<p>Thermal Energy Storage (TES) systems, particularly those utilizing phase change materials (PCMs), are gaining traction as a solution to enhance energy efficiency. However, the fresh and mechanical properties of mortars can be significantly affected by industrial PCMs. Accordingly, the present study investigates employing tobacco stem ash as a novel and sustainable agricultural waste to replace ground granulated blast-furnace slag (GGBFS) in alkali-activated mortar mixtures at 1%, 3%, and 5%. Various mechanical and thermal properties were tested in the present study using mortar samples containing paraffin, raw tobacco stem ash, and thermally calcined tobacco stem ash. Results confirmed that using raw and calcined tobacco waste improved mechanical properties by 44.3% and 59.3% compressive strength enhancement compared to paraffin-containing mortar for raw and calcined tobacco stem ash, respectively. A comparable thermal performance was also observed. The specific heat values of mortar infused with waste tobacco stem ash ranged from 1.05 to 1.23 J/g°C for the raw type and from 1.03 to 1.19 J/g°C for the calcined type. Regarding thermal conductivity, the mortar samples containing both raw and calcined tobacco stem ash exhibited comparable characteristics, albeit with a slightly higher coefficient. Overall, the results demonstrate that raw and calcined tobacco stem ash can effectively replace industrial paraffin in the mortar, providing comparable TES performance while significantly improving mechanical properties and mitigating the strength degradation typically associated with paraffin-based PCMs.</p>