Long-term performance of lime-treated soil and chemical reaction identification

Lime is widely used to modify clayey soils to enhance their physical and chemical properties, and lime-treated soil has become a key material in transportation infrastructure. Chemical reactions were identified through laboratory tests from field samples collected from the subgrade after 30 years of operation to understand its long-term performance evolution. Exchangeable calcium, carbonated calcium, and total calcium were quantified using ethylenediaminetetraacetic acid (EDTA) titration, gasometric analysis, and the strong acid extraction method, respectively. These measurements enabled the evaluation of calcium transformation during the pozzolanic reaction, providing a quantitative characterization of pozzolanic progression in the lime-treated clay matrix. Evolutions in pH, electrical conductivity, and salinity were also tracked. Mechanical performance was assessed through maximal shear modulus ( G max ) and unconfined compressive strength (UCS) tests. Then, the microstructure and mineral composition were analyzed via scanning electron microscopy (SEM) and X-ray diffraction (XRD). Furthermore, with an extended curing period, the pH, electrical conductivity, salinity, and exchangeable calcium content were found to decrease gradually. In contrast, the carbonation-related calcium content increased, and the clay mineral structures were significantly altered. The significant increase in G max and UCS is attributed to the formation of calcium-aluminate-silicate-hydrate (C-(A)-S-H) for pozzolanic and carbonation reactions where the clay mineral is involved. SEM reveals the curled edges of clay minerals and the formation of a 3D network. Additionally, XRD patterns further confirm the presence of increasing amounts of amorphous phases within the 2 θ range of 15° to 32°, indicating the progression of the pozzolanic reaction.