# Soil Stabilization Using Waste Paper Fly Ash: Precautions for Its Correct Use
## 3 Results and Discussion
### Strength in Stabilized Soil  
The different variable effects, such as delay time, water reduction, and curing time on compressive strength are shown in Figure 11. Results obtained from mix 1 show that adding WPFA to the soil will improve its properties. However, it is not enough for it to fulfill PG3 requirements (1.5 MPa in compressive strength). In mix 2, by curing the samples for 14 days, their strength improved over the second 7 days of curing by 10%. Likewise, adding a one-hour delay time before compaction in mix 3 improved the properties of the soil significantly over mixes 1 and 2 (improved over 50% more than mixes 1 and 2), but it is not adequate to pass the PG3 requirement. Likewise, by lowering the water content in the mixture and adding a one-hour delay time in mix 4, the strength improved significantly and reached around 2.16 MPa, which is adequate according to PG3. This fact was attributed to the effects of slaking and water reduction in the mix, which led to a better gain in strength. Comparing the results of mix 3 and mix 4, by decreasing the water from 10.5% to 8.2%, the results showed a growth in strength of over 1 MPa. This is believed to be due to how water reacts with WPFA, which besides producing the expansion, makes the mix more porous and causes it to have weak strength in mix 3. In mix 5, instead of an hour delay, a 30 min delay was applied. This decision was made because in practice, a one-hour delay in the job site is counterproductive. This mix showed the best improvement over the other ones by reaching 3.04 MPa in compressive strength. The experiment shows that it is crucial to add a delay time to the mix, and it can improve the strength of the soil. However, the delay time depends on factors like particle size and free lime content in WPFA. On the other hand, because the WPFA hydrates at a fast rate, a delay time of more than one hour should be avoided. Figure 10: Expansion in stabilized soil after curing for (**a**) 1 day and (**b,c**) 7 days. It was found that applying a delay time of 30 min and reducing the water content by one point of Proctor gained the most strength over other properties. As mentioned by [PERSON] [13], when GGBS is added to the mix, the water-to-WSA ratio improves, showing greater strength. In this case, by reducing the water in the mix by a point of Proctor, it led to improving the water-to-WPFA ratio and gaining more strength. However, to further study the effect of WPFA and its curing on the samples, the compressive strength test was carried out at different ages, as shown in Figure 12 with the optimum mix (mix 5). The mix gained most of its strength in the first seven days of curing, and in the later ages, from seven days to twenty-eight days, the strength improvement was only 0.18 MPa; it was more or less constant over the other periods (a small variation in compressive strength could be caused by the variability of WPFA properties). However, in the case of using 3% cement (mix 6), the strength improved over time. As the cement used in this study had pozzolanic materials, the hydration process was slower than normal cement. This type of cement tends to gain a small amount of strength on its first days of hydration, and when the pozzolanic reactions starts, an improvement in strength happens, as shown in Figure 12. Furthermore, the strength in samples with 5% WPFA was nearly equal to cement samples only with a curing time of  
Figure 11: Compressive strength with different conditions. Figure 12: Compressive strength development. 7 days, and at the rest of the ages, the cement samples improved significantly, whereas WPFA samples stayed nearly constant. As it observed in the mineralogical phases of WPFA (see Figure 4), the presence of lime and calcium silicate, which are reactive phases when mixed with water, leads to an increase in strength. Time present in WPFA hydrates at a fast rate and forms portlandite, giving strength at early ages. At later ages, calcium silicate hydrates due to its slow kinetic reaction, improving the strength by a tiny amount at 14 days.