## 3 Results and Discussion
### Carbonation Resistance  
The mean carbonation depths (Cd) of all the mixes after 7 d, 28 d, 56 d or 90 d in an accelerated carbonation chamber and associated standard deviations are listed in Table 6. Carbonation resistance was intensely affected by the porous microstructure of the new recycled concrete mixes with GRC and/or MRA. The use of GRC raised 28 d Cd by +0.8 mm in mix N10/0 and 5.7 mm in mix N25/0 relative to NAC and by 1.0 mm in mix R10/50 and 3.0 mm in mix R25/50 relative to R0/50. Similar findings were observed in mixes prepared with additions such as 5% to 25% marble slurry [53], 25% masonry CDW [12], and 25% to 50% fly ash from biomass-fired power plants [54]. The data in Table 7, in turn, denote the very close (correlation coefficients, R\({}^{2}\), of over 0.89) linear relationship between the Cd and GRC content, irrespective of exposure time and family (GRC-NA or GRC-MRA). The incorporation of 50% MRA (R0/50), in turn, induced an increase in Cd of 3.0 mm in the 28 d results, relative to the NAC mix. Such deeper carbonation penetration was associated with higher water absorption and porosity in MRA than in NA [6; 49]. This was the same pattern as observed in O\({}_{2}\) penetration discussed in Section 3.1, depicted in Figure 3b and observed by other authors [55; 56] in mixes with RA. The linear relationship in the 28 d and 90 d results between carbonation depth and oxygen permeability is depicted in Figure 5a, and between Cd and the chloride diffusion coefficient in Figure 5b. The high R\({}^{2}\) values (all lying between 0.895 and 0.965) denoted a close correlation between those properties and showed that both the O\({}_{2}\) permeability coefficient and the Cl\({}^{\ast}\) diffusion coefficient may serve to predict the carbonation depth [57; 58]. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{2}{*}{**Conctex Mix**} & \multicolumn{4}{c}{**Carbonation Depth (mm)**} & \multicolumn{4}{c}{**Carbonation Coefficient**} \\ \cline{2-7}  & **7 d** & **28 d** & **56 d** & **90 d** & **K\({}_{\text{ac}}\) (mm/year\({}^{\text{0.5}}\))** & **R\({}^{2}\)** \\ \hline NAC & 2.2 \(\pm\) 0.4 & 5.4 \(\pm\) 0.8 & 6.5 \(\pm\) 1.1 & 9.2 \(\pm\) 1.8 & 17.95 & 0.988 \\ N10/0 & 3.1 \(\pm\) 0.5 & 6.2 \(\pm\) 0.4 & 7.2 \(\pm\) 1.4 & 9.9 \(\pm\) 1.7 & 20.55 & 0.993 \\ N25/0 & 4.9 \(\pm\) 0.3 & 11.1 \(\pm\) 0.4 & 14.8 \(\pm\) 1.7 & 17.8 \(\pm\) 2.1 & 30.0.8 & 0.994 \\ R0/50 & 3.9 \(\pm\) 0.2 & 8.4 \(\pm\) 0.8 & 10.5 \(\pm\) 1.4 & 13.5 \(\pm\) 1.5 & 27.67 & 0.987 \\ R10/50 & 4.1 \(\pm\) 0.6 & 9.4 \(\pm\) 1.1 & 10.9 \(\pm\) 1.2 & 14.9 \(\pm\) 1.6 & 29.67 & 0.994 \\ R25/50 & 5.1 \(\pm\) 0.9 & 11.4 \(\pm\) 0.5 & 16.6 \(\pm\) 1.7 & 18.0 \(\pm\) 2.1 & 38.96 & 0.982 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Carbonation depth and coefficients in concrete mixes with 7 d, 28 d, 56 d or 90 d exposure to accelerated carbonation (\(\pm\), standard deviation). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Exposure Time (Days)**} & \multicolumn{4}{c}{**Type of Mix**} \\ \cline{2-7}  & \multicolumn{2}{c}{**GRC-NA**} & \multicolumn{4}{c}{**GRC-MRA**} \\ \cline{2-7}  & **m** & **a** & **R\({}^{2}\)** & **m** & **a** & **R\({}^{2}\)** \\ \hline
7 & 0.11 & 2.13 & 0.994 & 0.04 & 3.79 & 0.938 \\
28 & 0.24 & 4.81 & 0.924 & 0.12 & 8.32 & 0.994 \\
56 & 0.34 & 5.78 & 0.944 & 0.25 & 9.69 & 0.882 \\
90 & 0.35 & 8.11 & 0.892 & 0.18 & 13.34 & 0.990 \\ \hline \hline \end{tabular}  
* **Note.** Cd= mx + a, where Cd is mean carbonation depth; m, slope on the regression line; x, GRC replacement ratio; R\({}^{2}\), correlation coefficient.  
\end{table} 
Table 7: Correlation between mean carbonation depth and GRC replacement ratio. Earlier studies with mixes with additions such as fly ash, GGBFS [40,41] or BBA [46] as cement replacements [51,52] together with 25% to 100% RCA [59] yielded similar findings.