## 3 Results and Discussion
### Unconfined Compressive Strength and Curing Period  
Figure 6 represents the variations of UCS for the original soil and the mixtures compacted at different optimum moisture contents (Table 2) with curing periods of 0, 2, 8, and 28 days at room temperature (30 \(\pm\) 3 \({}^{\circ}\)C). No special treatment was done to the samples for curing other than just air-drying in the lab. This condition could be simulated in the field by covering the subgrade with a tarp and allowing it to air-dry for the desired curing period. Figure 5 shows a typical specimen appearance before and after the UCS test. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Properties** & **0\% Fly Ash** & **40\% Fly Ash** & **50\% Fly Ash** & **60\% Fly Ash** \\ \hline OMC \({}^{1}\) (\%) & 20.5 & 15.5 & 16.5 & 15.0 \\ MDD \({}^{2}\) (kg/m\({}^{3}\)) & 1648.3 & 1822.9 & 1770.0 & 1754.0 \\ \hline \hline \end{tabular} \({}^{1}\) OMC—Optimum Moisture Content, \({}^{2}\) MDD—Maximum Dry Density.  
\end{table} 
Table 2: Variations of OMC and MDD with fly ash content. Figure 4: Variations of specific gravity with fly ash content. The results showed that the pozzolanic effects of fly ash on increasing the compressive strength of the soil were consistent throughout the curing period for up to 8 days. The compressive strength started to drop from day 8 to day 28. The mixture with 50% fly ash showed the highest values in all curing periods, followed by the mixture with 40% fly ash, the mixture with 60% fly ash, and the original soil. However, the mixture with 60% fly ash content did not show any significant change in the compressive strength in comparison with the original soil. This is because fly ash became predominant in the mixture and it started behaving like fly ash without any effect of the soil. The increase in compressive strength with the increase in fly ash content could be due to the increase in the bonding between the soil and fly ash particles and due to the contribution of the angular glassy spheres of fly ash grains that increase the bonding between soil particles. It could also be because of the chemical reaction of the fly ash with the soil, represented by the deposition of some minerals, such as calcium carbonate, inside the pores of soil-fly ash matrix, which resulted in plugging the pores in the mixture, resulting in reducing the soil permeability and increasing its strength. The decrease in the compressive strengths for all samples in the curing period of 28 days was probably due to the development of pozzolanic reactions and the insufficient content of CaO in the Class **F** fly ash that was needed to sustain the formation of significant cementitious products. The decrease in the UCS could also be attributed to the changes in outside humidity, since the samples were kept in the lab and the indoor temperature and humidity levels were related to those outdoors. The optimum curing period seemed to be 8 days because no increase in strength was observed and, in some cases, a decrease in strength was observed after this period (Figure 6). Therefore, an optimum curing period of 8 days can be considered. Figure 5: Typical specimen appearance (**a**) before and (**b**) after the unconfined compression strength (UCS) test. Figure 6: Variation of UCS with curing period. The limitation of the study was that the UCS tests were not performed for 14 and 21 days to make sure that UCS did not peak in one of these curing periods. However, to avoid this limitation, polynomial trend lines were added to see any shift of max UCS, presented Figure 7. Identifying and using the right model for the data at hand require a combination of experience, knowledge about the underlying process, and statistical interpretation of the fitting outcomes. While the former is a somewhat individual choice, there is a need to quantify the validity of a fit by some form of measure that discriminates a \"good\" from a \"bad\" fit.