## 3 Results and Discussion
### Microstructural Analysis  
For a deeper explanation of the stabilization mechanism, Figure 7 shows FESEM micromorphology pictures of the fractured surface of 28 day-cured untreated samples (CL and CH) and POFA-based geopolymer stabilized samples (CL-G40 PA and CH-G40 PA) since they achieved the best UCS performance. It is claimed that the higher strength geopolymer is associated with a firmer internal microstructure [45]. As can be seen in Figure 7, untreated CL has shown a considerable open structure with weak or no bonds between soil particles, in which pores can be clearly seen in Figure 7a, whereas the cluster shape particles in Figure 7b refer actually to the isolated soil particles of untreated CH, which have a flaky and weak structure with some arrangement among them (particles became close to each other) due to the compaction effect. However, the FESEM pictures in Figure 7c,d indicate that the activator solution stimulated the POFA precursor because of chemical changes that occurred internally within the particles. The microcracks shown  
Figure 6: Parameters of shear strength for soils treated with G10 PA, G20 PA, G30 PA and G40 PA: (**a**) CL, (**b**) CH.  
in the treated samples may be due to increased stress during the UCS test or shrinkage throughout the curing process. Treated samples (CL-G40 PA and CH-G40 PA) have experienced a significant enhancement in the soil structure. POFA-based geopolymer has filled interparticle voids leading to a dense and compact structure. The enhancement mechanism is explained by the production of the main geopolymer gel, which is sodium aluminate silicate hydrate (N-A-S-H) that forms well-developed 3-D networks of Si-O-Al and Si-O-Si (cementation products) and binds the soil particles together throughout the curing time [46; 47; 48]. However, the mixture of CH-G40 PA indicated some roughness and particles are not well coated with the gel compared to CL-G40 PA, which has shown a smooth structure. This indicates that geopolymer's efficiency in improving soil properties is highly dependent on the soil's type.  
This can also be confirmed by the results of EDX where the chemical composition indicates detection of Si and Al elements in the samples treated, as explicitly shown in Figure 7, which implies that the POFA-based geopolymer has taken place in treated claye  
Figure 7: Field emission electron microscopic (FESEM) micrographs: (**a**) CL, (**b**) CH, (**c**) CL-G40 PA, (**d**) CH-G40 PA.  
soils. Table 5 demonstrates the weight percentages and the ratios of the main elements found in the samples tested using EDX at the selected points (1, 2 and 3) in Figure 7. Point 1 is relatively smooth in both treated soils, and the EDX detected major Si, Al and O elements and minor Ca, Na, Mg and Fe elements. These elements are the basic main components of the natural soil. At points 2 and 3, the highest percentage of elements was for silicon, which likely represents the geopolymer and pozzolanic reactions. The sudden increase in the percentage of Na indicates the possibility of geopolymer gel formation (NASH) [49]. It was proved that low calcium fly ash-based geopolymer results in N-A-S-H formation as a dominant gel, and very low and slower development of calcium silicate hydrate (C-S-H) gel [50]. The decline in the percentages of Si and Al (from point 1 to 3) was mainly a result of the geopolymer reaction with the time elapsed via dissolving alumina and silica, confirming the significant growth in strength [49]. However, the calculated ratios of Si:Al explain that there were changes in the composition of soils due to the coating effect or the formation of new minerals as a result of alkaline activation reactions [51].