The production of greenhouse gases has provoked discussion on global warming and motivated the scientific community to seek alternative options to Portland cement that are capable of reducing the emission of CO₂, such as supersulfated cements (SSCs). SSCs are comprised of blast furnace slag (up to 90%), calcium sulfate (up to 20%), and an alkaline activator (up to 5%), which form, as primary reaction products, ettringite and calcium-silicate-hydrate (C–S–H). A higher alumina content is desired in blast furnace slag since the compound enhances ettringite formation and, consequently, increases the mechanical strength of the system. In the present study, metakaolin (MK), which is rich in alumina, was used to replace slag. The results showed that MK was an efficient raw material for use in supersulfated cements, capable of partially replacing blast furnace slag containing low alumina content. A small level of MK replacement (5%) reduced the absorption values and increased the compressive strength from 23 to 44 MPa of the cement. This result was attributed to the greater formation of ettringite, which led to a consequent reduction in porosity.