Phase states of the gypsum thermal-annealing derivatives according to electron spin resonance spectra

The electron spin resonance (ESR) spectra of SO ₃ ^? and SO ₂ ^? radical ions with a linewidth ΔH ≈ 2.7 G and SO ₃ ^? (A ₁) and SO ₃ ^? (A ₂) centers with superhyperfine splitting due to the interaction with protons in platelike gypsum single crystals under X-ray irradiation have been analyzed at 25°C. Dehydrated regions with a radius >4 Å are revealed in gypsum. The ESR spectra of SO ₃ ^? radical ions and atomic hydrogen with ΔH ≈ 0.3 G are found in the products of isothermal annealing of gypsum kept for 30 min after X-ray irradiation at 25°C. The dependences of the intensities of these spectra on the annealing temperature are studied in the range of 100–450°C. The temperature range of formation of α- and β-phase states of bassanite and γ-anhydrite are determined. The process of residual water redistribution between the channel systems of the α- and β-phase types of γ-CaSO₄ in gypsum thermal derivatives is established.