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 usage of CO₂ for producing low-carbon concrete. Different carbonation mechanisms of binders and other concrete components and the commercialized technologies employing these to produce sustainable concrete are discussed.

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 at moderate pressures, swiftly forming bicarbonate, hydrogen, and carbonate ions. These may interact with other components in cement concrete, such as binders or aggregates, to create various hydrates. Thus, cementitious materials are a foundational medium for absorbing and utilizing CO₂.

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 with cement composite manufacturing mainly through pre-carbonation of concrete ingredients, carbonation curing, and CO₂ mixing. Carbonation of concrete ingredients involves pre-treatment with CO₂ before mixing to initiate carbonation reactions, resulting in carbonated supplementary cementitious materials (SCMs), fillers, and aggregates. The CaCO₃ precipitated in this process can be incorporated into several binder systems.

Alternatively, CO₂ is introduced directly during the concrete mixing in the CO₂ mixing method to promote carbonation, generating nano-sized CaCO₃ as nucleation sites and catalyzing early-age hydration. This method allows for more uniform dispersion of nanoparticles in the concrete mix. Carbonation curing is popular in the precast concrete industry to speed production and ease equipment turnover by simulating carbonation reactions during the curing process.

Carbonation of Binders

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

Cement hydration and carbonation processes work together during the early carbonation curing process. Carbonation reactions can release bound water from hydration products, stimulating further cement hydration. This phenomenon can enhance the strength of the concrete material. Simultaneously, the reaction between CO₂ and hydration products can modify the microstructure of fresh cement composites, affecting their performance and hardened concrete’s properties.

Carbonation adjustment and carbonation curing are commonly employed to treat RCA concrete. While the former involves introducing CO₂ into RCA using a sealable carbonation chamber, the latter encompasses reacting cement paste with CO₂ and applying it to the entire concrete block after mixing. Additionally, while RCA can be conducted before concrete mixing, carbonation curing must be performed after mixing, limiting its use before casting.

Commercialized Technologies

Over the past two decades, several companies have successfully demonstrated carbon footprint reduction while promoting CO₂ utilization technologies in cement concrete production. For instance, CarbonCure, based in Nova Scotia since 2007, has developed an innovative method of injecting a small amount of CO₂ directly into green concrete mixes during production.

Solidia Technology, established in the United States in 2008, offers green solutions to produce environmentally friendly building materials using CO₂. It integrates low-carbon processes, carbon mineralization, and efficient operations to enhance concrete performance and supply chain resilience and reduce the carbon intensity of concrete.

CarbiCrete, headquartered in Montreal, has an innovative process of using blast furnace slag from steel mills as a binder for concrete instead of cement. Alternatively, United Kingdom-based Novacem has been dedicated to developing negative carbon cement technology since 2007 using magnesium silicate. Furthermore, Heidelberg Materials in Germany since 1990 collaborates with partners and customers to create future-oriented building materials.

Overall, the utilization of CO₂ in cement concrete production has gained significant momentum over the past decade, offering substantial environmental benefits. The existing challenges can be overcome through comprehensive life cycle analyses and economic assessments 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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