Optimization of Recycled Fine Aggregate Content for All-Solid-Waste-Based Flowable Solidified Soil: Performance and Microstructure

To promote the high-value utilization of construction and industrial solid wastes, this study prepared an all-solid-waste-based flowable solidified soil (FSS) using soft clay and recycled fine aggregate (RFA) as the main constituents. The binder system comprised ground-granulated blast-furnace slag (GGBS), carbide slag (CS), and desulfurization gypsum (DG), while fly ash (FA) was incorporated to improve workability. The primary objective was to identify an appropriate RFA content for this FSS system through a combined evaluation of workability, mechanical performance, durability, and microstructural characteristics. The results showed that increasing the RFA content increased flowability and shortened the setting time. Unconfined compressive strength (UCS) and ultrasonic pulse velocity (UPV) both increased initially and then decreased as the RFA content increased, and relatively favorable mechanical performance was observed at RFA contents of 40–60%. In the durability tests, the mixture containing 40% RFA exhibited the lowest mass loss and UCS loss after both wetting–drying and freeze–thaw cycles within the investigated range. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses suggested that a moderate RFA content was associated with the development of C-(A)-S-H-gel-related phases and ettringite (AFt), together with a denser and more continuous microstructure. The improved strength and durability at moderate RFA contents were therefore interpreted as the combined results of hydration-product development and the physical skeleton effect provided by RFA. By contrast, the performance decline at excessive RFA contents appeared to be related to a less favorable internal structure, as indicated by SEM observations. Overall, when workability, mechanical performance, durability, and microstructural observations are considered together, 40% RFA is recommended as the most suitable content for the material system and test conditions investigated in this study. These findings demonstrate the potential of RFA to regulate the performance of all-solid-waste-based FSS and to improve the resource efficiency of multiple solid-waste streams.