# Effects of Aging on the Dry Shrinkage Cracking of Lime Soils with Different Proportions
## 4 Discussion
### Effect of the Particle Distribution on the Dry Shrinkage Cracking of the Samples  
To further explore the internal mechanism involved in the effect of the particle distribution on the cracking of the lime soil, nine groups of representative samples were selected for an SEM analysis at a magnification of 500 and 2000X. The SEM was performed to observe the effects of the aging time, lime content, and particle size on the microstructure of the samples. The SEM test results are shown in Figure 15. Figure 15a-f show the SEM images of the samples with a lime content of 20 and 30% and particle size of 0.01-0.074 mm at aging times of 6, 72, and 120 h. Figure 15g-i shows the SEM images of the samples with a lime particle size of 0.5-2 mm and lime content of 20% at aging times of 6, 72, and 120 h. The SEM observation revealed that:  
Figure 14: Fitting curve of the correlation between the pH and Ca(OH)\({}_{2}\) content of the samples.  
Figure 15: _Cont._  
1. The aging time had a significant effect on the structures of the particles in the sample (Figure 15). At an aging time of 6 h, the digested Ca(OH)2 exhibited a disordered morphology. In addition, during digestion, CaO generated tremendous heat, which resulted in a sudden increase in the temperature of the samples, and an uneven heat distribution, which affected the growth on the surface of the Ca(OH)2 crystals. Furthermore, from a macro-scale perspective, the soil was susceptible to cracking. With an increase in the aging time, the soil particles exhibited a gradually regular shape, a notable agglomeration effect, and the aggregates and clay fine particles were wrapped by calcified crystals and silicates. In addition, small particles were cemented together to form larger particles, and a relatively stable bond was formed between these particles. This is one of the important reasons for the weakening of the degree of cracks in the soil [38; 39].  
2. With an increase in the lime content, there was a general increase in the particle size of the lime soil (Figure 15a-f). Particularly, the particle size of the sample with 30% lime content was larger than that of the sample with 20% lime content. With an increase in the lime content, the soil particles exhibited a notable agglomeration reaction with lime. In addition, some Ca(OH)2 grains were wrapped on the surface of CaO, which was not completely hydrated during digestion, thus controlling the hydration reaction of CaO.  
With an increase in the aging time to 120 h, the Ca(OH)\({}_{2}\) crystals gradually dissolved and recrystallized, and the crystals exhibited a uniform and regular distribution.  
3. An increase in the lime particle size affected the size distribution of lime soil particles during aging (Figure 15a-c,g-i). The aged lime soil particles with a large particle size were mainly large and medium particles, and there were few fine particles. During the hydration of CaO to Ca(OH)\({}_{2}\), CaO with a large specific surface area formed Ca(OH)\({}_{2}\) particles in the liquid phase system, after which the particles were deposited on the surface of the large grains. Some Ca\({}^{2+}\) reacted with CO\({}_{2}\) in the liquid phase to form CaCO\({}_{3}\), which was attached to the surface of Ca(OH)\({}_{2}\) and blocked a part of the surface pore structure, which resulted in an increase in the particle size of the soil samples.