## 3 Results and Discussion
### Effect of Curing Time on Mechanical and Physical Behavior
The results demonstrated that accelerated carbonation is effective in improving the physical properties because of pore filling with carbonate products resulting in a decrease of apparent void volume (AVV) by 2.9% and 6.5%, a decrease of water absorption (WA) by 4.2% and 9%, and an increase of bulk density (BD) by 1.2% and 2.5% after 5 and 9 h carbonation time at 28 days, respectively. This is again the consequence of the increase of CaCO\({}_{3}\) in the composites, which reduces the porosity due to the higher molar volume of CaCO\({}_{3}\) compared to Ca(OH)\({}_{2}\)--the main carbonated phase in the cementitious matrix [40]. However, it was noticed that the precipitation of CaCO\({}_{3}\) crystals has a negative role on the carbonation degree due to pore clogging. The porosity decrease (represented by AVV in this study) and bulk density increase have also been observed by many researchers such as in the study of [PERSON] et al. [41], who showed a reduction of 15% in porosity and approximately 6.5% increase in the bulk density of the composites subjected to accelerated carbonation for 14 days. The bulk density evolution (\(W\), calculated as relative weight change in %) provides a quantitative estimation of the amount of CO\({}_{2}\) uptake during carbonation. The weight increase in this study was 1.2% and 2.5% (as shown earlier) after 5 and 9 h carbonation under semi-adiabatic conditions, respectively. If only portlandite is carbonated, we can estimate the CO\({}_{2}\) uptake as follows:  
\[\text{Ca(OH)}_{2}+\text{CO}_{2}\rightarrow\text{CaCO}_{3}+\text{H}_{2}\text{O} \tag{7}\]  
Figure 4: Average values of physical properties with different carbonation time (\(\times\) NCC, \(\square\) 3 h ACC, \(\blacklozenge\) 5 h ACC and \(\Delta\) 9 h ACC): (**a**) Apparent void volume (AVV); (**b**) bulk density (BD) and (**c**) water absorption (WA). \[\mathrm{Uptake}_{\mathrm{CO_{2}}}=\left(\frac{\mathrm{M}_{\mathrm{CO_{2}}}}{\mathrm{ M}_{\mathrm{CaCO_{3}}}-\mathrm{M}_{\mathrm{Ca(OH)_{2}}}}\right)\times W \tag{8}\]  
where \(\mathrm{M}_{\mathrm{CO_{2}}}\), \(\mathrm{M}_{\mathrm{CaCO_{3}}}\), and \(\mathrm{M}_{\mathrm{Ca(OH)_{2}}}\) (g/mol) denote the molar weight of CO\({}_{2}\), CaCO\({}_{3}\), and Ca(OH)\({}_{2}\), respectively. The CO\({}_{2}\) uptake is then 2.0% and 4.2% for 5 and 9 h carbonation curing, respectively. [PERSON] et al. [23] showed that the CO\({}_{2}\) uptake in the composites subjected to 4 h and 10 h of accelerated carbonation in a climate chamber at around 60 \({}^{\circ}\)C and 90% relative humidity (RH) was 10% (equivalent to around 0.5 kg of CO\({}_{2}\) uptake/m\({}^{2}\) roof with 4 mm thickness) and 18% (equivalent to around 0.8 kg of CO\({}_{2}\) uptake/m\({}^{2}\) roof with 4 mm thickness), respectively. Those results are higher than our results because of the fact that they carbonated the samples at higher initial temperature and the RH in the chamber was maintained at 90%, while in our case, the temperature started at 25 \({}^{\circ}\)C and increased up to a maximum temperature of 60 \({}^{\circ}\)C in 3 h (as seen in Figure 5) and the RH may be much higher than 90% as water accumulated in the chamber during carbonation. Therefore, higher temperature could accelerate the hydration and carbonation, while lower RH could induce faster carbonation. Furthermore, our estimation does not take into account the carbonation of C-S-H [10] and other minor phases, which could result in an underestimation of CO\({}_{2}\) uptake.