Volume-changing soils are among the most challenging geomaterials commonly met on the planet during infrastructural development. This study investigates the change in the geotechnical and microstructural characteristics of expansive soils treated with cinder fine and fly ash. The expansive soil used in this study has a high plasticity and a sizable potential for swelling. The California Bearing Ratio (CBR) tests' results show a very weak/poor soil (0.88%), which makes the soil unsuitable for pavement subgrade layers in accordance with ERA-2013 geotechnical design specifications. With varying amounts of fly ash and cinder particles, expansive soil can be stabilized. First, different amounts of cinder fines were added to the expansive soil to stabilize it (4%, 8%, 12%, 16%, 20%), and then fly ash was added in increments of 5% from 5 to 25%. Additionally, 20% Cinder Fine and a variety of 5% to 25% at 5% intervals of Fly Ash were added. For a soil treatment using 20% cinder fines and 20% fly ash, the liquid and plasticity index values dropped by 66.2% and 95.2%, respectively, while the unconfined compressive strength (UCS) value increased by 86.6%. Further investigations were carried out using Scanning Electron Microscopy (SEM) imaging equipment to assess the change in microstructural properties of stabilized expansive soil employing the soil stabilizer mix ratio that resulted in strength improved quality sample. The results show that the fabric and morphology of the stabilized sample have changed. Additionally, the dispersed raw expansive soil was transformed into a more cohesive and flocculated mass after being treated with a mixture of 10% Class-C fly ash and 20% cinder fines. The SEM image further reveals that the mechanical bonding of the cinder particles results in well-developed floccules and a porous character. Keywords: Expansive soil, Class-C Fly Ash, Cinder fines, Microstructural, SEM.