# Soil Stabilization Using Waste Paper Fly Ash: Precautions for Its Correct Use
## 3 Results and Discussion
### Water Reduction and Delay Time Verification  
As the WPFA is very sensitive to the amount of the water, and as free lime reaction is expansive, the homemade test was carried out with different water ratios and delay times. Figure 9a represents the expansion in the mixture with Proctor water content (10.5%) for 0, 15, and 60 min delay times. By adding a delay time, the expansion was decreased significantly, from 0.89% to 0.64% and 0.53% (0, 15, and 60 min delay times, respectively).  
Figure 9b shows the result of decreasing the water ratio to 8.2%. The applied delay times were equal to the previous test, and the only variation was the reduction in water by one point of Proctor. The expansion did not exceed the 0.2% in any case: with a 15 min delay, the expansion was around 0.2%, and with a 60 min delay, the expansion was close to zero.  
Variation in the quantities of water can be easily generated in a work site, and if they are higher than those of the Proctor, and since WPFA is a material sensitive to the amount of water, special care should be taken. To evaluate this, more water was added to the mix. When adding more water to the mix (12%), even with a 180 min delay, the expansion rose significantly to around 1.5%, as shown in Figure 9c. Even when adding 3 h of delay time, the expansion was still considerable. The results show that WPFA was very sensitive to the amount of water and delay time. Similarly, high water content in a mix is one of the causes of poor strength development, due to an increase in porosity in the WPFA and reduction of the density of the mix.  
It should be noted that the test was carried out with a soil-cement sample, and no significant expansion values were obtained (no delay time was applied for cement samples).  
As shown, a delay time can reduce the expansion of the mixture by nearly half, and even adding a 15 min delay to the mixture can have a positive effect on samples. Consequently, the water reduction in the mixture leads to even greater decrease in expansion. On the other hand, by adding more water, the expansion, even with a 3 h delay, increases significantly.  
According to the results of the slaking and expansion, it was observed that a reduction in the amount of water (from 10.5% to 8.2%) reduced the expansion, and by adding a delay time of 60 min, the expansion decreased significantly to nearly zero. However, in real work scenarios, waiting 60 min  
Figure 9: Expansion in soil containing 5% WPFA: (**a**) Proctor water content, (**b**) reducing the water content by one point of Proctor, (**c**) increasing the water content by one point of Proctor.  
may be counterproductive or unusual. Therefore, it was decided to use a delay time of 30 min for the long-term expansion test.  
Finally, to verify that the slaking occurred entirely, and see the delay time and water reduction effect over a long period on the stabilized soil, the samples were placed in continuous contact with water for 180 days, and vertical displacement was measured. The samples cured for 1 day are shown in Figure 10a, and in both cases--water absorption by suction or water poured from top--the expansion was less than 0.5%. The results of samples cured for seven days by pouring the water from the top are shown in Figure 10b. The expansion in one of the samples was slightly higher than the other one. However, in both samples it was still less than 0.5%. Finally, Figure 10c shows the 7 days cured sample using water suction. In both samples, the expansion was similar and less than 0.5%. It should be mentioned that [PERSON] [14] found that stabilizing a pretreatment clayey soil by adding 2 wt % quicklime and 6 wt % of a binder containing 80 wt % WSA and 20 wt % gypsum is acceptable in terms of expansion, and no significant expansion were observed.  
The results obtained from cement samples did not show any expansion; hence, to avoid confusion, their results are not included in Figure 10.