Foamed lightweight soil (FLS) is a cementitious geotechnical material prepared via mixing cement paste with preformed foam and sometimes incorporating additional aggregates. Its density and mechanical strength can be tailored via adjusting the foam content. FLS is widely employed in embankment construction and backfill applications due to its capacity to substantially reduce fill load and settlement. The strength of FLS generally exhibits an exponential relationship with density, which is affected by its internal pore structure. During casting, unstable bubbles can lead to non-uniform pore distribution. Enhancing foam stability is therefore crucial for pore structure optimization. Additives such as stabilizers and viscosifiers are commonly used to improve foam stability, but they can concurrently reduce foaming capacity. SiO₂ nanoparticles have a potential in modifying interface properties and inhibiting foam decay. However, its influence on the pore structure of FLS and a systematic comparison with conventional FLS remain to be insufficiently investigated. This study was to examine how SiO₂ nanoparticles could affect foam stability and enhance the mechanical properties and pore structure of FLS. The effects of morphology and content of SiO₂ nanoparticles were analyzed through bubble density measurements, foam bleeding tests, and optical microscopy. The mechanical performance and pore structure characteristics were evaluated by uniaxial compression tests and X-ray computed tomography, providing theoretical and practical insights for the application of nanomaterials in FLS.

FLS specimens were prepared with P·C 42.5 composite Portland cement, plant-protein-based foaming agent, nanoparticles of SiO₂, and tap water. SiO₂ nanoparticles were incorporated at different mass ratios (i.e., 1‰, 3‰, and 5‰) by cement mass. Three target wet density groups were designed at 1.5, 1.1 g/cm³, and 0.6 g/cm³. The mixtures were cast in cubic molds with the size of 70.7 mm, demolded after 24 h, and cured under standard conditions (i.e., (20 ± 2) ℃, RH > 95%) for 7, 28 d, and 60 d, respectively. The foam bleeding rate was determined via measuring the volume of liquid expelled from freshly prepared foam standing in air at specified intervals, calculated as a ratio of expelled liquid to initial foam volume. The bubble morphology and foam decay were analyzed by optical microscopy. The mechanical properties of hardened specimens were evaluated by uniaxial compression tests, while the pore structure was characterized using X-ray computed tomography (X-CT). All the experiments were repeated at least three times to ensure accuracy and reproducibility.

The results of foam stability test demonstrate that the incorporation of SiO₂ nanoparticles significantly reduces the foam bleeding rate and prolongs the bubble dissipation duration in FLS, while simultaneously promoting more uniform bubble size distribution and regular morphological characteristics. These results confirm the effective enhancement of bubble stability through the modification of SiO₂ nanoparticles. Note that FLS specimens incorporating irregularly-shaped SiO₂ nanoparticles exhibit superior comprehensive performance due to the larger contact area between irregular particles and bubble interfaces, compared to their spherical counterparts. For FLS specimens with irregular SiO₂ nanoparticles, the optimal incorporation ratio is 3‰ (in mass), achieving a 1 h bleeding rate of 23% in the foam system, representing a 70% reduction, compared to the control group without SiO₂ nanoparticles. This enhancement mechanism involves the adsorption of surfactant molecules onto the surfaces of the nanoparticles, where appropriate concentrations typically form monolayer adsorption films that convert the surface of originally hydrophilic to hydrophobic. However, the excessive surfactant adsorption leads to a bilayer formation, reverting the nanoparticle surface to hydrophilic and consequently diminishing a stabilization effectiveness. The results of uniaxial compression tests reveal that the peak strength of FLS in different density groups generally follows an initial increase and a subsequent decrease trend with increasing the content of SiO₂ nanoparticles, reaching the maximum values at an incorporation ratio of 3‰. The results of X-CT scanning further indicate that the incorporation of SiO₂ nanoparticles generally reduces the internal pore volume of FLS, while demonstrating that the pore volume has a decreasing-then-increasing pattern as the content of SiO₂ nanoparticles increases. Moreover, the addition of SiO₂ nanoparticles contributes to more regular pore morphology and more uniform pore spacing distribution in FLS. The specimens incorporating the SiO₂ nanoparticles of 3‰ and 5‰ exhibit more regular internal pore structures, compared to that with SiO₂ nanoparticles of 1‰.

The incorporation of SiO₂ nanoparticles enhanced foam density, while reducing foaming volume and bleeding rate. The microscopic analysis revealed that SiO₂ nanoparticles could thicken the liquid film, increase foam sphericity, and reduce the connectivity of Plateau borders, thereby inhibiting gas escape. The 3‰ content of irregular SiO₂ nanoparticles demonstrated an optimal foam quality. At identical curing ages, the uniaxial compressive strength of FLS in different density groups exhibited a characteristic trend of initial increase and subsequent decrease as the content of SiO₂ nanoparticles increased, obtaining the maximum value at the content of 3‰. Compared to the specimens without SiO₂ nanoparticles, the 28 d and 60 d compressive strengths in three density groups with SiO₂ nanoparticles could increase from 7% to 51%.For the specimens without SiO₂ nanoparticles, the 28 d FLS specimens with SiO₂ nanoparticles displayed reduced individual pore volumes in all density groups, and the average pore volume was decreased by 28%-46%. Concurrently, the pore sphericity increased and accompanied with a narrow size distribution, and the pore spacing became more uniform. The modification of SiO₂ nanoparticles could prove particularly effective in optimizing the pore structure of low-density FLS.