castellbisbal, Barcelona, has a subsidiary in Cardiff, Wales, with which Dunant and his team work. With Celsa’s electric kiln, they are putting cement recycling into practice on an industrial scale, and the researchers have already done enough tests to consider this process a real option for treating cement.
‘We have also approached other companies because the aim is to convince steel manufacturers that this is a good technology,’ says the scientist. Thus, the expectation is that the recycling method will expand in the industry and demonstrate that cement, although irreplaceable, can be given a new life.
EL PAIS Journal.
