IMPACT OF ULTRA-FINE CEMENT ON EARLY COMPRESSIVE STRENGTH OF CEMENT STABILIZED SOFT SOIL

In order to investigate the effect of ultra-fine cement on the early compressive strength development of cement stabilized soil, the composite cement binder is proposed by adding different contents of ultrafine cement into ordinary cement binder. Factors influencing the early strength of the composite cement stabilized soft soil were investigated. They include the content of the ultra-fine cement, the water content of soft soil and the curing stress. The curing equipment was designed and fabricated to apply the curing stress on the composite cement stabilized soil sample. The mechanical and micro-structural properties of the composite cement stabilized soft soil were obtained and analyzed via the unconfined compressive strength(UCS)test, the X-ray diffractometer(XRD)test, electron microscopy scanner(SEM)test, and proton nuclear magnetic resonance(NMR)test. The experimental results showed that at the same curing stress, the early compressive strength and the stiffness of the composite cement stabilized soft soil increased with the higher replacement contents of ultra-fine cement, regardless of the initial water content in the soft soil. The active particles with the smaller size in the composite cement binder could hydrate with free water in the stabilized soil, and then form the gelled products which would bind the soil particles and fill the pores of the soft soil. While, the inert particles with the coarser size in the composite cement binder were only served to fill the pores of the soil. With the higher initial water content of soft soil, the more amount of meso-scale pores were observed in the stabilized soil, which might induce the lower early compressive strength of the stabilized soil. With the increase of curing stress, the compressive strength and the stiffness of the composite cement stabilized soft soil enhanced in the 7-days curing period. The compactness of the stabilized soil became denser due to the higher curing stress. The function of the curing stress was found to develop the pre-stress state in the gel skeleton of the stabilized soil, and hence increase the compressive strength of the stabilized soil. Based on the test results, the formula was established to predict the early strength of the composite cement stabilized soil. At the same time, the hydration model of the soft soil stabilized by the composite cement binder was proposed.