PSI - Issue 70

C. Raghu Rami Reddy et al. / Procedia Structural Integrity 70 (2025) 223–230

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Fig 1. Flow chart of experimental methodology

4. Results and discussion

4.1 Mechanical properties

As shown in Figure 2, the use of mineral admixtures and polypropylene (PP) fibers greatly improved the mechanical performance of foamed concrete. All mixes showed a progressive increase in compressive, flexural, and split tensile strengths with curing age because of the ongoing pozzolanic reactions of additional components and the ongoing hydration of cementitious phases (Chen et al., 2025). M4 (silica fume + PP fibers) consistently showed the highest strength values throughout all ages, possesses a split tensile strength of 2.5 MPa after 90 days, a flexural strength of 3.9 MPa, and a compressive strength of 12.7 MPa. The silica fume's huge surface area, which serves as a micro-filler and promotes the nucleation of hydration products, greatly improves matrix densification, and refines the pore structure, is largely responsible for this exceptional performance (Ma, 2014) (Wang et al., 2024). M2 (metakaolin + PP fibers) also showed considerable strength enhancement compared to the reference mix. The early-age strength gain in M2 was more pronounced than in M3, attributed to metakaolin's rapid pozzolanic reactivity, which accelerates C – S – H gel formation and improves early matrix integrity (Li et al., 2024). After 28 days, M2 had with tensile values of 8.9 MPa for compression, 2.9 MPa for flexure, and 2.1 MPa for split tensile, surpassing M3. However, the presence of some unreacted metakaolin particles at later stages (as seen in SEM) indicates that not all pozzolanic potential was utilized, slightly limiting long-term strength compared to M4. M3 (fly ash + PP fibers) exhibited the slowest strength development. Although fly ash improved workability and fiber dispersion due to its spherical morphology, its lower reactivity delayed the formation of secondary hydration products (Zhang et al., 2023). Compressive strength in M3 increased from after 7 days, and 11.2 MPa after 90 days, while split tensile and flexural strengths reached 2.0 MPa and 3.4 MPa, respectively. The gradual strength gain highlights the extended pozzolanic reaction period typical of fly ash. Additionally, the relatively weaker interfacial transition zone (ITZ) in M3 limited the efficiency of PP fibers in crack bridging, contributing to lower tensile and flexural performance compared to M2 and M4. At all curing ages, the reference mix M1, devoid of mineral admixtures and fiber reinforcement, exhibited the lowest strength values. At 90 days, its compressive, flexural, and split tensile strengths were limited to 9.1 MPa, 2.7 and 1.7 MPa, respectively. The inferior mechanical performance is attributed to a highly porous and brittle matrix, the absence of secondary C-S-H formation, and the lack of crack-bridging mechanisms under tensile loading, as noted by Narayanan and Ramamurthy (2000). The results underscore the critical role of polypropylene (PP) fibers in enhancing tensile properties and the importance of mineral admixtures in improving hydration and microstructural refinement. The combination of silica fume and PP fibers in mix M4 delivers optimal performance, offering a balance of strength, ductility, and durability. These findings confirm that hybrid modifications are vital for tailoring foamed concrete for structural and semi structural uses, this is similar based on the work of Chen et al. (2025) and Wang et al. (2024).