*3.3. Sulfate Attack*

### 3.3.1. Visual Assessment

The performance of cubic concrete samples immersed in the MgSO<sup>4</sup> solution was visually inspected monthly in this investigation. The degradation scale used to assess the damage is shown in Table 4. Figure 4 depicts the appearances of specimens after 12 months of exposure, while Figure 3 depicts the deterioration degree of the OPC and POFA specimens in a MgSO<sup>4</sup> solution for the entire exposure period. The plain OPC mix without WPFT fibers was the only mix that showed initial signs of deterioration, such as small cracks along the corners of specimens. POFA-based specimens, on the other hand, showed the first marks of degradation after three months of immersion. As shown in Figure 5, the deterioration of all specimens, with and without fibers, accelerated over time. The evolution of the OPC-based mixtures was faster than that of the samples containing 30% POFA. The specimens containing a higher dose of WPFT fibers were less severely deteriorated after 12 months of exposure than the plain concrete mixes.

Since CaCO<sup>3</sup> and Ca(OH)<sup>2</sup> are the most vulnerable elements of the cement hydrate, degradation was more intense in the OPC specimens, which have higher contents of these components. This finding suggests that sulfate-induced degradation is exacerbated by the low resistance of CaCO<sup>3</sup> and Ca(OH)<sup>2</sup> to sulfate assault, resulting in the formation of calcium sulfate (CaSO4) [21]. The small cracks and slight deterioration that occurred at the corners of samples were the initial evidence of attack in all cases, which appears as small cracks, slow expansion of samples, and the spalling of edges. A soft white substance was also applied to the interior surfaces of the fractures. The presence of fibers was discovered to influence the degree of deterioration, as well as the progression. As can be seen, the deterioration of the specimens studied progressed more slowly in combinations reinforced with WPFT fibers, and greater fiber volume fractions showed stronger resilience. It is also worth noting that the final degree of deterioration in POFA-based combinations was lower than in the OPC mixtures for all specimens.

### 3.3.2. Mass Gain

Figure 6 depicts the variation in the masses of the concrete cube samples when immersed in a MgSO<sup>4</sup> solution for 12 months, revealing that all mixtures gained mass after the end of the exposure period due to sulfate particle absorption by the specimens [21]. The results revealed rises in masses by 1.3 and 0.8% for the control mixes of OPC and POFA, respectively, after 12 months of exposure. The addition of WPFT fiber to the mixture, on the other hand, drastically reduced the mass increase of the specimens. The addition of fiber in the mixes appears to have contributed to the reduction in mass gain, as shown in Figure 6. For fiber contents of 0.2, 0.4, 0.6, 0.8, and 1%, the mass gains of OPC fibrous mixes were 0.48, 0.56, 0.96, 1.27, and 1.57%, respectively. The results also demonstrated that

adding 30% POFA to the fibrous composites resulted in lesser mass gains when compared to the OPC mixes when submerged in a sulfate solution. 8 Extensive deterioration and washout 9 Sample breakdown 8 Extensive deterioration and washout 9 Sample breakdown

1 Slight deterioration at edges in the form of microcracks

1 Slight deterioration at edges in the form of microcracks

3 Cracking at corners and along the edges of samples 4 Severe cracks and expansion along the edges 5 Widespread cracks and sample expansion 6 Further expansion and side deterioration of samples

3 Cracking at corners and along the edges of samples 4 Severe cracks and expansion along the edges 5 Widespread cracks and sample expansion 6 Further expansion and side deterioration of samples

also applied to the interior surfaces of the fractures. The presence of fibers was discovered to influence the degree of deterioration, as well as the progression. As can be seen, the deterioration of the specimens studied progressed more slowly in combinations reinforced with WPFT fibers, and greater fiber volume fractions showed stronger resilience. It is also worth noting that the final degree of deterioration in POFA-based combinations

also applied to the interior surfaces of the fractures. The presence of fibers was discovered to influence the degree of deterioration, as well as the progression. As can be seen, the deterioration of the specimens studied progressed more slowly in combinations reinforced with WPFT fibers, and greater fiber volume fractions showed stronger resilience. It is also worth noting that the final degree of deterioration in POFA-based combinations

*Crystals* **2021**, *11*, x FOR PEER REVIEW 9 of 22

*Crystals* **2021**, *11*, x FOR PEER REVIEW 9 of 22

was lower than in the OPC mixtures for all specimens.

was lower than in the OPC mixtures for all specimens.

**Deterioration** 

**Deterioration** 

**Table 4.** Assumed degree of deterioration and the detected damages.

**Table 4.** Assumed degree of deterioration and the detected damages.

**Scale Observed Damages** 

**Scale Observed Damages** 

0 No noticeable deterioration

0 No noticeable deterioration

2 Deterioration at edges and corners

2 Deterioration at edges and corners

7 Wide expansion and spalling

7 Wide expansion and spalling

**Figure 4.** The appearance of concrete specimens exposed to a MgSO4 solution for 12 months. **Figure 4.** The appearance of concrete specimens exposed to a MgSO<sup>4</sup> solution for 12 months. **Figure 4.** The appearance of concrete specimens exposed to a MgSO4 solution for 12 months.

**Figure 5.** Deterioration degree of concrete specimens reinforced with WPFT fibers exposed to MgSO4 solutions. **Figure 5.** Deterioration degree of concrete specimens reinforced with WPFT fibers exposed to MgSO<sup>4</sup> solutions.

the end of the exposure period due to sulfate particle absorption by the specimens [21]. The results revealed rises in masses by 1.3 and 0.8% for the control mixes of OPC and POFA, respectively, after 12 months of exposure. The addition of WPFT fiber to the mixture, on the other hand, drastically reduced the mass increase of the specimens. The addition of fiber in the mixes appears to have contributed to the reduction in mass gain, as shown in Figure 6. For fiber contents of 0.2, 0.4, 0.6, 0.8, and 1%, the mass gains of OPC fibrous mixes were 0.48, 0.56, 0.96, 1.27, and 1.57%, respectively. The results also demonstrated that adding 30% POFA to the fibrous composites resulted in lesser mass gains

The variations in the masses of POFA-based mixes were recorded as 0.36, 0.47, 0.77, 1.1, and 1.35% for the similar fiber levels, respectively, which indicates a gain in mass for all specimens. The development of a grid structure by fibers, which restricts particle penetration and disruption into specimens, might be attributed to the drop in the mass gain of specimens reinforced with WPFT fiber. As a result of the irregular pore arrangement of the POFA concrete and the quantity of calcium hydroxide contained in the matrix, the use of POFA in mixtures is favorable when in direct contact with a sulfate attack. Microcracks are the primary entry points for hazardous chemical ions, such as sulfate ions, into the concrete interior. The exposure of concrete specimens, either plain concrete or fiber-reinforced concrete, followed several steps, as shown in Figure 7, including: (1) initial exposure, (2) chemical reactions, (3) stress development in the concrete specimens, (4) crack formation, (5) crack development and infiltration, and (6) substantial damage in various forms. It can be seen that there were no noteworthy changes in the process of sulfate attacks amongst the plain mix and mixes reinforced with fibers, with gypsum and ettringite development producing expansion tension to cause concrete degradation [24]. The bridging action of the fiber, which arrests microcracks, reduces the permeability and spalling of the concrete significantly. Furthermore, the fibers prevent the passage of disturbance particles into specimens by arresting microcracks induced by excessive stress [25]. Consequently, the concrete mixtures comprising fibers gained less mass than conventional plain

when compared to the OPC mixes when submerged in a sulfate solution.

3.3.2. Mass Gain

concrete.


**Table 4.** Assumed degree of deterioration and the detected damages.

**Figure 6.** The effects of the WPFT fibers on the mass gain of concrete samples exposed to a MgSO4 **Figure 6.** The effects of the WPFT fibers on the mass gain of concrete samples exposed to a MgSO<sup>4</sup> solution.

solution. The variations in the masses of POFA-based mixes were recorded as 0.36, 0.47, 0.77, 1.1, and 1.35% for the similar fiber levels, respectively, which indicates a gain in mass for all specimens. The development of a grid structure by fibers, which restricts particle penetration and disruption into specimens, might be attributed to the drop in the mass gain of specimens reinforced with WPFT fiber. As a result of the irregular pore arrangement of the POFA concrete and the quantity of calcium hydroxide contained in the matrix, the use of POFA in mixtures is favorable when in direct contact with a sulfate attack. Microcracks are the primary entry points for hazardous chemical ions, such as sulfate ions, into the concrete interior. The exposure of concrete specimens, either plain concrete or fiber-reinforced concrete, followed several steps, as shown in Figure 7, including: (1) initial exposure, (2) chemical reactions, (3) stress development in the concrete specimens, (4) crack formation, (5) crack development and infiltration, and (6) substantial damage in various forms. It can be seen that there were no noteworthy changes in the process of sulfate attacks amongst the plain mix and mixes reinforced with fibers, with gypsum and ettringite development producing expansion tension to cause concrete degradation [24]. The bridging action of the fiber, which arrests microcracks, reduces the permeability and spalling of the concrete significantly. Furthermore, the fibers prevent the passage of disturbance particles into specimens by arresting microcracks induced by excessive stress [25]. Consequently, the concrete mixtures comprising fibers gained less mass than conventional plain concrete.

(**a**) Plain concrete

solution.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Mass gain (%)

**Figure 6.** The effects of the WPFT fibers on the mass gain of concrete samples exposed to a MgSO4

0 0.2 0.4 0.6 0.8 1

Fiber volume fraction (%)

OPC 30% POFA

(**b**) Fiber-reinforced concrete

**Figure 7.** Diagrammatic representation of sulfate attack against (**a**) plain concrete and (**b**) fiber-reinforced concrete. **Figure 7.** Diagrammatic representation of sulfate attack against (**a**) plain concrete and (**b**) fiber-reinforced concrete.

### 3.3.3. Compressive Strength Loss 3.3.3. Compressive Strength Loss

Generally, concrete performance often degrades when exposed to a chemical attack and extends the immersion period, owing to physical destruction and undesirable chemical breakdown of the matrix. The inclusion of WPFT fibers improved the resistance to magnesium sulfate assault, as seen in Figure 8. After 12 months of exposure, the compressive strength of concrete mixtures exposed to sulfate attacks was evaluated, and the attained outcomes were associated with the water-cured mixes. The strength loss was described as the variation in strength among water-cured mixes and those exposed to Generally, concrete performance often degrades when exposed to a chemical attack and extends the immersion period, owing to physical destruction and undesirable chemical breakdown of the matrix. The inclusion of WPFT fibers improved the resistance to magnesium sulfate assault, as seen in Figure 8. After 12 months of exposure, the compressive strength of concrete mixtures exposed to sulfate attacks was evaluated, and the attained outcomes were associated with the water-cured mixes. The strength loss was described as the variation in strength among water-cured mixes and those exposed to MgSO4. The

MgSO4. The data in Figure 8 demonstrate that all of the blends had lost strength. When

MPa were detected for fiber dosages of 0, 0.2, 0.4, 0.6, 0.8, and 1%, correspondingly, whereas the values of 7.5, 5.7, 3.6, 8.4, 10.7, and 12.5 MPa were recorded as strength losses for the same fiber dosages, respectively, in the POFA mixtures. It was observed that the obtained values are lower in POFA mixtures than those of OPC, which indicates the better

The tensile expansion strain from the development of ettringite and gypsum in matrix induces cracking and reduces the strength characteristics of concrete because of its low tensile strength. Furthermore, the magnesium attack causes calcium compounds to be released from C-S-H gels, reducing matrix stiffness and causing the specimens to break down [26]. By microscopic investigation, Honglei et al. [43] reported the formation of gypsum with double layers on the surface of the concrete samples, with only a slight amount of ettringite and monosulfate. Magnesium sulfate attack is characterized by a loss of strength and adhesion rather than cracking and expansion [44]. Furthermore, WPFT fibers inhibited the creation of tiny cracks and failure of specimens due to their bridging effect. The strength loss factor (SLF), following Equation (2), was used to indicate the decline of compressive strength of concrete mixtures containing WPFT fibers after sulfate attack. Figure 9 illustrates the SLF after 12 months of exposure to the MgSO4 solution. Sulfate reactions altered all of the mixes and resulted in strength reduction. Under the same circumstances, the SLF values were much higher in the OPC fibrous composites, but the POFA specimen containing 0.4% WPFT fibers had the lowest SLFs; however, a further

performance of POFA concrete when exposed to chemical attacks.

data in Figure 8 demonstrate that all of the blends had lost strength. When the OPC mixes were associated with those of POFA mixes, the loss was found to be greater. For example, in the OPC mixes, strength losses of 9.5, 6.3, 5.2, 9.6, 12.5, and 14.6 MPa were detected for fiber dosages of 0, 0.2, 0.4, 0.6, 0.8, and 1%, correspondingly, whereas the values of 7.5, 5.7, 3.6, 8.4, 10.7, and 12.5 MPa were recorded as strength losses for the same fiber dosages, respectively, in the POFA mixtures. It was observed that the obtained values are lower in POFA mixtures than those of OPC, which indicates the better performance of POFA concrete when exposed to chemical attacks. rise in the dosage of fibers resulted in a minor surge in the SLF values. There has been no research conducted so far on the combined effect of POFA and WPFT fibers on the sulfate resistance of concrete. Nevertheless, the study results are comparable to those of Behfarnia and Farshadfar [34] when considering the effective combination of PP fibers and pozzolanic ashes in concrete bare to chemical attacks. They reported that the combination of PP fibers and pozzolanic ashes has a significant effect on improving concrete resistance against sulfate attacks.

*Crystals* **2021**, *11*, x FOR PEER REVIEW 13 of 22

**Figure 8.** The variation in the 365-day compressive strength of concrete mixtures containing WPFT fibers exposed to a MgSO4 solution. **Figure 8.** The variation in the 365-day compressive strength of concrete mixtures containing WPFT fibers exposed to a MgSO<sup>4</sup> solution.

The tensile expansion strain from the development of ettringite and gypsum in matrix induces cracking and reduces the strength characteristics of concrete because of its low tensile strength. Furthermore, the magnesium attack causes calcium compounds to be released from C-S-H gels, reducing matrix stiffness and causing the specimens to break down [26]. By microscopic investigation, Honglei et al. [43] reported the formation of gypsum with double layers on the surface of the concrete samples, with only a slight amount of ettringite and monosulfate. Magnesium sulfate attack is characterized by a loss of strength and adhesion rather than cracking and expansion [44]. Furthermore, *Crystals* 

WPFT fibers inhibited the creation of tiny cracks and failure of specimens due to their bridging effect.

The strength loss factor (SLF), following Equation (2), was used to indicate the decline of compressive strength of concrete mixtures containing WPFT fibers after sulfate attack. Figure 9 illustrates the SLF after 12 months of exposure to the MgSO<sup>4</sup> solution. Sulfate reactions altered all of the mixes and resulted in strength reduction. Under the same circumstances, the SLF values were much higher in the OPC fibrous composites, but the POFA specimen containing 0.4% WPFT fibers had the lowest SLFs; however, a further rise in the dosage of fibers resulted in a minor surge in the SLF values. There has been no research conducted so far on the combined effect of POFA and WPFT fibers on the sulfate resistance of concrete. Nevertheless, the study results are comparable to those of Behfarnia and Farshadfar [34] when considering the effective combination of PP fibers and pozzolanic ashes in concrete bare to chemical attacks. They reported that the combination of PP fibers and pozzolanic ashes has a significant effect on improving concrete resistance against sulfate attacks. **2021**, *11*, x FOR PEER REVIEW 14 of 22

**Figure 9.** The effects of WPFT fibers on the strength loss factor of concrete exposed to a MgSO4 **Figure 9.** The effects of WPFT fibers on the strength loss factor of concrete exposed to a MgSO<sup>4</sup> solution.

### solution. *3.4. Sulfate Attack*

months.

### 3.4.1. Visual Assessment

*3.4. Sulfate Attack*  3.4.1. Visual Assessment The cubic samples submerged in a H2SO4 solution were visually inspected monthly for 12 months, similar to the sulfate resistance test. Figure 10 reveals the appearance of concrete specimens with and without WPFT fibers when exposed to the acid solution after 12 months. The initial stage in measuring the deteriorating consequence of acidic attack is commonly visual assessment through analyzing the color change, subsurface cracks, the deposition of additional components on the surface, and the spalling of specimens. As illustrated in Figure 11, the appearance and color of the concrete specimens provides a comprehensive indicator of the acid's effect on the material, regardless of whether the The cubic samples submerged in a H2SO<sup>4</sup> solution were visually inspected monthly for 12 months, similar to the sulfate resistance test. Figure 10 reveals the appearance of concrete specimens with and without WPFT fibers when exposed to the acid solution after 12 months. The initial stage in measuring the deteriorating consequence of acidic attack is commonly visual assessment through analyzing the color change, subsurface cracks, the deposition of additional components on the surface, and the spalling of specimens. As illustrated in Figure 11, the appearance and color of the concrete specimens provides a comprehensive indicator of the acid's effect on the material, regardless of whether the color reflects the original or acid-attacked surface. It was detected that the addition of WPFT fibers and POFA in concrete mixtures results in slight changes in the shape and texture of concrete samples exposed to acid, although both the plain and fiber-reinforced OPC samples were fragmented and distorted after the 12 months of exposure.

color reflects the original or acid-attacked surface. It was detected that the addition of WPFT fibers and POFA in concrete mixtures results in slight changes in the shape and texture of concrete samples exposed to acid, although both the plain and fiber-reinforced OPC samples were fragmented and distorted after the 12 months of exposure. The damage shown in Figure 12 was measured using the degradation scale provided in Table 4. As illustrated in Figure 12, all specimens, with and without fibers, deteriorated The damage shown in Figure 12 was measured using the degradation scale provided in Table 4. As illustrated in Figure 12, all specimens, with and without fibers, deteriorated faster over time. The evolution of the OPC-based mixtures was faster than that of the 30% POFA samples. For example, after 12 months of exposure, the plain OPC mix had a deterioration degree of 8, whereas the plain POFA mix had a deterioration degree of 6. For the entire testing period, the degree of degradation decreased as the fiber volume percentage increased. It is worth noting that a larger fiber content exhibited

faster over time. The evolution of the OPC-based mixtures was faster than that of the 30%

rioration degree of 8, whereas the plain POFA mix had a deterioration degree of 6. For the entire testing period, the degree of degradation decreased as the fiber volume percentage increased. It is worth noting that a larger fiber content exhibited an excellently increased acid resistance and lower deterioration level. When exposed to acid, concrete specimens containing WPFT fibers showed less spalling than the control specimens without fibers.

**Figure 10.** The appearance of various concrete specimens exposed to a H2SO4 solution for 12

solution.

0

10

20

Strength loss factor (%)

30

40

50

months.

*Crystals* 

*3.4. Sulfate Attack* 

3.4.1. Visual Assessment

an excellently increased acid resistance and lower deterioration level. When exposed to acid, concrete specimens containing WPFT fibers showed less spalling than the control specimens without fibers. color reflects the original or acid-attacked surface. It was detected that the addition of WPFT fibers and POFA in concrete mixtures results in slight changes in the shape and texture of concrete samples exposed to acid, although both the plain and fiber-reinforced OPC samples were fragmented and distorted after the 12 months of exposure.

### 3.4.2. Mass Loss The damage shown in Figure 12 was measured using the degradation scale provided

*Crystals* **2021**, *11*, x FOR PEER REVIEW 14 of 22

OPC 30% POFA

0 0.2 0.4 0.6 0.8 1

Fiber volume fraction (%)

The fiber reinforcement had a considerable influence on mass loss, as demonstrated by the results. This could be ascribed to the fiber bridging action, which reduced concrete components from spalling and improved performance in an acidic environment. The mass loss vs. fiber volume fraction test results are shown in Figure 13. At the end of 12 months of exposure to H2SO<sup>4</sup> solutions, all concrete specimens showed the same mass reduction behavior. The plain OPC specimens were found with a comparatively higher rate of deterioration in acidic solutions. As OPC contains about 62.5% CaO, it results in faster chemical reactions with acid and forms increased levels of gypsum and ettringite, which are washed out quickly and result in a higher mass rate loss spalling. in Table 4. As illustrated in Figure 12, all specimens, with and without fibers, deteriorated faster over time. The evolution of the OPC-based mixtures was faster than that of the 30% POFA samples. For example, after 12 months of exposure, the plain OPC mix had a deterioration degree of 8, whereas the plain POFA mix had a deterioration degree of 6. For the entire testing period, the degree of degradation decreased as the fiber volume percentage increased. It is worth noting that a larger fiber content exhibited an excellently increased acid resistance and lower deterioration level. When exposed to acid, concrete specimens containing WPFT fibers showed less spalling than the control specimens without fibers.

**Figure 9.** The effects of WPFT fibers on the strength loss factor of concrete exposed to a MgSO4

The cubic samples submerged in a H2SO4 solution were visually inspected monthly for 12 months, similar to the sulfate resistance test. Figure 10 reveals the appearance of concrete specimens with and without WPFT fibers when exposed to the acid solution after 12 months. The initial stage in measuring the deteriorating consequence of acidic attack is commonly visual assessment through analyzing the color change, subsurface cracks, the

illustrated in Figure 11, the appearance and color of the concrete specimens provides a comprehensive indicator of the acid's effect on the material, regardless of whether the

**Figure 10.** The appearance of various concrete specimens exposed to a H2SO4 solution for 12 **Figure 10.** The appearance of various concrete specimens exposed to a H2SO<sup>4</sup> solution for 12 months. **2021**, *11*, x FOR PEER REVIEW 15 of 22

**Figure 11.** The effects of H2SO4 exposure on the texture and color of the concrete matrix. **Figure 11.** The effects of H2SO<sup>4</sup> exposure on the texture and color of the concrete matrix.

0% 0.20% **OPC**

1 2 3 4 5 6 7 8 9 10 11 12

0% 0.20% **30% POFA**

1 2 3 4 5 6 7 8 9 10 11 12

Time (months)

Time (months)

**Figure 12.** Deterioration degree of concrete specimens comprising WPFT fibers exposed to H2SO4

The fiber reinforcement had a considerable influence on mass loss, as demonstrated

by the results. This could be ascribed to the fiber bridging action, which reduced concrete components from spalling and improved performance in an acidic environment. The mass loss vs. fiber volume fraction test results are shown in Figure 13. At the end of 12 months of exposure to H2SO4 solutions, all concrete specimens showed the same mass reduction behavior. The plain OPC specimens were found with a comparatively higher rate of dete-

0.40% 0.60% 0.80% 1%

0.40% 0.60% 0.80% 1%

solutions.

Deterioration degree

Deterioration degree

3.4.2. Mass Loss

**Figure 11.** The effects of H2SO4 exposure on the texture and color of the concrete matrix.

**Figure 12.** Deterioration degree of concrete specimens comprising WPFT fibers exposed to H2SO4 solutions. **Figure 12.** Deterioration degree of concrete specimens comprising WPFT fibers exposed to H2SO<sup>4</sup> solutions. respectively. According to the findings, the incorporation of WPFT fibers and POFA led to the enhancement of concrete against acid attack and reduced spalling and mass loss.

**Figure 13.** The effects of the WPFT fibers on the mass variation of the concrete specimens after 12 months of exposure in a H2SO4 solution. **Figure 13.** The effects of the WPFT fibers on the mass variation of the concrete specimens after 12 months of exposure in a H2SO<sup>4</sup> solution.

and specimens submerged in a H2SO4 solution for 365 days. The extensive immersion in the acid solution was thought to be the cause of the strength reduction in all mixes. It was discovered that as the amount of WPFT fibers in the concrete increased, the strength loss reduced, whereas the rate of strength loss was higher in the plain concrete mix. When comparing the OPC specimens to 30% POFA mixes, the loss was shown to be greater. For OPC mixes comprising 0, 0.2, 0.4, 0.6, 0.8, and 1% WPFT fibers, values of 31.4, 27.5, 24.7, 23.4, 21.7, and 20.8 MPa were recorded as strength losses, respectively. For the same amounts of fiber, the losses in the strength of POFA mixes were recorded as 28.4, 25.4,

3.4.3. Strength Loss

22.8, 21.7, 20.5, and 19.9 MPa.

In contrast, the CaO content is only about 5% in POFA and provides it good acid resistance as a material in concrete production [22]. The reinforcement of concrete with WPFT fibers reduced the rate of mass loss in the OPC mixtures. In the OPC mixes reinforced with 0, 0.2, 0.4, 0.6, 0.8, and 1% WPFT fibers, mass loss values of 17.5, 12.7, 9.7, 7.8, 6.6, and 5.4% were recorded, respectively, indicating that the levels of mass loss in fibrous mixtures was much lower than those in plain mixtures. Acid penetrated the concrete and dissolved the cement paste binder, producing soft and soluble gypsum that subsequently interacted with Ca(OH)<sup>2</sup> to form ettringite, leading to mass losses. The presence of fibers helped the concrete constituents to join via a bridging action, resulting in less breakage and deterioration of the matrix. Another harmful consequence of acid attack was the reactions among aluminate and calcium components in the OPC, leading to the formation of soluble products. These exceptionally expansive and soluble products caused internal pressures in the matrix, and microcracks and voids then formed and initiated the deterioration of the concrete and a loss in the strength of the concrete components [21].

It is worth noting that the mass loss rates in the 30% POFA mixes were far lower than the OPC specimens. This might be due to a lower concentration of reactive compounds, like CaO, which slowed the degradation process and prevented ettringite's creation, which would have caused the concrete to expand [23]. The replacement of cement with POFA resulted in a more remarkable performance for the concrete. For the exact dosages of WPFT fibers, the mass losses of the POFA mixes were 15.6, 10.5, 8.3, 5.8, 3.5, and 3.2%, respectively. According to the findings, the incorporation of WPFT fibers and POFA led to the enhancement of concrete against acid attack and reduced spalling and mass loss.

### 3.4.3. Strength Loss

Figure 14 shows a comparison of the strength values among water-cured samples and specimens submerged in a H2SO<sup>4</sup> solution for 365 days. The extensive immersion in the acid solution was thought to be the cause of the strength reduction in all mixes. It was discovered that as the amount of WPFT fibers in the concrete increased, the strength loss reduced, whereas the rate of strength loss was higher in the plain concrete mix. When comparing the OPC specimens to 30% POFA mixes, the loss was shown to be greater. For OPC mixes comprising 0, 0.2, 0.4, 0.6, 0.8, and 1% WPFT fibers, values of 31.4, 27.5, 24.7, 23.4, 21.7, and 20.8 MPa were recorded as strength losses, respectively. For the same amounts of fiber, the losses in the strength of POFA mixes were recorded as 28.4, 25.4, 22.8, 21.7, 20.5, and 19.9 MPa.

Due to the good pozzolanic activity of POFA and a lower CaO content, the combined effect of WPFT fiber and 30% POFA caused a minor rate of strength loss. This is because OPC's CaO content reacts with POFA's reactive SiO<sup>2</sup> during the hydration process, resulting the formation of additional products, which helps to enhance the acid resistance of the matrix [22]. The use of POFA, on the other hand, has permitted the production of concrete that is very resistant to acid attacks. The presence of WPFT fibers reduced the creation of fine cracks and specimen spalling due to their linking effect. As such, the reinforced specimens immersed in the acid solution maintained their ductile behavior with a similar mode of failure, almost identical to that of the water-cured materials under compressive loads. Nevertheless, the control specimens without any fibers were completely distorted, and their failure modes were not the same as typical of water-cured concrete. In addition, as shown in Figure 15, the obtained SLF values of OPC mixtures were slightly higher than those noted for POFA mixes after 12 months of exposure to the acid solution. This behavior can be explained as follows: Adding 30% POFA to concrete specimens resulted in more Ca(OH)<sup>2</sup> being consumed throughout the hydration procedure, resulting in secondary C-S-H gels in the matrix, which reduced the micro-sized void density in the matrix [25]. The lower SLF values of reinforced POFA mixes signify the positive interaction of POFA and WPFT fiber in enhancing concrete performance against acid attacks.

*Crystals* 

WPFT fiber in enhancing concrete performance against acid attacks.

Due to the good pozzolanic activity of POFA and a lower CaO content, the combined effect of WPFT fiber and 30% POFA caused a minor rate of strength loss. This is because OPC's CaO content reacts with POFA's reactive SiO2 during the hydration process, resulting the formation of additional products, which helps to enhance the acid resistance of the matrix [22]. The use of POFA, on the other hand, has permitted the production of concrete that is very resistant to acid attacks. The presence of WPFT fibers reduced the creation of fine cracks and specimen spalling due to their linking effect. As such, the reinforced specimens immersed in the acid solution maintained their ductile behavior with a similar mode of failure, almost identical to that of the water-cured materials under compressive loads. Nevertheless, the control specimens without any fibers were completely distorted, and their failure modes were not the same as typical of water-cured concrete. In addition, as shown in Figure 15, the obtained SLF values of OPC mixtures were slightly higher than those noted for POFA mixes after 12 months of exposure to the acid solution. This behavior can be explained as follows: Adding 30% POFA to concrete specimens resulted in more Ca(OH)2 being consumed throughout the hydration procedure, resulting in secondary C-S-H gels in the matrix, which reduced the micro-sized void density in the matrix [25]. The lower SLF values of reinforced POFA mixes signify the positive interaction of POFA and

**Figure 14.** The variation in the 365-day compressive strength of concrete mixtures containing WPFT fibers exposed to a H2SO4 solution. **Figure 14.** The variation in the 365-day compressive strength of concrete mixtures containing WPFT fibers exposed to a H2SO<sup>4</sup> solution. **2021**, *11*, x FOR PEER REVIEW 18 of 22

**Figure 15.** The effects of WPFT fibers on the strength loss factor of concrete mixtures exposed to a H2SO4 solution. **Figure 15.** The effects of WPFT fibers on the strength loss factor of concrete mixtures exposed to a H2SO<sup>4</sup> solution.

ages of water-cured concrete samples and those immersed in MgSO4 and H2SO4 solutions for 365 days. Figure 16a shows the homogeneous spreading of C-S-H gel for water-cured specimens after 365 days. The POFA-based mixes had enhanced performance under chemical attacks due to the consumption of Ca(OH)2 in high amounts, which is a soluble component in OPC during the pozzolanic reactions between POFA and OPC particles along the exposure period [22]. The SEM results of conventional OPC and POFA concrete mixtures treated with a MgSO4 solution are shown in Figure 16b. The SEM image demonstrates how the morphology of the matrix changed when immersed in a sulfate solution. The spherical pores in the OPC concrete mix gradually filled with more precipitated particles due to sulfate exposure. It can be detected that the pore volume of the OPC matrix varied during the exposure period, which affects both the mechanical properties and the durability of the concrete. Consequently, due to the pozzolanic activity of POFA and the development of extra C-S-H gel, the majority of the voids in the mixtures containing 30% POFA filled up with these products, especially at later ages, leaving less space for the freshly harmful particles formed during the exposure period in the sulfate [41]. Accordingly, the strength and durability of concrete improved with the addition POFA as partial

Besides, the microstructures of the OPC and POFA samples after 12 months of exposure to a severe H2SO4 solution are shown in Figure 16c. The production of gypsum at a high level can be seen in the OPC specimens. In the POFA matrix, the generation of gypsum was marginally lower. This was attributed to the existence of a greater amount of Ca(OH)2 particles in the OPC, which caused the formation of a soluble product such as gypsum in the matrix. Generally, gypsum with a weak structure is the main product of a chemical reaction between sulfuric acid and cement particles, which causes the specimen to expand and split [36]. During the hydration process, the Ca(OH)2 in the POFA matrix was consumed, generating C-S-H gels, which have a greater ability to resist acid attacks. Besides, the reactions between other components of OPC, such as calcium aluminate and gypsum, may result in the creation of ettringite, which expands the specimens and con-

*3.5. Scanning Electron Microscopy Analysis* 

cement replacement [45,46].

tributes to the formation of more cracks [47,48].

### *3.5. Scanning Electron Microscopy Analysis*

Chemical attacks are primarily manifested by losing strength and adherence rather than cracking and expansion. Figure 16 shows scanning electron microscopy (SEM) images of water-cured concrete samples and those immersed in MgSO<sup>4</sup> and H2SO<sup>4</sup> solutions for 365 days. Figure 16a shows the homogeneous spreading of C-S-H gel for watercured specimens after 365 days. The POFA-based mixes had enhanced performance under chemical attacks due to the consumption of Ca(OH)<sup>2</sup> in high amounts, which is a soluble component in OPC during the pozzolanic reactions between POFA and OPC particles along the exposure period [22]. The SEM results of conventional OPC and POFA concrete mixtures treated with a MgSO<sup>4</sup> solution are shown in Figure 16b. The SEM image demonstrates how the morphology of the matrix changed when immersed in a sulfate solution. The spherical pores in the OPC concrete mix gradually filled with more precipitated particles due to sulfate exposure. It can be detected that the pore volume of the OPC matrix varied during the exposure period, which affects both the mechanical properties and the durability of the concrete. Consequently, due to the pozzolanic activity of POFA and the development of extra C-S-H gel, the majority of the voids in the mixtures containing 30% POFA filled up with these products, especially at later ages, leaving less space for the freshly harmful particles formed during the exposure period in the sulfate [41]. Accordingly, the strength and durability of concrete improved with the addition POFA as partial cement replacement [45,46].

Besides, the microstructures of the OPC and POFA samples after 12 months of exposure to a severe H2SO<sup>4</sup> solution are shown in Figure 16c. The production of gypsum at a high level can be seen in the OPC specimens. In the POFA matrix, the generation of gypsum was marginally lower. This was attributed to the existence of a greater amount of Ca(OH)<sup>2</sup> particles in the OPC, which caused the formation of a soluble product such as gypsum in the matrix. Generally, gypsum with a weak structure is the main product of a chemical reaction between sulfuric acid and cement particles, which causes the specimen to expand and split [36]. During the hydration process, the Ca(OH)<sup>2</sup> in the POFA matrix was consumed, generating C-S-H gels, which have a greater ability to resist acid attacks. Besides, the reactions between other components of OPC, such as calcium aluminate and gypsum, may result in the creation of ettringite, which expands the specimens and contributes to the formation of more cracks [47,48].

**Figure 16.** *Cont*.

solutions.

**30% POFA** 

**Figure 16.** SEM images of plain OPC and POFA matrices at 12 months of exposure in (**a**) water, (**b**) MgSO4, (**c**) and H2SO4 **Figure 16.** SEM images of plain OPC and POFA matrices at 12 months of exposure in (**a**) water, (**b**) MgSO<sup>4</sup> , (**c**) and H2SO<sup>4</sup> solutions.
