Chloride and carbonate immobilisation by monosulfoaluminate: study of the solid solutions in the CO3~(2-) Cl-SO4~(2-) AFm systems.

摘要

Cementitious matrices are being used worldwide as a containment medium for radioactive and non-radioactive waste to retard the mobility of contaminants. AFm phases are hydrates precipitated during the hydration process of Portland and CSA cements. The most important AFm phase is the monosulfoaluminate (i.e. the sulphate AFm phase). These phases, of general formulae Ca2Al(OH)6·X·nH2O (X is either one singly charged anions or half a doubly charged anion), are layered compounds. Their structure is composed of positively charged main layer [Ca2Al(OH)6]+ and negatively charged interlayer [X-·nH2O]. The mains anions encountered in AFm chemistry are carbonate CO3 2, sulfate SO4 2-and chloride Cl-. Nevertheless numerous others anions can insert the AFm interlayer region such as nitrate NO3-, bromide Br-, iodide I-, chromate CrO4 2-… The chemistry of AFm phases have been widely studied, and their interesting anionic exchange capacity make these compounds are good candidates for the binding of anionic radio-nuclides present in nuclear wastes like chloride and carbonate (36Cl and 14C). The aim of the present study was to focus on the possibility to trap these three anions (sulfate, carbonate and chloride) by considering the existence of solid solutions and definite compounds. The crystallographic structures of two AFm mixed compounds were fully characterised: the chloro-carboaluminate compound [Ca2Al(OH)6]·[Cl1/2·(CO3)1/4·2.25H2O] from single crystal data and the chlorosulfoaluminate compound, called Kuzel’s salt [Ca2Al(OH)6]·[Cl1/2·(SO4)1/4·2.5H2O], from synchrotron powder data. The synthesized [Ca2Al(OH)6]·[Cl1-x·(CO3)x/2] samples have shown the existence of two polymorphs for chlorocarboaluminate (monoclinic and rhombohedral) at 25°C and only the rhombohedral polymorph at 85°C. In the cases of the two other poles Cl-SO4 and CO3-SO4, only definite compounds were detected, with a new carbo-sulfoaluminate compound observed by powder X-ray diffraction and Raman spectroscopy.