## 3 Results and Discussion
### Characterization of Mortars Containing Cork Waste  
A good and homogeneous distribution of cork within mortars was detected for all the design mortar mixes through optical microscopy. A comparison between the reference mortar without cork (Figure 4a) and mortar with 4 wt% (Figure 4b) is reported as a representative comparison for the other mixes in Figure 4. In Figure 4b, the cork particles, recognizable by their characteristic color, are well distributed within the matrix and have both fine and coarse particles. Moreover, it can be observed that the maximum size of cork particles is smaller than that of natural aggregate (gray grains). Figure 4: Optical microscopy images at 8 magnification of mortars containing 0 wt% (a) and 4 wt% (b) of cork waste. Figure 3: Optical microscopy images of cork waste: (a) 8 magnification, (b) 35 magnification. The introduction of increasing cork content involved a color variation in mortars. This phenomenon was visually and macroscopically observed (Figure 5a) and also confirmed through the colorimeter test (Figure 5b and Table 3). Table 3 shows the results obtained by the colorimetry test in terms of a*, b*, L* values, where:  
a* > 0 predominance of red; a* < 0 predominance of green, b* > 0 predominance of yellow; b* < 0 predominance of blue, L*: 0 = dark, 100 = bright [31]. It can be observed that at increasing content of cork, mortars increase in yellow component (increasing positive a* values), in red one (increasing positive b* values) and become darker (decreasing L* values). Representative color comparison between the samples containing 0 wt% and 4 wt% of cork is graphically reported in Figure 5b within the CIELAB color space. The thermal conductivity test showed that, although the cork is regarded as an insulating material [24], the introduction of increasing cork content did not make the mortar an insulator. Indeed, thermal conductivity values above 0.1 W/mK, limit not to be exceeded for the definition of thermally insulating material according to UNI EN 998-1, were recorded for all mortar mixes (Figure 6). The mortars containing up to 3 wt% of cork displayed similar thermal conductivities to the reference mortar (0 wt%). Interestingly, the thermal conductivity of mortars tends to increase with cork addition and increased by 12% for the mortar with 4 wt%. This phenomenon resulted from the promotion of binder hydration by the progressive and slow release in time of the water absorbed by cork (wextra), which contributed to reducing the matrix porosity with hydration products. So, even if an insulating material was added, the mortar with more cork (4 wt%) was more thermally conductive than the other. On the other hand, the reduction of matrix porosity  
Figure 5: Color variation of mortar samples induced by the addition of cork waste: visual macroscopic evidence of mortars containing increasing cork content from left to right (a) and colorimetry values of representative samples containing 0 wt% and 4 wt% of cork (b). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cork Content (wt%)** & **a* & **b* & L* \\ \hline
0 & 1.86 & 6.79 & 83.22 \\
1 & 2.16 & 6.90 & 82.26 \\
3 & 2.87 & 11.17 & 74.52 \\
4 & 3.09 & 11.29 & 74.35 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Colorimetry test results for mortars. stands as a highly valuable consequence of cork addition, and that could contribute to the prevention of chemical degradation at equal or enhanced lightweight conditions. Since the cork is much lighter than mortar ( 87%), as expected it was observed a gradual decrease in the apparent mortar density with the increase of cork content (Figure 6). So, even though adding higher cork content increased thermal conductivity due to the lower porosity of the lime-based matrix, the designed mortars were lighter because the cork was an extremely lightweight material. Despite this, the designed mortars can not be defined as lightweight mortars because they all exceeded the apparent density threshold value of 1100 kg/m\({}^{3}\) according to UNI EN 206-1.