# Experimental Study on the Mechanical Properties of Xinyang Red Clay Improved by Lime and Fly Ash
## 5 Conclusions  
This study focused on improving Xinyang red clay using three methods: single lime, single fly ash, and compound lime-fly ash. Various tests were conducted, including the limiting water content test, compaction test, direct shear test, unconfined compression test, compression test, and microscopic test, to analyze the physical and mechanical properties, as well as the microscopic structure of the improved red clay. The main conclusions drawn from this study are as follows:  
1. The liquid limit, plastic index, and maximum dry density of red clay decrease with the increase of the proportion of lime and fly ash, while the plastic limit and optimal water content increase with the increase of lime and fly ash content. This suggests  
Figure 9: Energy-dispersive spectroscopy images: (**a**) red clay, (**b**) 1:2:7, (**c**) 1:3:6, and (**d**) 1:4:5.  
that adding lime and fly ash reduces the plasticity and increases the compaction characteristics of the red clay.
2. The addition of lime and fly ash has a significant influence on the cohesion (c) of the shear strength index, but has no significant influence on the internal friction angle (\(\varphi\)). This indicates that lime and fly ash primarily affect the cohesive properties of the red clay.
3. When the content of lime is 5%, fly ash is 10%, and the ratio of lime to fly ash is 1:2:7, the shear strength index and unconfined compressive strength of the soil are improved, and the compressibility of the soil is reduced. This suggests that a specific combination of lime and fly ash, with appropriate proportions, yields the best results in terms of improving the mechanical properties of the red clay.
4. When the red clay is improved by adding lime-fly ash, the improved soil may become brittle to some extent when the fly ash content is high. Therefore, it is important to control the fly ash content appropriately, to avoid detrimental effects on the mechanical properties of the soil.
5. Scanning electron microscope and X-ray diffraction analysis revealed that the modified red clay particles, with a ratio of 1:2:7 of lime to fly ash, were coated with Ca(OH)\({}_{2}\), calcium silicate hydrate (C-S-H), calcium aluminate hydrate (C-A-H), and other gel substances. These substances cemented the particles together, increasing the bonding force between the particles, and improving the overall structure of the red clay.  
The study in this paper is still relatively shallow due to a variety of factors, such as experimental conditions. More experiments are needed in the future to explore the optimal combination of fly ash and lime content to obtain a high-performance modified Xinyang red clay. Moreover, more microstructure testing techniques need to be utilized to reveal the mechanism of the effect of different admixtures on the soil. Additionally, it is also important to characterize the natures of different phases under microstructures, and analyze the effects of phase natures on soil properties.  
H.T. Writing--original draft, methodology; Z.Y.: methodology, conceptualization; H.Z.: revised manuscript, formal analysis; H.D.: formal analysis, resources. All authors have read and agreed to the published version of the manuscript.  
Key Scientific and Technological Research Projects of Henan Province (23210220004).  
**Data Availability Statement:** Some or all data during the study are available from the corresponding author by request.  
**Conflicts of Interest:** The authors declare no conflict of interest.