The sequestration of CO2 in concrete and artificial aggregates through a process known as carbonation could be a key factor in the development of a sustainable construction industry.

Carbonation is a natural process involving the reaction of carbon dioxide (CO2) with alkali materials containing calcium or magnesium. These can be naturally occurring or manmade, such as Portland cement, concrete and various industrial by-products.

In nature, it occurs over geological timescales and is responsible for the formation of limestone (CaCO3) and dolomite (MgCO3). Recent development has focused on speeding up the reaction in various applications as a method of sequestering CO2. Potential applications include:

  • The manufacture of artificial aggregates.
  • Curing concrete.
  • Carbonation of recycled concrete.

Artificial aggregates

Placing natural minerals or (ideally) industrial/construction waste materials in high concentrations of CO2 and optimum reaction conditions (e.g. high temperature and pressure) produces artificial carbonates that can be used in as aggregate in the construction industry.

Originally pioneered as a method of carbon sequestration by Novacem, using natural magnesium-silicon rocks, a number of companies have developed processes to produce a manufactured limestone from industrial and construction waste streams that is suitable for construction applications, including Blue Planet in the US and Carbon8 Systems in the UK.

Although these applications remain relatively small, the use of carbonation in the production of artificial building material has significant potential as an effective measure for carbon capture and utilisation, provided sufficient quantities of feedstock and CO2, as well as a market for the products.  

Carbonation of concrete

The carbonation of concrete has traditionally been considered undesirable, due to its potential to increase the risk of concrete reinforcement corrosion. Durability of reinforced concrete structures remains of primary importance and proper design and workmanship to prevent corrosion must not be compromised.  However, in parallel, the fact that concrete does absorb CO2 is of interest from an environmental perspective.  The ability of concrete to absorb CO2 offers a potentially important method of reducing the construction industry’s carbon footprint.

Two areas are the subject of research: firstly, the curing of concrete in CO2-rich environments and secondly the carbonation of concrete structures through their lifetime – and particularly at end of life.

Carbon curing

Both traditional binders (e.g. lime and Portland cement) and alternatives (such as wollastonite) cure quickly in moist CO2-rich environments. The concretes formed have shown equivalent or higher performance in terms of resistance to freeze-thaw cycling, sulfate attack, ASR expansion, and chloride corrosion. Meanwhile, the development of fibre-reinforced polymer (FRP) concrete reinforcement has the potential to negate the risk posed to steel-based reinforcement.

Currently suitable for only precast applications with large surface areas and shallow depths, further development is required before the technology is suitable for more widespread deployment. It does however present a cost-effective solution for carbon sequestration, with the potential to recover much of the CO2 produced during the manufacture of cement.

Carbonation of recycled concrete

All concrete carbonates over its lifetime: according to one estimate, 4.5 billion tonnes of CO2 have been sequestered in concrete between 1930 and 2013, offsetting [43%] of the CO2 produced by the cement industry. Much of this occurs during demolition, when more of the concrete is exposed.

With increased recycling of concrete waste being encouraged in a number of regions, there is the opportunity to enhance carbonation at demolition through optimised handling and processing. Both crushing and grading the concrete have been shown to facilitate CO2 uptake, while also creating a more usable product for onward use (e.g. as a base course in road construction).

The properties of the concrete also play a role in its ability to absorb carbon. Higher water-to-cement ratios in the original concrete have been shown to positively impact the rate of carbonation. Other parameters controlling carbonation include humidity, porosity, temperature, binder content, particle size, partial pressure of CO2, and the present and type of supplementary cementitious materials.

Whether the carbonatability of a concrete at demolition should be considered when specifying at construction is a question that may need to be addressed by the industry in future, particularly when it comes into conflict with other construction trends, e.g. the requirement for more durable concrete made with low water-to-cement ratios. But as concrete is recycled in ever larger amounts, its uptake of carbon should be an important element in assessing its sustainability.