## 4 Results and Discussion
This is further expected since specimen volume and water to cement ratio are kept constant for all specimens; therefore, paste samples contain more cement compared to mortar samples, which have also aggregates. Figure 4 presents a comparative graph of cement paste and mortar specimens as concerns the materials' strength during their hardening process. These specimens have  
Figure 3: Strength values for different carbon nanotube concentrations for: (**a**) cement paste and (**b**) mortar. Figure 2: Sound velocity for all the carbon nanotubes concentrations for: (**a**) cement paste and (**b**) mortar. no nanoreinforcement and the results are derived from a previous study [20]. Comparing Figures 3 and 4, it is graphically proved the prior acceptance that the algorithm that ConRef-700 equipment uses does not take into consideration at the strength results the existence of the nanoinclusions. Both the form and the values of the strength are extremely similar. In materials research, the monitoring of hardening process of fresh cement-based materials is an essential for the quality assessment of the material. A sharp rise in temperature, which lasts for about 9 h, was observed during the first stage of the hardening process. The temperature difference was computed by the difference between the temperature of the cement paste (or mortar) and the ambient temperature. According to previous investigations [20], the clustering between the different particles reaches a peak during this stage. First, the smallest particles are created, then follow the largest particles. It was observed that the maximum temperatures for the cement paste specimens were greater than those for the mortar samples. In addition, the pattern of temperature difference was found to be similar for samples of the same type (Figure 5) due to the same value of water to cement ratio for all specimens and the fact that that ConReg's 700 algorithm does not take into account the CNT concentrations in the mixture, as also mentioned in previous paragraph. In addition, CNTs do not take part in the exothermic chemical reaction. Figure 4: Strength values for cement paste and mortar with absence of nanoinclusions. Figure 5: Difference in temperature for all carbon nanotube concentrations for: (**a**) cement paste and (**b**) mortar. The higher temperature value in the cement paste specimens indicates that, for a given period more cement powder reacts with water, resulting to higher energy release. This result was expected because for producing the cement paste double amounts of cement powder are used, comparing to mortar samples of same volume. Figure 6 presents a comparative graph of cement paste and mortar specimens regarding the materials' released temperature during their hardening process. These specimens have no nanoreinforcement and the results were derived from a previous study [20]. Comparing Figures 5 and 6, it is evident that the algorithm used by the ConRef-700 device does not take into consideration the presence of nanoinclusions. Both the form and the values of the deference in temperature look similar for the released temperatures of both the cement paste and mortar. The use of linear and stable in time experimental devices should be the prerequisite of any reliable measurement of nonlinear acoustic behavior. This is necessary for the material inherent nonlinear elastic behavior to be reflected rather than the artificial harmonic generation that is produced by the measuring electronics. To demonstrate the long-term stability of the experimental setup for performing nonlinear measurements, the system was assessed over a period of 48 h.  
The system was also tested to verify its linearity. If a measurement system is not linear, it cannot be ascertained whether the nonlinear data are due to the material under testing or from the system itself. This test was performed for 48 h on a 28-day hardened specimen (Figure 7a).