## 3 Results and Discussion
### Effect of the Combined Use of Virgin and Recycled Fillers and Fibres on Mortars' Properties  
#### 3.2.1 Mechanical Strength  
The second part of the experimentation was focused on the study of GCH added both alone, and together with VCF and RCF in cement mortars. The obtained results are also reported with those obtained for REF and GNP mortars, for comparison.  
The flexural strength of mortars is shown in Figure 6.  
As already explained, the GCH addition only slightly decreases the R\({}_{\text{f}}\) of mortar, whereas the addition of commercial GNP increases the R\({}_{\text{f}}\) value of about 10%. The enhancement of flexural strength in GNP up to 1.2 vol% has already been reported by [PERSON] and [PERSON] [35]. The increase in flexural strength has been related to the capacity of almost 2D sheets, such as GNP, to behave as reinforcing materials that bridge cracks [4].  
On the other hand, considering carbon fibres, the increase in flexural strength is higher the lower the amount of fibres addition. Moreover, RCF significantly increase the R\({}_{\text{f}}\) of mortars compared to that of VCF. When GCH is added to mortars containing VCF, the flexural strength is similar to that of mortars containing GCH alone, whereas a slight increase in flexural strength compared to the REF mortar is registered when RCF are used.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Mortar**} & **R\({}_{\text{f}}\)** & **R\({}_{\text{c}}\)** & **DC a** & **AC b** \\  & **(MPa)** & **(MPa)** & **\(\rho\)** & **\(\rho\)** \\  & & & **(\(\Omega\cdot\)cm)** & **(\(\Omega\cdot\)cm)** \\ \hline REF & 8.0 \(\pm\) 0.0 & 47.6 \(\pm\) 0.9 & 7615 \(\pm\) 334 & 8202 \(\pm\) 214 \\ UFS & 7.0 \(\pm\) 1.1 & 44.9 \(\pm\) 1.3 & 10,863 \(\pm\) 542 & 10,464 \(\pm\) 371 \\ GCH & 7.4 \(\pm\) 0.1 & 47.6 \(\pm\) 0.5 & 5316 \(\pm\) 159 & 4727 \(\pm\) 29 \\ \hline \hline \end{tabular}  
* Direct current,b alternating current.  
\end{table}
Table 3: Mechanical and electrical results of REF, UFS and GCH mortars after 28 days of curing.  
The superior behaviour of mortars with 0.05 vol% of fibres compared to those containing 0.2 vol% is probably related to their better dispersion. It is known that an excessive content of carbon fibres causes both their agglomeration and the formation of air voids, which reduce the mechanical compressive strength of the material [12; 13]. On the other hand, the higher flexural strength of mortars manufactured with RCF is related to the better interface of RCF with the cement paste compared to VCF. In fact, in the SEM images of broken mortars it is clear that the surface of VCF appears smooth (Figure 7a) whereas the surface of RCF is covered by the cement paste (Figure 7b).  
Results for the compressive strength of the mortars are shown in Figure 8. GCH maintains the compressive strength of mortar, whereas GNP decreases the \(\mathrm{R_{c}}\) value of mortar of by about 19%. The decrease in the mechanical properties of GNP mortar is related to the nature of GNP; the high specific surface area and the hydrophobicity of GNP [36] tend to create agglomerates that are barely dispersible in polar liquids like water. For this reason, GNP agglomeration results in the difficult compaction of mortar specimens [15] and a subsequent increase in the mortar's porosity [16; 17; 37].  
Figure 6: Flexural strength (\(\mathrm{R_{f}}\)) of mortars after 28 days of curing.  
Figure 7: SEM images of mortars containing (**a**) VCF and (**b**) RCF.  
The addition of carbon fibres generally does not significantly modify the compressive strength of mortars, which remains around 44 MPa, even if at high dosage they provide a slightly better behaviour than at low dosage.  
The combined use of GCH and carbon fibres, regardless of their nature and amount, does not have any beneficial effect on the flexural and compressive strength of mortars, which instead suffer a slight decrease of mechanical performance.