# Effects of Aging on the Dry Shrinkage Cracking of Lime Soils with Different Proportions
## 4 Discussion  
### Effect of pH on the Dry Shrinkage Cracking of the Samples  
For the insufficiently aged sample, during the dry-wet cycle, CaO reacted with water to produce Ca(OH)\({}_{2}\), and this process was accompanied with the release of tremendous heat, which resulted in the uplifting and cracking of the sample. During aging, the change in the pH of the sample was closely related to the occurrence of the CaO hydration reaction and the content of Ca(OH)\({}_{2}\). To further explore the internal mechanism involved in the effect of pH on the cracking of lime soil, nine groups of representative samples were selected for an XRD analysis to explore the formation and change in the internal products of the samples with a different lime content and particle size under a different aging time. For this analysis, samples with a lime content of 20 and 30%, lime particle size of 0.01-0.074 mm, and aging time of 6, 48, 72, and 168 h, were selected. In addition, samples with a lime content of 20%, lime particle size of 0.5-2 mm, and aging times of 6, 48, 72, and 168 h were prepared. Figure 12 shows the XRD results, and Figure 13 shows the fitting curve of the XRD results.  
1. At the initial aging stage, a large amount of CaO rapidly reacted with water to form Ca(OH)\({}_{2}\). Accordingly, the CaO content decreased with an increase in the aging time. With an increase in the lime content, the time required for the consumption of CaO increased. The CaO content of the 20%-0.01-0.074 mm sample after 72 h aging was 0.8%, whereas that of the 30%-0.01-0.074 mm sample decreased to 0.1% after 120 h aging. With an increase in the lime particle size, the time required for the consumption of CaO increased. At an aging time of 120 h, the CaO content of the 20%-0.5-2 mm sample decreased to 0.2%.  
2. With an increase in the aging time, the Ca(OH)\({}_{2}\) content first increased and then decreased. In addition, with an increase in the lime content and particle size, the required aging time for the peak Ca(OH)\({}_{2}\) content increased. For example, the aging times required for the peak Ca(OH)\({}_{2}\) content in the 20%-0.01-0.074 mm, 30%-0.01-0.074 mm, and 20%-0.5-2 mm samples were 48, 72, and 72 h, respectively.  
3. Ca(OH)\({}_{2}\) carbonized with the CO\({}_{2}\) in air to form CaCO\({}_{3}\), which was attached to the surface of Ca(OH)\({}_{2}\)[37]. CaCO\({}_{3}\) crystals exhibited a low diffraction peak intensity, low CO\({}_{2}\) content in natural air, slow carbonization reaction, and limited carbonization products. In addition, CaO, SiO\({}_{2}\), and Al\({}_{2}\)O\({}_{3}\) reacted to form albite, which consumed part of CaO and SiO\({}_{2}\), resulting in the reduction of the diffraction peaks of CaO and SiO\({}_{2}\) in the sample.  
Figure 12: X-ray diffraction test results of the samples.  
Figure 13: Fitting curve of the X-ray diffraction test results.  
Figure 14 shows the fitting curve of the correlation between the pH and Ca(OH)\({}_{2}\) content of the samples. There was a good positive correlation between the pH and Ca(OH)\({}_{2}\) content of the samples during aging (Figure 14). With an increase in the aging time, the pH and Ca(OH)\({}_{2}\) content of the samples increased, and then decreased. In addition, the amount of Ca(OH)\({}_{2}\) produced by the hydration reaction of CaO and the pH of the sample increased. Further, when the peak pH was attained, a large amount of CaO was transformed into Ca(OH)\({}_{2}\). With an increase in the aging time, a small amount of CaO continued to react to form Ca(OH)\({}_{2}\). Consequently, a part of Ca(OH)\({}_{2}\) was carbonized to form CaCO\({}_{3}\), which consumed a part of Ca(OH)\({}_{2}\), thus decreasing the pH of the sample.