The production of cement is responsible for as much as 5% of the CO2 emitted each year. It's mostly because of chemistry; when limestone is cooked to about 1400 degrees Celsius, it drives off water and CO2 and becomes the main component of cement.
A lot of companies are trying to develop new technologies and replace conventional cement to reduce its carbon footprint, but CarbonCure takes a really simple, logical approach to getting rid of CO2: It puts it back into the concrete. Then the magic of chemistry happens again, as it bonds with the cement and essentially turns it back into limestone. Or as company founder Robert Niven tells Carbontalks,
If you look at concrete, cement starts off as CaC03, it’s heated in cement kilns, and this release one molecule of C02 for every molecule of lime. We’re reversing that reaction – we’re providing a supplementary curing reaction.
It sounded silly to me when it was first explained by Marketing Manager Christie Gamble: The CO2 is collected from industrial processes like power plants or factories, purified and compressed into liquid form, the same stuff that is sold to Coke and Pepsi to carbonate their soda pop, then trucked to concrete block factories and squirted into the concrete that is formed into blocks. But they have done the life cycle analyses and a whole lot more CO2 is sequestered than is produced through this processing and shipping. It reduces carbon emissions by up to 20%.
But wait, there's more. CO2 is part of the chemistry of curing cement, so the blocks have higher early compressive strength, and less cement is required to make the block. It seems like a great idea for all concrete, not just block, but they are starting with concrete masonry units because it is a controlled environment and if there is a problem, it's not yet in the building. However the process will scale so that it can be used in poured concrete as well. Founder Robert Niven told the Financial Post:
“We make concrete that is really indistinguishable from other concrete products,” Mr. Niven said in an interview at his Halifax office. “We make a better quality product, at a lower price. And it’s green. It’s that triple-win that makes it really attractive to producers.”
Of course if there was a price on carbon, then everyone would be running around trying to do this, to get better concrete and sequester CO2 at the same time. Right now, it's popular with architects who can specify it and get credit in green building programs like LEED for recycled content, carbon impact and innovation in design. As John Crace of WHW architects said after using it on a high school:
Concrete masonry units (CMU) that sequester carbon dioxide have less embodied energy and provide greater strength than conventional CMUs. This offers an irresistible combination to any design team.
I complain a lot about concrete and its carbon footprint, but below grade it is often hard to find affordable alternatives that will get past the tough plan examiners in this town. These guys are standing in my future office right now, building the foundations for my sort of green renovation with standard CMUs. CarbonCure blocks look and perform the same as the standard ones, meet the same UL and CSA standards, and the trades, plan examiners and building inspectors aren't going to give anyone any trouble; it's just a greener block.
I wish I had learned about CarbonCure a few weeks earlier.
UPDATE: I should have remembered CarbonCure, it was one of BuildingGreen's top ten products in 2012 and I wrote about it here and it is available from a local block supplier. Which shows what happens when you don't do your research on TreeHugger.