The microstructure of inorganic polymers (IP) formed from fayalite slag was investigated as a function of the composition of different activating solutions. The starting slag was 80 wt% amorphous, and after activation using sodium silicate solutions with varying SiO2/Na2O molar ratios, the amorphous phase dissolved and a binder phase was formed. The morphology of this binder, including the population and size of remnant particles and pores, was dependent on the particular activating solution used, and became denser as the level of silicate rose. 57Fe Mössbauer spectroscopy revealed that the IP synthesis reaction is combined with the oxidation of Fe2+ from the fayalite slag to Fe3+ in the inorganic polymer binder. The reaction extent varied and could be quantified using the absorption areas of these ions. Data corroborate that the Fe2+ ions in the amorphous part of the fayalite slag and the Fe3+ ions in the new binder phase had an average oxygen‐coordination number of 5.

Belonging to the family of alternative cementitious materials, inorganic polymers are rising in importance because of the drive to decrease CO 2 emissions of concrete production. A synthetic Fe-rich slag resembling industrial copper or lead slags, was mixed with a sodium silicate activating solution. 57 Fe Mössbauer spectra analyses indicate that the oxidation reactions are taking place simultaneously with the polymerization reactions. The slag contains Fe 2+ states and a small amount of Fe 3+. During polymerization a new octahedral Fe 2+ state is formed, while oxidation is manifested through the appearance of an additional Fe 3+ state. The reactions continue after setting, lowering the relative contributions of the slag in the Mössbauer and FTIR spectra of the samples. The Na + /Fe 3+ molar ratio in the mixture that makes up the binder after 28 days is ∼1, suggesting the participation of tetrahedral Fe 3+ in the silicate framework, charge balanced by Na + .

The molecular structures of CaO–FeOx–SiO2 slags and their inorganic polymer counterparts were determined using neutron and X‐ray scattering with subsequent pair distribution function (PDF) analysis. The slags were synthesized with approximate molar compositions: 0.17CaO–0.83FeO–SiO2 and 0.33CaO–0.67FeO–SiO2 (referred to as low‐Ca and high‐Ca, respectively). The PDF data on the slags reasserted the predominantly glassy nature of this iron‐rich industrial byproduct. The dominant metal‐metal correlation was Fe–Si (3.20‐3.25 Å), with smaller contributions from Fe–Ca (3.45‐3.50 Å) and Fe–Fe (2.95‐3.00 Å). After inorganic polymer synthesis, a rise in the amount of Fe3+ was observed via the shift of the Fe–O bond length to shorter distances. This shortening of the Fe–O distance in the binder is also evidenced by the apparent rise of the Fe–Fe correlation at 2.95‐3.00 Å, although this feature may also suggest a potential aggregation of FeOx clusters. In general, the atomic arrangements of the reaction product was shown to be very similar to the precursor structure and the dominance of the Fe–Si correlation suggests the participation of Fe in the silicate network. The binder was shown to be glassy, as no distinct atom‐atom correlations were observed beyond 8 Å.

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