Abstract A great variety of silica and alumina-rich materials have been investigated in the production of alkaline activated binders (or geopolymers) with low environment impact, e.g. low CO2 emissions. Geopolymer mortars and concretes may be more environmentally friendly if not only Portland cement but also natural aggregates (sand, gravel) are replaced with waste materials. This article presents results of geopolymers made with tailings from the iron mining industry. The geopolymer matrices are composed of metakaolin (MK) activated with a sodium silicate + sodium hydroxide solution. Two different iron ore tailings replaced a natural quartz aggregate in the geopolymers’ formulation. The hardened properties assessed were compressive strength, water absorption, apparent density and porosity. Scanning electron microscopy (SEM) and x-ray micro computed tomography (µ-CT) were used to assess the changes in the microstructure when the tailings were incorporated into the geopolymers. Results show that the employment of iron ore tailings is not detrimental to the mechanical strength of geopolymers; however, geopolymers containing those alternative aggregates may present higher water absorption and porosity, and durability studies are required.