In order to utilize reclaimed gypsums (gypsum board waste) derived from waste plasterboards as geo-materials, thermal behaviors of the reclaimed gypsums and reagent gypsum are investigated in this paper. Furthermore, simple density measuring method of judging quality of the reclaimed gypsums is examined to determine the gypsum hemihydrate content. As the results, it was found that the dihydrate gypsum is changed into the hemihydrate gypsum under thermal condition of 90 °C and the hemihydrate gypsum is changed into anhydrite under that of 120 °C with heating of 24 h. The followings were clarified in this paper. The cement density measuring method was available in order to measure the density of reclaimed gypsums; that density depended on the drying conditions of the reclaimed gypsums; and the density measuring method was appropriate to estimate the gypsum hemihydrate content in reclaimed gypsums. Furthermore, it was found that the quality control method based on the density was effective by calculating the gypsum hemihydrate content from the density values of reclaimed gypsums.
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