This paper investigates the immobilization behaviors of Cd2+, Pb2+ and Hg2+ ions in a geopolymer based on water treatment sludge (WTS) and dealuminated metakaolin (DaMK) solid waste from alum industry. For synthesis of WTS/DaMK based geopolymer, five mixes of WTS/DaMK (75/25, 80/20, 85/15, 90/10 and 95/5, respectively) are tested to obtain the optimum synthesis condition based on the compressive strength. Results showed that the geopolymer mortar with 85/15WTS/DaMK has the highest compressive strength (8 MPa). In addition, X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) techniques are used to characterize the mineral and chemical compositions of WTS and DaMK. WTS/DaMK based geopolymer mortar have been tested for leaching to study its immobilization behavior under the optimum condition. The Cd2+, Pb2+ and Hg2+ ions were used and effectively immobilized in this study. The heavy metals with concentrations of 100, 200 and 300 ppm were used in the geopolymer matrix giving about 98% of immobilization efficiency. The study showed that Pb2+ has the best immobilization efficiency followed by Hg2+ and Cd2+ with high concentrations.