We studied the effect of a new kind of retarder

hydroxy carboxyl compound (KHPT)

on setting time and 28d strength of building gypsum. We used a noncontacting electrical resistance device and a scanning electric microscope to study the change of the hydration process and the crystal morphology of the gypsum. The results indicate the following: With increasing addition of new retarder

gypsum setting time extends. A new retarder content of 0.05% not only affects gypsum setting but also enhances gypsum strength. At a retarder content of 0.10%

the retarding effect is quite good with the setting time nearly reaching 1 hour. With increased new retarder content

the abrupt change stage of hydration resistivity of building gypsum is delayed

the resistivity stabilization stage decreases

the crystal enlarges

and the ratio of length to diameter decreases.