Treatment of the sludge from water-purification plants is becoming more and more urgent due to the inability to increase its storage area. To avoid CO2 emissions, the use of non-Portland cement binders is recommended. The application of geopolymerization of waste sludge (WS) from water-purification plants is a novel solution. Curing conditions including high temperature, pressure or microwaves enhance the formation of geopolymer bonds. This paper presents the results of a research on the treatment of the WS of the Thu Duc water-purification plant (Vietnam) with the geopolymerization method. Solid phases were prepared by mixing the WS and fly ash (FA). The FA proportions of the solid phases were (10, 40, 70) w/%. The alkali-activated solution (AAS) was a mixture of a 40 w/% NaOH 6M solution and 60 w/% water glass (WG: Na2O.nSiO2 with n = 1.75 and volumetric density r  = 1.40 kg/L). The geopolymer materials were mixtures containing an 80 w/% solid phase and a 20 w/% liquid phase of the AAS. Geopolymer samples were formed in a cylindrical steel mold with a diameter of 10 mm at a high pressure. The samples were cured in a 112 W microwave oven for 30 s or in a dryer at 110 °C for 24 h. The compressive strength and volumetric density of both sample groups were determined and compared to each other. The formation of geopolymer bonds was investigated using XRD, FTIR and SEM.