Utilizing CO2 to Revolutionize Concrete Sustainability

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Oct 7 2024

A recent article published in the Journal of Building Engineering explored the current use of CO₂ for producing low-carbon concrete. The article discussed the different carbonation mechanisms of binders and other concrete components and the commercialized technologies employing these to produce sustainable concrete.

Background

Carbon capture, utilization, and storage (CCUS) technologies have emerged as a viable solution to curtail anthropogenic CO₂ emissions and attain the ambitious carbon neutrality target by 2050.

Cementitious materials, the most extensively used man-made material, account for a substantial carbon footprint throughout their lifecycle. Thus, cement concrete is a promising opportunity for CCUS through carbonation treatment. This includes carbonating binders, industrial wastes, and recycled aggregates from demolished structures, which can be incorporated into new concrete.

CO₂ readily dissolves in water under moderate pressure, forming bicarbonate, hydrogen, and carbonate ions. These ions interact with other components in cement concrete, such as binders or aggregates, to create various hydrated compounds. Consequently, cementitious materials provide an effective medium for CO₂ absorption and utilization, offering substantial potential for reducing carbon footprints in the construction industry.

Carbonation of Cement Concrete

Carbonation is a chemical process that can impact cement concrete’s structural integrity. During this process, CO₂ infiltrates the concrete, reacts with the alkaline substances, and forms carbonates and water. While such natural carbonation can enhance the durability of plain concrete, it can initiate rusting in steel-reinforced concrete depending on factors such as CO₂ concentration, relative humidity, and permeability of cementitious materials.

Accelerated carbonation of cement concrete has greater potential in the short term for mitigating climate change than natural carbonation. Moreover, it can be applied at various stages beyond the concrete’s service life. Thus, early-age carbonation, involving accelerated carbonation for curing freshly mixed concrete, is becoming popular.

Alternatively, mineral carbonation is viable for sequestering substantial amounts of CO₂ in basic calcium and magnesium compounds. Sources of such minerals include fresh concrete, recycled concrete aggregates (RCA), industrial wastes like steel slags and cement kiln dust, municipal waste, and natural. In the long run, integrated methods could result in significant carbon sinks.

CO₂ Utilization in Concrete Production

Accelerated carbonation is integrated into cement composite manufacturing through three primary methods: pre-carbonation of concrete ingredients, carbonation curing, and CO₂ mixing. In the pre-carbonation process, concrete ingredients are treated with CO₂ before mixing, initiating carbonation reactions. This results in the formation of carbonated supplementary cementitious materials (SCMs), fillers, and aggregates. The CaCO₃ precipitated during this process can then be incorporated into various binder systems.

Alternatively, the CO₂ mixing method introduces CO₂ directly into the concrete mix, promoting carbonation and generating nano-sized CaCO₃ as nucleation sites. This approach catalyzes early-age hydration and ensures a more uniform dispersion of nanoparticles throughout the concrete mixture. Carbonation curing, widely used in the precast concrete industry, accelerates production and improves equipment turnover by simulating carbonation reactions during the curing phase.

Carbonation of Binders

Accelerated carbonation is applied to cement composites in two key ways: pre-carbonating industrial wastes before mixing and early-stage carbonation curing of cement composites after mixing.

During early carbonation curing, cement hydration and carbonation processes occur simultaneously. Carbonation reactions can release bound water from hydration products, which in turn stimulates further cement hydration, potentially increasing the strength of the concrete. Additionally, the interaction between CO₂ and hydration products can alter the microstructure of fresh cement composites, influencing both their performance and the properties of the hardened concrete.

In the case of RCA concrete, carbonation adjustment and carbonation curing are widely used. Carbonation adjustment involves introducing CO₂ into RCA within a sealable carbonation chamber, while carbonation curing involves reacting cement paste with CO₂ and applying it to the entire concrete block after mixing. Although RCA carbonation can be carried out before mixing, carbonation curing is limited to post-mixing, restricting its application before casting.

Commercialized Technologies

Over the past two decades, numerous companies have successfully reduced their carbon footprint while advancing CO₂ utilization technologies in cement concrete production. For instance, CarbonCure, based in Nova Scotia since 2007, has developed an innovative method of injecting small amounts of CO₂ directly into fresh concrete mixes during production.

In the United States, Solidia Technologies, founded in 2008, provides green solutions by leveraging CO₂ to produce eco-friendly building materials. Its approach combines low-carbon processes, carbon mineralization, and operational efficiency to enhance concrete performance, boost supply chain resilience, and reduce the carbon intensity of concrete.

CarbiCrete, headquartered in Montreal, uses a novel method of replacing cement with blast furnace slag from steel mills as a binder in concrete. Similarly, Novacem, established in the United Kingdom in 2007, has been working on negative carbon cement technology using magnesium silicate. Heidelberg Materials, based in Germany since 1990, collaborates with partners and customers to develop innovative, future-oriented building materials.

Overall, the integration of CO₂ utilization in cement concrete production has gained significant traction in the last decade, yielding substantial environmental benefits. The remaining challenges can be addressed through comprehensive life cycle analyses and economic evaluations of these technologies.

Journal Reference

Lu, D. et al. (2024). Innovative Approaches, Challenges, and Future Directions for Utilizing Carbon Dioxide in Sustainable Concrete Production. Journal of Building Engineering, 110904. DOI: 10.1016/j.jobe.2024.110904, https://www.sciencedirect.com/science/article/pii/S2352710224024720

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