Decarbonisation Technology - November 2023 Issue

2018). Controlled carbonation of cementitious material was first proposed in the 1970s. However, it has recently gained attention due to the potential to mitigate against the significant GHG emissions from concrete production (Qin, et al., 2019). Between 1950 and 1990,

Civil acronym

Full chemical

Chemical formula

Percentage composition

name

C3S CS2 C3A

Tricalcium silicate Dicalcium silicate Tricalcium aluminate

3CaO·SiO 2 2CaO·SiO 2 3CaO·Al 2 O 3 4CaO.Al 2 O 3

61% 13% 8.9% 8.9% 8.2%

C4AF

Tetracalcium aluminoferrite

Gypsum

Calcium sulphate

CaSO 4

Table 1 Composition of ordinary Portland cement (CPA 32.5)

carbonation of steel-reinforced concrete was an area of major concern. The carbonation neutralised the alkalinity of the concrete and caused the steel to corrode. Hence, a qualitative method of analysing carbonation was developed by applying a phenolphthalein indicator to the concrete, which provides a measure of the pH of the concrete. It was identified that increasing the density of the concrete reduced the porosity and prevented significant carbonation. This change in the production of reinforced concrete was advised with the introduction of European standard EN 206-1 in 2000, enabling structures to have an expected service life of 50-100 years. However, the process of carbonation actually increases the strength of the concrete due to the formed calcium carbonate having a larger crystal size and occupying the unfilled porosity of the concrete (Hussain, et al., 2017). Calcium hydroxide (Ca(OH)₂) is the hydrate most readily reactive with CO 2 within hardened cement (Savija, et al., 2016). This readily reacts with CO₂ in the atmosphere to reform calcium carbonate (CaCO₃), as shown in Equation 2 . Other hydrated salts in the concrete can also uptake CO₂. If produced from calcium hydroxide or other calcium families, the produced calcium carbonate is air and temperature-stable and, therefore, will not release the trapped CO₂ back into the atmosphere. Cement has a theoretical maximum carbonation capacity of 50% (Lippiatt, et al., 2020). If the cement within concrete could be carbonated to this maximum capacity, this would sequester 2 Gt of CO₂ annually.

in Table 1 alongside the traditional nomenclature and full chemical name. It is fair to say that for most people who have laid concrete, water is added to the initial ready mix of cement, followed by the addition of aggregate to make a ‘wet’ cement, which then ‘dries’ to form solid concrete. This is a misnomer as the water does not evaporate from the mixture at all; if significant evaporation were allowed to occur, this would lead to weak, undesirable concrete. Instead, there is a chemical reaction between the compounds in the cement (see Table 1) and the water. This hydration reaction transforms dicalcium silicate and tricalcium silicate into calcium silicate hydrate and hydrated lime (also known as portlandite), as shown in Equation 1 . The hydration reaction is the essential process that enables cement to harden, and hence concrete to transform from a liquid paste into a hardened product suitable for construction: 2[2CaO ∙ SiO₂] + 4H₂O → 3CaO ∙ 2SiO₂ ∙ 3H₂O + Ca(OH)₂ Carbonation of concrete and cement Carbon sequestration directly into concrete or cementitious material is one of the potential mitigation pathways to offset (and reduce) the total GHG emissions of concrete production. While other pathways exist targeting the manufacturing stage (for example, carbon capture at the kiln and replacement of cement composition), this article focuses on carbon sequestration within concrete and cementitious materials. Essentially, the carbon sequestration process reverses the initial cement production (Eq 1) process where CO 2 is split from calcium carbonate (limestone), and is referred to as carbonation or mineralisation (Stefanoni, et al.,

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(Eq 2)

Carbonation of concrete, as a naturally occurring process, is slow and problematic when not controlled. As previously highlighted, natural carbonation of steel-reinforced concrete