## 3 Results and Discussion
### Microstructures of Geopolymer Concrete  
XRD analysis results of geopolymer concrete specimens are shown in Figure 7. Most of the crystalline phases of quartz were detected as sharp peaks in all mixtures. Similar XRD patterns were observed for all geopolymer concrete mixes, indicating FCC residue can be used as a replacement of fly ash precursor in geopolymer concrete.  
To determine the differences between specimens consisting of fly ash and FCC residue and explain the results from previous sections, four geopolymer mixtures AL:GS = 0.65 (G1,5) and AL:GS = 0.5 (G7,11) were selected for SEM observations. The reason for choosing the aforementioned mixtures instead of AL:GS = 0.4 mixtures was that the 7-day compressive strength of AL:GS = 0.5 and AL:GS = 0.4 was similar. Results of microstructural observation of AL:GS = 0.65 (G1,5) and AL:GS = 0.5 (G7,11) specimens are illustrated in Figure 8, whereas Figure 9 shows void volume percentages of those specimens and their binary images using ImageJ at a threshold value at 76. Although image processing by ImageJ software is probably affected by the vagueness of the objects, such as aggregate particles and cementitious matrix, this technique provides an easy method to determine void percentage in geopolymer microstructures. These studies [42,43] in which fuzzy divergence was used in image detection are recommended for improvement of quality and quantity of image processing in future research. For specimen G1 (0% FCC residue), it was apparent that its microstructure with 29.7% void (by volume) was not as dense as specimen G5 (80% FCC residue) with 24.8% void by volume. However, for the case of AL:GS = 0.5 mixtures, the microstructure of the G7 (15.8%) mixture with only fly ash was denser than the G11 (22.7%) mixture with FCC residue. This might be possibly explained by the fact that for the mixtures with a low ratio of AL:GS (0.5), with the presence of FCC residue particles, the amount of alkali liquid was not enough for both fly ash and FCC residue particles to transport and arrange for producing geopolymer products. As a result, the compressive strength of hardened geopolymer concrete consisting of FCC residue (G11) was reduced due to the presence of a great number of large voids compared to fly ash-based geopolymer concrete (G7), as shown in Figure 8c,d. There were numerous unreacted FCC  
Figure 7: X-ray diffraction (XRD) patterns of geopolymer concrete with various FCC contents (% by binder mass).  
residue particles and voids inside the concrete structure in Figure 8d. On the contrary, AL:GS = 0.65 mixtures with a high ratio of AL:GS showed contrasting performances in terms of compressive strength. With a higher ratio of AL:GS, finer and smoother fly ash particles could overcome obstacles caused by large and angular FCC residue particles that filled the internal voids inside the concrete structure. Therefore, pores and unreacted fly ash particles in AL:GS = 0.65 mixtures were less than AL:GS = 0.5 mixtures, leading to improved compressive strength.  
Figure 8: SEM images of geopolymer concrete specimens. (**a**) G1; (**b**) G5; (**c**) G7; (**d**) G11.  
Figure 9: Percentage of voids in binary SEM images of geopolymer concrete specimens after image acquisition procedures. (**a**) G1; (**b**) G5; (**c**) G7; (**d**) G11.