Synergistic Enhancement of Sulfate-Resistant Polymer Repair Concrete Incorporating Supplementary Cementitious Materials and Polypropylene Fibers

Concrete remains the most widely used construction material due to its strength, versatility, and affordability. However, the deterioration of aging infrastructure, particularly in sulfate-rich environments, poses serious durability and safety challenges. Conventional cement-based repair materials often fail to provide long-term performance under such aggressive conditions, highlighting the need for advanced, durable, and sustainable repair solutions [1].

Recent advances have demonstrated that polymer incorporation into cementitious systems significantly improves bonding, crack resistance, and resistance to chemical attack [2]. Polymer-modified concrete (PMC) and polymer-cement composites exhibit enhanced durability under harsh exposure conditions [3]. At the same time, supplementary cementitious materials (SCMs) such as silica fume and ground granulated blast furnace (GGBS) refine the pore structure, reduce permeability [4], and support sustainability through reduced cement demand and the reuse of industrial by products [5]. Several studies have investigated polymers or SCMs individually; however, limited research has explored their combined use with fibers to create high-performance repair concrete tailored for sulfate-rich environments [6]. This gap restricts the development of reliable repair guidelines for modern infrastructure.

To address this gap, the present study systematically examines the synergistic and complementary effects of SCMs, polymer, and polypropylene fibers on repair concrete. The objectives are to:

1. Analyze material interactions and their influence on microstructural refinement, 2. Evaluate compressive, flexural, and bond strength as well as water absorption, and 3. Propose optimized mixture designs that balance sustainability with superior performance. The findings provide practical insights and implementation strategies for durable repair systems suitable for aggressive sulfate conditions.

Concrete mixes were produced following the designed proportions as shown in Table 6. A high-range water-reducing admixture (superplasticizer) was used at a fixed ratio of 1% of the binder weight to achieve the desired workability. Materials were mixed consistently, ensuring uniform fiber distribution and homogeneous integration of polymer.

Table 6. Mix proportions of polymerized repair concrete

5.1 Slump test

The workability of the fresh concrete mixes, as shown in Figure 9 for (M1 to M9) was assessed using the slump test, following the standard procedure outlined in ASTM C143/C143M [12]. For each mix, a fresh concrete sample was prepared and poured into a standard slump cone in three layers. Each layer was tamped 25 times using a standard tamping rod to ensure uniform compaction. After filling, the cone was slowly lifted vertically, and the decrease in the height of the concrete (slump) was measured immediately. The slump values recorded for all mixes fell within the 11.1 cm to 12.6 cm range, which is considered moderate and appropriate for repair applications requiring good workability without segregation or bleeding. However, the results of the slump are illustrated in Figure 10.

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Figure 9. Slump test of polymer concrete

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Figure 10. Results of the polymer concrete slump test

The lowest slump values were observed for mixtures M3 (11.1 cm) and M1 (11.2 cm). These mixtures contained lower proportions of SCMs, which resulted in reduced fines content and consequently a stiffer fresh concrete consistency. As the contents of silica fume and GGBS increased, as observed in mixtures M7 to M9, a gradual increase in slump was recorded. The maximum slump value of 12.6 cm was achieved for mixture M8. This improvement can be attributed to enhanced particle packing, improved paste cohesiveness, and the lubricating effect of the polymer modifier. Although the incorporation of 0.2% polypropylene fibers generally tends to reduce workability due to fiber-induced water retention and restricted flow, all mixtures maintained acceptable workability levels. This indicates that the selected water-to-binder ratio and superplasticizer dosage were sufficient to offset the workability reduction associated with fiber addition.

The moderate slump values achieved across all mixtures ensured adequate flowability for placement and consolidation while minimizing the risk of segregation. Such characteristics are particularly important for repair applications, where proper surface finishing, interfacial bonding, and long-term durability are critical.

5.2 Water absorption

To fully understand, or perhaps more realistically, attempt to understand the water absorption characteristics of the developed PMC mixtures (M1-M9), the absorption test was rigorously conducted. Porosity and permeability, undeniably critical yet inherently ambiguous characteristics, were evaluated strictly according to the procedures outlined in ASTM C642 [16]. For each mixture, three cubic specimens (100 mm × 100 mm × 100 mm) were prepared and cured underwater for 28 days. After curing, the samples were carefully removed from their immersion environment and gently surface-dried at ambient room conditions for precisely 24 hours. To achieve certainty, or at least to approach it more confidently, further drying was performed in a controlled oven environment at 100 ± 5℃. This drying phase lasted 24 hours and was continued meticulously until a constant mass was achieved. Once cooled thoroughly in a desiccator, ensuring no reabsorption of atmospheric moisture, this stable dry mass (A) was carefully recorded. Subsequently, the specimens were immersed again, this time at room temperature, for 48 hours. After this period, they were removed, wiped, and weighed to obtain the saturated mass (B). Finally, the ultimate saturated mass post-boiling (C) was determined and carefully documented.

Water absorption percentages were calculated according to the Eq. (1):

$Water \,\,absorption\,\, \%=\frac{C-A}{A} \times 100$                   (1)

For each mix, average values from the three tested samples were reported clearly and systematically in Figure 11.

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Figure 11. Water absorption of polymer concrete

The water absorption values varied between 5.12% and 3.61%, indicating a gradual but meaningful reduction in porosity across the mix series. The general trend suggests that increasing the dosage of SCMs and integrating polymer contributed to the refinement of the microstructure and improved resistance to water ingress. The highest absorption was observed in M1 (5.12%) and M3 (5.06%), both of which had lower SCM content and represent the baseline performance of the system. These relatively high absorption values reflect a more open pore structure, typical of mixes without optimized particle packing or secondary hydration reactions. The limited polymer and pozzolanic contribution in these mixes likely led to capillary continuity and higher water uptake. Additionally, minor surface voids and inconsistent compaction may have contributed to these elevated values factors not uncommon in site or lab repair conditions. In contrast, M8 (3.61%), M7 (3.72%), and M9 (3.76%) showed the lowest water absorption. These mixes contained higher levels of silica fume and GGBS, which enhanced pozzolanic reactivity and reduced capillary porosity. The polymer likely filled part of the pore network and created a partial hydrophobic barrier, consistent with observations in previous studies [17, 18]. However, even in these well-optimized mixes, the absorption did not drop below 3.6%, indicating that total elimination of porosity is not practical and is influenced by curing efficiency, mixing energy, and air content. Intermediate mixes such as M4 to M6 showed reasonable values (4.13-4.45%), reflecting a balanced design. Slight variations in absorption among them may be attributed to inconsistencies in curing temperature, mix homogeneity, or fiber distribution, all of which can affect the interconnected porosity and therefore the absorption result. The absorption test results confirm that higher SCM content combined with polymer resin improves impermeability, but field-relevant variability in results remains due to practical factors like compaction, surface finish, and fiber orientation. This highlights the importance of mix optimization and quality control, particularly when designing repair mortars for durability in aggressive exposure conditions.

5.3 Compressive strength

The compressive strength results of mixes M1 to M9, illustrated in Figure 12, show a clear and progressive enhancement in strength from 28 days to 90 days, which is typical in blended cementitious systems where continued hydration and pozzolanic reactions contribute to long-term performance. All mixes exhibit strength development over time, with an average increase of approximately 2.5-3.2 MPa, indicating the ongoing contribution of SCMs. The lowest performance was recorded in M1 and M3, which contained minimal SCM content, indicating limited refinement of pore structure and reduced secondary hydration. This aligns with the study [17], which reported that polymer modification alone provides limited strength improvement without the synergistic effect of reactive mineral admixtures.

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Figure 12. Compressive strength of polymer concrete

In contrast, M8 achieved the highest compressive strength (30.1 MPa at 90 days), followed by M7 and M9. This superior performance can be attributed to the higher silica fume and GGBS content, which refined the pore structure and generated additional calcium silicate hydrate (C-S-H), thereby densifying the matrix. The inclusion of polymer (5%) improved bonding at the interface, while polypropylene fibers (0.2%) provided crack-bridging and stress redistribution. These microstructural mechanisms are supported by Guo et al. [18] and Shill et al. [19], who observed enhanced cohesion and reduced microcracking when polymer and fibers were combined with SCMs.

Intermediate mixes (M4-M6) showed moderate improvements (28 MPa), reflecting the incremental role of SCMs in strengthening the matrix. Overall, the results confirm that the integration of SCMs with polymer and fibers promotes microstructural refinement, reduced porosity, and enhanced internal cohesion, producing a synergistic improvement in compressive strength compared to conventional repair concretes.

5.4 Flexural strength

The flexural strength results illustrated in Figure 13 for mixes M1 through M9 indicate a consistent and progressive improvement from 28 to 90 days, with strength gains ranging between 0.4 MPa to 0.5 MPa across all mixes. This trend reflects the continuous hydration of cementitious materials and the beneficial role of polymer in enhancing tensile stress resistance and crack-bridging capacity within the concrete matrix. According to the study [18], the lowest values were observed in M1 and M3 (3.51 and 3.93 MPa at 28 and 90 days, respectively), which can be attributed to their limited SCM content, resulting in weaker pozzolanic activity and reduced refinement of the interfacial transition zone (ITZ). Such behavior is consistent with findings by the study [12], who emphasized that a polymer alone cannot significantly enhance tensile properties without reactive mineral admixtures.

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Figure 13. Flexural strength of polymer concrete

In contrast, M8 achieved the highest flexural strength (4.52 MPa at 90 days), followed by M7 (4.45 MPa) and M9 (3.41 MPa). This superior performance can be explained by the optimized synergy between silica fume, GGBS, and polymer, which improved particle packing, densified the ITZ, and promoted additional C-S-H formation. Polymer contributed to micro-level adhesion, distributing tensile stresses more evenly across the matrix, while polypropylene fibers (0.2%) acted as crack arresters and enhanced post-cracking toughness. These mechanisms are supported by Shill et al. [19] and Liang et al. [20], who reported that combining polymers with SCMs and fibers significantly improves resistance to crack propagation under flexural loading.

Intermediate mixes (M4-M6) exhibited moderate improvements (4.18-4.32 MPa at 90 days), confirming that incremental SCM dosage and polymer inclusion lead to steady enhancements in tensile performance [21]. Overall, the combined effect of SCMs, polymer, and fibers produced a denser, more cohesive microstructure with improved crack-bridging and energy absorption capacity, distinguishing the optimized mixes from conventional repair concretes [22].

5.5 Slant shear bond strength

The slant shear strength results, as illustrated in Figure 14 for the repair concrete mixes M1 to M9, demonstrate a consistent and notable improvement from 28 to 90 days, emphasizing the progressive development of bond strength at the interface between the repair material and the existing substrate. This enhancement is largely attributed to the combined effect of SCMs and polymer, which synergistically improve both the microstructure and adhesion characteristics of the matrix.

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Figure 14. Slant shear strength of concrete

The lowest bond strengths were recorded in M1 and M3 (7.0 MPa at 28 days, 7.9 MPa at 90 days), reflecting their limited SCM content and correspondingly weaker pozzolanic reactivity, which restricted ITZ refinement and left the matrix more porous. Such performance aligns with the study [17], who emphasized that a polymer alone cannot ensure optimal bonding without adequate mineral admixture integration.

In contrast, M8 achieved the highest slant shear strength (9.1 MPa at 90 days), followed closely by M7 (8.9 MPa) and M9 (8.8 MPa). These mixes contained higher proportions of silica fume and GGBS, which refined the ITZ, reduced porosity, and stimulated secondary C-S-H formation, thereby improving cohesion across the repair interface. Polymer (5%) further strengthened adhesion by penetrating microcracks and forming a continuous polymeric network that anchored the cement paste to the substrate. This mechanism is consistent with Shill et al. [19] and Liang et al. [20], who reported that polymer-SCM systems exhibit superior resistance to delamination and shear failure. Moreover, polypropylene fibers contributed indirectly by controlling crack initiation and restraining stress concentration along the interface, supporting long-term bond stability [23].

Intermediate mixes (M4-M6) demonstrated moderate strengths (8.4-8.7 MPa at 90 days), indicating that incremental increases in SCM dosage progressively improved interfacial performance. Overall [24], the results confirm that the integration of SCMs, polymer, and fibers generates a denser microstructure, refined ITZ, and stronger chemical and mechanical adhesion, producing a synergistic improvement in slant shear strength compared to polymer-only repair concretes [24, 25].