Phosphate sludge is a ﬁne by-product produced with large quantities during ore phosphate beneﬁciation(washing and ﬂotation), and deposited in surface impoundment without treatment, which createenvironmental issues. The main objective of this study was to investigate the potential reuse of phos-phate sludge as a partial replacement of ﬂy ash and metakaolin for the production of geopolymers.Geopolymers were prepared by mixing phosphate sludge with metakaolin or with ﬂy ash in a proportionof 50 wt%. The effects of the main experimental factors (NaOH concentration, curing temperature andcuring time) on the main physical characteristics (density, water absorption and compressive strength)were assessed using an experimental design of response surface methodology (DOE). The microstructureof geopolymer samples were investigated using X-ray diffraction, scanning electron microscopy andFourier-transform infrared spectroscopy. Analysis of variance conﬁrmed that the adopted DOE modeldescribed well the experimentally observed properties. The results of the microstructural analysisshowed that the development of a gel-like phase was favored by the increase of sodium hydroxideconcentration (from 8 to 14M) for the two kinds, ﬂy ash and metakaolin-based geopolymers. The physicalproperties of the geopolymers indicated that an increase of temperature and NaOH concentration led tostrengthen and dense materials, while the addition of phosphate sludge resulted in a reduction on themechanical properties. However, substitution of ﬂy ash by phosphate sludge resulted in strong speci-mens (62 MPa), which indicated that the use of phosphate mine tailings in geopolymers can be a viabletechnological solution for the sustainable management of these by-products.