Belonging to the family of alternative cementitious materials, inorganic polymers are rising in importance because of the drive to decrease CO₂ emissions of concrete production. A synthetic Fe-rich slag resembling industrial copper or lead slags, was mixed with a sodium silicate activating solution. ⁵⁷Fe Mössbauer spectra analyses indicate that the oxidation reactions are taking place simultaneously with the polymerization reactions. The slag contains Fe²⁺ states and a small amount of Fe³⁺. During polymerization a new octahedral Fe²⁺ state is formed, while oxidation is manifested through the appearance of an additional Fe³⁺ state. The reactions continue after setting, lowering the relative contributions of the slag in the Mössbauer and FTIR spectra of the samples. The Na⁺/Fe³⁺ molar ratio in the mixture that makes up the binder after 28 days is ~1, suggesting the participation of tetrahedral Fe³⁺ in the silicate framework, charge balanced by Na⁺.