Carbon-absorbing concrete, also known as carbon capture and utilization (CCU) concrete, is an innovative material designed to reduce the environmental impact of the construction industry by actively absorbing carbon dioxide (CO₂) during its production or lifecycle.
The cement industry contributes approximately 8% of global CO₂ emissions, and this technology offers a sustainable solution by sequestering CO₂ and lowering emissions.
Below is a detailed explanation of the principles, mechanisms, and technologies behind carbon-absorbing concrete, supported by credible international sources.
Principles of Carbon-Absorbing Concrete
1. Natural Carbonation Process
- Mechanism: Concrete naturally absorbs CO₂ from the atmosphere through carbonation, a chemical reaction where CO₂ reacts with calcium hydroxide (Ca(OH)₂) or other calcium compounds in the cement paste to form calcium carbonate (CaCO₃).
The reaction is:
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
This process sequesters CO₂ over the concrete’s lifetime, typically decades, and enhances durability by filling pores with calcium carbonate, reducing permeability and improving resistance to environmental degradation (e.g., sulfate or chloride attack).
Impact: Research indicates that concrete can absorb 5–10% of the CO₂ emitted during its production through natural carbonation, depending on factors like surface exposure and concrete composition.
2. Accelerated CO₂ Curing
- Mechanism: To significantly increase CO₂ absorption, accelerated carbonation involves injecting captured CO₂ into concrete during the curing process. CO₂ reacts with calcium-based compounds, such as calcium silicates (CaSiO₃), to form calcium carbonate and silica:
CaSiO₃ + CO₂ → CaCO₃ + SiO₂
This reaction strengthens the concrete by creating a denser microstructure, often increasing compressive strength by 10–30% compared to traditional water-based curing. - Technologies:
- Solidia Technologies: Solidia produces a low-carbon cement using wollastonite (calcium silicate) instead of limestone-based clinker, reducing CO₂ emissions from production by up to 30%. During curing, CO₂ is injected into precast concrete products, where it mineralizes into calcium carbonate, permanently storing CO₂. This process reduces the carbon footprint by up to 70% compared to traditional Portland cement concrete.
- Carbicrete: Carbicrete eliminates cement by using steel slag, a byproduct of steel production, as a binder. CO₂ is injected during curing, reacting with calcium compounds in the slag to form calcium carbonate, binding the concrete. This results in a carbon-negative product, as the CO₂ absorbed exceeds emissions from production.
3. Use of Supplementary Cementitious Materials (SCMs)
- Mechanism: SCMs, such as ground granulated blast furnace slag (GGBS) and fly ash, are used to partially replace Portland cement, reducing CO₂ emissions by 50–80% due to their lower energy requirements. These materials contain reactive silica and alumina, which form calcium silicate hydrates (C-S-H) during hydration, similar to cement, and enhance CO₂ sequestration during carbonation.
4. Structural and Environmental Benefits
- Enhanced Durability: Carbonation reduces concrete porosity by forming calcium carbonate, improving resistance to chemical attacks and extending service life, which is particularly valuable for infrastructure like bridges and buildings.
- Reduced Emissions: Technologies like Solidia’s lower emissions by reducing energy use in cement production and incorporating CO₂ during curing. Carbicrete’s cement-free process eliminates cement-related emissions entirely.
- Carbon Neutrality or Negativity: In optimal conditions, carbon-absorbing concrete can achieve carbon neutrality (emissions equal absorption) or carbon negativity (absorption exceeds emissions). Carbicrete’s products, for instance, are carbon-negative due to high CO₂ uptake during curing.
6. Challenges and Limitations
- Applicability: CO₂-curing technologies are currently limited to precast concrete products (e.g., blocks, panels) produced in controlled environments. Applying these methods to ready-mix concrete for on-site construction is challenging due to logistical issues with CO₂ delivery and curing control.
- Scalability: Widespread adoption requires significant investment in CO₂ capture, transport, and injection infrastructure, as well as regulatory frameworks to certify carbon-negative concrete.
- Performance Trade-offs: High SCM replacement rates can reduce early-age strength, potentially affecting structural applications. Accelerated carbonation can mitigate this by enhancing early strength, but careful mix design is required.
7. Conclusion
Carbon-absorbing concrete leverages natural carbonation, accelerated CO₂ curing, and low-carbon materials like SCMs to sequester CO₂, enhance material performance, and reduce emissions.
This technology represents a significant step toward sustainable construction, with the potential to transform the industry. Continued advancements in scalability, infrastructure, and regulatory support are essential to maximize its impact.
Sources
- Xi, F., Davis, S. J., Ciais, P., et al. (2016). "Substantial global carbon uptake by cement carbonation." Nature Geoscience, 9(12), 880–885. doi: 10.1038/ngeo2840.
- Monkman, S., & MacDonald, M. (2017). "On carbon dioxide utilization as a means to improve the sustainability of ready-mixed concrete." Journal of Cleaner Production, 167, 365–375. doi: 10.1016/j.jclepro.2017.08.194.
- Zhang, D., Ghouleh, Z., & Shao, Y. (2017). "Review on carbonation curing of concrete: Mechanism, performance, and implementation." Construction and Building Materials, 155, 870–883. doi: 10.1016/j.conbuildmat.2017.08.116.