The advancement of cement alternatives in the construction materials field is fundamental to sustainable development. Geopolymer is the optimal substitute for ordinary Portland cement, which produces 80% less CO2 emissions. Metakaolin was used as one of the raw materials in the geopolymerization process. This research examined the influence of three different percentages of sulfate (0.00038, 1.532, and 16.24)% in sand per molarity of NaOH on the absorption and density of metakaolin-based geopolymer mortar (MK-GPM). Samples were prepared with two different molarities (8M and 12M) and cured at room temperature. The best results obtained for geopolymer mortar in the absorption and density test were (3.89%) and (2280 kg/m3), respectively, recorded with 12M with the lowest sulfate content (0.00038%) at 90 days. Moreover, it has been observed that the absorption percentage increased along with sulfate content in the sand, and an inverse relationship was recorded between the increasing sulfate percentages in the sand and density values of (MK-GPM).