Crossover from insulating into solid electrolyte behavior in bulk CaSO4⋅0.5H2O material due to ion exchange processes induced by high-temperature treatment with orthophosphoric acid

CaSO 4 -based citrogypsum was used to prepare bulk samples of calcium sulfate hemihydrate (CaSO 4 ·0.5H 2 O). To alter composition and improve conductivity, the samples were treated at 95 °C in 85 wt% orthophosphoric acid for 1 to 15 min. Within <1 min, CaSO 4 ·0.5H 2 O transforms into CaSO 4 ·2H 2 O. At longer times, partial dehydration converts part of the CaSO 4 ·2H 2 O. back into CaSO 4 ·0.5H 2 O, yielding a two-phase mixture. Acid treatment induces cation (2H + ↔ Ca 2+ ) and anion ((HPO 4 ) 2− ↔ (SO 4 ) 2− ) exchange, producing a transition from insulating to solid-electrolyte behavior. Weakly bound H + ions, incorporated either by Ca 2+ substitution or via (HPO 4 ) 2− residues, act as mobile charge carriers and enable proton conductivity. Residues that release protons convert to (PO 4 ) 3− groups, which can be displaced in an alternating field, generating ionic polarization and relaxation currents. The combined effects of H + mobility and (PO 4 ) 3− polarization produce two arcs in AC impedance spectra. Conductivity parameters extracted with the Cole model strongly depend on sample composition. • An increase in electrical conductivity was provided by an 85 wt% aqueous solution of orthophosphoric acid. • Cation and anion exchange processes occurring during acid treatment lead to changes in the elemental composition of samples. • Forming of the mobile ions results in appearance of two arcs in the AC impedance spectra of the acid treated samples.