Biopolymer and nanosilica application in stabilizing sand-clay soil: An investigation of strength and microstructural improvements

Motivated by environmental concerns associated with conventional stabilizers, recent research has focused on sustainable alternatives for soil stabilization. However, the combined influence of biopolymers and nanoparticles on soil behavior under freeze–thaw (F–T) conditions remains insufficiently understood. This study investigates the mechanical performance, durability, and microstructural behavior of a hybrid corn starch–silica nanoparticle stabilization system for sand–kaolinite mixed soil (MSK). Corn starch-based biopolymer (CST) was incorporated at 1.5%, 3%, and 4.5% by dry soil weight, while silica nanoparticles (NSP), were added at 0.5%, 1%, and 1.5%. Specimens were cured for 1, 7, 14, and 28 days and subsequently subjected to repeated F–T cycles. Mechanical performance was evaluated using unconfined compressive strength (UCS), indirect tensile strength (ITS), ultrasonic pulse velocity (UPV), California bearing ratio (CBR), and F–T durability tests, while SEM, XRD, and FTIR analyses were conducted to examine microstructural changes. The results demonstrated that the optimum mixture containing 3% CST and 1% NSP produced the highest overall performance among all tested mixtures. Compared with untreated soil, the optimum blend increased UCS by 465% (up to 1195.6 kPa), ITS by 435.7% (up to 310 kPa), UPV by 58.7%, dynamic modulus by 156%, and CBR by 261%, indicating substantial improvements in strength, stiffness, load-bearing capacity, and internal structural integrity. The stabilized specimens also showed significantly improved durability under repeated F–T cycles, with UCS and ITS reductions limited to only 15.09% and 6.42%, respectively, after 10 cycles, whereas untreated soil experienced severe deterioration due to crack propagation and loss of particle bonding. In addition, the incorporation of NSP enhanced matrix densification and improved stress transfer efficiency within the CST-treated soil. SEM observations revealed the formation of a dense hydrogel-bonded structure with reduced pore connectivity, while XRD and FTIR analyses confirmed that the stabilization mechanism was primarily governed by physical and intermolecular interactions, including hydrogen bonding, nanoparticle filler effects, and enhanced interparticle adhesion. The results further indicated that excessive CST content reduced performance because of gel-dominated behavior and reduced effective soil–soil contact, highlighting the importance of dosage optimization. Overall, the findings demonstrate that the CST–NSP hybrid system provides an effective, durable, and environmentally sustainable approach for improving soil performance in cold-region geotechnical applications.