Synthesis and thermal characterization of glass bonded Ca-chloroapatite matrices for pyrochemical chloride waste immobilization

Abstract

Ca-chloroapatite (CaApCl), glass-bonded CaApCl compositions loaded with 16–32wt.% simulated pyrochemical chloride waste were prepared by mixing and heating (773–1023K) apatite and borosilicate glass (BSG) forming reagents in appropriate ratios. The compositions were characterized by XRD, TGA/DTA, SEM, and EDAX. Among the products, 16–27wt.% chloride waste loaded composition yielded phase pure Ca-chloroapatite and were resistant to leaching of Cl− and other ions. In case of 28–32wt.% waste loaded compositions, even though formation of phase pure Ca-Chloroapatites was observed by XRD, the leaching of Cl− and other ions was found to be significant. Bulk thermal expansion behavior of the samples was studied by dilatometry. The 16wt.% chloride waste loaded matrix showed nearly the same thermal expansion compared to pure Ca-Chloroapatites. The % linear thermal expansion of the matrices decrease on increasing the chloride waste loading; however, Ca-chloroapatite mixed with 20wt.% BSG mixed matrix showed slightly higher thermal expansion. The coefficient of thermal expansion of borosilicate glass is the lowest among all the matrices measured. The coefficient of thermal expansion (CTE) is found to be 12.76±0.64×10−6K−1 for CaApl and 12.18±0.63×10−6K−1 for 16wt.% waste loaded BSG-encapsulated CaApl in the temperature range of 298–780K. The glass transition temperature of the waste loaded matrices is lower than that of the bare BSG and 20wt.% BSG encapsulated Ca-chloroapatite.