*3.3. Ultrasonic Pulse Velocity*

Figure 6 shows the results of the ultrasonic pulse velocity test. This test consisted of obtaining the velocity (m/s) that the ultrasonic waves needed to cross the longest length of the specimen. The test was performed for mortars maintained in nonaggressive conditions (Figure 6a) and the ultrasonic velocity obtained was similar for all mortars in both t56 (from 4248 to 4340 m/s) and t118 (from 4198 to 4364 m/s). For mortars exposed to acid attack (Figure 6b), the ultrasonic velocity decreased for all mortars with respect the velocity obtained for a nonaggressive environment for both t56(28) (from 4091 to 4206 m/s) and t118(90) (from 3525 to 3794 m/s). To obtain a measure of the effect of the acid attack on the propagation velocity, the difference between ultrasonic pulse velocity at t118 for the mortars with and without attack was calculated. The measurements were taken after 90 days of acid attack, which is an action time long enough for the effects to manifest. It was found that the mortars with the admixture

had less velocity decrease (B mortar: 13.0%, C mortar: 14.3% and D mortar: 11.9%) than the reference mortar (16.0%). In addition, the longer the acid exposure time, the lower the ultrasonic pulse velocity. The results demonstrate that the ultrasonic technique reflects the deterioration caused by sulphuric acid attack, providing information about the internal condition and quality of the mortars [56,57]. This is due to the large amount of cracks in specimens [58]. Indeed, in this work, two relationships between the involved variables were found: (i) correlation between the decrease in ultrasonic pulse velocity and the mass loss, and (ii) correlation between the ultrasonic pulse velocity and the compressive strength (Figure 6c,d). Recent studies have demonstrated [59,60] that the variation of the ultrasonic pulse velocity indicates a variation in Young's modulus. Lower speed means lower Young's modulus and greater deterioration of mortars.

**Figure 6.** (**a**) Ultrasonic pulse velocity of mortars kept in a nonaggressive environment at t56 and t118. (**b**) Ultrasonic pulse velocity of mortars exposed to a sulphuric acid solution for 28 and 90 days. (**c**) Correlation between ultrasonic pulse velocity (m/s) and mass loss (%) after the acid attack.

(**d**) Correlation between ultrasonic pulse velocity (m/s) and compressive strength (MPa).

#### *3.4. Capillary Water Absorption Coefficient*

The results of the capillary water absorption test are shown in Figure 7. For nonaggressive conditions and early ages (56 days from its manufacture), the absorption coefficient of the mortars with the crystalline admixture was slightly lower (B mortar: 0.20; C mortar: 0.19; D mortar: 0.20) than the absorption coefficient of the reference mortar (A mortar: 0.21). At t118, the absorption coefficients for all the mortars reduced but without reaching the values obtained for a water-repellent mortar [25]. These results agree with other studies which found that concretes treated with crystalline materials had slightly lower depth of pressure water penetration than the reference concrete in the short term [36,61] and negligible effects in terms of water vapor permeability [33]. In a sulphuric acid exposure, all mortars slightly reduced their capillary coefficients when compared to the values obtained for those same times in a nonaggressive environment. This reduction could be due to the formation of a gypsum

coating on the surface so that the gypsum crystals obstructed the capillary net, reducing the absorption coefficient [24].

**Figure 7.** (**a**) Capillary water absorption coefficients of mortars kept in a nonaggressive environment at t56 and t118. (**b**) Capillary water absorption coefficients of mortars exposed to a sulphuric acid solution for 28 and 90 days.
