PSI - Issue 70

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

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4 2. Microstructural characterization

The microstructural evolution of foamed concrete mixes was analyzed at 28 and 90 days via Scanning Electron Microscopy (SEM) To assess the impact of mineral admixtures on polypropylene (PP) fibers on hydration product formation, fiber-matrix bonding, and pore structure refinement. Morphological changes observed through SEM are presented in Figure 3. The reference mix (M1) exhibited a highly porous, loosely packed matrix with large, interconnected pores exceeding 500 µm. The presence of weakly bound Calcium-Silicate-Hydrate (CS-H) gel and the absence of crack-bridging processes led to brittle fracture features. This demonstrates the poor structural integrity and explains why M1 has a low mechanical strength. In contrast, M2 (metakaolin + PP fibers) revealed a denser microstructure with reduced pore size in the range of 200 – 400 µm, reflecting metakaolin’s high reactivity and its ability to consume portlandite through pozzolanic reactions, producing additional C – S – H. The SEM images showed partially embedded PP fibers within the matrix, with visible fiber-matrix interlocking and microcrack bridging. However, some unreacted metakaolin particles were also detected, indicating that the pozzolanic reaction was not entirely complete, especially in deeper matrix zones with limited moisture availability. The microstructure of M3 (fly ash + PP fibers) demonstrated more uniform pore distribution compared to M1, although the pores remained relatively coarser (200 – 500 µm) than those in M2 and M4. Fly ash’s spherical morphology facilitated improved workability, leading to better fiber dispersion throughout the matrix. Nevertheless, the delayed pozzolanic activity unreacted particles were found in fly ash. loosely packed hydration products. The fiber matrix interface appeared weaker compared to M2 and M4, contributing to inferior tensile and flexural strength. This is further supported by the higher degree of fiber pull-out observed under fracture, indicating suboptimal stress transfer. M4 (silica fume + PP fibers) exhibited the most refined and compact microstructure among all mixes. When silica fume was added, consistently dispersed, discontinuous holes smaller than 100 µm were formed. This improved matrix homogeneity and drastically decreased porosity. Full involvement in hydration and pozzolanic reactions was demonstrated by SEM images, which verified the formation of an ultra-dense C – S – H gel with no discernible unreacted silica fume particles. With a strong interfacial transition zone (ITZ) and little indications of fiber debonding or pull out, PP fibers in M4 were firmly incorporated into the matrix. This confirms M4's better mechanical performance and suggests efficient load transfer across the matrix-fiber contact. Furthermore, the densified matrix in M4 improved strength and durability by limiting the spread of microcracks. According to SEM study, mineral admixtures refine the pore network, encourage denser C-S-H formation, and promote fiber-matrix interaction, all of which greatly improve the microstructural The qualities of foamed concrete. In line with its improved mechanical behaviour, M4 showed the best microstructural characteristics among the mixes under study. These results highlight how highly reactive pozzolans and PP fibers work together to create foamed concrete composites that are both structurally sound and long lasting.

Table 4. Compressive, Split tensile and flexural strengths of the CLC for varying densities of different mixes Mix Type Compressive strength (MPa) Split Tensile strength (MPa)

Flexural strength (MPa) 7D 14D 28D 56 90 7D 14D 28D 56 90 7D 14D 28D 56D 90D

M1(RM)

3

5

6.8 8 9.1 1 0.9 1.2 2 2 1 1.5

2

2

3

M2(FA10)

4 6.6 8.9 1012 1 1.1 1.6 22 1

2

2.9

3

4

M3(MK10)

4 6.2 8.6 9.8 11 1

1

1.5 2 2 1 1.8 2.6

3

3

M4(SF10)

5

7

9.6 11 13 1 1.3 1.9 2 3 2 2.3 3.2

4

4