Decarbonisation Technology - November 2023 Issue

Initial concrete

Concrete after natural carbonisation. External porosity is blocking access of CO

Increased carbonation of concrete due to accelerated carbonation

Carbonation using in - situ CO release

Figure 2 Representation of changes in the porosity of concrete with different types of carbonation, natural, accelerated chamber, and in-situ carbonation

CO₂ releasing material was implemented in concrete with 10 wt% cement, this could enable the sequestration of 15 kg of CO₂ per tonne of concrete. If all the 30 Gt of concrete produced yearly were replaced with a mixture able to accomplish this, 0.45 Gt of CO₂ could be sequestered annually. Even if modest sequestration like this were achieved, it would reduce cement-related emissions by 17.3%. Combined with the predicted 50% reduction from electrification, this would reduce GHG emissions by 67.3%. It is estimated that 3 billion tonnes of construction and demolition waste is produced annually (Pu, et al., 2021). It is possible to recover a portion of the cementitious material from this waste and reuse it in the production of fresh concrete. When this is performed, the materials are widely referred to as recycled concrete aggregate (RCA). However, when RCA is used in concrete production, significant issues are often present, such as high energy and cost, micro-cracking, and production of waste solutions/fine aggregates. This is where controlled carbonation comes into play. This RCA has the same chemical composition as fresh concrete, meaning the RCA can permanently sequester CO₂ through mineralisation following the same reaction pathway. Furthermore, carbonated RCA can mitigate some of the existing problems when used in fresh concrete due to reduced water absorption and increased stability to leaching. In terms of carbon sequestration, plain crushed concrete was reported to absorb ~11 kg CO₂ per tonne, and RCA (5-20 mm) sequestered 7.9 kg of CO₂ per tonne (Xuan, et al., 2016), (Kikuchi, et al., 2011). However, using carbonated RCA also avoids the emissions from the displaced aggregate.

One study produced 1 m 3 concrete blocks with reportedly 5.53% lower GHG emissions due to the 43% RCA content (Guo, et al., 2018). If the carbon sequestration was also considered for carbonated RCA in this instance, it would have reduced the GHG emissions by a further 2.28%. When the quality of RCA is not good enough, it can still be used as down-cycled material in floor slap applications. Conclusion Concrete is a fundamental material in the construction industry. Concrete production poses many environmental challenges, with direct carbon emissions arising from not only extracting raw materials but also the calcination of limestone. In this article, carbon sequestration by concrete is discussed as a potential mitigation pathway to reduce emissions from concrete production. Carbon sequestration in concrete by controlled carbonation using controlled carbonation chambers and/or carbon-rich materials to produce concrete may be a viable strategy. It is reasonable to state that by acting as a carbon sink during the use stage, concrete can partially self-offset the carbon emissions generated during the manufacturing stage. There are also potential applications in carbonating end-of-life concrete and reusing it as a filler material to further increase carbon sequestration.

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Gareth Davies garethdavies@tunley-environmental.com Luan Ho luanho@tunley-environmental.com