While two−dimensional microstructure models are adequate for demonstrating hydration and examining microstructural features such as interfacial zones (Garboczi and Bentz 1991), three−dimensional microstructures are needed to accurately examine properties such as phase percolation and transport coefficients. Generating realistic starting microstructures (images) in three dimensions is a non−trivial task. One approach is to simplify the problem by considering the cement to be a single pure phase, similar to C3S, the major component of Portland cement, and generating digitized spheres of a specific size distribution to represent the starting cement powder in water. This simplified approach has been shown to produce realistic microstructures and has provided new insights into percolation and diffusivity properties of cementitious materials (Bentz and Garboczi 1991b, Garboczi and Bentz 1992). More realistic chemical phase information must be built in to accurately simulate early−time hydration (before the set−point) in high aluminate cements, which is controlled by the aluminate phases (Brown 1991, Chen and Odler 1992, RILEM Technical Committee 66-MMH 1986). A fast and simple way of making the spheres multi-phase is by simply converting all solid pixels which are in each nth plane to C3A, where 1/n is the desired proportion of aluminate.