*2.3. Testing Methods*

Concrete cubes of a 100 mm size were cast for the entire experimental work in accordance with the specifications of BS EN 12390-2: 2009 and BS EN 12390-3: 2009 at various curing periods up to 12 months. Chemical immersion experiments were utilized in this research work to investigate the performance of concrete composites comprising WPFT fibers and POFA against magnesium sulfate and sulfuric acid attacks. Using scanning electron microscopy (SEM), microstructural investigation of several concrete mixtures was undertaken to evaluate the morphological and degradation mechanisms. Overall, 36 concrete cubes of a 100 mm size were cast and coated with plastic to minimize fast vaporization for each test. The samples were then remolded after 24 hours and submerged in water for 28 days to cure. Afterward, the samples were withdrawn, wiped dry, and weighed to the nearest 0.1 g with an electronic weighing balance, recording the initial weight with the succeeding weight throughout the exposure period. Next, the concrete cubes were immersed in 10% MgSO<sup>4</sup> and 5% H2SO<sup>4</sup> solutions for the sulfate and acid resistance tests.

The concrete sample cubes were entirely submerged in the chemical solutions for 12 months, and the pH values of the solutions changed quickly in both tests. Nevertheless, the pH for the sulfate resistance test was restricted to a concentrated value of 8.5 and 2.5 for the acid solution. The pH was controlled through refreshing the solutions and was then maintained constantly during the immersion duration. No standardized approach is currently in place to test concrete resistance against sulfuric acid and sulfate assaults. Furthermore, the testing techniques of ASTM C1012-15, ASTM C267-01-2012, and ASTM C452-15 guide testing the chemical resistance of mortar and polymer concrete. Consequently, following the standard specifications and the current literature, the concentration levels of 10% and 5% were selected in this study for the magnesium sulfate and sulfuric acid, respectively.

At the end of the exposure period (12 months), to evaluate the consequences of chemical attacks on the performance of the concrete samples, the samples were taken from the testing tanks and cleaned with plain water, then dried in a testing room at

ambient temperature for about 30 minutes. After the specimens had sufficiently dried, visual inspection was carried out to observe the degradation degree for each mixture after 12 months of exposure in the MgSO<sup>4</sup> and H2SO<sup>4</sup> solutions. The evaluation considered the specimen edges, texture, color, size, and geometry. After that, the average masses of specimens for each group were measured to calculate the variation in the mass by following Equation (1):

$$ML\_t = \frac{M\_t - M\_i}{M\_i} \times (100) \tag{1}$$

where *M<sup>t</sup>* is the average mass of samples after 12 months exposure to chemical solutions (gr), *M<sup>i</sup>* is the original mass earlier absorption (gr), and *ML<sup>t</sup>* is the cumulative mass loss after 12 months of exposure.

An evaluation of the strength loss factor (*SLF* %) after 12 months of exploration was carried out to indicate the level of deterioration in concrete mixtures in terms of reduction in compressive strength, and the calculation was made with the following formula (Equation (2)):

$$SLF = \frac{F\_{cw} - F\_{cs}}{F\_{cw}} \times 100\% \tag{2}$$

where *SLF* indicates the percentage of loss in compressive strength of concrete cubes exposed to H2SO<sup>4</sup> and MgSO<sup>4</sup> solutions for 12 months, *Fcw* is the obtained strength values of concrete specimens after 12 months cured in plain water, and *Fcs* is the residual strength of the cubic samples immersed in H2SO<sup>4</sup> and MgSO<sup>4</sup> solutions for the same period.

### **3. Results and Discussion**

### *3.1. Workability*

A slump test was carried out to investigate the effects of WPFT fibers on the fluidity of concrete mixtures and the results are presented in Figure 2. WPFT fiber addition led to a reduction in the slump values, with the highest slump value being 185 mm for the OPC control mix. Slump values of 140 mm and 95 mm were noted for mixes comprising 0.2% and 0.4% WPFT fibers, respectively. According to the ACI 211-02 standard requirements, these concrete mixtures have fluid and plastic workability and can be used in structural members with congested and regular reinforcing. A minimum slump value of 35 mm was measured for the mixture containing 1% WPFT fibers. The results also demonstrate that the POFA mixes had less workability than the OPC-based mixes. Increases in the dosage of fibers had a comparable consequence on the flowability of the POFA mixtures. The slump of mixes with fiber dosages of 0, 0.2, 0.4, 0.6, 0.8, and 1%, for example, were measured as 165, 115 mm, 70 mm, 55 mm, 40 mm, and 25 mm. Previous research has shown that the PP types of waste plastic fiber harm the workability of concrete [37]. This could be attributed to the fact that fibers with a large surface area absorb more cement paste, which causes the viscosity to rise, resulting in a lower slump value. Furthermore, the high fiber content and wide surface area of short fibers results in reduced workability [38].

### *3.2. Water-cured Compressive Strength*

Cubic compressive strength testing for the concrete samples cured in plain water at various curing periods was carried out and the results are illustrated in Figure 3. The obtained results show a decrease in the compressive strength values as the fiber volume fractions increased. Nevertheless, the reduction in strength was minor, and the strength values were within the acceptable range for structural applications. After 365 days of curing in plain water, the strengths of OPC mixtures reinforced with 0, 0.2, 0.4, 0.6, 0.8, and 1% WPFT fibers were 48.5, 47.2, 44.8, 43.2, 39.7, and 37.4 MPa, accordingly. Generally, the modulus of elasticity of PP types of fibers ranged between 3.5–4.9 GPa, and these comparatively low values classified the PP fiber as a soft material, and the matrix was treated as a soft composite, and, therefore, caused a drop in the compressive strength of reinforced concrete specimens [39,40].

*Crystals* **2021**, *11*, x FOR PEER REVIEW 7 of 22

**Figure 2.** Effects of the WPFT fibers at various dosages on the slump of fresh concrete mixtures. **Figure 2.** Effects of the WPFT fibers at various dosages on the slump of fresh concrete mixtures. *Crystals* **2021**, *11*, x FOR PEER REVIEW 8 of 22

**Figure 3.** The variation in the water-cured compressive strength of OPC and POFA concrete mixtures containing WPFT fibers. **Figure 3.** The variation in the water-cured compressive strength of OPC and POFA concrete mixtures containing WPFT fibers.

ually 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 MgSO4 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

Since CaCO3 and Ca(OH)2 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 CaCO3 and Ca(OH)2 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

after 12 months of exposure than the plain concrete mixes.

*3.3. Sulfate Attack* 

3.3.1. Visual Assessment

Furthermore, replacing 30% of OPC with POFA resulted in greater compressive strength values, particularly after 12 months of curing in water. The strength value for the plain POFA mix was recorded as 52.9 MPa, which is approximately 10% greater than the 48.5 MPa compressive strength value observed for the plain OPC mixture without any fibers. The testing results showed that the POFA mixes obtained higher strength values at extended curing periods than the OPC mixes. According to Zeyad et al. [41], the higher strength of POFA mixtures could be attributed to pozzolanic activity at ultimate ages, leading to increased concrete strength by creating supplementary C-S-H gels. POFA mixtures incorporating WPFT fibers showed a similar trend to the OPC mixtures, where the inclusion of fibers and rises in fiber volume fractions resulted in slight decreases in compressive strength values [42]. After 365 days of immersion in water, the compressive strength values of POFA mixes containing 0, 0.2, 0.4, 0.6, 0.8, and 1% WPFT fiber were 52.9, 51.2, 49.3, 47.6, 43.9, and 42.4 MPa, respectively, as shown in Figure 3 These values were higher than those recorded for the OPC mixtures with the exact fiber dosages.
