Synergistic stabilization of expansive clay using cement kiln dust, calcium carbide residue, and polypropylene fibers for pavement subgrades

The major problems associated with expansive clay are its high swelling potential, plasticity, moisture-sensitivity, and low load-bearing capacity, all of which challenge the subgrade of pavements. The main challenge is to simultaneously improve the mechanical performance with volumetric instability and minimize the amount of conventional stabilizing materials needed to achieve this. This study is aimed to solve this challenge by systematically using the chemical–mechanical stabilization method with cement kiln dust (CKD), calcium carbide residue (CCR), and polypropylene fibers (PPF). Differential free swell (DFS), Atterberg limits, compaction, unconfined compressive strength (UCS), soaked California Bearing Ratio (CBR), and microstructural analysis using scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) were used to analyze individual, binary, and ternary stabilization systems. The stabilized mixtures were then evaluated for pavement-subgrade performance in terms of resilient modulus determined from CBR and flexible pavement thickness design as per IRC:37–2018. The untreated clay had a DFS of 75.38%, a plasticity index of 32.2%, a 28-day UCS of 250 kPa, and a soaked CBR of just 1.97%, which were not satisfactory for subgrade materials. Systematic optimization showed that the combination of 20% CKD, 10% CCR and 1.5% PPF resulted in a most balanced chemical and mechanical stabilization. This ternary system lowered the DFS to 1.10%, made the soil nearly non-plastic and increased UCS to 1178 kPa at 28 days and to 1318 kPa at 90 days. The soaked CBR increased to 20.25%, while the resilient modulus increased from 19.70 MPa for untreated clay to 120.68 MPa. The SEM–EDS observations verified that the soil had a denser structure, fewer pores, better bonding of the particles and the formation of calcium-based cementitious products, as well as the presence of fiber-mediated crack bridging. The optimum ternary mixture resulted in a pavement thickness of 480 mm at 5 MSA and 575 mm at 50 MSA, which is 55 mm less than with PPF-only stabilization at high traffic levels, as determined by pavement analysis. The results show that the combined use of calcium rich industrial by-products and fiber reinforcement is a synergistic method for the preparation of high strength and stability pavement subbase from highly expansive clay.