In Japan, large amount of waste gypsum board is generated from demolition works, and most of it has placed in the landfill. Many companies in Japan have attempted to recycle waste gypsum board to solidification treatment of waste slurry from construction works. However, some waste gypsum board contains fluoride and some chemical impurities. These impurities in the waste gypsum have possibility of soil pollution. Calcium hydrogen phosphate dihydrate (DCPD) reacts with fluoride in the aqueous

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... tion and forms fluoroapatite (FAp). We have applied the reaction to immobilize fluoride in the environment. In this study, we have investigated immobilization of fluoride in waste gypsum by using DCPD, and apply the recycle gypsum to solidification of waste slurry. DCPD controlled release of fluoride from waste slurry less than 0.8 mg/L (Japanese environmental regulation of soil pollution). To improve reaction rate of DCPD, we attempted to surface modification of DCPD particles. By mixing DCPD with warm water, nano-scale particle formed on surface of DCPD particles. The surface-modified DCPD ("nano-etching DCPD") reacted fluoride with high reaction rate and immobilized fluoride in waste gypsum effectively.