The foundry industry produces millions of castings each year, generating large volumes of industrial waste, much of which is still destined for landfills. Among these byproducts is paraffin wax waste, a residue from the investment casting whose reuse potential remains largely underexplored, especially when compared to well-studied materials like waste foundry sand. Paraffin wax waste offer a benefit: it is an organic phase change material (PCM), capable of storing and releasing thermal energy in response to temperature variations. These properties make it particularly attractive for use in energy-efficient construction materials. This study investigates the innovative use of paraffin wax waste in cement mortars, aiming to repurpose it as a partial or full replacement for natural aggregates. Experimental work was carried out with replacement levels of 20%, 40%, 60%, 80%, and 100%. The results demonstrated that mortars incorporating paraffin wax waste maintained adequate physical and mechanical performance, particularly in terms of water absorption and compressive strength. This research brings the opportunity of reducing environmental impact from industrial waste while contributing to the development of thermally responsive and resource-efficient building materials.