PSI - Issue 67

Shohana Iffat et al. / Procedia Structural Integrity 67 (2025) 1–7 Iffat et al./ Structural Integrity Procedia 00 (2024) 000–000

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Samples from the failed specimens were utilized for scanning electron microscopy (SEM) analysis to examine the incorporation and dispersion of GONRs in concrete. The sample size was less than 5-mm in length in any direction. To remove moisture, the selected samples were air-dried for 48 hours and then placed in a vacuum-suction chamber for one hour. Then, the samples were gold-sputtered before being examined under a scanning electron microscope (model Ultraplus Thermal Field Emission Scanning Electron Microscope, Zeiss). 3. Results and discussions 3.1. Dispersion of oxidized GONRs in aqueous solution The effective dispersion of GONRs in the aqueous suspensions was assessed based on zeta potential measurements. For an oxygen content of 32.3 O%, the zeta potential was –44.5±4.86 mV and –41±0.99 mV for a GONR concentration of 0.0125 g/L (~0.0005 wt% in concrete) and 1.25 g/L (~0.05 wt% in concrete), respectively. For an oxygen content of 41.3 O%, the zeta potential was –58.1±3.21 mV and –37.1±1.41 mV for a GONR concentration of 0.0125 g/L and 1.25 g/L, respectively. All GONR suspensions were characterized as stable since their zeta potential is smaller than – 30 mV (Freitas and Muller 1998). 3.2. Compressive strength For each specimen configuration, the mean and standard deviation of the 7-day and 28-day compressive strength values are summarized in Fig. 2b. These results show that the incorporation of small amounts of GONRs in stable suspensions result in significant compressive strength enhancements after 7 days of curing. Namely, compared to the benchmark (unamended) specimens, the average strength increase was +22% and +45% for a GONR concentration of 0.0005 wt% and oxygen content of 32.3 O% and 41.3 O%, respectively, which highlights the effectiveness of the higher degree of oxidation. The smaller standard deviation also suggests that the incorporation of GONRs did not result in the formation of defects (e.g., because of agglomeration) to an extent that curtailed the effectiveness of the nanoamendments. Instead, less significant enhancements in compressive strength, namely +27% and +32% on average, were observed for a GONR concentration of 0.05 wt% and oxygen content of 32.3 O% and 41.3 O%, respectively. The variability of these results was clearly higher compared to those obtained for the lower GONR content (0.0005 wt%). This evidence is significant since it indicates that a 100× increase in amount of GONR is inconsequential, and possibly counterproductive, as far as short-term strength enhancement. The results after 28 days of curing are also valuable. In fact, they show that the strength enhancement resulting from the incorporation of oxidized GONRs in stable suspensions is statistically insignificant, irrespective of GONR concentration (whether 0.0005 wt% or 0.05 wt%) and oxygen content (whether 32.3 O% or 41.3 O%). 3.3. Concrete microstructure SEM analysis of concrete samples was used to inspect the microstructure and generate supporting evidence to explain the results presented in Fig. 2b. Representative SEM micrographs for unamended concrete and concrete with a GONR concentration of 0.05 wt% after 7 and 28 days of curing are presented in Fig. 3 and Fig. 4, respectively. This evidence suggests that after 7 days of curing the incorporation of GONRs in stable suspensions resulted in an accelerated and preferential formation of amorphous C-S-H (Fig. 3b) compared to the unamended concrete (Fig. 3a). In fact, oxidized graphitic nanoparticles may act as preferential nucleation sites for hydrating cement particles (Makar and Chan 2009), which may explain the more evident presence of C-S-H phases in the cementitious matrix, also including interfacial transition zone (ITZ) regions, in GONR-amended concrete (Fig. 3b). An additional contribution to strengthening may be due to the physical embedment of the GONRs in the cementitious matrix, which is facilitated by the chemical affinity between edge functional groups of oxidized carbon nanoparticles and cement hydrates (e.g., C-S-H and Ca(OH) 2 ) (Li et al. 2005).