Putting concrete in an environmental perspective
Cement and concrete are intrinsically low impact materials. “The reason cement and concrete have big impact is because we use them in such large amounts. They have in fact far lower impact than any substitute. So substituting cement or concrete with other materials that have higher environmental impact will not improve the situation.” Dr Karen Scrivener, head of the Laboratory of Construction Materials at Ecole Polytechnique Fédérale de Lausanne, points out.
“We have to remember that it’s only because concrete- or cement-based materials make up over half of all the construction materials we use that the overall impact in terms of CO2 emission is quite high.”
The embodied CO2 in concrete is paid off early in the lifetime of concrete buildings because of greater energy efficiency. Concrete is made from a mixture of aggregates, cement, water and air. The typical concrete mix contains between 10-15 percent cement. According to The Concrete Initiative, a project led by CEMBUREAU (the European Cement Association), BIBM (the European Federation of Precast Concrete), ERMCO (the European Ready-Mixed Concrete Organisation) and UEPG (the European Aggregates Association), when the CO2 emitted during the extraction, transport and manufacturing of these ingredients are taken into account, a typical concrete mix has an embodied CO2 of around 50-150 kg per ton. Studies show that the payback period for concrete buildings can be as short as 11 years.
Conventional buildings use 150-200 kWh/m2/year of energy. By contrast, modern concrete buildings can be designed to use 50 kWh/m2/year or even less energy. Concrete contributes to greater energy efficiency due to its durability, air-tightness and thermal mass. Concrete buildings can last over 100 years. Compare this to buildings made of other construction materials that last 50 years, that would need to be rebuilt twice in order to last the same amount of time and it’s easy to see that concrete buildings save both resources and have lower embodied CO2. Concrete structures also have very few joints that help keep buildings airtight. Thermal mass allows concrete to absorb heat and helps prevent overheating. The stored heat is then released in the evening when it is cooler outside. This keeps room temperatures relatively stable throughout the year, which means less energy is required to heat or cool rooms.
A lot of research is currently being undertaken into greener cement. “Ninety percent of the CO2 emissions associated with concrete comes from cement clinker production. Clinker is an intermediate product during the manufacture of Portland cement that is produced when limestone and aluminosilicates like clay are heated to a high temperature in the rotary cement kiln . There is currently a lot of research going on into developing greener cement that aim to reduce the clinker content as far as possible in the final cement,” Prof. Christian John Engelsen, Senior Scientist at SINTEF, an independent research institute in Norway, points out.
Dr Scrivener and her team have also developed a new kind of cement called LC3 (Limestone Calcined Clay Cement) that can reduce CO2 emissions by 30 percent. LC3 is made from limestone and low-grade clays which are abundantly available, making it a cost-efficient alternative to Portland cement as it doesn’t require capital intensive modifications to existing plants.
Carbonation, a naturally occurring process that has been known for thousands of years, allows concrete structures to absorb CO2 from the atmosphere. “When cement is added to make concrete, the natural carbonation process starts since carbonation is the product of lime and water and air coming together. The speed of carbonation depends on a number of factors, such as concrete quality, humidity, surface finishing, among others,” Engelsen says.
He cites a study his team did a few years ago on calculating the CO2 absorption in Norwegian building stock. “We calculated that over a service life of 100 years with a 10 percent level of recycling concrete that the level of CO2 rebound was about 15 percent of the CO2 emitted during the production of the cement. This is considered a conservative figure as we didn’t want to overestimate the amount of CO2 being bound from the atmosphere.”
Jussi Mattila is CEO of the Confederation of Finnish Construction Industries RT (CFCI), a joint interest organization of building contractors, special contractors and the construction product industry. Mattila is actively involved in Concrete Solutions, a project run by the Finnish Environment Institute to look into the possibilities of concrete as a carbon sink. “We have been manufacturing cement and concrete for 150 years. So we have produced a lot of concrete structures that are taking CO2 from the atmosphere all the time. We are carrying out research on quantifying just how big of a carbon sink concrete acts as, but it’s still in its early stages.”
Mattila adds that research carried out on the potential of concrete as a carbon sink is going on all over the world. Concrete Solutions, for example, is a €1.1 million research project. “There are a lot of considerations we have to note when talking about the possibilities that concrete offers as a carbon sink. It depends on a combination of factors. In Finland, for example, where we have a different climate, we use frost-resistant concrete, which might behave differently than concrete used in other parts of the world.”
According to The Concrete Initiative, carbonation can be especially relevant after a concrete building is demolished and the concrete pieces crushed, when the surface area exposed to air increases dramatically.
Concrete is 100 percent recyclable. When calculating concrete’s environmental impact, what happens once a concrete building is demolished also needs to be taken into account. Concrete can be easily reused and recycled: aggregates from demolished buildings can be used in unbound applications such as road base and even as aggregates for new concrete.
“Scrapped precast from concrete element factories can be used to replace up to five percent of materials since scrap from production is high quality concrete. Also, since you have control of the concrete recipe, it’s easy to crush it and use it again in new production.” Engelsen explains.
The net CO2 emissions associated with concrete products over their lifecycle are less than if only the CO2 emitted during their production is taken into account. “There is a tendency to focus on the CO2 produced while concrete is being manufactured. We have to look at the whole life of a concrete structure to get an accurate perspective on its environmental impact. Concrete performs better than other building materials in terms of energy efficiency, lower maintenance costs and longer service life,” Mattila contends. “As long as a concrete building is properly maintained, it basically lasts forever. And as long as it is standing, it will bind CO2 from the atmosphere.”
It’s important to be realistic about concrete
Dr Karen Scrivener is a material chemist known for her groundbreaking work on cementitious materials, an area she has researched for over four decades. She heads the Laboratory of Construction Materials at Ecole Polytechnique Fédérale de Lausanne, where she is Full Professor.
We need to be realistic about the role that concrete plays in our lives and how we can mitigate its effects on the environment. One thing we do know is that because we use so much concrete – it’s about half of all the material we use – we also have a big opportunity to improve things. To put it in perspective, we manufacture about 4 billion tons of cement, which translates to about 10 times that amount in concrete. If we can make a few changes in the amount of CO2 emissions, for example, when you multiply that by the amount of material that we use, it can have a very substantial impact.
There has been a lot of discussion around what can be done to lower the impact of concrete. Some have proposed tweaking its carbonation capacity to enable it to capture more CO2. Although there is some margin there, it’s always going to be just a fairly modest proportion of the CO2 that’s being emitted. And there are going to be applications that cannot be used in all situations.
See beyond the hype
There have also been suggestions about making concrete blocks with bacteria. The fact is that doing this will not actually solve anything, because the calcium these bacteria use has to come from decarbonizing limestone in the same way used for the manufacture of cement. It might actually make it worse since you’d probably need bigger amounts of calcium. There is a tendency in media, especially, to focus on things that sound more innovative than they really are.
We have to keep in mind that 90 percent of concrete production happens in emerging or developing economies. We have to make sure that the solutions we come up with can really be used in the field. We also have to be practical. We have to think about how we can improve the whole chain.
Optimize technology we use now
We wrote a report a couple of years ago for the European Climate Foundation, where we estimated that just using technologies that are available today, we can lower emissions by up to 80 percent, by optimizing these different links in the chain.
The first level we need to work at is the level of cement. This is why we developed LC3, because it could lead to savings of between 30-40 percent. And then you look at the concrete level. We are very wasteful and we tend to put much more cement into concrete than we need to. We can save another 40-50 percent at that level. And then we look into the actual construction. Again, we use more concrete in structures than needed and the wastage rate can reach to about 50 percent or about twice what is actually needed. If you add all these levels up, you have very substantial reductions for this material.
We need to implement practical things while working on longer term solutions like carbon capture. But we have to realize the urgency of the situation. The concrete industry is very conscious about the issue of environmental degradation and it’s important to act now.