## 4 Conclusions  
This research aimed to investigate the effect of POFA-based geopolymer on clayey soil stabilization in order to solve the issues corresponding to weak soil, protecting the environment by reducing energy consumption and using locally available agricultural waste in an efficient way.  
* Based on the findings, the prepared geopolymer consisting of POFA as source material and NaOH along with Na\({}_{2}\)SiO\({}_{3}\) as alkali activators was successful in enhancing the properties of the soil. However, the geotechnical properties of soil treated using POFA-based geopolymer depend mainly on the dosage of the binder, the host soil type and the curing conditions.
* POFA-based geopolymer-treated clayey soil specimens have been shown to have higher MDD values than untreated samples, with a drop in the accompanying OMC as the geopolymer dosage rises, which is an indication of improvement.
* The UCS results demonstrated that the maximum strength was accomplished by the geopolymer mixture consisting of the highest POFA percentage (G40 PA) at both curing times. At 28 days, the effect of POFA-based geopolymer was more obvious due to the development of greater strength compared with 7 days curing. In addition, the shear strength parameters--especially cohesion of soil--were improved due to the addition of POFA-based geopolymer even without curing.
* Apart from the positive outcomes shown by the current research, the application of POFA-based geopolymer to low and high plasticity clayey soils played a significant role in changing the soils' structure where the detached particles of soils in treated mixes have shown that they become more closely tied together, with the voids apparently filled, leading to a significant improvement in strength, as depicted in FESEM-EDX photographs. In addition, EDX confirmed that NASH gel is the most likely formed gel in the treated soils  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & **Point Number** & **Si** & **Al** & **Na** & **O** & **Ca** & **Si/Al** \\ \hline \multirow{3}{*}{CL-G40 PA} & 1.00 & 21.53 & 8.75 & 0.31 & 36.20 & 0.06 & 2.46 \\  & 2.00 & 17.44 & 6.08 & 0.22 & 40.79 & 0.09 & 2.87 \\  & 3.00 & 9.60 & 2.90 & 4.27 & 33.56 & 1.38 & 3.31 \\ \hline \multirow{3}{*}{CH-G40 PA} & 1.00 & 23.14 & 14.46 & 0.49 & 44.37 & 0.24 & 1.60 \\  & 2.00 & 37.08 & 18.17 & 0.43 & 41.00 & 0.49 & 2.04 \\  & 3.00 & 21.95 & 8.34 & 5.10 & 39.82 & 2.32 & 2.63 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Chemical composition and ratios of elements in the selected specimens.  
**Author Contributions:** All stages of the paper including conceptualization, design, research, analysis, conclusions and writing were carried out by I.A.K. and N.N.N.D. N.A.M.N. was involved in the supervision, editing and reviewing of the final version of the paper. All authors have read and agreed to the published version of the manuscript.  
**Funding:** No external funding was received for this research.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy issues.  
**Acknowledgments:** Authors would take thank and express their deep gratitude and appreciation for the technical and partial financial aid provided by the Civil Engineering Department, Engineering Faculty, Universiti Putra Malaysia (UPM).  
**Conflicts of Interest:** The authors have not reported any potential conflicts of interest. Also, in the design of the study, in the collection, analysis and interpretation of data and the decision to publish the results, the funders had no role.