The Krieger-Dougherty equation allows calculation of viscosity as a function of volume fraction for suspensions of noninteracting particles. For model suspensions (of spherical, monosized particles), it has been shown to provide excellent agreement between calculated and measured viscosities. In the present study, this equation was applied to portland cement pastes, also with good correlations between calculated and measured viscosities. Because cement has a broad particle size distribution and its particles are angular and elongated, the two constants in this equation (the maximum volume fraction and the intrinsic viscosity) were estimated using nonlinear optimization techniques. The equation provides an excellent fit to measured viscosity data. However, the nature of the equation makes the estimation somewhat difficult, and the solutions are not well-defined.