CarbonCure-Capturing Carbon in Concrete Blocks
By Brent Ehrlich
Could injecting carbon dioxide (CO2) directly into concrete masonry units (CMUs) during production be a tool for lowering carbon emissions from construction? The makers of the CarbonCure Block System say that their system both lowers theand improves their overall strength.
The production of portland cement, a key ingredient in concrete, results in approximately 5% of the world’s anthropogenic CO2 emissions. Reducing the amount of this CO2 in concrete products has long been an industry goal, but CarbonCure is the first company to bring to market a viable, mass-produced product that “sequesters” significantly more CO2 from the atmosphere than standard concrete without requiring a radical change in technology.
How carbon is injected
CarbonCure CMUs are produced using a specially designed mold that is attached via a hose to a tank of CO2 supplied from local industrial sources—usually high emitters such as refineries or fertilizer plants. The CO2 is injected at a carefully controlled rate as the concrete mix is pressed in the mold. Robert Niven, CEO and founder of CarbonCure, says one of the advantages of CarbonCure is that it does not fundamentally change how the CMUs are produced. Because the system can basically be bolted onto existing CMU production facilities, “We can commission a plant in half a day at very low cost.”
More than half the CO2 created during the production of portland cement is generated by the chemical reaction called calcination, during which calcium carbonate (limestone) is converted to calcium oxide under high heat, producing CO2 as a byproduct. When the cement is later mixed with water, aggregate, and admixtures to form concrete, some CO2 is absorbed back into the concrete during curing in a process called carbonation. According to Niven, while standard concrete continues to undergo carbonation over time, the process gradually becomes less effective as the CO2 hardens the surface and the concrete becomes less permeable. “Simple carbonation is limited by depth; at most you get about 2 mm of carbonation,” claims Niven. “It would take about 1,000 years to get the same amount of carbonation as you would get from our process in around six seconds.”
CMUs are an ideal fit for CarbonCure technology because blocks use a relatively dry, porous cement mix, which allows the CO2 to penetrate deep into the block. Depending on size, weight, and density, a CarbonCure block will convert from 1.75 ounces (50 g) to 3.50 ounces (100 g) of CO2, according to data from the company’s pilot work. Nevin says that the total CO2 kept out of the atmosphere is actually higher and is closer to 7 ounces (200 g) per block because the CMUs are 10%–48% stronger than standard products. The added strength allows block manufacturers to use 10% less portland cement, and they require less time (about 38% less) in the steam-curing kiln. A stronger block also leads to 20% fewer defective blocks. It’s the combination of carbon reductions and early strength gains that make the product especially attractive.
Taken together, the company estimates that for every 50,000 CarbonCure CMUs produced, CO2 emissions are reduced by 13 tons (about 12 metric tons) and product waste by 11 tons (about 10 metric tons). To put that in perspective, in 2010, 119 U.S. block manufacturers produced an average of 6.1 million CMUs each, according to concrete block trade groups. If all those CMUs were made using CarbonCure, it would sequester 160,000 tons (145,000 metric tons) of CO2 from the atmosphere. While that is less than half a percent of the 46,000,000 tons (42 million metric tons) released by cement production in 2008 (the last year data was available through the U.S. Energy Information Administration), it could be part of an overall carbon reduction strategy.
Full production runs
Three major CMU producers, Shaw Brick, Atlas Block, and Basalite, have incorporated CarbonCure technology, but none are currently making regular, full production runs of the blocks. Shaw and Basalite have, however, already sold CarbonCure blocks for projects. Brady Hawley, operations and sales manager at Shaw Group, oversaw three years of CarbonCure CMU testing. “We have now accomplished full-scale runs and have made a lot of strides towards making this a full-scale operation, including a run of 5,000 blocks that are going to be used in a local school.” Hawley said the latest round of CarbonCure blocks looked and performed as well as standard blocks, and he claims the technology is “potentially revolutionary.”
Niven says CarbonCure and the companies using the technology are being cautious rolling out these CMUs, citing market demand and capacity, and are currently supplying block on a project-by-project basis. But he sees a big opportunity for the CMU industry because the technology provides a clear, marketable green CMU that is cost-competitive with standard blocks and doesn’t require a radical technology change. Though CarbonCure is currently being used for CMUs, in time Niven expects the technology to be adapted to other precast concretes as well.
For more information:
CarbonCure Technologies, Inc.