Comparison of four techniques for monitoring the setting kinetics of gypsum

Setting time of gypsum depends on the method of measurement.

In this study, four methods for ascertaining the setting time of gypsum were compared.

Gypsum setting 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. Setting expansion was monitored until equilibrium was achieved. For scanning electron microscopy, samples of mixed stone were immersed into liquid nitrogen to stop the reaction. The water was subsequently removed by freeze-drying. Samples were then examined in a scanning electron microscope. For x-ray diffraction, the percentages of dihydrate and hemihydrate crystals were monitored every 1.5 minutes.

Setting times for the fast-setting type III stone and for the regular-setting type IV stone were approximately 3 and 10 minutes, respectively, with the Gillmore needle indentation tests; 10 and 20 minutes with scanning electron microscopy; and 20 and 60 minutes with x-ray diffraction.

Scanning electron microscopy, setting expansion, and x-ray diffraction indicated changes that occurred at times after a clinically relevant set was obtained in the gypsum setting reaction.