Study on Strength and Micromorphology of Expansive Soil Improved by Cement–Coal Gangue under Dry–Wet Cycle

Abstract

In order to improve the poor engineering characteristics of the expansive soil, such as cracking resulting from dry–wet swelling and shrinkage. In this study, coal gangue and cement were used as improving materials to improve the expansive soil. The research focuses on testing the efficacy of various coal gangue additions in enhancing the soil properties through compaction, liquid–plastic limit, expansion rate, unconfined strength, direct shear tests, as well as the dry–wet cycle stability of cement–coal gangue composite improved the samples. The findings suggest that as the coal gangue content raise, there is a corresponding decrease observed in both the maximum dry density and the expansion rate of the expansive soil. Simultaneously, the unconfined compressive strength (UCS) an increment from 0.07 MPa for the untreated soil to 0.6 MPa. Additionally, the inclusion of cement as a compound improvement agent substantially improves the UCS of the composite soil samples, with the highest recorded value reaching 3.14 MPa. The UCS of the composite improved soil is observed through three distinct stages: small increase, followed by a rapid decline, and ultimately reaching a state of gradual stabilization. Following 15 cycles of dry–wet cycles, the strength of the enhanced soil undergoes a reduction ranging from 26% to 57%. Scanning electron microscopy (SEM) analysis of the internal structure of the improved soil sample reveals that the cementitious material generated by cement hydration encapsulates the expansive soil particles and establishes connections with the coal gangue particles. However, fine cracks are observed in the cement hydration products near the surface of the coal gangue particles.