Green Concrete: a Contradiction in Terms?
I’ve previously written a post about “green steel” and the challenges involved in reducing the emissions associated with steelmaking. Now the turn has come for concrete, which is the most widely used construction material in the world, and the second most used substance of any kind –after water. Understandably, it has an even worse reputation than steel in terms of its impact on the natural environment and climate. Is green concrete therefore a contradiction in terms, or can concrete become part of a sustainable economy?
Concrete is made from cement (a binder), water and aggregate (crushed rock, gravel and sand), sometimes combined with chemical additives. The idea of using a binder to hold construction materials together isn’t new. Mortar – a mixture of cement, water and sand – has been used for thousands of years, including in the Great Wall of China and the pyramids at Giza. However, the large-scale use of concrete – containing gravel as well as the finer aggregates found in mortar – dates back to Roman times, when it was used in monumental buildings like the Colosseum and Pantheon. The latter still holds the record for the largest unreinforced concrete dome in the world.
Concrete has many virtues – it is relatively cheap, it can be poured into shapes, and when it sets it becomes very strong and durable, particularly if reinforced with steel “rebars”. That explains why it has become so ubiquitous in modern construction. There is an environmental price to be paid, however.
Environmental Impacts
The good news is that the aggregate needed to make concrete can often be sourced locally with relatively little impact on the environment. Water is slightly more of an issue: concrete production is responsible for close to 10 percent of water used for industrial purposes1, and a few percent of global water consumption. In drier climates, this can put significant additional pressure on limited water supplies.
Nevertheless, cement is the ingredient with by far the biggest environmental footprint. Cement use has grown from next to nothing before the 1930s to around 4,000 million tonnes per year now. And just like steel, it is thought to be responsible for approximately 8% of global CO₂ emissions2.
Modern concrete generally uses Portland cement as its binder. This is made by crushing limestone, clay and other materials and then heating them to very high temperature in a kiln to produce clinker. After that, the clinker is ground with gypsum to make the final cement product. One-third of the CO₂ emissions come from burning fossil fuels to heat the kiln; chemical reactions during the cement-making process are responsible for the other two thirds.
Green(er) Cement?
The need to reduce the emissions associated with concrete has long been recognised. Back in 2010, a spinoff of Imperial College London called Novacem generated a lot of interest in its “green” cement based on magnesium oxides and carbonates. However, the company was unable to obtain sufficient funding and eventually folded.
Another option is to incorporate a type of mineral called a pozzolan into the cement. This is what the Romans did – in their case the pozzolan was volcanic dust, whereas the most popular modern pozzolans are fly ash and silica fume, but the principle remains the same. One of the key benefits of Portland cement is that it sets and hardens quickly, but with modern additives, pozzolanic cements can achieve similar properties to Portland cement.
Makers of pozzolanic cement claim that it has over 90 percent lower emissions than Portland cement, and it also provides a use for fly ash, which is a waste product of coal-fired power stations that would otherwise go to landfill. As ever, there is a catch: the pozzolanic cement has to be mixed roughly half and half with traditional cement when you make concrete, so the total energy saving is significantly lower. In fact, fly ash has been mixed into cement for decades – typically in a proportion of around 30% – so the newer “green” products are arguably just a way of repackaging what is already done.
Another material that can used as cement replacement is ground-granulated blast-furnace slag (GGBS or GGBFS), a by-product from the production of iron. GGBS, which is generally mixed half and half with Portland cement, can reduce the embedded CO₂ emissions in concrete by as much as 40-50% compared with pure Portland cement. It has the added benefits of being cheaper, making the concrete more workable and reducing early thermal cracking. However, like fly ash this is not a new product.
Using renewable energy could allow further emissions reductions. In the past, it has been almost impossible to use renewable energy to generate the high temperature – close to 1,500 °C – needed in the kiln where the clinker is produced. Encouragingly, recent technological breakthroughs have now made it possible to use concentrated solar energy to heat the kiln to a sufficiently high temperature. This could completely eliminate the one third of emissions associated with heating the kiln. Moreover, eliminating the use of fossil fuels reduces the impurities in the flue gases, which makes it easier to potentially capture carbon from the CO2 produced.
Other companies are trying to reduce their carbon footprints by incorporating construction waste and calcined clays into their cement. Since calcined clays activate at a much lower temperature than clinker, and have different chemical properties, they have the potential to reduce CO₂ emissions by 40 percent in comparison with traditional cements. The lower temperature needed also makes it easier to shift to renewable energy.
Model Projects
From an environmental perspective, the products discussed above are a huge improvement over pure Portland cement, but in spite of their widespread use, cement still produces eight percent of global CO₂ emissions. Clearly, there is a need to do much more. So are there any model projects that are going the extra mile to reduce the emissions associated with concrete use? The disappointing truth is that I have been unable to find many major projects that are adopting truly “green” concrete.
One luxury block of flats in Chicago is replacing 60% of the Portland cement in its concrete with fly ash, slag and silica fume, which is significantly better than the more normal 30-50%. Apparently, this adds just 1-2 percent to the overall cost of the project, so price shouldn’t be an insurmountable barrier to others following its example.
The athletes’ village for the 2024 Summer Olympics in Paris also claims to be using low-carbon concrete, but it is unclear whether this entails any more than mixing in some fly ash or GGBS.
Painfully Slow Progress
The construction industry is facing growing pressure from governments and clients to reduce its environmental footprint, including demands to adhere to green standards like BREEAM and LEED. Since cement is one of the two biggest sources of greenhouse gas emissions from the construction industry (along with steel), there is a strong incentive to come up with greener alternatives.
Cement’s pervasiveness makes it impossible to eliminate from the industry any time soon, so there is an urgent need to reduce the emissions that each tonne of cement generates as much as possible. Some improvement can be achieved through wider adoption of current best practice: by no means all concrete incorporates significant amounts of waste products like fly ash, silica fume and GGBS.
Nevertheless, the emissions from these lower carbon cements are incompatible with net zero, so they should only be a first step towards a more sustainable construction industry. Moreover, none of the new cements solve the other big problem: producing cement generates lots of small particles (so-called PM2.5 and PM10), which can exacerbate the risk of pulmonary disease, including lung cancer, and heart attacks. Cement dust in the final product is also a health hazard to construction workers on building sites.
With respect to the cements marketed as being green, it is reasonable to ask whether they are intended as genuine solutions or are just part of an elaborate greenwashing exercise. In some cases it is unclear whether they represent a significant improvement over existing products, and in others they are not widely used.
What, then, are the long-term solutions? One possibility is that incremental improvements will enable a gradual reduction in the GHGs and negative health impacts from cement. Another option is to replace concrete with alternative materials, such as timber. As with steel, the fundamental difficulty with that approach is the sheer volume of concrete currently used: no substitute is available in the required quantities. A more promising approach may be to use intelligent design processes to cut the overall use of construction materials. This area is something I intend to look at in more detail in a future blog post.
In general, the more research I’ve done for these blog posts, the more optimistic I have become. While the challenges associated with decarbonising the world economy remain enormous, greener technologies and practices are rapidly gaining ground. I wish I could say the same about concrete production, but unfortunately that is not the case. Whether due to intrinsic difficulties or the construction industry’s deep-rooted conservatism, progress in this area is painfully slow.
With other sectors shrinking their carbon footprints, concrete risks becoming a roadblock to a more sustainable future for the planet. With many of the major infrastructure projects that consume most concrete being directly or indirectly in the hands of the public sector, governments have a real opportunity to make sure the change goes beyond symbolic gestures.
https://www.nature.com/articles/s41893-017-0009-5
Estimates vary from 5%-11%. Taken from https://www.nature.com/articles/d41586-021-02612-5