## 4 Conclusions  
The conclusions that may be drawn from this study are set out below:  
Figure 6: Service life model proposed in Spanish concrete code EHE-08 for carbonation spread in concrete mixes. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{6}{c}{**Natural Carbonation Coefficient, \(K_{n}\) (mm/year\({}^{0.5}\))**} \\ \hline NAC & N10/0 & N25/0 & R0/50 & R10/50 & R25/50 \\ \hline
2.07 & 2.37 & 4.28 & 3.20 & 3.46 & 4.50 \\  & & **Carbonation Resistance Class** & & & \\ \hline RC2 & RC3 & RC4 & RC5 & RC6 & RC7 \\ \hline
1 \textless{} Kn \(\leq\) 2 & 2 \textless{} Kn \(\leq\) 3 & 3 \textless{} Kn \(\leq\) 4 & 4 \textless{} Kn \(\leq\) 5 & 5 \textless{} Kn \(\leq\) 6 & 6 \textless{} Kn \(\leq\) 7 \\  & & R0/50, & N25/0, & & \\  & NAC, N10/0 & R10/50 & R25/50 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Natural carbonation coefficients and carbonation resistance classes. * Incorporating GRC and/or MRA induces an increase in O2 permeability associated with the greater porosity of these new materials than that found in their conventional counterparts, OPC and NA. Nonetheless, all the O2 permeability coefficients were below the \(4.5\times 10^{-17}\) m2 ceiling for quality concrete;
* CI permeability was not significantly affected (<8%) by the replacement of 10% OPC with GRC, irrespective of the aggregate type present (NA or MRA). The resulting 90 d mixes exhibited the same'moderate' risk of corrosion as conventional concrete of the same age;
* The high linear correlation between the O2 permeability and CI' diffusion coefficients, irrespective of cement and aggregate type, can be interpreted as proof that these transport mechanisms are governed by both pore structure and interconnectivity;
* The mean carbonation depth in mixes with 10% GRC, separately or jointly with 50% MRA, was 15% to 75% greater than that in mixes prepared with conventional cement and natural aggregate;
* All mixes, irrespective of the GRC replacement ratio and aggregate type, exhibited CO2 penetration coefficients lower than or similar to 4 mm/year[9, 5];
* The high correlation between O2 permeability and carbonation depth suggests that the former may be a good indicator for predicting the latter;
* According to the service life prediction model proposed in Spanish structural concrete code EHE-08, incorporating up to 10% GRC as a cement replacement and/or 50% MRA as an NA substitute does not compromise the reinforcement's passivity. [PERSON] performed the experiments; [PERSON] and [PERSON] analysed the data and wrote the paper; [PERSON], [PERSON] and [PERSON] supervised the research work and revised the paper. All the authors contributed to the experiment design and analysed and discussed the findings. All authors have read and agreed to the published version of the manuscript. This research was funded by Spanish Ministry of Science, Innovation and Universities mobility grant ETS18/00313, awarded in connection with pre-doctoral university professor scholarship 16/02693, under which [PERSON] conducted research at the Lisbon University Instituto Superior Tecnico construction laboratory. European Regional Development Fund (ERDF) funding was also received for an Interreg-POCTEP research grant (0008_ECO2 CIR_4_E, 'Project for cross-border cooperation to introduce the eco- and circular economy by reducing waste generation and enhancing waste recycling, management and valorisation in Central Spain, Extremadura and Alentejo) and, in conjunction with the Government of Extremadura, under grant GR 18,122 awarded to the MATERIALS research group. Data Availability StatementData sharing not applicable. This study benefitted from the Spanish Ministry of Science, Innovation and Universities mobility grant ETS18/00313, awarded in connection with pre-doctoral university professor scholarship 16/02693, under which [PERSON] conducted research at the Lisbon University Instituto Superior Tecnico construction laboratory.