Effects of a New Retarder on Building Gypsum and Its Mechanism

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.