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 heterogene-ous 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].