Biomedical ash is the residual matter from biomedical waste incinerators. Despite its superior characteristics as a construction material, biomedical ash is usually averted as an additive due to the mobility of toxic heavy metals. Arresting heavy metal mobility has gained the interest of scientific communities due to the ever-increasing waste and continuous demand for construction materials. This research investigated the application of modified ash by calcium carbonate biomineralization as a soil stabilizing agent in highly plastic clays. Initially, the nutrient medium for the indigenous Bacillus cereus bacteria was optimized for maximum urease activity. The ability of biomineralization to arrest mercury, chromium, zinc, lead, iron, copper, cadmium, barium, arsenic, titanium, and selenium in biomedical ash by calcium carbonate biomineralization was determined through a leaching test. The characteristics of modified ash were determined by FESEM, XRD, FTIR, and TG analyses. Adding modified ash correlates with the increasing soil strength, suggesting the suitability of calcium carbonate biomineralization in immobilizing toxic heavy metals and simultaneously enhancing soil strength.