COMPARISON OF 4 TECHNIQUES FOR MONITORING THE SETTING KINETICS OF GYPSUM

Statement of problem. Setting time of gypsum depends on the method of measurement. Purpose. In this study, four methods for ascertaining the setting time of gypsum were compared. Material and methods. Gypsum se tting was evaluated with Gillmore needles, setting expansion, scanning electron microscopy, and x-ray diffraction. Both die stone (Vel-Mix) and fast-setting dental stone (Snap-Stone) were investigated. By using Gillmore needles, both initial set and final set were recorded. Setti ng expansion was monitored until equilibrium was achieved. For scannin g electron microscopy, samples of mixed stone were immersed into liqui d nitrogen to stop the reaction. The water was subsequently removed by freeze-drying. Samples were then examined in a scanning electron micr oscope. For x-ray diffraction, the percentages of dihydrate and hemihy drate crystals were monitored every 1.5 minutes. Results. Setting time s for the fast-setting type III stone and for the regular-setting type TV stone were approximately 3 and 10 minutes, respectively, with the Gillmore needle indentation tests; 10 and 20 minutes with scanning ele ctron microscopy; and 20 and 60 minutes with x-ray diffraction. Conclu sion. Scanning electron microscopy, setting expansion, and x-ray diffr action indicated changes that occurred at times after a clinically rel evant set was obtained in the gypsum setting reaction.