The production of alkali-activated materials and geopolymers from the finer fraction of tailings is being increasingly investigated, as they could be used for building and public works, or to retain heavy metals in a consolidated solid matrix. The present study proposes a Pitzer-based geochemical model to reproduce the alkaline leaching for two mine tailing samples and the polymerization reactions. The model fits the results of the leaching tests quite well over the reaction duration and up to NaOH 10 M and 60 °C. Discrepancy for the dissolved Mg concentration indicates that the database can handle ionic strength up to 5 mol.kg−1. A geopolymer solid solution was developed for setting reactions modeling. The results compare favorably with compressive strength tests results, with C-S-H, M-S-H and geopolymers as the main hydration products. The study proposes an innovative application of Pitzer database developments with application to the mining industry.