The effects of gypsum and an additive (Mighty 150 from Kao Soap Co.) on the time dependence of the apparent viscosity ηa of cement paste were studied by the use of a coaxial cylinder rheometer. The apparent viscosity decreased first and then remarkably increased. The decrease in apparent viscosity is related to the breakdown of aggregation structure formed by hydration reaction, and hence its initial value varies with the rest period for cement paste without gypsum. In the presence of gypsum the time dependence of the apparent viscosity was not affected by the rest period, because gypsum retards the hydration of C3A(3CaO·Al2O3) and C4AF(4CaO·Al2O3·Fe2O3). Moreover, gypsum had a tendency to reduce the rheopectic characteristics. The addition of Mighty led to a significant decrease in apparent viscosity.
