Hydrocalumite-type materials: 1. Interest in hazardous waste immobilization

This study investigated the incorporation of Cr, V and Si oxoanions into hydrocalumite. Hydrocalumite together with ettringite form during the hydration of Portland cement and the use of these materials in hazardous waste immobilization is of current concern. The anion uptake was examined in terms of the structural characteristics by means of thermogravimetric analysis, infrared spectroscopy, X-ray diffraction (XRD) and in situ high temperature X-ray diffraction (HTXRD). The syntheses were carried out by coprecipitation or exchange reaction. The short interlamellar distances observed for Ca2Al–V2O7 and Ca2Al–SiO4 phases, 7.90 and 10.93 Å, respectively, are explained by a grafting process onto the hydroxide layers. The good crystallinity of Ca2Al–CrO4 allowed the refinement of the cell parameters in the P-3 space group with a=5.7507(3) Å and c=20.164(2). A structural model is proposed with CrO4 species close to the seventh coordination position of Ca atoms. In situ HTXRD measurements in the temperature range 25–350 °C reveal different thermal behaviors depending on the oxoanion. At low temperature, a dehydration–hydration process partly reversible was observed for Ca2Al–V2O7 and Ca2Al–CrO4 phases, between 100–200 and 50–120 °C, respectively. At higher temperature, strong interlayer contractions were observed due to a pillaring process: Δd/d=−13% for Ca2Al–V2O7 at 200 °C, Δd/d=−32% for Ca2Al–CrO4 at 250 °C and Δd/d=−35% for Ca2Al–SiO4 at 135 °C.