# Experimental Study on the Mechanical Properties of Xinyang Red Clay Improved by Lime and Fly Ash
## 1 Introduction  
Red clay is a highly plastic clay of brown-red and yellow-brown color, formed by intense weathering of carbonate rocks in a temperate and humid subtropical climate [1, 2]. Its mineral composition is dominated by kaolinite, illite, and chlorite, and it is characterized by a high natural water content, a high porosity ratio, and low density [3]. As a wide range of engineering materials, such as roadbed filler and slope filling, red clay is prone to appear with a large number of cracks, expansion and contraction, and other adverse engineering characteristics in the project if it is not improved, leading to road subsidence and slope sliding [4]. The most commonly used method for improving red clay is to add lime, fly ash, cement or lime-cement, fly ash-cement, and other materials in a certain proportion [5, 6, 7, 8]; there are also other engineering soft soil improvement methods [9, 10]. At present, the research on improving engineering soft soil mainly focuses on three aspects. The first is to analyze the incorporation of different improving materials macroscopically, test the physical and mechanical property indexes through geotechnical experiments, and compare and analyze the indexes before the improvement to determine the optimal incorporation ratio of the improving materials. For example, [PERSON] et al. [11] used fly ash, water glass mortar, and NaOH solution as an activator to improve highly plastic clay. Through experiments, it was found that 25% of the improving material was the most effective in improving the compressive strength of the soil. [PERSON] et al. [12] used jute fiber and lime to improve argillaceous soil. Jute fiber has a certain elongation, and the optimal addition amount of jute fiber is 1% based on the unconfined compressive strength and ductility of the soil. [PERSON] et al. [13] mixed 60% construction waste particle material into the sediment silt, and the soil load ratio, resilience modulus, and unconfined compressive strength achieved satisfactory results. [PERSON] et al. [14] added 8% waste concrete into silty and and crushed it into fine material and 6% lime, which showed the maximum bearingratio of soil and significantly increased shear strength. [PERSON] et al. [15] added 9% lime to silty soil. After curing for 180 days, the tensile strength and unconfined compressive strength of the silty soil reached the maximum. [PERSON] et al. [16] used ordinary silicate cement and fly ash to improve dredged silt. The content of ordinary silicate cement is 5% and that of fly ash is 10%, and the mechanical property reached the maximum. [PERSON] et al. [17] added 20% coffee shell ash to expansive soil, and the bearing capacity of the soil increased by three times. [PERSON] et al. [18] used palm oil fuel ash as a precursor and mixed sodium silicate solution and sodium hydroxide solution as alkaline stimulation. [PERSON] et al. [19] used fly ash and blast furnace slag to improve silty clay, and found that the strength of the soil is affected by the interaction between soil particles, mainly from chemical bonding strength, and the optimal ratio is 20:10. [PERSON] et al. [20] added bagasse ash and fly ash into clay to improve the clay. At the same time, a mixed sodium silicate solution and sodium hydroxide solution were added as an alkaline activator, and the appropriate dosage was 5-10%. [PERSON] et al. [21] modified soft marine clay with fly ash and slag, and different combinations of sodium- and potassium-based liquid alkaline activators (L) were added to the soil to improve soil strength. The strength of the combination of 30% NaOH with 70% Na\({}_{2}\)SiO\({}_{3}\) composite is the highest. [PERSON] et al. [22] also improved soft clay with high water content by adding a polymer made of fly ash and fine blast furnace slag. The comparison analysis showed that when the content of polymer and cement was 20%, and the curing time was 28 days, the polymer-improved soil had higher strength and more acid-resistant properties.