# Experimental Study on the Mechanical Properties of Xinyang Red Clay Improved by Lime and Fly Ash
## 4 Microscopic Test
### Microstructure Analysis of Improved Soil  
A field emission scanning electron microscope was used to conduct scanning electron microscopy on the original red clay and the improved soil [27]. The microstructure is shown in Figure 8, as well as the analysis of the main components of the improved soil. Among them, (a) is red clay, (b) is 1.2:7 mixed with lime-fly ash improved soil sample, (c) is 1.3:6 mixed with lime-fly ash improved soil sample, and (d) is 1:4:5 mixed with lime-fly ash improved soil sample.  
As can be seen from Figure 8, the particle size of red clay is small, the shape of red clay is mostly flat and irregular, and there are many small pores among the particles. The red clay is improved by adding lime and fly ash. The main reason is that the active substance CaO reacts with the free water in the red clay to produce Ca(OH)\({}_{2}\), and the generated Ca(OH)\({}_{2}\) ionizes Ca\({}^{2+}\) and (OH)\({}^{-}\). When Ca\({}^{2+}\) reaches a certain concentration, it replaces the low-price cations on the surface of the red clay particles, changing the electrical properties of the surface of the soil particles, thus enhancing the interparticle cohesion. At the same time, Ca(OH)\({}_{2}\) further reacts with the SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) in clay to produce calcium silicate hydrate (C-S-H) and calcium aluminates hydrate (C-A-H), respectively [7; 8]. The  
Figure 8: Remolding red clay and improving its microstructure (5000 times): (**a**) red clay, (**b**) 1:2:7, (c) 1:3:6, and (**d**) 1:4:5.  
resulting calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) have strong cementation and stable water stability. (C-S-H), (C-A-H), and Ca(OH)\({}_{2}\) are covered on the surface of soil particles and filled in pores, and wrapped on the surface of soil particles. The loose particles are consolidated into small particles, and the small particles are reconsolidated into large particles to increase the internal cohesion of the soil, so as to improve the engineering characteristics of the red clay. Adding an appropriate amount of fly ash to increase the content of SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) in the improved soil promotes the hydration reaction to proceed forward, and increases the content of calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H). As a result, the particles are more fully coated, the pores are dense, the interaction between particles is stronger, and the engineering characteristics of the red clay are improved.  
The comparative analysis of (b)(d) in Figure 8 shows that the surface of modified red clay particles, with a ratio of 1:2:7, is basically coated with the generated gel material and cemented together with each other. When the ratio is 1:4:5, the lime content is low, resulting in insufficient Ca(OH)\({}_{2}\), high fly ash content, and a high content of SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\), which cannot react with Ca(OH)\({}_{2}\) to form calcium silicate hydrate (C-S-H) and calcium aluminates hydrate (C-A-H). It can only be filled in the pores between soil particles or wrapped in gel materials, which cannot reach the goal of soil improvement. This is consistent with the results of the composition analysis of modified red clay in Figure 9, and the test results of the mechanical property index of the improved soil.