## 3 Results and Discussion
### Mechanical Properties of Mortars
The mechanical resistances of the specimens of the mixtures containing 5% (M5) and 10% of marble powder (M10) are quite similar, respectively 53.9 MPa and 52.7 MPa, as well as those of the mixtures made with 15% (M15) and 20% (M20) of marble powder, respectively 49.8 MPa and 48.1 MPa. It is also interesting  
Figure 7: Compressive strength histogram of the specimens of the 0 MS, G10, G20, FF10, FF20 at 2, 7, and 28 days of curing. to note that the uniaxial compressive strength (\(\sigma\)) vs. cement replacement (r) with marble powder can be explained by the following inverse linear relationship:  
\[\sigma=-0.5531\cdot\text{r}+57.981,\text{R}^{2}=0.97.\]  
[PERSON] et al. [33] and [PERSON] et al. [21], by using Portland cement 42.5 grade, obtained at 28 days a decrease in the compressive strength values with the increase of replacement percentage of cement. The trend of the compressive strength versus the percentage of marble powder replacing cement observed by analyzing the data of this study is quite like that obtained by [PERSON] et al. [21], but differs substantially from the values obtained by [PERSON] et al. [33] that, for example, by replacing 20% by weight of the cement with marble powder, achieved a 5% decrease in resistance compared to the reference samples, while in this study and in that one conducted by [PERSON] et al. [21] the decrease in mechanical resistances was 18% and 20%, respectively. Table 6 shows the propagation speed of the ultrasonic pulse (UPV--Ultrasonic Pulse Velocity) measured on the mortar samples of the MS, M5, MP10, M15, M20, and M25 mortar mixtures at 28 days of curing. The mean of the standard deviations of the ultrasonic pulse propagation speed is 56 m/s. Figure 8 shows the uniaxial compressive strengths and the UPVs of the mortar specimens for MS, M5, M10, M15, M20, and M25 mortar mixtures at 28 days of curing. From the graph, it is possible to observe that the UPV linearly decreases as the mechanical compressive strength of the specimens decreases (it decreases as the percentage of replacement of cement with marble powder M increases). The best equation describing the relationship between UPV and compressive strength is:  
\[\text{UPV (m/s)}=14.344\cdot\sigma_{\text{c}}\text{ (MPa)}+3516,\text{ R}^{2}=0.98.\] \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **UPV (m/s)** \\ \hline \multicolumn{3}{c}{Mean SD = 56 m/s} \\ \hline MS & 4356 \\ M5 & 4287 \\ M10 & 4261 \\ M15 & 4245 \\ M20 & 4214 \\ M25 & 4128 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Ultrasonic pulse velocity of the specimens MS, M5, M10, M15, M20, and M25 at 28 days of curing. Figure 8: Compressive strength vs. ultrasonic pulse velocity (UPV) of the MS, M5, M10, M15, M20, and M25 mortar mixtures at 28 days of curing. Table 7 reports the apparent density, the water absorption coefficient by capillarity, the water absorption by total immersion referred to weight and to volume, measured on the mortar specimens of the MS, M5, M10, M15, M20, and M25 mortar mixtures at 28 days of curing. The average of the standard deviations of apparent density, water absorption coefficient by capillarity, water absorption by capitalarity, water absorption by total immersion referred to the weight, and to the volume, are 0.02 g/cm\({}^{3}\), 1.2 g/m\({}^{2}\) s\({}^{0.5}\), 0.2 wt.% and 0.5% by volume, respectively. The highest apparent density value is that relating to specimens made with 5% marble powder (M5, 2.19 g/cm\({}^{3}\)), while the lowest value is relative to specimens made with 25% marble powder (M25, 2.13 g/cm\({}^{3}\)). The highest value of the water absorption coefficient by capillarity is that measured on specimens made with cement replacement of 25% marble powder (M25, 18 g/m\({}^{2}\) s\({}^{0.5}\)), while the lowest is that relating to mortar mixture made with 5% marble powder (M5, 13 g/m\({}^{2}\) s\({}^{0.5}\)).