# Experimental Study on the Mechanical Properties of Xinyang Red Clay Improved by Lime and Fly Ash
## 3 Test Results and Analysis  
### Limitation Water Content Test  
The plastic limit and liquid limit of soil samples with different modified materials and different mixing amounts are tested by the liquid-plastic limit tester, and the plasticity index is calculated. The results are shown in Figure 2. The liquid limit, plastic limit, and plasticity indexes of remolded red clay are 38.4%, 20.2%, and 18.2. It can be seen from Figure 1 that the liquid-plastic limit and plasticity index of the soil mass can be improved by adding lime and fly ash individually or in combination. The liquid limit and plasticity index decreased with the increase of the proportion of lime and fly ash, while the plastic limit increased with the increase of the amount of lime and fly ash. When the lime content is 5%, the liquid limit drops to 35.5%, the plastic limit rises to 25.7%, and the plasticity index drops to 9.6. At 0-2% and 8-11%, the plastic limit, liquid limit, and plasticity index have little change. It can be seen that it is appropriate to use only lime to improve the red clay,  
Figure 1: Red clay soil sample.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Experimental Items** & **Experimental Instruments** & **Testing Parameters** \\ \hline Limit water content experiment & Liquid–plastic combined tester & Plastic limit \\  & Liquid limit \\ \hline Compaction experiment & Standard portable compaction instrument & Optimal water content \\  & & Maximum dry density \\ \hline Direct shear experiment & Strain-controlled direct shear apparatus & Cohesion \\  & & Friction angle \\ \hline Unconfined compression experiment & Strain-controlled unconfined pressure & Unconfined compressive strength \\  & gauge & \\ \hline Consolidation experiment & Fully automatic consolidation instrument & Compression coefficient \\ \hline \hline \end{tabular}
\end{table}
Table 3: Experimental items and recorded data.  
with a mixing amount of 5%. When the content of fly ash is 10%, the liquid limit decreases to 33.5%, the plastic limit increases to 23.5%, and the plasticity index decreases to 10. When the content of fly ash is 0-5% and 15-20%, the plastic limit, liquid limit, and plasticity index have little change. When the red clay is improved by adding lime and fly ash, the ratio of 1:2:7 is better according to the change of plastic limit, liquid limit, and plastic index.  
After adding lime, CaO in lime reacts with the water in red clay to generate Ca(OH)\({}_{2}\), which consumes part of the free water; the Ca\({}^{2+}\), ionized from Ca(OH)\({}_{2}\), displaces K\({}^{+}\) and Na\({}^{+}\) on the surface of red clay particles, reducing the thickness of the bound water film and increasing the attraction between particles. The addition of lime reduces the proportion of clay particles in the soil, reducing the hydrophilicity of the soil. After the fly ash is added, the active substances in the fly ash and the free water in the red clay will undergo hydrolysis and hydration reactions, and the active SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) will be released to generate compounds such as silicic acid, aluminic acid, and ferritic acid. The liquid limit and plasticity index of the improved red clay decreased, while the plastic limit increased.