Experimental Investigation on Mechanical Properties and Microstructure of Fiber-Reinforced Solid Waste-Based Foamed Lightweight Soil

Conventional cement-based foamed lightweight soil (FLS) faces cost and environmental challenges. This study develops a sustainable polyvinyl alcohol (PVA) fiber-reinforced solid waste-based FLS (PVA-SWFLS) by entirely replacing cement with a ternary system of red mud, granulated blast furnace slag, and fly ash. PVA fibers were incorporated to mitigate inherent brittleness and cracking. The effects of fiber content (0–0.9 vol%), length (3–15 mm), water–binder ratio (0.35–0.55), and wet density (550–950 kg/m3) on the fluidity and compressive strength were evaluated, along with analyses of microstructure and pore characteristics using scanning electron microscopy and mercury intrusion porosimetry. Findings reveal that fiber addition reduces flowability (up to 34.9%) but significantly bolsters compressive strength, depending on fiber content and length. For 0.3% and 0.5% contents, optimal fiber lengths of 12 mm and 9 mm were observed, respectively; the 28-day compressive strength reached a maximum of 2.97 MPa at the 0.3% content with 12 mm fibers. Beyond these optimal points, and particularly for higher contents (0.7–0.9%), strength decreased monotonically with increasing fiber length due to fiber agglomeration and reduced compactness. Furthermore, strength correlated positively with wet density and negatively with the water–binder ratio, while fluidity increased with both. The hierarchy of influence was identified as: fiber content > fiber length, and wet density > water–binder ratio, while all four parameters significantly governed fluidity. The stress–strain behavior under different parameter combinations was analyzed, and a parametric constitutive model was established to support practical applications.