Immobilization Mechanism of Fluorine in Aqueous Solution with Calcium Aluminates

Abstract

Immobilization of fluorine in aqueous solution with 3CaO · Al₂O₃ (C₃A) and 12CaO · 7Al₂O₃(C₁₂A₇) has been investigated under the shaking and static conditions. The hydration products are identified by XRD and the microscopic observation has been made for the hydrated particles using electron microprobe analysis. It is found that the degree of immobilization of fluorine increases in the order of CA₂<CA<C₁₂A₇<C₃A and the immobilization of fluorine occurs through the formation of Ca₃Al₂(OH)_12-xF_x and C₃A · Ca(OH)_2-xF_x · 18H₂O. The former which contains a larger amount of F (x = 0–1.2) is the major hydration product for hydrated C₃A, and the latter which contains a smaller amount of F (x = 0–0.12) is the dominant phase for hydrated C₁₂A₇. The conversion of the hexagonal hydrates of C₂A · 8H₂O and C₃A · Ca(OH)₂ · 18H₂O to the cubic hydrate of Ca₃Al₂(OH)₁₂ occurs rapidly in the case of C₃A, while it occurs slowly in the case of C₁₂A₇. During this conversion the F^- ion is incorporated into (OH^-) site in the Ca₃Al₂(OH)₁₂ phase.