The hydration of hemihydrate to gypsum can be categorized as heterogeneous and secondary nucleation [5]. The gypsum is formed by heterogeneous nucleation onto hemihydrate particles. But in a later phase of the hydration the gypsum nucleates on the present gypsum particles, which is secondary nucleation. Unfortunately there are no general laws for heterogeneous and secondary nucleation available. Although the secondary nucleation rate will decrease with an increase in the seed size or in the number of seeds of a given size [10].