Concrete is one of the most common construction materials in the world, and yet it’s also not a topic that gets a lot of discussion time in the sustainable sphere. Attention is often focused on intensive animal agriculture, fossil fuels and plastic production as huge problems (as it should be), but as we move forward and attempt to decarbonise as rapidly as possible over the next decade, we have to make sure we’re looking at every area of society. That has to include construction and development and, therefore, concrete.
At the moment, concrete is the most consumed man-made material in the world and the second most consumed material on the planet after water. It is long-lasting and dependable, affordable, and adaptable to many designs. Plus, concrete’s thermal stability can also create more energy-efficient buildings. However when it comes to materials, only coal, oil and gas are a greater source of greenhouse gases.
All the plastic produced over the past 60 years amounts to 8bn tonnes. The cement industry pumps out more than that every two years. But though the problem is bigger than plastic, it is generally seen as less severe. Concrete is not derived from fossil fuels. It is not being found in the stomachs of whales and seagulls. Doctors aren’t discovering traces of it in our blood. Nor do we see it tangled in oak trees or contributing to subterranean fatbergs. We know where we are with concrete. Or to be more precise, we know where it is going: nowhere. Which is exactly why we have come to rely on it.
Modern-day concrete is created by a process that was designed in the early 19th century by Joseph Aspdin, a bricklayer from Leeds. First cement is made from a mixture of limestone, clay and sand. This is ground into a powder and heated in a kiln, usually at 1500°C, to form pellets known as clinker. The clinker is mixed with a small amount of gypsum and ground into a fine powder, known as portland cement, which is one of the key ingredients of concrete. Portland cement is mixed with sand, water and aggregate (rock fragments) to form concrete: the water reacts with the cement, causing it to harden, which is known as curing. Overall, about 1.6 tons of raw materials are used to make 1 ton of cement.
Portland cement is a key component of almost all modern concrete, accounting for more than 95% of the cement market. Concrete is typically made up of 41% crushed rock, 26% sand, 16% water, 11% portland cement, and 6% entrained air. But, despite being made from natural materials, this material carries a heavy footprint.
The eco issues with concrete
Unfortunately, portland cement has a high environmental cost. Cement production is said to be the third-largest producer of CO2 in the world after transport and energy generation. According to think tank Chatham House, it is the source of about 8% of the global CO2 emissions. If the cement industry were a country, it would be the third-largest emitter in the world after China and the US. For the cement industry to fall in line with the Paris agreement, its annual emissions need to fall by 16%.
There are a few reasons why cement production is intense for the environment. It is responsible for almost a 10th of the world’s industrial water use, often in water-stressed areas, and its production involves quarrying, which causes air pollution by creating dust, exacerbating respiratory diseases.
The dust from wind-blown stocks and mixers contributes as much as 10% of the coarse particulate matter that chokes Delhi, where researchers found in 2015 that the air pollution index at all of the 19 biggest construction sites exceeded safe levels by at least three times… At this scale, even the acquisition of sand can be catastrophic – destroying so many of the world’s beaches and river courses that this form of mining is now increasingly run by organised crime gangs and associated with murderous violence.
Production also requires huge kilns, which need large amounts of energy. In the US, portland cement production accounts for about 0.33% of the annual energy consumed, which is equivalent to the amount of energy in around 13 million tons of coal.
It is also true that cement has a lower embodied energy (the sum of all the energy required to produce something) than materials such as steel or aluminium, meaning that it’s not a simple solution of changing to another material. However, the fact that remains that, within the construction industry, cement production is a huge source of emissions. Some of this CO2 comes from producing heat for the kilns that run non-stop, however, half of portland cement’s emissions come from the process of calcining limestone, which produces CO2 during the chemical reaction. In 2016, world cement production generated around 2.2 billion tonnes of CO2, with over 50% coming from the calcination process.
What is being done
There are multiple factors to be considered when it comes to developing more sustainable options. Any alternative that is adopted needs to be produced from plentiful resources while creating minimal waste and without needing huge amounts of energy. It also needs to be durable and ensure buildings are energy efficient.
The dangers are recognised. A report last year by Chatham House calls for a rethink in the way cement is produced. To reduce emissions, it urges greater use of renewables in production, improved energy efficiency, more substitutes for clinker and, most important, the widespread adoption of carbon capture and storage technology – though this is expensive and has not yet been deployed in the industry on a commercial scale.
The sector has been slowly moving towards progress. So far they’ve seen improvements in the energy-efficiency of new plants, including using waste materials for energy instead of fossil fuels, which has seen average CO2 intensity of cement production drop by 18% in the last 30 years. In other regions it has been higher, as Poland recorded a 42% decrease in this time, proving that change can be made.
Low carbon alternatives
A lot of work has been done to develop more sustainable alternatives to portland cement, which is where the real change is needed. CEM II is currently the most common low carbon alternative, as it substitutes clinker for other materials such as fly ash, which would usually go to landfill. Another alternative is Ground Granulated Blastfurnace Slag, also known as GGBS, which was first pioneered in 1999.
GGBS, a by-product from the production of iron, involves no quarrying and requires very little energy to manufacture. This gives Ecocem GGBS an extremely low carbon footprint of 42kgCO2/tonne, which is a 95 per cent reduction when compared with CEM II.
It is estimated that 90 per cent of the embodied carbon of concrete is attributed to the cement – consequently replacement with 50 per cent GGBS offers an immediate 52 per cent saving and a quick win for carbon reduction.
Solidia, a company specialising in sustainable cement and concrete are doing particularly promising work. So far they have developed two approaches to reduce emissions. For cement, they use the same raw materials in a 50/50 blend (rather than two parts limestone to one part sand/clay), heated to a lower temperature of 1200°C. This uses less energy and less limestone, cutting CO2 emissions in both areas, while also making more cement with less raw materials. When it comes to concrete, instead of a reaction between water and cement, they pump carbon dioxide in for the curing process instead. This is cheaper, quicker (24 hours rather than the usual 28-day process), and uses less water. All water that is used is also able to be recovered, causing less waste. It also uses the same manufacturing processes, equipment, materials and production lines, so no new costs or expensive machinery is needed.
“You’ll actually see the [concrete] pick up about 5 per cent weight and that’s the CO2 converting to, essentially, limestone,” says Schuler.
While still a small movement, this also represents another potential future option for carbon sequestration. If concrete production is located near places where CO2 is readily available, this creates the option to design closed-loop systems. I’ve often been asked about emissions that come from creating renewable technology, particularly when it comes to wind farms. My answer has always been to also have a carbon capture plan in conjunction with renewable development, this seems like a brilliant option seeing as concrete is often used to anchor the base of offshore wind turbines.
The even better news is that Solidia’s target market is huge: the $1 trillion global concrete and $300 billion global cement industries have the power to make a huge impact incredibly quickly by adopting this kind of technology. If widely adopted Solidia believe they have the potential to eliminate 1.5 gigatons of CO2, save 3 trillion litres of freshwater, eliminate 100 million tonnes of concrete landfill, half the emissions of mercury, nitrogen oxide and sulphur oxide, and reduce energy consumption by the equivalent of 67 million tonnes of coal. This would potentially exceed the industry goals for carbon reduction, which is an incredibly exciting thought.
Beyond this, there is also BioMason, a US-based start-up that uses bacteria to grow bio-concrete bricks. They do this by placing sand in moulds and injecting it with microorganisms, mimicking the way coral is created.
The bacteria wrap themselves around grains of sand. Each grain acts as a nucleus and calcium carbonate crystals form around it… The bricks are then fed nutrient-rich water via an irrigation system for the next couple of days, speeding up the growth of the crystals which then fill the gaps between grains of sand. After four days, the bricks are strong, durable and ready to be used on the construction yard, and the water is then reused for the next set.
The process takes four days at room temperature, meaning it doesn’t require large amounts of energy and there is no calcination to produce high emissions.
Moving away from concrete altogether, there are also multiple developments looking at using alternative materials for transport infrastructure. Companies around the world have looked at utilising plastic waste, household waste, plant polymer by-products from the paper industry, and even solar panels to create roads without using concrete at all. While these are all in the research stage, they present an exciting, less concrete-dependant future.
Ultimately, concrete is a tricky subject. As parts of the world continue to develop there is a demand for more infrastructure including buildings, sanitation and energy services, all of which require construction. In order to meet other UN Sustainable Development Goals, it’s expected that $60 trillion will be invested in this infrastructure in Global South countries. At a time when it needs to dramatically reduce emissions, the concrete and cement industries will actually be dealing with massive expansion.
It seems like a multi-pronged approach across the sector is what will work best:
Architects believe the answer is to make buildings leaner and, when possible, to use other materials, such as cross-laminated timber. It is time to move out of the “concrete age” and stop thinking primarily about how a building looks
This, in part, is a fair assessment. However, we also know that other materials, such as steel, are more energy-intensive than concrete. Plus, in a world already struggling with deforestation, a shift to timber en-masse would probably be very dangerous. It would be impossible to sustainably manage all the timber required, seeing as we’re already not doing this right now.
“The raw materials are virtually limitless and it will be in demand for as long as we build roads, bridges and anything else that needs a foundation,” he said. “By almost any measure it’s the least energy-hungry of all materials.”
Instead, he calls for existing structures to be better maintained and conserved, and, when that is not possible, to enhance recycling. Currently most concrete goes to landfill sites or is crushed and reused as aggregate. This could be done more efficiently, Purnell said, if slabs were embedded with identification tags that would allow the material to be matched with demand.
A combination of circular approaches, such as more concrete reuse and recycling, implementing alternative materials like timber when it’s appropriate, reducing concrete demand through design, and mass adoption of sustainable options like Solidia’s concrete, seems to me to be the best combination of approaches. So what does it take to get there?
According to Chatham House, a large barrier is a lack of carbon-pricing (aka a carbon tax) on the industry. Without the economic incentive, the small handful of large producers who dominate the sector are cautious to adopt new technologies that challenge their business models. I think this is where citizens can come in, as there are things we can do.
Firstly, we can talk to our local representatives and demand a cement tax as part of a global carbon tax, which will push the industry to actually make a change. While this may seem impossible under centralised conservative governments, this is also where getting involved with local authorities can be powerful. Local authorities are predominantly in charge of planning permission, therefore making these decisions, and those who make them, much more accessible to the local community. By getting involved in local elections (council elections are happening in the UK this May) and talking to your local representatives, we can request less concrete and ask for buildings in our communities to have stricter standards around low carbon alternatives. If it becomes a priority in our local areas, we can hopefully get it on the national political agenda too. Options like Solidia can be widely adopted, we just have to incentivise it.
Ultimately, like with many of these issues, the solutions are there. People just need to demand them, and those in charge need to stand up and actually do something.
Beyond this, the Guardian also makes a compelling closing argument around changing our perspective to create the world we want, which I will leave you with here:
Arguably more important still is a change of mindset away from a developmental model that replaces living landscapes with built environments and nature-based cultures with data-driven economies. That requires tackling power structures that have been built on concrete, and recognising that fertility is a more reliable base for growth than solidity.