Capillary water absorption behavior and long-term mechanical response of lightweight soil-based foamed concrete

Lightweight soil-based foamed concrete presents a sustainable solution for geotechnical applications, but its long-term performance in moisture-rich environments remains a critical concern. This study systematically investigates the capillary water absorption behavior and long-term mechanical response, combining capillary water absorption tests, uniaxial compression testing and microstructural characterization. The results demonstrate a distinct three-stage capillary absorption behavior: an initial phase of rapid penetration (quantified by coefficient A 1 ), a subsequent period of matrix-governed absorption ( A 2 ), and a final stage dominated by diffusion ( A 3 ). The initial absorption coefficient A 1 exhibits significant positive correlations with foam volume fraction φ f , soil mass fraction δ s , and fly ash content δ a , whereas it demonstrates an inverse correlation with the water-to-solid ratio δ w . The second coefficient A 2 is predominantly determined by the distribution and connectivity of matrix pores. Two primary mechanisms compete to shape the mechanical response: strength weakening occurs in cementation-driven systems subjected to long-term water absorption, while enhanced ductility arises from the delayed drainage effects in air-voids-dominated systems. Microstructural analyses reveal a triphasic deterioration sequence initiated by water infiltration through interconnected pores, progressing to pore solution modification and matrix softening, and ultimately resulting in increased porosity and fracture network development. The study deepens a fundamental understanding of moisture-mechanics coupling in lightweight soil-based foamed concrete while providing practical mix design guidelines for infrastructure applications exposed to water.