PSI - Issue 67

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Hwan Lee et al. / Procedia Structural Integrity 67 (2025) 107–114 Author name / Structural Integrity Procedia 00 (2024) 000 – 000

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1.2.1. Experimental Methods

A dispersion study was conducted to effectively disperse the CNTs in water before their use in the cement paste and mortar sample. Ultrasonication was utilized for dispersing the CNTs. The energy of 787.5 J/ml was applied to a mixture of CNTs, water, and superplasticizer. This process involved cycles of 10 seconds of operation followed by 5 seconds of rest to prevent overheating (SIQUEIRA and GLEIZE 2020). The selection of this ultrasonication energy level was based on our preliminary research, which identified it as the optimum for achieving effective dispersion. This dispersed solution with CNT and superplasticizer was added as mixing water according to the test procedure. This was not required for the NMK, as its efficacy in concrete without needing dispersion has been shown by researchers (Abo-El-Enein et al. 2014; Morsy et al. 2014). Three experimental analyses were conducted: alkali-silica reaction (ASR), compressive strength, and bulk resistivity. The test protocol for the study included casting mortar bars for ASR with mix proportions shown in Table 1 and cubes for compressive strength with mix proportions shown in Table 2. The mortar cubes were also used for conducting resistivity tests, before the compressive strength test, as the resistivity test is non-destructive. The mortar samples were prepared per ASTM C305 (ASTM C 305 1998). In the case of the CNT_0.05% dosage mix, due to the hydrophobic tendency of CNTs and the higher dosage adopted, a modified mixing protocol was used. The final mixing time was increased from 90s to 120s to incorporate the CNTs better and prevent segregation and bleeding. The workability of all the mixes was adjusted to reach 100 mm flow by adding a superplasticizer. A mini-slump test was used to determine the workability (Tan et al. 2017). The accelerated ASR expansion test was conducted per the ASTM C 1567 (ASTM C1567-23 2005). Compressive strength tests were conducted at 7, 28, and 90 days according to the ASTM C109 (ASTM C109 1993). The bulk resistivity of the samples was measured at 28 and 90 days based on the ASTM C1876 (ASTM C1876 2019). The bulk resistivity of cube samples was tested with a 10 kHz frequency. 2. Results and discussions 2.1. ASR expansion Fig. 2 displays the ASR expansion results for the mortar bars. The acceptable expansion limit per ASTM C1567 is 0.1%, which is indicative of a low risk for ASR expansion (ASTM C1567-23 2005). While all the NMK dosages showed a reduction in ASR expansion, only the 10% replacement dosage was able to control the expansion made with the highly reactive siliceous aggregate below the prescribed 0.1% criteria (see Fig. 2a). The replacement of cement with 5 wt%, 7.5 wt%, and 10 wt% NMK reduced the expansion by 40.9%, 83.2%, and 86.0%, respectively. This reduction could be attributed to multiple factors. The replacement of 5wt%, 7.5wt%, and 10wt% of cement with NMK reduces the alkali content of the binders by 5%, 12.1%, and 20.9%, respectively, may lower the alkali load on the mortar bars (Moser et al. 2010). Likewise, a reduction in calcium hydroxide (CH) can lead to the formation of a less expansive gel (Rajabipour et al. 2015). Additionally, the increased pozzolanic reaction, which produces more C-S-H can potentially trap alkali ions within their interlayer structures (Luo et al. 2022). The pozzolanic reaction and reduced CH content mechanisms will be investigated with the resistivity tests and TGA analysis. In the case of CNTs, adding 0.01 wt% resulted in a 12.2% increase in ASR expansion (see Fig. 2b). However, the addition of 0.03 wt% and 0.05 wt% of CNTs led to reductions in expansion by 11.9% and 67.7%, respectively. This reduction in ASR expansion is likely due to the synergistic effect of CNTs (a) inhibiting crack propagation and thus reducing the expansion caused by ASR, and (b) refinement of the pore structure within cement composites, contributing to this reduction in expansion (Ramezani et al. 2022). However, the results show that the 0.01% dosage of CNT was not sufficient to prevent the ASR expansion and thus, there appears to be a minimum dosage of CNTs needed to restrain expansion. It should be noted that only one study has been published examining CNTs for ASR mitigation. In this study by Ramezani et al, 0.1 wt% of CNT was able to reduce expansion by 73% when compared to the control (Ramezani et al. 2022). In their work the 0.3% dosage did not perform as well as the 0.1% dosage, indicating that simply increasing the CNT dosage does not ensure ASR resistance and that there is an optimal value. However, neither dosage was able