# Pathway to Carbon Neutrality in the Cement Industry: CO2
###### Abstract  
Cementious materials can capture CO2 through carbonation reaction during their service life and post-demolition. Indeed, construction and demolition waste (CDW) still have some potential for carbonation as they contain concrete and cement-based mortars. This research consists of an experimental programme to evaluate the CO2 capture of recycling aggregates (RAs) from CDW. Two types of CDW were studied, namely mixed recycled aggregates (MRAs) and recycled concrete aggregates (RCAs). The recycled aggregates were submitted to forced and accelerated carbonation at 23 \({}^{\circ}\)C, 60% relative humidity and 25% of CO2 concentration. This study contributes to the existing literature by investigating more realistic RA sources that have already absorbed atmospheric CO2 during their service life. From the experimental campaign, the results show that RCAs have higher carbonation potential when compared to MRAs due to the higher cementitious material content (Rc) and to the degree of natural carbonation. The recycled aggregates' maximum CO2 capture was assessed by thermogravimetric analysis (TGA) at different CO2 exposure times. It was verified that the maximum CO2 capture, respectively, for MRAs and RCAs, occurred after 5 h and 12 h of exposition. In short, CDW captured from 5 wt.% to 35 wt.% of CO2 per tonne of cement paste, which corresponds to 0.6% to 4.1% per tonne of aggregate. It was concluded that the carbonation process of CDW has the potential to sequester from 123 kg to 225 kg of CO2 per tonne of cement paste for MRAs and 52 up to 491 kg of CO2 per tonne of cement paste for RCAs. CO2 uptake; carbonation reaction; construction and demolition waste; recycled aggregates; carbonated aggregates; cement paste; TGA +
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of calcium carbonate is dominant in these materials (Equation (1)), and it is an exothermic reaction [4]. The formation of CaCO\({}_{3}\) within the pores of the cement matrix causes an increase in density and a decrease in water absorption [5; 6]. In the presence of water and CO\({}_{2}\), carbonic acid is formed (Equation (2)), decreasing the pH of the interstitial solution and causing the release of Ca\({}^{2+}\) ions from hydrated cement compounds, leading to the formation of CaCO\({}_{3}\) (Equation (3)). The reaction with CO\({}_{2}\) also occurs with other hydrated phases of the cementitious material. At the limit, the carbonation of calcium silicate hydrate (C-S-H) leads to the formation of calcium carbonate and amorphous silica gel (Equation (4)). Other hydrated compounds like ettrinigite and calcium aluminates can be carbonated as well (Equations (5)-(9)) [7; 8; 9; 10; 11; 12; 13; 14; 15]. Ettrinigite in contact with CO\({}_{2}\) forms calcium carbonate, gypsum and alumina gel. In addition, anhydrous compounds, like tricalcium silicate (alite) and dicalcium silicate (belite), can also be carbonated, producing calcium carbonate and, at the limit, silica gel. \[Ca(OH)_{2}+CO_{2}\to CaCO_{3}+H_{2}O \tag{1}\]  
\[H_{2}O+CO_{2}\to H_{2}CO_{3} \tag{2}\]  
\[H_{2}CO_{3}+Ca(OH)_{2}\to CaCO_{3}+2H_{2}O \tag{3}\]  
\[C-S-H+CO_{2}\to CaCO_{3}+SiO_{2}\cdot\alpha H_{2}O \tag{4}\]  
\[3 CaO.Al_{2}O_{3}.CaSO_{4}.32H_{2}O+32H_{2}CO_{3}\to 3 CaCO_{3}+2 Al_{2}(OH)_{3} \cdot 3 CaSO_{4}.2H_{2}O+23H_{2}O \tag{5}\]  
\[CAH_{10}+CO_{2}\rightarrow\ CaCO_{3}+AH_{3}+7H_{2}O \tag{6}\]  
\[C_{3}AH_{6}+3 CO_{2}\rightarrow\ 3 CaCO_{3}+AH_{3}+3H_{2}O \tag{7}\]  
\[3 CaO.SiO_{2}+H_{2}O+3 CO_{2}\to 3 CaCO_{3}+SiO_{2}+H_{2}O \tag{8}\]  
\[2 CaO.SiO_{2}+H_{2}O+2 CO_{2}\to 3 CaCO_{3}+SiO_{2}+H_{2}O \tag{9}\]  
Utilizing Construction and Demolition Waste (CDW) for CO\({}_{2}\) sequestration can contribute to mitigating greenhouse gas emissions from the cement industry and also enhance the RA properties. This research aims to investigate the carbonation of several types of recycled aggregates (RAs) from different sources: recycling plants, in situ demolitions, and concrete industries (waste).