Study on compressive strength characteristics of lime-fly ash composite improved mottled red clay

To address the susceptibility to softening instability in mottled red clay subgrade fillers from Jiangxi Province, a lime-fly ash composite modification approach was proposed. Systematic experimental investigations were conducted through a series of unconfined compressive strength (UCS) tests with designated fly ash (0%-25%) and lime (0%-8%) dosages, combined with immersion tests, wet-dry cycles, and scanning electron microscopy (SEM) analyses, to elucidate the strength evolution patterns and micro-mechanisms of modified soils. The test results show that: (1) the soil's UCS strength significantly increased after adding lime and fly ash to the mottled red clay. The strength increment from composite improvement with both lime and fly ash was more significant than the sum of the individual improvements, demonstrating a "1 + 1 > 2" synergistic effect; (2) After water immersion, the UCS of the soil modified with 3% lime and 15% fly ash and cured for 7 days, reached 538.22 kPa, meeting the railway subgrade fill specification of 350 kPa. After 28 days of curing, the UCS can be as high as 1176.85 kPa. Furthermore, the water stability and softening coefficients after five dry–wet cycles were 1.44 times and 2.25 times those of pure mottled red clay; (3) SEM analysis results indicate that while filling the pores, fly ash also acts as a network of solidified particle structures, forming rod-like and three-dimensional mesh structures with enhanced bonding forces between soil particles; and (4) An optimal ratio of 3% lime and 15% fly ash is recommended based on this study, which can be used as a reference for the modified mottled red clay field engineering application.