## 3 Results and Discussion
### Effect of the Combined Use of Virgin and Recycled Fillers and Fibres on Mortars' Properties
#### 3.2.2 Electrical Resistivity Measurements  
Results obtained for the DC measurements are displayed in Figure 9. As already reported, GCH decreases the \(\rho\) value by 31% compared to REF mortar. On the contrary, the addition of GNP increases the electrical resistivity by 12%. Carbon fibres do not modify the electrical resistivity of mortars, which always remains similar to that of REF regardless of the type and amount of fibres. Finally, the combined use of GCH and fibres shows that when VCF are used the \(\rho\) value decreases by 21% and 12% at 0.05 and 0.2 vol%, respectively, whereas it increases by 15% and 23% when RCF are used at the same two dosages. Figure 8: Compressive strength (R\({}_{\rm c}\)) of mortars after 28 days of curing. Figure 9: Electrical resistivity (\(\rho\)) of mortars after 28 days of curing measured in DC. The electrical measurements carried out in AC are given in Figure 10. In the figure, the results are divided into three main ranges corresponding to the resistivity at \(10\div 50\) Hz, \(50\div 100\) Hz and \(100\div 10^{6}\) Hz, hereafter referred to as \"low\", \"medium\" and \"high\" frequencies, respectively, in order to improve the readability and interpretation of the experimental data. As already reported, the REF mortar has a resistivity of 8202 \(\Omega\cdot\)cm and the addition of 1.0 vol% of GCH lowers the \(\rho\) value by 42% (both measurements are calculated in the range of medium frequencies). However, the addition of commercial GNP does not make possible to analyse the electrical resistivity at medium frequencies, but only at low and high frequencies (see \"Alternating Current (AC)\" in Section 2.2.3 for details). This filler at low frequencies gives a \(\rho\) value similar to that of REF, whereas at high frequencies it has a 62% lower \(\rho\) value. The addition of fibres modifies the electrical resistivity of mortars both at low and high frequencies. In general, at low frequencies, mortars with fibres show \(\rho\) values slightly different from that of REF mortar whereas the electrical resistivity values are always lower than that of the reference when high frequencies are considered. In particular, in the high frequency range, the mortars manufactured with VCF show \(\rho\) values that are 55% and 70% lower than REF when fibres are added at 0.05 and 0.2 vol%, respectively. Mortars containing RCF, in the same frequency range, show \(\rho\) values that are 47% and 67% lower than that of REF mortar at the same two dosages, respectively. The obtained results show that VCF are slightly better than RCF in reducing the electrical resistivity of mortars at high frequencies, even if both behave as very good electrical conductors in cement-based mortars. Anyway, the higher the percentage of fibre addition, the lower the electrical resistivity of mortars, as was found by the present authors in [13] and also by others in [12; 38]. The effect of the combined use of fibres and GCH filler in mortars is different in the case of VCF and RCF. In particular, when GCH is used with VCF, the electrical resistivity of mortars at low frequencies is around 5450 \(\Omega\cdot\)cm for both fibre dosages (33% lower than REF). At high frequencies, the resistivity becomes 68% and 76% lower than REF when fibres are added at 0.05 and 0.2 vol% amounts, respectively. On the contrary, at low frequencies, the addition of RCF to GCH causes an increase in electrical resistivity equal to 8% if compared to that of REF at any dosage. Instead, at high frequencies, RCF at a 0.05 vol% decrease the \(\rho\) value by 68% compared to REF at 0.2 vol%, which decrease it by around 80%. Figure 10: Electrical resistivity (\(\rho\)) of mortars after 28 days of curing measured in AC at low, medium and high frequencies. The obtained results show that GCH is a good sustainable micro filler for decreasing the electrical resistivity of mortars in AC, since alone it provides a decrease of 42%.