## 3 Results and Discussion
### Unconfined Compressive Strength  
Figure 4 illustrates the stress-strain relationships of CL and CH soils before and after treatment with POFA-based geopolymer at curing periods 7 and 28 days. The highest strain/deformation before failure was presented by untreated samples at a strain value of nearly 3.5% and 3% for CL and CH, respectively. However, there is an axial stress increase via the addition of POFA-based geopolymer to the soils accompanied by a strain failure reduction.  
Figure 3: \(Cont.\)Figure 4: Stress-strain curves of POFA-based geopolymer-treated soils: (**a**) 7 days CL, (**b**) 28 days CL, (**c**) 7 days CH, (**d**) 28 days CH.  
Figure 3: Moisture content-dry density relationships for soils treated with POFA-based geopolymer: (**a**) CL, (**b**) CH.  
The POFA-based geopolymer impact on the compressive strength (UCS) of both soils is depicted in Figure 5a,b. Accordingly, the results show that the UCS of both soils is highly dependent upon the geopolymer dosage and curing time. The higher the dose of POFA-based geopolymer applied to the soils, the higher is the maximum compressive strength attained. Treated soil samples with G40 PA show the maximum strength developed among geopolymer mixtures with some deviations considering the curing time, which resulted in very strong and dense mixtures. The results show that a moderate strength change was developed by the geopolymer treated specimens in the initial stages (7 days) and a greater strength was achieved after 28 days of curing. The improvement in UCS values with time can be addressed by two mechanisms. The first one is the development of highly cross-linked sodium aluminate silicate hydrate gel (N-A-S-H), which becomes stronger with time and was reported as the main mechanism by which the strength is enhanced [34]. The second minor mechanism is the pozzolanic reaction producing calcium silicate hydrate (C-S-H) due to the reaction of soluble calcium and silica found in the precursor (POFA) [22]. For the CL sample, the UCS increased from 0.26 MPa for untreated soil to 1.71 MPa and 4.18 MPa for soil treated with G40 PA (CL-G40 PA) at 7 and 28 days, respectively. Regarding CH soil, the UCS of untreated soil was 0.13 MPa and reached 1.06 MPa and 2.86 MPa when treated with G40 PA (CH-G40 PA) at 7 and 28 days, respectively. The Department of Public Works in Malaysia stated that a minimum strength of 0.8 MPa for stabilized subgrade soil should be achieved [35]. Based on this, most samples stabilized with the geopolymer in this study meet the requirement after 28 days of curing. This improvement indicates the high impact of POFA as a precursor, which is rich with reactive Al and Si ions in the existence of alkali cations, forming a strong and dense soil mixture by developing Al-O-Si and Si-O-Si chains associated with geopolymerization [36]. The fly ash reactivity is also represented by the small size of particles since it has a high influence on the strength of the geopolymer produced [37] and the amount of residual carbon that may hinder the process of alkali activation [38; 39]. Moreover, the noticeable improvement in strength between 7 and 28 curing days confirms that the geopolymer reaction is time-dependent and soil particles bonding with POFA-based geopolymer progresses with time. However, the dependence of soils' UCS on the dosage of geopolymer refers to the increase in the formation of the cementitious products accompanied by the geopolymer content increase in the soil [40].  
Figure 5: Compressive strength (UCS) of untreated and treated samples: (**a**) CL, (**b**) CH.