# Pathway to Carbon Neutrality in the Cement Industry: CO2
## 2 Research Significance  
### Environmental Parameters Affecting Carbonation Reaction  
The carbonation of RCAs is mainly affected by relative humidity, temperature, CO\({}_{2}\) concentration and CO\({}_{2}\) pressure. In the carbonation reaction, the CO\({}_{2}\) is first dissolved (solvation) in water, forming carbonic acid. Then, the carbonic acid is dissociated into hydrogen ions and bicarbonate ions, which will react with calcium cations forming calcium carbonate [13; 14]. However, a high water content leads to the saturation of capillary pores, reducing the diffusion rate of CO\({}_{2}\) and, hence, the degree of carbonation [14]. Therefore, there is an optimal range of relative humidity (RH) in which the carbonation rate is maximized. From the literature, the maximum carbonation rate of concrete is reported at 50-70% RH [16; 17; 18]. In fact, various studies point to circa 60% of relative humidity as an ideal value [19; 20]. Below 50% RH, the carbonation reaction is unlikely to occur [21], and above 70%, the small pores are likely to be filled with water, preventing carbonation from occurring [22; 23]. Taking into account a wide range of concrete strength classes, [PERSON] et al. [20] and [PERSON] et al. [18] found that maximum carbonation rates are obtained at 65-70% RH. According to the results of [PERSON] et al. [20], this optimal range was independent of the strength class of concrete. The maximum carbonation depths were achieved at 70% RH, for a temperature of 20 \({}^{\circ}\)C and a CO\({}_{2}\) concentration of 20%. Naturally, concretes of lower strength classes, with higher porosity and less carbonatable products, presented the highest carbonation depth [20]. A similar trend was reported by [PERSON] et al. [18], for concretes with CEM I 42.5 N and CEM III/ A 42.5 N, with and without silica fume. [PERSON] et al. [24] evaluated the forced carbonation of RCAs under 5%, 50% and 95% relative humidity (RH) and 100% CO\({}_{2}\) concentration. It was found that the highest CO\({}_{2}\) capture of RCAs was obtained at 50% RH. In this case, the CO\({}_{2}\) capture after 24 h was around 12 kg per 1 tonne of RCAs. The authors have evidenced that the carbonation reaction did not occur at 5% nor at 95% relative humidity. Temperature affects carbonation and hydration reactions. In the work of [PERSON] et al. [20], the effect of temperature was investigated between 10 \({}^{\circ}\)C and 30 \({}^{\circ}\)C on the carbonation process of cement pastes. The authors reported that the carbonation depth of concrete increased with temperature, i.e., a higher carbonation rate as temperature increases [25]. [PERSON] et al. [26] developed an elementary model to predict the carbonation of mortars under various CO\({}_{2}\) partial atmospheric pressures, namely 0.03%, 10%, 25% and 50%. Three types of normalized mortars at seven exposure times were considered. The results of the X-ray diffraction (XRD) analysis indicated that the carbonation of portlandite, ettrinigite and hydrated calcium aluminates were extensively carbonated at low CO\({}_{2}\) pressure. However, it was found a significant reduction in the porosity of the carbonated zone when higher pressures were implemented, meaning a higher carbonation degree. It was also found that the C-S-H carbonation rate is widely dependent on the pressure. [PERSON] et al. [27] studied the C-S-H and cement paste carbonation at different air pressure (0.1, 0.15 and 0.2 MPa). It was concluded that, under the same carbonation time, the carbonation depth increased with the air pressure. [PERSON] et al. [28] studied the influence of temperature on the forced carbonation speed of lime, considering the effect of different CO\({}_{2}\) pressures (above atmospheric values). It was concluded that with the increase in pressure the effect of temperature in the carbonation reaction becomes less relevant. Nevertheless, the carbonation rate increased with temperature.