Assessment of Geotechnical and Microstructural Properties of Expansive Clay amended with Bauxite Residue using Sodium Silicate

This study investigates the geotechnical and microstructural behaviour of expansive Black Cotton Soil (BCS) stabilized with Red Mud (RM) using 10 M sodium silicate as an alkaline activator. A series of laboratory tests, including Atterberg limits, Free Swell Index (FSI), compaction, UCS, XRF, XRD, and SEM, were performed on BCS–RM blends containing 10–30% RM. The incorporation of RM significantly reduced plasticity and swelling, lowering the PI from 26.63% to about 20% and the FSI from 52.17% to 16.67%. Compaction behaviour improved, with maximum dry density increasing from 1.82 g/cc to 1.89 g/cc at 25% RM, while OMC decreased. Under sodium silicate activation, the UCS rose sharply from 142.71 kN/m² for natural BCS to 329.80 kN/m² for the 75BCS25RM mix. Microstructural analysis confirmed the formation of N-A-S-H and C-A-S-H gels and a denser, cemented matrix responsible for strength enhancement. Based on IRC:36-2010 criteria, mixes containing 25–30% RM were found suitable for embankment and subgrade applications. The results highlight that red mud, when chemically activated, offers an effective and sustainable solution for stabilizing expansive soils.