Abstract
In the present study, fluorine in recycled gypsum was focused on with regard to the morphology in gypsum as well as the leaching behavior when gypsum is mixed with soil as a recycling measure of soil improvement. The analytical results using an electron probe micro analyzer indicated that the chemical form of fluorine in recycled gypsum is calcium fluoride and fluoride apatite, and the form does not change after a leaching test on the soils according to the Japanese Leaching Test No.46 (JLT-46). JLT-46 was adopted for not only recycled gypsum but for mixtures of recycled gypsum and decomposed granite soil, exceeding the environmental standard limit value of 0.8 mg/L for both, by 6.8 and 2.4 mg/L, respectively. This means that countermeasures are prerequisite in the utilization of recycled gypsum as a soil improvement material. A pH-dependence test indicated that leaching of fluorine increases along with the decrease of pH in acidic conditions. A parallel batch leaching test using water indicated that release of fluorine occurs relatively slowly compared with calcium sulfate which reaches chemical equilibrium in less than 0.5 h. Decomposed granite soil has the effect of holding fluorine from leaching.
