Use of a form-stable phase change material to improve thermal properties of phosphate sludge-based geopolymer mortar

•A novel PCM was prepared by encapsulating fatty acids in polyurethane foam.•Geopolymer mortars based on phosphate by-products with incorporated PU@PCM were studied experimentally.•New potential composite materials were developed to improve the energy efficiency of buildings.•The incorporation of PU@PCM resulted in a slight decrease in the compressive strength of the geopolymer mortars.•The compressive strength of the rich PU@PCM-geopolymer mortar is still sufficiently high for building applications.