Sustainable soil stabilization: The role of nano-iron oxide in self-healing improvement

• Effect of nano iron oxide on the self-healing properties of collapsible soil, has been investigated. • The samples stabilized with 1.5 % nano iron oxide had 18 %, 30 %, and 42 % strength healing after 7, 14, and 28 days. • Nano iron oxide can increase the self-healing ability of soils. • The non-destructive UPV tests showed a strong correlation (R² = 0.92) between UPV and UCS. In this research, the impact of nano-iron oxide (Fe₂O₃) on the mechanical properties and self-healing ability of this soil type was investigated. In addition, the effect of curing time was considered as an important factor, and the specimens were subjected to different curing and self-healing periods to evaluate time-dependent changes, particularly during the self-healing stage. Samples with varying nano-iron oxide contents (0–4 %) were prepared and subjected to a series of laboratory tests, including Atterberg limits, unconfined compressive strength (UCS), large-scale direct shear and ultrasonic pulse velocity (UPV). The results showed that adding 1.5 % nano iron oxide increased UCS by 32 % after 14 days of curing, whereas higher contents led to a relative decrease in strength. Furthermore, during the self-healing stage, the effect of curing time was clearly observed, as the cracked specimens treated with 1.5 % nano iron oxide recovered 18 %, 30 %, and 42 % of their initial strength after 7, 14, and 28 days, respectively. The increase in UPV values during the same periods indicated a decrease in porosity and an improvement in the internal structure of the samples. The high correlation between the UPV and UCS test results (R² = 0.92) highlights that UPV can effectively monitor the healing process. Microstructural analyses further confirmed this trend by revealing reduced surface roughness, improved particle bonding, and partial closure of microcracks in nano–Fe₂O₃–treated specimens. These findings demonstrate that nano iron oxide not only provides initial enhancement to collapsible soils but also enables the gradual healing of mechanical properties after cracking.