## 3 Results and Discussion  
### Oxygen Permeability  
The graphs in Figure 3 of the 28 d and 90 d O\({}_{2}\) permeability coefficient and standard deviation (error bars) values denote a decline in permeability with curing age, irrespective of the GRC and MRA content. At the same time, the incorporation of the new recycled components induced a rise in O\({}_{2}\) permeability due to (i) the higher porosity of the new cement matrices, attributable to the dilution due to the presence of non-reactive particles in GRC (see item 2.1.1) and (ii) the greater porosity of MRA than NA, induced by the bound mortar and masonry materials found in the former (Table 2). Those findings were consistent with, and within, the range (1.0 \(\times\) 10\({}^{-17}\) to 8.0 \(\times\) 10\({}^{-17}\)) reported for mixes prepared with 50% RCA [35], with 25% masonry aggregate [36], and mixes with 20% masonry waste (in the form of fired clay brick, gloss- or bisque-fired white stoneware or sanitary ware) used as a cement substitute [8]. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multicolumn{1}{c}{**Material**} & \multicolumn{5}{c}{**Proportion (\% or kg/m\({}^{3}\))**} \\ \hline
**Concrete Mix** & **NAC** & **N10/0** & **N25/0** & **R0/50** & **R10/50** & **R25/50** \\ \hline MRA replacement & & 0\% & & & 50\% & \\ GRC replacement & 0\% & 10\% & 25\% & 0\% & 10\% & 25\% \\ OPC & 300 & 270 & 225 & 300 & 270 & 225 \\ GRC & - & 30 & 75 & - & 30 & 75 \\ Total water & 168 & 174 & 180 & 205 & 211 & 217 \\ [w/b(OPC+GRC)] & 0.56 & 0.58 & 0.60 & 0.59 & 0.61 & 0.63 \\ NS-F & 154 & 150 & 154 & 154 & 154 & 154 \\ NS-C & 755 & 755 & 755 & 755 & 755 & 755 \\ NG-M & 367 & 367 & 367 & 184 & 184 & 184 \\ NG-G & 653 & 653 & 653 & 327 & 327 & 327 \\ MRA 16-22 mm & - & - & - & 109 & 109 & 109 \\ MRA 11.2–16 mm & - & - & - & 182 & 182 & 182 \\ MRA 8–11.2 mm & - & - & - & 85 & 85 & 85 \\ MRA 5.6–8 mm & - & - & - & 59 & 59 & 59 \\ MRA 4–5.6 mm & - & - & - & 14 & 14 & 14 \\ \multicolumn{1}{c}{**Fresh and hardened concrete properties (\(\pm\): standard deviation).**} \\ \hline
**Slump (mm)** & 65 \(\pm\) 2.8 & 74 \(\pm\) 2.5 & 65 \(\pm\) 3.7 & 75 \(\pm\) 3.1 & 61 \(\pm\) 3.7 & 63 \(\pm\) 4.2 \\
**Density (kg/m\({}^{3}\))** & 2367 \(\pm\) 8 & 2340 \(\pm\) 9 & 2309 \(\pm\) 10 & 2251 \(\pm\) 11 & 2244 \(\pm\) 12 & 2219 \(\pm\) 10 \\
**Strength class 1** & C35/45 & C30/37 & C20/25 & C25/30 & C25/30 & C12/15 \\
**P\({}_{0,28}\) a (vol%) 2** & 14.0 \(\pm\) & 14.8 \(\pm\) 0.2 & 15.6 \(\pm\) 0.4 & 16.3 \(\pm\) 0.4 & 17.4 \(\pm\) 0.5 & 18.6 \(\pm\) 0.4 \\
**P\({}_{0,90}\) a (vol%) 3** & 13.3 \(\pm\) & 14.2 \(\pm\) 0.2 & 15.0 \(\pm\) 0.4 & 14.9 \(\pm\) 0.3 & 16.3 \(\pm\) 0.4 & 17.5 \(\pm\) 0.3 \\ \hline \hline \end{tabular} Note. 1 Strength class in EC-2 is designated CX/Y, where X is the characteristic 28 day compressive strength value in 15 \(\times\) 30 cm cylindrical specimens and Y the characteristic compressive strength in 15 \(\times\) 15 cm cubic specimens. 2\({}^{3}\) Po, 28 d/90 d: 28 d/90 d open porosity (porosity accessible to water).  
\end{table} 
Table 4: Concrete mixes’ compositions. The use of GRC raised 28 d O\({}_{2}\) permeability by 47.0% in mix N10/0 and 97.3% in mix N25/0 relative to NAC, and by 25.7% in mix R10/50 and 51.9% in mix R25/50 relative to R0/50. These increases were less steep at 90 d: 7.5% in mix N10/0 and 84.1% in mix N25/0 relative to NAC, and 14.9% in mix R10/50 and 47.8% in mix R25/50 relative to R0/50. Analogously, the use of 50% MRA (mix R0/50) raised 28 d O\({}_{2}\) permeability by 92.7% and the 90 d value by 67.5% relative to the conventional mix with NA. That rise was primarily associated with the effect of the presence of bound mortar and masonry material in MRA (more porous than NA) on O\({}_{2}\) permeability and the more readily accessible ingress pathways generated by microcracks in the MRA microstructure [37; 38].