Monitoring and characterization of the polymer-filled pores in sand using nuclear magnetic resonance during air-drying

The polymer has been widely used in soil stabilization, but the microstructure and the polymer film formation process are seldom investigated. This paper investigated the time-dependent changes in microstructure and strength of the polymer-stabilized sand during the air-drying process using nuclear magnetic resonance (NMR) and unconfined compressive strength tests. Different polymer contents (1%, 2%, 3% and 4%, by the weight of dry sand) and curing time (0, 24, 48, 72 and 96 h) have been considered to highlight the ongoing polymer air hardening reaction. The results indicate that the polymer-occupied porosity decreased exponentially with the increase of curing time and increased by the rise in polymer content. The elastic modulus and compressive strength increased gradually with polymer content and curing time increment. The air-hardening process of polymer can be classified as the rapid air-hardening stage (first 48-hour curing) and slow air-hardening stage. The polymer-filled pores can be divided into micropores, mesopores, and macropores. Based on the corresponding relaxation time and amplitudes of the peak value and envelope area of the relaxation time curve in NMR, it can be found that the size of polymer-filled pores and the proportion of macropores decreases and the proportion of micropores increases during the air-drying process. During the air-drying process, the water in the polymer film evaporated; therefore, the shrinkage occurred in the polymer film, and the polymer-occupied space decreased. Additionally, the polymer film has higher strength and toughness under the action of air-hardening, which results in higher elastic modulus and compressive strength of polymer-stabilized sand. The present work may be helpful to understand the polymer stabilization mechanism and shed light on the new stabilization techniques of the problem soil.