Exposing Sustainable Mortars with Nanosilica , Zinc Stearate , and Ethyl Silicate Coating to Sulfuric Acid Attack

Obtaining durable materials that lengthen the service life of constructions and thereby contribute to sustainability requires research into products that improve the durability of cementitious materials under aggressive conditions. This paper studies the effects of sulfuric acid exposure on four mortar types (control mortar, mortar with nanosilica, mortar with zinc stearate, and mortar with an ethyl silicate coating), and evaluates which of them have better performance against the acid attack. After 28 days of curing, the samples were exposed to a sulfuric acid attack by immersing them in a 3% w/w of H2SO4 solution. Physical changes (mass loss, ultrasonic pulse velocity, open porosity, and water absorption), and mechanical changes (compressive strength) were determined after the sulfuric acid exposure. A scanning electron microscope (SEM) was used to characterize the morphology of the surface mortars after the exposure. The control mortar had the highest compressive strength after the acid attack, although of the four types, the zinc stearate mortar showed the lowest percentage of strength loss. The zinc stearate mortar had the lowest mass loss after the acid exposure; moreover, it had the lowest capillary water absorption coefficient (demonstrating its hydrophobic effect) both in a non-aggressive environment and acid attack.


Introduction
Nature and the environment are not an inexhaustible source of resources, and their protection and rational use are necessary. Therefore, it is vital to work with durable materials that extend the service life of constructions. This contributes to the saving of existing resources and avoids causing harm to the environment. It is especially difficult to stretch the service life of cement-based materials when they are exposed to aggressive chemical environments. This is the case of some infrastructures that are affected by chemical attacks, such as sanitation networks, foundations [1,2], and infrastructures in contact with groundwater or in agricultural and farm facilities. These types of attacks are one of the most severe mechanisms in the deterioration of cementitious materials, so it is important to research products that contribute to prolonging the durability of these materials. This is one of the reasons why the use of admixtures and additives in cement-based materials has become very popular [1,[3][4][5].
External sulfate exposures have a detrimental impact on cementitious materials [6,7], such as concretes and mortars. An externally-sourced sulfate attack is a slow process that can take years to manifest itself, causing internal changes in the microstructure. These changes are manifested in physical and mechanical alterations such as strength loss, mass variation, cracking, softening,
Limestone fine aggregate sand was used. Figure 1 shows the particle-size distribution of the limestone fine aggregate carried out according to the EN 933-1 standard [34]. The limestone fine aggregate to binder ratio was 3:1 as prescribed by standard EN 196-1 [35], and the water to binder ratio was 0.45. To maintain the workability of all four mortars, superplasticizer (Sika ViscoCrete-5980, Sika SAU, Madrid, Spain) amounting to 1.5% of the cement weight was added. The NS mortar was manufactured by adding amorphous nanosilica suspension (BASF MasterRoc MS 685, BASF Construction Chemicals España, S.L. Barcelona, Spain), amounting to 2% of the cement weight. The ZS mortar was manufactured by adding zinc stearate (Alfa Aesar, Ward Hill, MA, USA), amounting to 2% of the cement weight. The ES mortar was carefully spray-coated with ethyl silicate (Silikat Tes 40 WN, Wacker Chemie AG, Munich, Germany). Chemical-Physical Characterization of the Binder An ordinary Portland cement CEM I 52.5 R (CEMEX, Madrid, Spain) according to the standard EN 197-1 [36], was used to produce the mortars. The crystallographic phases were identified in the cement by using X-ray diffraction (XRD) carried out with a Bruker D8-Advance diffractometer (Bruker Española S.A., Madrid, Spain). The X-ray tube was operated at 40 kV and 40 mA, and the spectra were registered with angles from 4° to 60° at 0.05° stepping intervals in Ө-Ө mode. In addition, the chemical composition of the cement was obtained using X-ray fluorescence (XRF) with an X-ray sequential spectrometer PHILIPS MAGIC PRO (Philips Ibérica, Madrid, Spain). The XRD analysis in Figure 2 shows the following major phases in Portland cement: (C3S), (C2S), (C4AF) and calcite. The XRF results (Table 1) show the binder was mainly composed of CaO and SiO2 as expected for calcium silicates (C3S and C2S). The percentage of SO3 can be explained by the addition of gypsum (as shows the XRD in Figure 2), which is the set retarder for ordinary Portland cement. Chemical-Physical Characterization of the Binder An ordinary Portland cement CEM I 52.5 R (CEMEX, Madrid, Spain) according to the standard EN 197-1 [36], was used to produce the mortars. The crystallographic phases were identified in the cement by using X-ray diffraction (XRD) carried out with a Bruker D8-Advance diffractometer (Bruker Española S.A., Madrid, Spain). The X-ray tube was operated at 40 kV and 40 mA, and the spectra were registered with angles from 4 • to 60 • at 0.05 • stepping intervals in Ө-Ө mode. In addition, the chemical composition of the cement was obtained using X-ray fluorescence (XRF) with an X-ray sequential spectrometer PHILIPS MAGIC PRO (Philips Ibérica, Madrid, Spain). The XRD analysis in Figure 2 shows the following major phases in Portland cement: (C 3 S), (C 2 S), (C 4 AF) and calcite. The XRF results (Table 1) show the binder was mainly composed of CaO and SiO 2 as expected for calcium silicates (C 3 S and C 2 S). The percentage of SO 3 can be explained by the addition of gypsum (as shows the XRD in Figure 2), which is the set retarder for ordinary Portland cement.  The tests were carried out on prismatic specimens with standardized dimensions (40 × 40 × 160 mm) that were manufactured according to the standard EN 196-1 [35]. A total of 24 specimens were manufactured, 6 per each type of mortar. After the first 24 h, the specimens were de-molded and were kept in a humidity chamber for 28 days at 95 ± 2% of relative humidity and 20 ± 2 °C. The specimens that had to be coated with ethyl silicate (ES mortars) were taken out from the humidity chamber in the midst of the curing process only for the time necessary to carry out the spray coating. The specimens were carefully spray-coated with ethyl silicate at a controlled distance of 10 cm, using a small airbrush (Figure 3d). After completing the coating, the specimens were returned to the humidity chamber to continue the curing process. After 28 days of curing, twelve specimens were exposed to a sulfuric acid attack simulation (three specimens per each type of mortar), and the other twelve were stored in standard laboratory conditions to use as reference samples.
The sulfuric acid attack was performed by immersing the specimens into a H2SO4 solution (3% w/w) (Figure 3a,b) in hermetically closed containers, this procedure has been used in previous studies [30]. A high concentration of sulfuric acid was chosen in order to accelerate their effects on the mortars, and obtain the same degradation in less exposition time [1]. The H2SO4 solution was changed weekly to reduce the variation of the concentration of sulfuric acid. Finally, the containers

Manufacturing and Curing Process of the Mortars and Sulfuric Acid Attack Simulation
The tests were carried out on prismatic specimens with standardized dimensions (40 × 40 × 160 mm) that were manufactured according to the standard EN 196-1 [35]. A total of 24 specimens were manufactured, 6 per each type of mortar. After the first 24 h, the specimens were de-molded and were kept in a humidity chamber for 28 days at 95 ± 2% of relative humidity and 20 ± 2 • C. The specimens that had to be coated with ethyl silicate (ES mortars) were taken out from the humidity chamber in the midst of the curing process only for the time necessary to carry out the spray coating. The specimens were carefully spray-coated with ethyl silicate at a controlled distance of 10 cm, using a small airbrush (Figure 3d). After completing the coating, the specimens were returned to the humidity chamber to continue the curing process. After 28 days of curing, twelve specimens were exposed to a sulfuric acid attack simulation (three specimens per each type of mortar), and the other twelve were stored in standard laboratory conditions to use as reference samples.
The sulfuric acid attack was performed by immersing the specimens into a H 2 SO 4 solution (3% w/w) (Figure 3a,b) in hermetically closed containers, this procedure has been used in previous studies [30]. A high concentration of sulfuric acid was chosen in order to accelerate their effects on the mortars, and obtain the same degradation in less exposition time [1]. The H 2 SO 4 solution was changed weekly to reduce the variation of the concentration of sulfuric acid. Finally, the containers were kept in a controlled-temperature room. The procedure for changing the solution was done as follows: 1.
The specimens were extracted from the H 2 SO 4 solution and were gently brushed under a weak flow of tap water to eliminate material residue that may have adhered to the surface.

2.
The H 2 SO 4 solution was replaced with a new solution. 3.
The specimens were reintroduced into the new solution.
Sustainability 2018, 10, x FOR PEER REVIEW 5 of 14 were kept in a controlled-temperature room. The procedure for changing the solution was done as follows: 1. The specimens were extracted from the H2SO4 solution and were gently brushed under a weak flow of tap water to eliminate material residue that may have adhered to the surface. 2. The H2SO4 solution was replaced with a new solution. 3. The specimens were reintroduced into the new solution. (c) Specimens before testing at t56. Upper row: specimens exposed to sulphuric acid; lower row: specimens with no acid attack. (d) Airbrush used to spray the specimens.

Physical and Mechanical Properties of the Mortars After the Sulfuric Acid Attack
The mass variation of the specimens exposed to the sulfuric acid was obtained by weighing the specimens six times during the acid attack period: (i) before starting the sulfuric acid exposure and hence at 28 days after their manufacture (t28), (ii) at 7 days of acid exposure (t35), (iii) at 14 days (t42), (iv) at 21 days (t49), (v) at 28 days (t56), and (vi) at 90 days of acid exposure (t118). The percentage of mass loss was calculated in relation to the initial weight, i.e., at t28.The open porosity of the specimens exposed to the sulfuric acid attack was determined following the procedure of the standard EN 1936 [37], and it was calculated at 28 days (t56) and 90 days (t118) of acid exposure.
The ultrasonic pulse velocity test was performed following the EN 12504-4 standard [38]. The test consisted of measuring the propagation time of the ultrasonic waves along the longest dimension of the specimen (160 mm). Contact transducers emitting ultrasonic pulses at 54 kHz were coupled to the end sides of the specimens. Three determinations were made per sample. The test was performed for the specimens that had been exposed to the sulfuric acid attack and for those that had not been. The ultrasonic test was carried out at 28 days (t56) of acid exposure, i.e., at 56 days from their manufacture. Upper row: specimens exposed to sulphuric acid; lower row: specimens with no acid attack. (d) Airbrush used to spray the specimens.

Physical and Mechanical Properties of the Mortars After the Sulfuric Acid Attack
The mass variation of the specimens exposed to the sulfuric acid was obtained by weighing the specimens six times during the acid attack period: (i) before starting the sulfuric acid exposure and hence at 28 days after their manufacture (t 28 ), (ii) at 7 days of acid exposure (t 35 ), (iii) at 14 days (t 42 ), (iv) at 21 days (t 49 ), (v) at 28 days (t 56 ), and (vi) at 90 days of acid exposure (t 118 ). The percentage of mass loss was calculated in relation to the initial weight, i.e., at t 28 .The open porosity of the specimens exposed to the sulfuric acid attack was determined following the procedure of the standard EN 1936 [37], and it was calculated at 28 days (t 56 ) and 90 days (t 118 ) of acid exposure.
The ultrasonic pulse velocity test was performed following the EN 12504-4 standard [38]. The test consisted of measuring the propagation time of the ultrasonic waves along the longest dimension of the specimen (160 mm). Contact transducers emitting ultrasonic pulses at 54 kHz were coupled to the end sides of the specimens. Three determinations were made per sample. The test was performed for the specimens that had been exposed to the sulfuric acid attack and for those that had not been. The ultrasonic test was carried out at 28 days (t 56 ) of acid exposure, i.e., at 56 days from their manufacture. The compressive strength tests were conducted according to the EN 196-1 standard [35] and were carried out at 28 (t 56 ) and 90 (t 118 ) days of sulfuric acid exposure (three samples per each exposition time and type of mortar).
The capillary water absorption coefficient was calculated for the reference samples (kept in standard laboratory conditions) and for the samples exposed to the sulfuric acid attack for 90 days (t 118 ), with the aim of comparing the difference of behavior. According to the standard EN 1015-18 [39], the coefficient was computed using the formula: where: C = capillary water absorption coefficient, kg/(m 2 ·min 0.5 ) M 1 = specimen mass after the immersion for 10 min, kg M 2 = specimen mass after the immersion for 90 min, kg A = surface of the specimen face submerged in the water, m 2 t 2 = 90 min t 1 = 10 min

Scanning Electron Microscopy (SEM) Examination
All the samples were examined with a scanning electron microscope Hitachi S3000N (Hitachi High-Technologies Canada, Inc., Toronto, Canada) in order to perform a microstructural analysis. The microscope was equipped with an X-ray detector Bruker XFlash 3001 (Bruker Española S.A., Madrid, Spain) for microanalysis EDX (energy-dispersive X-ray spectroscopy). To study the samples, small fragments were carefully removed from the surface and dried at 60 • C. For each sample, three EDX analyses were performed in three different zones, operating in variable pressure mode at 20 kV. The surface was studied in secondary electrons mode at an ultra-high vacuum using an accelerating voltage of 15 kV and the working distance was variable. Despite EDX being a semi-qualitative technique and it has some uncertainty, in this study it has been used to approximate the sample composition.

Compressive Strength
The progress of a sulfuric acid attack was evaluated by measuring the compressive strength. Figure 4 shows the compressive strength of the mortars tested under two curing conditions: non-aggressive environments, and aggressive environments with sulfuric acid attack. In the first case, non-aggressive environments, the strength of the mortars at t 118 increased with respect the strength at t 56 ; this increase is common in mortars that cure under standard conditions. The mortar with the ethyl silicate coating had the highest strength for t 56 and t 118 , and the zinc stearate mortar had the lowest strength, having lower strength at t 56 than the control mortar, although at t 118 , it was slightly higher. Figure 4c shows that for a non-aggressive environment, the control mortar maintained its strength; however, the other three mortars augmented their strength from t 56 to t 118 , possibly because the compressive strength increase of these mortars occurred over a long period.
In the second case, aggressive environments, the reduction in compressive strength was a direct effect of the sulfuric acid exposure due to the migration of sulfate ions into concrete accompanied by a gradual dissolution of Portlandite and decomposition of the C-S-H phase [40]. Simultaneously, the formation of more expansive compounds [41,42] and consequent volumetric strains in the hardened material were also considered to be responsible for expansive forces and micro-cracking [43]. As can be seen in Figure 4, the compressive strength for the mortars exposed to the sulfuric acid attack was lower than the compressive strength of the mortars that were not attacked, both at t 56 and t 118 . All the mortars presented strength loss in the time interval from t 56 to t 118 . The zinc

Mass Loss Due to the Sulfuric Acid Exposure
The analysis of the mass loss as a function of the exposure time to the sulfuric acid attack ( Figure 5) showed that the resistance to the attack of the nanosilica mortar and the mortar treated with an ethyl silicate coating was similar to the control mortar. This result is consistent with other authors [8,13] who state that the sulfate resistance is generally higher at a lower water/cement ratio. However, the zinc stearate mortar was the one that best resisted the acid attack as its mass loss was lower than for the other mortars at t118. The increment in mass loss and compressive strength loss observed when the exposure time increased are consistent with the results obtained in previous studies [26].

Mass Loss Due to the Sulfuric Acid Exposure
The analysis of the mass loss as a function of the exposure time to the sulfuric acid attack ( Figure 5) showed that the resistance to the attack of the nanosilica mortar and the mortar treated with an ethyl silicate coating was similar to the control mortar. This result is consistent with other authors [8,13] who state that the sulfate resistance is generally higher at a lower water/cement ratio. However, the zinc stearate mortar was the one that best resisted the acid attack as its mass loss was lower than for the other mortars at t 118 . The increment in mass loss and compressive strength loss observed when the exposure time increased are consistent with the results obtained in previous studies [26].

Mass Loss Due to the Sulfuric Acid Exposure
The analysis of the mass loss as a function of the exposure time to the sulfuric acid attack ( Figure 5) showed that the resistance to the attack of the nanosilica mortar and the mortar treated with an ethyl silicate coating was similar to the control mortar. This result is consistent with other authors [8,13] who state that the sulfate resistance is generally higher at a lower water/cement ratio. However, the zinc stearate mortar was the one that best resisted the acid attack as its mass loss was lower than for the other mortars at t118. The increment in mass loss and compressive strength loss observed when the exposure time increased are consistent with the results obtained in previous studies [26].  The formation of ettringite and gypsum is common in cementitious systems exposed to sulfuric acid solutions and is observed in numerous field and laboratory studies [11,12,44]. Deleterious expansion occurs in mortars due to an excessive amount of these formations. The expansion is usually accompanied by the development of cracks in the mortar, initiated near the surface, and gradually evolving towards the central portion of the mortar [11]. As mentioned in Section 2.2.1, the specimens were brushed and cleaned weekly to remove the gypsum adhered to the specimen surface. As can be seen in Figure 5, the mass loss was mainly associated to the degradation of the mortar due to the formation of gypsum in the surface removed during the process.

Ultrasonic Pulse Velocity
Ultrasonic pulse velocity is based on a longitudinal wave pulse and it can be simply performed [45]. The ultrasonic propagation velocity obtained for all the mortars was similar except for the zinc stearate mortar that was the lowest. Some experimental studies have demonstrated that the measurement of the P-waves is suitable for estimating the changes in the dynamic modulus of concrete depending on the sulfate attack damage [46,47]. When the mortars were exposed to the sulfuric acid attack, a significant reduction of the ultrasonic pulse velocity was observed ( Figure 6). The reduction reflects the deterioration caused by sulfuric acid attack, which provides information about the internal condition and quality of mortars [48][49][50]. The formation of ettringite and gypsum is common in cementitious systems exposed to sulfuric acid solutions and is observed in numerous field and laboratory studies [11,12,44]. Deleterious expansion occurs in mortars due to an excessive amount of these formations. The expansion is usually accompanied by the development of cracks in the mortar, initiated near the surface, and gradually evolving towards the central portion of the mortar [11]. As mentioned in Section 2.2.1, the specimens were brushed and cleaned weekly to remove the gypsum adhered to the specimen surface. As can be seen in Figure 5, the mass loss was mainly associated to the degradation of the mortar due to the formation of gypsum in the surface removed during the process.

Ultrasonic Pulse Velocity
Ultrasonic pulse velocity is based on a longitudinal wave pulse and it can be simply performed [45]. The ultrasonic propagation velocity obtained for all the mortars was similar except for the zinc stearate mortar that was the lowest. Some experimental studies have demonstrated that the measurement of the P-waves is suitable for estimating the changes in the dynamic modulus of concrete depending on the sulfate attack damage [46,47]. When the mortars were exposed to the sulfuric acid attack, a significant reduction of the ultrasonic pulse velocity was observed ( Figure 6). The reduction reflects the deterioration caused by sulfuric acid attack, which provides information about the internal condition and quality of mortars [48][49][50].

Open Porosity and Capillary Water Absorption Coefficient
Open porosity influences the durability of the cement-based materials because it affects the corrosion of reinforcing steel, which many authors consider to be the most grave pathology affecting reinforced concrete structures [1,51]. Figure 7 shows the open porosity evolution of the mortars attacked with the sulfuric solution from t56 to t118. All the mortars presented a similar behavior, as they increased their open porosity in similar proportions from t56 to t118, except for the ZS mortar, which had the highest increase in its open porosity. However, the open porosity percentage of the ZS mortar at t118 was the lowest open porosity compared with the other three mortars. The increment of the open porosity observed was related to the sulfate attack effects, which would involve the formation of expansive products [43], producing a loss of solid fraction in the mortar [2]. Figure 8 shows the capillary water absorption coefficients calculated according to the EN 1015-18 standard [39]. The coefficients were evaluated for the mortars at t118, both the non-attacked mortars and the mortars attacked with the sulfuric solution. For the non-attacked mortars, the highest coefficient was obtained for the NS mortar, and as expected, the lowest coefficient was obtained for the ZS mortar. The capillary water absorption coefficients after 90 days of sulfuric acid exposure of the mortars reduced its value in similar proportions, except the ZS mortar that increased its coefficient to 82.0%. The reduction of the absorption coefficients for the mortars (C mortar, ES mortar, and NS mortar) may be due to the network of gypsum crystals that was compact enough to obstruct the capillary network [7] and can be seen in the SEM micrographs of the mortars attacked ( Figure 9). It should be highlighted that the zinc stearate had a hydrophobic effect, and therefore, the ZS mortar had the lowest absorption coefficient, both in a non-aggressive environment and under

Open Porosity and Capillary Water Absorption Coefficient
Open porosity influences the durability of the cement-based materials because it affects the corrosion of reinforcing steel, which many authors consider to be the most grave pathology affecting reinforced concrete structures [1,51]. Figure 7 shows the open porosity evolution of the mortars attacked with the sulfuric solution from t 56 to t 118 . All the mortars presented a similar behavior, as they increased their open porosity in similar proportions from t 56 to t 118 , except for the ZS mortar, which had the highest increase in its open porosity. However, the open porosity percentage of the ZS mortar at t 118 was the lowest open porosity compared with the other three mortars. The increment of the open porosity observed was related to the sulfate attack effects, which would involve the formation of expansive products [43], producing a loss of solid fraction in the mortar [2]. Figure 8 shows the capillary water absorption coefficients calculated according to the EN 1015-18 standard [39]. The coefficients were evaluated for the mortars at t 118 , both the non-attacked mortars and the mortars attacked with the sulfuric solution. For the non-attacked mortars, the highest coefficient was obtained for the NS mortar, and as expected, the lowest coefficient was obtained for the ZS mortar. The capillary water absorption coefficients after 90 days of sulfuric acid exposure of the mortars reduced its value in similar proportions, except the ZS mortar that increased its coefficient to 82.0%. The reduction of the absorption coefficients for the mortars (C mortar, ES mortar, and NS mortar) may be due to the network of gypsum crystals that was compact enough to obstruct the capillary network [7] and can be seen in the SEM micrographs of the mortars attacked ( Figure 9). It should be highlighted that the zinc stearate had a hydrophobic effect, and therefore, the ZS mortar had the lowest absorption coefficient, both in a non-aggressive environment and under acid exposure. A possible reason for the increase of capillary water absorption found in ZS mortars is that the gypsum coating created on the surface was weaker and more external as compared to the C, ES, and NS mortars (Figure 9f). On the other hand, the observed differences were minimal considering the magnitude of the capillary water uptake.
Although the capillary water absorption coefficients were obtained according to the UNE EN 1015-18 standard (Figure 8), they were also obtained using linear regression for water absorption data as a function of time (Table 2). It was found that in almost all cases (except in the ZS mortar) the water absorption variation was practically linear, as shown by the values of the correlation coefficients (R 2 ) that are close to the unit. Moreover, the values of the regression line slopes obtained are almost the same as the obtained following the indications of the standard EN 1015-18 [39]. Therefore, as some authors show [52], the absorption coefficient can be obtained as the slope of the regression lines.
Sustainability 2018, 10, x FOR PEER REVIEW 9 of 14 acid exposure. A possible reason for the increase of capillary water absorption found in ZS mortars is that the gypsum coating created on the surface was weaker and more external as compared to the C, ES, and NS mortars (Figure 9f). On the other hand, the observed differences were minimal considering the magnitude of the capillary water uptake. Although the capillary water absorption coefficients were obtained according to the UNE EN 1015-18 standard (Figure 8), they were also obtained using linear regression for water absorption data as a function of time (Table 2). It was found that in almost all cases (except in the ZS mortar) the water absorption variation was practically linear, as shown by the values of the correlation coefficients (R 2 ) that are close to the unit. Moreover, the values of the regression line slopes obtained are almost the same as the obtained following the indications of the standard EN 1015-18 [39]. Therefore, as some authors show [52], the absorption coefficient can be obtained as the slope of the regression lines.

Scanning Electron Microscopy (SEM) Examination
Degradation is usually found as a result of the severe external damage. The sulfuric acid attack created on the mortars' surface a layer of gypsum that could be observed both with the naked eye and the SEM images. The surfaces of the specimens exposed to the acid attack had a white coating visible to the naked eye (Figure 3c). SEM micrographs were taken from the surface of the mortar samples (Figure 9). In the SEM images of the specimens attacked, typical gypsum acicular forms acid exposure. A possible reason for the increase of capillary water absorption found in ZS mortars is that the gypsum coating created on the surface was weaker and more external as compared to the C, ES, and NS mortars (Figure 9f). On the other hand, the observed differences were minimal considering the magnitude of the capillary water uptake. Although the capillary water absorption coefficients were obtained according to the UNE EN 1015-18 standard (Figure 8), they were also obtained using linear regression for water absorption data as a function of time (Table 2). It was found that in almost all cases (except in the ZS mortar) the water absorption variation was practically linear, as shown by the values of the correlation coefficients (R 2 ) that are close to the unit. Moreover, the values of the regression line slopes obtained are almost the same as the obtained following the indications of the standard EN 1015-18 [39]. Therefore, as some authors show [52], the absorption coefficient can be obtained as the slope of the regression lines.

Scanning Electron Microscopy (SEM) Examination
Degradation is usually found as a result of the severe external damage. The sulfuric acid attack created on the mortars' surface a layer of gypsum that could be observed both with the naked eye and the SEM images. The surfaces of the specimens exposed to the acid attack had a white coating visible to the naked eye (Figure 3c). SEM micrographs were taken from the surface of the mortar samples ( Figure 9). In the SEM images of the specimens attacked, typical gypsum acicular forms

Scanning Electron Microscopy (SEM) Examination
Degradation is usually found as a result of the severe external damage. The sulfuric acid attack created on the mortars' surface a layer of gypsum that could be observed both with the naked eye and the SEM images. The surfaces of the specimens exposed to the acid attack had a white coating visible to the naked eye (Figure 3c). SEM micrographs were taken from the surface of the mortar samples ( Figure 9). In the SEM images of the specimens attacked, typical gypsum acicular forms were observed (Figure 9, right). Moreover, the energy-dispersive X-ray spectroscopies (EDX) carried out (Table 3) revealed that the mortars exposed to the acid attack contained more sulfur than the non-attacked mortars. The percentage of sulfur found in the ZS mortars attacked with the sulfuric acid solution was the lowest, while for the rest of the mortars was similar. Calcium sulfate (gypsum) is formed in a reaction of the acid with calcium hydroxide and the C-S-H phase. The chemical reactions result in a profound degradation of the hydrated cement paste, and is associated with a loss of strength (Figure 4) [38]. All carbonate-based rocks, such as limestone (used in this research), dolomite, magnesite, etc., are vulnerable to this type of attack. The results are consistent with previous experiments conducted on adobe samples and rendering mortars attacked by sulfuric acid, where the formation of gypsum patina is due to the following chemical reaction [32,44,53]:  were observed (Figure 9, right). Moreover, the energy-dispersive X-ray spectroscopies (EDX) carried out (Table 3) revealed that the mortars exposed to the acid attack contained more sulfur than the non-attacked mortars. The percentage of sulfur found in the ZS mortars attacked with the sulfuric acid solution was the lowest, while for the rest of the mortars was similar. Calcium sulfate (gypsum) is formed in a reaction of the acid with calcium hydroxide and the C-S-H phase. The chemical reactions result in a profound degradation of the hydrated cement paste, and is associated with a loss of strength ( Figure 4) [38]. All carbonate-based rocks, such as limestone (used in this research), dolomite, magnesite, etc., are vulnerable to this type of attack. The results are consistent with previous experiments conducted on adobe samples and rendering mortars attacked by sulfuric acid, where the formation of gypsum patina is due to the following chemical reaction [32,44,53]: CaCO3 + H2SO4 + 2H2O → CaSO42H2O (coating) + CO2 + H2O

Conclusions
This work analyzed the effects of a sulfuric acid attack on the physical and mechanical properties of four different mortars: control mortar, nanosilica mortar, zinc stearate mortar, and a mortar with an ethyl silicate coating. Sulfuric acid attack has significant consequences on the microstructure and the physical and mechanical properties of the mortars. A marked loss in the mechanical properties was accompanied by microstructural alterations. In general, none of the treatments showed an obvious benefit when compared to the control sample. From the results presented in this study, the following conclusions can be drawn: • The compressive strength of the four mortars studied decreased when they were exposed to a sulfuric acid attack. The effect on the strength reduction of the mortars was greater with longer periods of acid exposure. The control mortar had the highest compressive strength after the acid attack. However, the zinc stearate mortar had the lowest percentage of compressive strength loss after 90 days of sulfuric acid attack.

•
All the mortars reduced their mass after the sulfuric acid exposure. The lowest mass loss of zinc stearate mortar proves that this mortar best resisted the acid attack.

•
The zinc stearate mortar demonstrated its hydrophobic effect with the lowest coefficient of capillary water absorption, both in a non-aggressive and aggressive environment. • Nanosilica mortar and the mortar with an ethyl silicate coating do not present positive effects when they are exposed to sulfuric acid.

•
As a consequence of the sulfuric acid attack, an increase in the open porosity of mortars makes them more vulnerable to aggressive environments. Moreover, a decrease in the ultrasonic pulse velocity is detected after the acid attack.

Conclusions
This work analyzed the effects of a sulfuric acid attack on the physical and mechanical properties of four different mortars: control mortar, nanosilica mortar, zinc stearate mortar, and a mortar with an ethyl silicate coating. Sulfuric acid attack has significant consequences on the microstructure and the physical and mechanical properties of the mortars. A marked loss in the mechanical properties was accompanied by microstructural alterations. In general, none of the treatments showed an obvious benefit when compared to the control sample. From the results presented in this study, the following conclusions can be drawn:

•
The compressive strength of the four mortars studied decreased when they were exposed to a sulfuric acid attack. The effect on the strength reduction of the mortars was greater with longer periods of acid exposure. The control mortar had the highest compressive strength after the acid attack. However, the zinc stearate mortar had the lowest percentage of compressive strength loss after 90 days of sulfuric acid attack.

•
All the mortars reduced their mass after the sulfuric acid exposure. The lowest mass loss of zinc stearate mortar proves that this mortar best resisted the acid attack.

•
The zinc stearate mortar demonstrated its hydrophobic effect with the lowest coefficient of capillary water absorption, both in a non-aggressive and aggressive environment. • Nanosilica mortar and the mortar with an ethyl silicate coating do not present positive effects when they are exposed to sulfuric acid.
• As a consequence of the sulfuric acid attack, an increase in the open porosity of mortars makes them more vulnerable to aggressive environments. Moreover, a decrease in the ultrasonic pulse velocity is detected after the acid attack.

•
The results support that an alternative and reliable method of obtaining the capillary water absorption coefficient for mortars is by calculating the slope of the linear regression lines for the water absorption data as a function of time.