Which of the following stages of gypsum expansion shows initial crystal growth?
Imagine that the initial mix is represented in the figure (stage I) by the three round particles of hemihydrate surrounded by water.
Under normal setting conditions, the crystals of the dihydrate begin to form on the nuclei. The water around the particles is reduced by hydration and these particles are drawn more closely together because surface tension of the water keeps the water surface area at a minimum (stage II).
As the crystals of dihydrate grow, they contact each other and the water around the particles again decreases (stage III).
Further dihydrate growth consumes more water and should draw the crystals together as before, but the outward thrust of the growing crystals opposes this contraction (stage IV).
Eventually, the crystals become intermeshed and entangled (stage V).

Key Concept
I
II
III
IV