## 4 Results and Discussion  
As it was also observed in previous research [20, 24], three stages of the cement paste hardening process occur. These three phases are quite apparent on the obtained diagram of strength as shifts in the slope of the curve. * On the initial stage, the ultrasonic velocity starts with a value close to the velocity in air (340 m/s), then increases progressively up to values close to 1500 m/s, which is close to the ultrasonic velocity in water. * The second stage appears between 1500 m/s and 3000 m/s. The rise of ultrasonic velocity is due to a decrease in water volume and an increase of hydration products with progress of cement paste hydration [20, 24, 49]. * Finally, stage 3 occurs after stage 2 is completed, until the end of measurements. It should be mentioned here that during the initial stage of hydration, during the first 3 to 4 h which is considered as the period needed for the connection of small particles, it was observed an inability to detect any ultrasound signal, a fact that has been also noticed in previous investigations [2, 26, 50, 51, 52, 53]. This repeatable behavior can be attributed to the high attenuation of the fresh cement-based products. Regarding the values of sound velocity as a function of time (Figure 2), it was observed that the specimens with 0.4 wt.% of cement CNTs in both the cement paste and mortar, present higher wave speeds in comparison with the rest of nanoinclusion concentrations. Previous studies reported on the correlation of sound velocity with strength of the cementitious materials [38, 54, 55]. Furthermore, the same trend follow the findings of a previous research regarding the relation between carbon nanotube concentrations and strength values [12]. The specimens reinforced with 0.6 wt.% CNTs present lower strength values than the specimens with 0.4 wt.% CNTs, due to agglomerate creation; they also present higher strength values in comparison with the specimens with 0.2 wt.% CNTs. In addition, according to Figure 2b, the recording of sound velocities began earlier for the mortar specimens in comparison to the cement paste, which took twice as long during the first stage of hydration, when the ultrasonic signals could not be observed (Figure 2a). The fine aggregates are responsible for this behavior because they absorb water and thereby bind a proportion of the water that would make the mixture waterier. Figure 1: Experimental setup. Additionally, the first ultrasonic waves for both types of mixtures were registered at the same velocities, at around 600 m/s (Figure 2). It was observed that there is a rapid rise in sound velocity during Stage 1 and Stage 2 of hydration. The chemical properties of the materials had unexpectedly altered, and the cement paste and mortar textures became denser, leading to an increase of the ultrasonic velocity [24]. Figure 3a,b show the strength over time curves for both types of mixtures. According to the above observations, it would be expected that the strength curve would have the similar trend with the velocity over time graph. However, it is observed that the strength does not follow the exact same trend as the sound velocity. This is due to the strength calculation by ConReg 700, the equipment for the maturity calculation. The algorithm takes various parameters into account for calculating the strength, such as the temperature released during cement hardening. The ConReg 700 algorithm does not consider the concentration of CNTs in the mixture. However, based on the sound velocity measurements it is implied that the samples of cement paste present higher strength values compared to the mortar samples, for all the carbon nanotube concentrations. These elevated strength values occur because the cement paste exhibits higher hydration rates than mortar, since in this case an amount of water is bounded by aggregates.