A Process-Based Framework for Early-Age Volumetric Stability in Foamed Controlled Low-Strength Material

Abstract

Early-age volumetric instability severely constrains the reliability of foamed controlled low-strength material (CLSM) when high foam fractions are required for lightweight backfilling. While prior studies emphasize compositional modification, this work demonstrates that geometric stability is primarily governed by shear history during mixing. A process-based framework was established to decouple process control from material stabilization in dredged-soil-based CLSM. Foam content (10–30%), mixing sequence (K, U1, U2), and rotational speed (30–120 rpm) were first evaluated to quantify shear-induced instability. As foam content increased from 10% to 30%, ΔV_Final increased from 1.47% to 19.82%, indicating a pronounced stability threshold beyond 25% foam. Under identical foam content, the staged-water method (U2) at 60 rpm reduced ΔV_Final to 5.91%, approximately 53–55% lower than the dry-mix condition, confirming the dominant role of mixing sequence and controlled shear. Material modifications acted as secondary stabilizers. Surfactant at 0.15% reduced ΔV_Final to 4.21% through interfacial reinforcement. FA replacement at 25% improved rheological cohesion, lowering ΔV_Final to 4.56%. Coconut fiber (0.20%) primarily enhanced structural integrity during setting (ΔV_Final = 4.89%). The integrated SFC configuration achieved ΔV_Final of 2.78% and 28-day compressive strength of 1.62 MPa, compared with 1.42 MPa for the baseline. The findings establish that early-age stability in foamed CLSM is process-dominant, with material strategies providing hierarchical reinforcement across interfacial, rheological, and structural scales. The proposed process–material integration framework establishes shear history as a governing design variable in foamed CLSM and provides a reproducible pathway for balancing lightweight performance with geometric stability in soft-ground backfilling applications.