Taking Concrete Steps to CO2 Sequestration

By Annabelle Wong, Member-at-Large for the GCI

With heightened concerns on greenhouse gas (GHG) emissions in recent years, scientists and engineers have come up with some innovative solutions to mitigate carbon dioxide emissions. One solution is to utilize and covert CO₂ to everyday products such as fuels and plastics. Recently I learned that CO₂ is now being converted into cement on an industrial scale.

Concrete is the most common construction material for buildings, roads, and bridges. Cement is one of the components of concrete and acts as a glue to hold concrete together. However, cement manufacturing is an energy-intensive process and the cement/concrete industry is one of the biggest CO₂ emitters. In fact, 5% of the global GHG emission stems from cement production.^1–3 To understand why so much CO₂ is released, let’s first take a look at how cement is produced.

To make cement, limestone (calcium carbonate, CaCO₃), silica (SiO₂), clay (containing mostly Al₂O₃), and water are mixed and heated. This reaction produces a significant amount of CO₂ and is called calcination. During calcination, at temperatures above 700 °C, limestone is decomposed to lime, or calcium oxide, and CO₂ (Reaction 1). Then, lime reacts with SiO₂ to form calcium silicates (C₂S in simplified cement chemist notation, where C = CaO, S = SiO₂) and tricalcium silicates (C₃S) as the temperature ramps up to 1500 °C (Figure 1). The final product, called clinker, is then cooled and milled into a fine power. Afterwards, minerals such as gypsum (CaSO₄) are added to make cement.⁴ A useful animation of cement making can be found here.⁵

CaCO₃ (s) → CaO (s) + CO₂↑ (g)                   (1)

Figure 1. Raw materials are heated up to 1500 degrees C to synthesize clinker. The ratios of products yielded at various temperatures are shown. [4]

CO₂ generated via calcination actually only consists of 50% of the total CO₂ emission from cement production, while 40% comes from fuel combustion for heating the reaction and 10% comes from electricity usage and transportation.^6,7

The idea of rendering the cement process more sustainable is to capture CO₂ from a cement plant’s flue gas and convert it to the starting material of cement, CaCO₃, creating a carbon neutral process. Scientists and engineers have been developing different technologies to achieve this goal. For example, at Calera, a company in California, CO₂ is first converted to carbonic acid. Then, Ca(OH)₂, which can be found in industrial waste streams, is added to convert carbonic acid to CaCO₃ and water. The overall reaction is shown in Reaction 2.⁸

CO₂ + Ca(OH)₂ → CaCO₃ +H₂O                     (2)

Iizuka et al.⁹ suggested that the Ca(OH)₂ and calcium silicates can be extracted from waste concrete, such as concrete from dismantled buildings, as a source of calcium ions. Their methodology is similar to Calera’s, but the carbonic acid is used for the extraction of calcium ions from waste cement (Figure 2).⁹ Furthermore, Vance et al. has shown that liquid and supercritical CO₂ can accelerate the formation of CaCO₃ from Ca(OH)₂.¹

Figure 2. Recycling CO2 and concrete to make limestone, the starting material of cement. [9]

On the other hand, CarbonCure, a Canadian company, takes a slightly different approach in CO₂ sequestration in the concrete industry. In their technology, CO₂ is incorporated in the concrete production process, rather than the cement production process. CO₂ is injected into the wet concrete mixture, where it is mixed with water to form carbonates (Reactions 1-3 in Figure 2). Then, the carbonates react with the existing Ca²⁺ in cement to form calcium carbonate nanoparticles, or limestone nanoparticles (Reaction 6 in Figure 2), which are well distributed in the concrete. This technique not only upcycles CO₂, but also increases the compressive strength of the material due to these limestone nanoparticles.¹⁰

As mentioned above, fuel combustion and use of electricity also contribute to the CO₂ emissions of cement production. In this way, other efforts to reduce CO₂ emissions include recovering heat from the cooled clinker,⁵ utilization of alternative fuels, reduction of clinker in cement,^3,11 and utilization of cement to absorb CO₂.²

With innovative research, development, and commercialization of CO₂ conversion technologies, I am optimistic that they will have a solid impact in the near future at the global scale. However, despite the current advances in CO₂ conversion technology, a collaborative effort on both CO₂ capture and utilization, along with the infrastructure to bridge these two technologies together, is essential to realize a carbon- neutral society.

References

(1)         Vance, K.; Falzone, G.; Pignatelli, I.; Bauchy, M.; Balonis, M.; Sant, G. 2015.

(2)         Torrice, B. M. Chemical and Engineering News. November 2016, p 8.

(3)         Crow, J. M. Chemistry World. 2008.

(4)         Maclaren, D. C.; White, M. A. J. Chem. Educ. 2003, 80 (6), 623–635.

(5)         Cement Making Process http://www2.cement.org/basics/images/flashtour.html.

(6)         Explore Cement http://www.wbcsdcement.org/index.php/about-csi/explore-cement?showall=&start=2.

(7)         Mason, S. UCLA scientists confirm: New technique could make cement manufacturing carbon-neutral http://newsroom.ucla.edu/releases/ucla-scientists-confirm:-new-technique-could-make-cement-manufacturing-carbon-neutral.

(8)         The Process http://www.calera.com/beneficial-reuse-of-co2/process.html.

(9)         Iizuka, A.; Fujii, M.; Yamasaki, A.; Yanagisawa, Y. Ind. Eng. Chem. Res. 2004, 43, 7880–7887.

(10)      Technology http://carboncure.com/technology/.

(11)      Cement Industry Energy and CO2 Performance: Getting the Numbers Right (GNR); 2016.