Experimental evidence on formation of ulexite in sulfoaluminate cement paste mixed with high concentration borate solution and its retarding effects

Abstract

Abstract Calcium sulfoaluminate (CSA) cement is one of the potential candidates for the solidification of low and intermediate level radioactive nuclear waste which comprises high borate concentrations. Borate is well known for acting as set retarder during the immobilization of radioactive waste by cement. In this study, for the first time, we report the direct observation of formation of dense amorphous layer of ulexite fully covered the surface of CSA clinker particles after 3 days of mixing with 0.5 M borate solution under pH of 7, which prevents the further dissolution of ye’elimite, resulting in the strong retardation of hydration. For CSA clinker powder mixed with deionized water, ettringite and amorphous alumina hydrate formed in the first hour of hydration, with large area of clinker surface exposed to the solution, which is beneficial for the further dissolution of ye’elimite and continuous hydration. For CSA clinker powder mixed with borate solution under pH of 7 simulating radioactive waste solution, no crystalline hydration products could be identified up to 28 days of hydration. No heat evolves for 72 h indicating the strong retardation of hydration by borate solution. The retardation is resulted from the formation of dense amorphous layer of ulexite fully covering the surface of CSA clinker particles. The amorphous ulexite layer shows foil-like morphology with thickness of approximately 100 nm. The dense amorphous ulexite layer stays stable up to 28 days under pH of 7.