Dynamic properties and microstructure of slag-modified loess

ObjectiveTo improve the performance of loess subgrade and promote the resource utilization of industrial solid waste, this study investigates the macro- and micro-characteristics of slag-modified loess, taking loess from Xining, Qinghai, as the study object.MethodFirst, the optimal slag content was determined through unconfined compressive strength test. On this basis, the cyclic loading was applied using a torsional shear apparatus with hollow cylinder. It simulated the stress on subgrade soil due to the rotation of principal stress axes with different dynamic stress amplitudes and torsional shear stress ratios; as well as analyzed the evolution of accumulated plastic strain and resilient modulus. Finally, the micro-mechanism of slag-modified loess was investigated from aspects of pore structure and fractal dimension.ResultThe slag content of 25% yielded the optimal improvement, enhancing the unconfined compressive strength of loess by 50%-200%. The incorporation of slag reduced the cumulative plastic strain by 40%-200% under cyclic loading. With a constant cyclic vertical stress ratio, the attenuation rate of resilient modulus accelerated with the increase of torsional shear stress ratio. When cyclic vertical stress ratio was 0.3, the mitigation of resilient modulus attenuation of the modified soil was improved by 84.6% and 184.5% with the torsional shear stress ratios of 1/3 and 1/2 respectively compared with plain loess. The combined SEM-CT observations revealed that the porosity ratio of macropores (>100 μm) and mesopores (30-100 μm) in the modified soil decreased by 15.3% and 22.1% respectively.ConclusionThe principal stress axis rotation accelerates the accumulation of vertical plastic strain and the degradation of resilient modulus under traffic cyclic loading. The addition of slag significantly inhibits this effect. The microscopic mechanism indicates that Ca2+ from slag promotes the formation of aggregates (80-150 μm) from clay particles through ion exchange. These aggregates effectively fill pores and establish a spatial network skeleton, thereby significantly enhancing the mechanical properties of soil. The findings provide a theoretical basis for the slag recycling in loess subgrade engineering.