Stabilization of Soft Clay by a Low-Calcium Fly Ash Geopolymer

Geopolymers are widely considered be low-carbon and cost-effective binders and are used for the stabilization of soft soils in recent years. The current study investigated the stabilization efficacy of low-calcium fly ash (FA) geopolymer for soft clay. The influence of FA content, initial water content, and curing time on stabilization efficacy was investigated. A range of laboratory tests were conducted to investigate the strength and permeability performance of the geopolymer-stabilized clay, including unconfined compressive strength (UCS), falling head permeability, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results indicated that the mechanical performance of the soft clay at high water content was greatly improved by low-calcium FA geopolymer, exhibiting obvious characteristics of continuous long-term strength development and more ductility compared with cemented clay. A linear empirical relationship was proposed to overcome the shortcomings of the traditional logarithmic function for predicting the long-term strength evolution of low-calcium FA geopolymer-stabilized clay. The UCS of the mixtures increased rapidly with increasing FA content from 16% to 30% and from 25% to 30% for clay water content at liquid limit (LL) and 1.4 LL, respectively. Thirty percent FA content is needed for stabilizing soft clay at 1.0 LL by the deep soil mixing (DSM) method but is obviously not enough at higher initial water content. The major reduction in permeability of the geopolymer-stabilized clay occurred within 28 days of curing while the FA content reached 12%. Despite the increase in curing time up to 90 days and the increase in FA content to 30%, permeability decreased quite slowly. Comparing with the cemented clay, the stabilized clay showed better hydraulic performance but slightly lower permeability.