Reducing the compressibility of flowable cement- and ground granulated blast-furnace slag (GGBS)-stabilized soft clay through low sand addition: behavior, mechanism, and prediction

Producing flowable filling materials from waste soft clay provides a sustainable reuse pathway, yet their application is limited by inherently high compressibility associated with the high water content required for flowability. Hence, this study investigates the effectiveness of low-content sand addition in reducing compressibility of the stabilized clay-based material while maintaining workability. Two binders including cement and ground granulated blast-furnace slag (GGBS) were adopted, and the effects of binder content, water content, and sand volume ratio were examined. Before yielding, the recompression index (CR) is jointly controlled by strength and void ratio. However, the compression index (CC) corresponds to the stage where the cemented microstructure has yielded, after which compressibility predominantly depends on void ratio. Although increasing binder content from 60 kg/m3 to 80 kg/m3 markedly leads to a severalfold increase in strength, it does not reduce the compression index (CC), while a sand volume ratio of 0.15 decreases CC by 30.8–39.4% across different binder systems. Notably, specimens with and without sand exhibited similar coefficients of consolidation (cv) and hydraulic conductivity (k). Furthermore, this study proves the reduction of porosity is the governing mechanism for compressibility reduction within the investigated sand content range. Based on the mechanisms, this study proposes predictive relationships for the compressibility of stabilized clay–sand mixtures, offering a practical approach for compressibility control through minimal sand incorporation.