**2. Experimental Procedure**

#### *2.1. Materials*

Fly ash was procured from NTPC, Kaniha, in Odisha. The chemical composition of the FA (fly ash) mostly contained SiO2 and Al2O3 with 60.34 and 30.83 wt.%, respectively. The particle size of FA was 19.18804 μm (median) and 36.73520 μm (mean). Low-cost GO synthesized via mechanical exfoliation with a layer thickness (>10 stacking layers) was given by CSIR-IMMT, Odisha. The particle size of the GO varied from 3 to 200 nm. NaOH flakes were of 99.6% purity, and the Na2SiO3 solution consisted of Na2O (15.85 wt.%), SiO2 (32.15 wt.%), and H2O (52 wt.%).

### *2.2. GRGC Fabrication*

A NaOH solution of 12M was prepared by mixing 480 g of NaOH flakes and 1000 mL of tap water, resulting in an exothermic reaction. GRGC specimens were prepared in 5 different batches with varying GO additions (0, 0.1, 0.2, 0.3 and 0.4) wt.% of FA, as shown in Table 1. The alkali activator solution was mixed via a magnetic stirrer at low rpm. GO was consequently introduced to the solution carefully to achieve maximum dispersion and to avoid the agglomeration phenomenon. Later, the solution was ultrasonicated for 30 min and mixed with FA for 10-15 min. The geopolymer slurry was poured into custom molds (10 × 3) mm<sup>3</sup> and cured at 25 ◦C (±<sup>3</sup> ◦C) for 24 hr. The GRGC specimens were demolded and cured in ambient conditions for 28 days. Figure 1a illustrates the cured GRGC specimens prior to SSIS (solid state impedance spectroscopy) characterization studies. The specimens were polished with emery paper at different grift sizes to obtain the necessary dimensions for the experimental setup, as shown in Figure 1a,b, for precise results.

**Table 1.** GRGC mixture composition.


**Figure 1.** GRGC characterization process: (**a**) specimen specifications and (**b**) SSIS measurement methodology.
