This article was first published in Castanet April 16, 2024
Some aspects of our economy are easy to decarbonize. We are well on our way switching cars from internal combustion to electric. We are rapidly adding carbon-free electricity from solar panels and wind turbines. However, there are areas where it is not easy to decarbonize. Top of that list is concrete. According to Our World in Data cement production is responsible for 3% of worldwide emissions, more emissions than aviation or landfills.
The world uses a lot of concrete -- it is estimated at 30 billion tons of concrete a year. Concrete is inexpensive, durable, strong, and resilient. We cannot do without concrete. As the population grows the amount of concrete we need is increasing. Global warming only makes this worse -- concrete plays a big role as we try to adapt to climate change, think of construction protecting communities from sea level rise.
Concrete is made up of sand, gravel, water and cement, where cement is the binder that makes the concrete harden and keeps it strong. While cement only makes up 15% of concrete, it is responsible for 80% of emissions. The fundamental problem today is how we make cement. Cement starts as limestone which is crushed and then heated to 1450 C in a kiln to create lime.
Concrete can be “recycled” where it is crushed and used to replace sand and gravel but while this prevents concrete from overwhelming landfills it doesn’t change the amount of greenhouse gases released in the production of cement.
Let’s take a look at two parts of this process, starting with the heat. The super high temperature needed to create lime from limestone is created using powdered coal, oil or gas (it is worth noting that in this context gas is indeed a low emission fuel, releasing about forty percent less CO2 than coal and oil). Electricity is poor at creating this kind of heat, and while it is possible to achieve these temperatures with concentrated solar power, fluctuations from nighttime and clouds remain a problem.
A study by the Canadian Energy Systems Analysis Research simulated using a mix of natural gas and alternative fuels to create cement, such as plastic waste or wood dust. The problem is that these energy sources have a lower heating value and a higher moisture content. Because of these and other differences a 50-50 mix of alternative fuel and natural gas requires a lot more oxygen and heating and pumping oxygen actually generated more CO2 than using gas alone by itself.
Now let's look at the chemical transformation from limestone to lime. Limestone is mostly calcium carbonate and lime is calcium oxide. As the “carbonate” in the name suggests, limestone contains carbon which is released in the form of carbon dioxide during the high-temperature chemical reaction in the kiln. Sixty percent of the carbon emissions from cement come from this chemical reaction.
So cement has two problems -- the high temperature needed to change limestone into lime and the chemical reaction that releases CO2 in that high temperature reaction. Is this hopeless? No. Project Drawdown, a collection of current-technology climate change solutions, takes aim at both the energy used generating the high temperatures and the limestone-to-lime CO2 emissions. First, as with so many areas, they suggest beginning by upgrading to more energy efficient equipment. Then they suggest switching from lime as a binder. Natural products such as calcined clay can be used. More exciting is the possibility to use waste such as granulated slag (a by product of steel) or recycled glass. There are several small companies today that produce alternative cement such as CarbonCrete in Montreal or Brimstone in California.
So the carbon footprint of concrete can be reduced by energy efficiency. It can be reduced by switching from limestone to an alternate material. However, concrete can also be a carbon storage solution. One of the hottest areas of research is using concrete to lock away carbon emissions. There are companies doing this including CarbonCure (from Dartmouth Nova Scotia) and CarbonBuilt (from California). Both these companies collect CO2 and inject it into the concrete. The great thing about pumping carbon into concrete, is that it is permanent storage. You can store carbon by planting a tree, but trees can die, be chopped down or burned in a forest fire, releasing their stored carbon back into the atmosphere. Carbon injected into concrete is there to stay.
Concrete is irreplaceable in modern construction and there’s a lot of carbon emissions from how we create cement today. But there is a three prong solution: energy efficiency, alternatives to cement, and carbon capture and storage. We can update our cement manufacturing, taking energy efficiency steps to create high temperatures with less fuel. We can switch the binder from cement to alternatives that don’t release as much CO2. And finally we can integrate carbon storage in all concrete production. Concrete’s problem is that it isn’t snazzy like solar panels or lovely like forests but it is literally the foundation of society. We have the tools, now let’s use them.
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