The concentration of CO2 in the atmosphere has increased dramatically, which is causing irreversible change to global climate systems. Carbon capture and storage is one course of action proposed to remove some of this excess CO2 and decrease the amount of new CO2 being emitted. It is known that calcium will react with CO2 under certain conditions and create calcium carbonate<!–[if supportFields]> XE “calcium carbonate (CaCO3)” <![endif]–><!–[if supportFields]><![endif]–> (CaCO2), a stable compound that sequesters the carbon in a way that it is not reintroduced into the atmosphere. Furthermore, there are several industrial, calcium-carrying materials that can participate in this reaction. Previous research has shown that concrete can take up CO2 as carbonate while it is curing. The incorporation of CaCO2 into the concrete makes the material stronger. It also makes cement more compatible with wood materials for the production of products like wood-cement particleboard. The carbonation also reduces shrinking by about 50% as cement sets and makes the material less permeable to water. Currently, this practice is not used because the cost of producing CO2 is so high that it is not economically feasible. However, it has been proposed that if recovered CO2 were used, then this option would become economically possible with the introduction of a carbon credits or carbon tax systems. Monkman et al. (2010) compared different scenarios for using emitted CO2for this process and looked at the environmental and economic viability. —Anna Fiastro
Monkman, S., Shao, Y., 2010. Integration of carbon sequestration<!–[if supportFields]> XE “carbon sequestration” <![endif]–><!–[if supportFields]><![endif]–> into curing process of precast concrete. Civil Engineering. DOI:10.1139/L09-140.
The authors compared recovered CO2to untreated industrial flue gas emissions in their ability to carbonate 4 different cement products. The recovered CO2 was imitated with highly concentrated and pressurized CO2. For each exposure, a continuous supply was used, causing constant pressure and CO2 concentrations to mimic flue gas which is emitted at a pressure higher than atmospheric pressure. The cement product was closed in a chamber with the pressurized gas and allowed to take up the CO2. After the designated period of time the chamber was emptied and refilled with flue gas. This process was repeated about seven times, and each time the carbon uptake gradually slowed down as it reached saturation. The amount of carbon taken up was quantified by the percent gain in mass of the cement.
Concrete masonry units (CMU) are one industrially produced product that was examined. CMU were found to be the ideal candidate for carbon uptake because it is porous and already cured in a closed camber, making the addition of a gas easy. Concrete paving stones were also looked at. They are not subjected to any special curing scheme so they could easily be placed in a large sealed room filled with gas for the carbonation treatment. Fiberglass mesh reinforced cement is a product that is cured with high pressure and moisture. Because if this, a large surface area to volume ratio is necessary, which also lends itself to carbon uptake. The last product examined in the study was cellulose<!–[if supportFields]>XE “cellulose” <![endif]–><!–[if supportFields]><![endif]–> fiberboard, used in place of asbestos cement. The curing steps for this material can also easily be replaced by carbonation curing, giving the benefit of hydration of the material and carbon sequestration<!–[if supportFields]> XE “carbon sequestration” <![endif]–><!–[if supportFields]><![endif]–>. Both the fiberglass mesh cement and cellulose fiberboard benefit from a lowered pH, protecting the material from alkali corrosion.
All four of these materials would benefit in strength and durability from carbonation, and their curing processes could easily be replaced or supplemented by carbonation curing. In all cases the uptake from recovered CO2 was greater than uptake from flue gas.
It is also important to compare the energy and CO2 penalties for recovering, compressing, and transporting the CO2 in order for it to be used. When this is taken into consideration, recovered CO2 is still the most viable option, because it is already at high concentrations. The compression and transportation of flue gas makes it only feasible if the curing process occurs on site. The transportation associated with recovered CO2 can also be compared to other capture and storage methods, such as geological storage, which would result in equal or more transportation emissions.
Part of the analysis of processing emissions is the comparison of carbonation curing with steam and autoclave curing, the methods predominantly used in the industry. The emissions from carbonation curing are less than one tenth of those associated with steaming and autoclaving. This method is also attractive if one takes into account the possible trading value of carbon and improved technologies reducing energy use in production phases.
One last analysis was done looking at the carbon uptake of ladle slag<!–[if supportFields]> XE “slag” <![endif]–><!–[if supportFields]><![endif]–> fines as a replacement for sand. This was done using a different CO2exposure process than the other materials. The particulates were exposed to gas made up of 50% CO2 at atmospheric pressure. This was meant to replicate flue gas without the compression step that is so energy expensive. The material was found to have a modest carbon uptake of about 10% after almost 60 days. This is not thought to be the best opportunity for carbon sequestration<!–[if supportFields]> XE “carbon sequestration” <![endif]–><!–[if supportFields]><![endif]–> but other calcium-rich slags may serve this goal more affectively.