Decarbonisation Technology - November 2023 Issue

can cause rusting of steel. In addition, natural carbonation can also cause the cracking of concrete and build-up of harmful chloride- based compounds (Xu, et al., 2022). Therefore, different processes for controlled carbonation to sequester CO₂ into the cementitious material and improve the properties of the concrete are being researched. A key advantage of controlled carbonation is that once the concrete is sufficiently carbonated, further natural carbonation is limited. Consequently, the existing issues occurring from natural carbonation are also mitigated. Concrete quality is typically assessed in terms of compressive strength, curing time, workability, tensile strength, and modulus of elasticity. Research on the carbonation of cement also aims to identify the rate of carbonation and the amount of carbonation (often measured by depth of carbonation). The depth of carbonation is the distance (mm) from the surface to the concrete that the CO₂ has penetrated and been mineralised to form calcium carbonate. One of the controlled carbonation methodologies involves utilising a carbonation chamber to carbonate pre-cast materials (Hussain, et al., 2017). These accelerated carbonation chambers can be fitted to allow control over CO₂ concentration, humidity, and temperature. Notably, commercial carbonation chambers typically do not utilise pressurised CO₂ and only use a percentage concentration of CO₂ at atmospheric concentration. The main parameters affecting the rate and depth of carbonation and structure of final carbonised concrete are permeability (porosity), pozzolanic content (beyond the scope of this article), moisture (humidity), CO₂ concentration, temperature, and duration of exposure (Hussain, et al., 2017). Control over these parameters is essential as accelerated carbonation chambers aim to maximise carbonation depth. However, the formation of calcium carbonate in the exterior pores blocks further carbonation in the rest of the concrete. For example, the temperature of the carbonation chamber affects solubility and transport as well as the hydration of the cement phases (Xu, et al., 2022). Higher temperatures decrease the solubility of both calcium ion and CO₂, which retards the carbonation reaction. However, the transport of

substances and the amount of available calcium hydrates for carbonation increases at higher temperatures. Therefore, there is an optimal temperature for carbonation to give a high carbonation rate and large carbonation depth. This optimum temperature depends on the cement composition, with temperatures varying between 0°C and 100°C. One significant observed advantage of this is that carbonation causes the accelerated curing of concrete. One study found that concrete bricks, which originally required 20 hours of steam curing, only needed two hours of carbonation curing to have comparable compressive strength (Liu, et al., 2022). Li et al. investigated the carbonation of cured concrete blocks in an accelerated carbonation chamber (Li, et al., 2019). They identified CO₂ uptake after 28 days at 15.8%. Additionally, the carbonated concrete blocks had a 10% higher compressive strength than the control moisture-cured concrete. Beyond this, the carbonated concrete blocks also had higher abrasion resistance and chloride ion permeability. Another emerging method of carbon sequestration in concrete is embedding CO₂-rich materials within the concrete mixture (Stefaniuk, et al., 2023). These materials can then release the CO₂ during the curing of the concrete, which can react with calcium hydrate salts to form calcium carbonate immediately. This process avoids the challenge of achieving the required CO₂ penetration depth in the concrete for sufficient mineralisation. However, it introduces many new challenges. One recent investigation by MIT incorporated sodium bicarbonate, which dissolved in the water and released the CO₂ slowly over time. This CO₂ then readily reacted with the calcium silicate hydrates in situ to create an extended structure of calcium carbonate with reduced porosity (see Figure 2 ). Another advantage of this process is that it mitigates the harmful consequences of late- stage carbonation, such as shrinkage and cracking. Early results demonstrate this method of carbonation to be very effective at carbon sequestration, with CO₂ incorporation listed at 15 wt%. This is based on TGA decomposition profiles of the cured cementitious material. Cement is typically used at 10-15 wt% in concrete, depending on the final properties required. If sodium hydroxide or a similar