PSI - Issue 70

R. Mohanraj et al. / Procedia Structural Integrity 70 (2025) 401–408

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reference continued to exhibit the highest increase, reaching 0.428 g at 300 minutes (an overall gain of 0.025 g or 6.2%). In contrast, the nano-SiO 2 5% mix showed the least gain of 0.008 g (2.1%), indicating reduced water absorption. Among all mixes, NS3% to NS5% demonstrated significantly lower water uptake compared to the reference, highlighting the effectiveness of nano-SiO 2 in enhancing concrete's resistance to moisture ingress. The reduced absorption is likely due to the pore-refining or hydrophobic effect introduced by nano-SiO 2 , with 5% emerging as the most efficient dosage in this study. 4.3 FTIR Test The FTIR spectrum of the control sample, which had no silica, revealed a prominent peak at about 3640 cm⁻¹, which is the O-H stretching vibration of calcium hydroxide. The existence of silicate phases was also indicated by a noticeable peak in the 1000 – 1100 cm⁻¹ region, which is typical of Si – O – Si stretching, Fig. 3. Furthermore, the existence of calcium carbonate due to carbonation was suggested by the identification of the carbonate bands at roughly 1400 – 1500 cm⁻¹. The FTIR spectra showed notable variations when the silica content rose to 2.5 percent. Additional C-S-H gel was formed because of the first pozzolanic interactions between silica and CH, as evidenced by the decrease in intensity of the calcium hydroxide peak at 3640 cm⁻¹. A small increase in silicate polymerization was indicated by the Si-O-Si stretching peak about 1000 – 1100 cm⁻¹ becoming significantly more pronounced. There appears to be little impact on carbonation at this point, as the carbonate absorption band stayed almost constant. For the 5% silica sample, a further reduction in calcium hydroxide absorption was noted, demonstrating an enhanced pozzolanic reaction.

Fig. 3 FTIR test results

The intensity of the Si-O-Si peak increased more significantly, confirming additional formation of C-S-H. The carbonate absorption band at 1400 – 1500 cm⁻¹ showed a slight shift, indicating reduced carbonation due to improved concrete densification. This significant structural and chemical alteration in the concrete matrix is shown by the