PSI - Issue 70

Shashikant Kumar et al. / Procedia Structural Integrity 70 (2025) 501–508

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An examination of the XRD pattern for HVFA-SCC with 40% fly ash (FA) and 8% SF showed that, after 28 days, the quartz content decreased from 11.4% to 7.27%, and mullite content reduced from 4.98% to 2.43%. Furthermore, quantitative analysis of the CH peak revealed a reduction in calcium hydroxide from 6.31 mass% to 4.53 mass% after 28 days. This reduction is attributed to the pozzolanic reaction of SF, where Al₂O₃ and/or SiO₂ rapidly react with Ca(OH) 2 , leading to its swift consumption. 3.3.2 Scanning electron microscopy (SEM) analysis Using scanning electron microscopy (SEM) at various ages, the microstructure of the HVFA-SCC containing SF is displayed here. The SEM micrographs may help to explain the microstructure's influence on the mechanical, rheological, and durability characteristics. To assess the alterations taking place in the microstructure of the concrete mixtures, SEM examination was performed. Fig. 6 displays the control mix's SEM micrograph. The ITZ development between the aggregates and mortar phase was found to be inadequate. Additionally, there were voids and cracks, which led to a mechanical performance that was worse than that of the binary and ternary mix of SCC. In the binary mix of HVFA-SCC, an excellent ITZ formed, as seen in Fig. 7. The SEM image of the binary mix of SCC confirmed a dense mortar mix and fewer voids. The ball bearing-shaped FA particles are dispersed throughout the HVFA-SCC paste, as shown in Fig. 7. These particles may help the paste flow, which would enhance the mixture's workability and rheological characteristics. Furthermore, Fig. 7 shows that at later ages, the HVFA-SCC without SF sample has a low amount of Ca(OH) 2 and ettringite but a denser appearance and significant CSH content. It is evident that the fiber like CSH gel aggregates to create even larger crystals by adhering to the plate-like CH crystals' surface. This tendency of pore structure evolution may account for the growth of compressive strength at higher ages. According to the SEM image, the pore structure of the HVFA-SCC mixtures becomes condensed and more compacted, particularly at older ages, and the pores become diminished, which may lead to increased resistivity and decreased capillary action, water absorption, and chloride ion permeability. As seen in Fig. 8, ternary mix of HVFA-SCC with SF displayed a very dense mortar matrix and the fewest number of voids. Improved mechanical performance was the outcome of proper HVFA, SF, and cement proportioning and distribution. The HVFA-SCC with SF appears to create an interconnected microstructure with dense C-S-H phase, as shown in Fig. 8, which suggests the presence of secondary C-S-H. By limiting the formation of Ca(OH) 2 crystal by nanoparticles of SF, the crystallization will be controlled to be in a desirable state if the nanoparticle content and spacing are suitable. Furthermore, because of their high activity, the nanoparticles found in cement paste as kernel might further encourage cement hydration.

As a result, the cement matrix becomes more compact and uniform. Porosity of HVFA-SCC can be considerably decreased by using nanoparticles of SF as a filler to increase the material's density. On the other hand, because of their high activation, SF nanoparticles can also act as a kernel in cement paste, shrinking the Ca(OH) 2 crystal. This enables them to function as an activator to accelerate the hydration of cement (Jalal et al., 2015). The HVFA-SCC blended with SF exhibits a significant improvement in compressive and splitting tensile strength due to the quick consumption of Ca(OH) 2 during the hydration of OPC, especially at initial ages, caused by the high reactivity of SF particles. Cement hydrates more quickly as a result, and more reaction products are produced in greater quantities. Fig. 6. SEM image of control SCC mix Fig. 7. SEM image of HVFA-SCC mix Fig. 8. SEM image of HVFA-SCC with SF