Treating desalination concentrates by precipitating sparingly soluble salts is a promising method for extending water recovery limits of such processes and thereby reducing concentrate volumes. One of more common technologies for carrying this out is the use of fluidized bed crystallizers, often with sand grains as the crystallizing surface. Most of the works done to date on such crystallizers for water treatment purposes have been on water softening, and little has been done on use of such crystallizers for precipitation of gypsum. This study was on the crystallization kinetics of gypsum at initial supersaturation levels typical of reverse osmosis (RO) concentrates, in the presence of silica sand grains or gypsum seeds, in a batch stirred tank crystallizer. The findings show that silica sand grains were efficient primary heterogeneous nucleators for gypsum. Crystallization kinetics increased with larger grain size or higher grain number concentration. Considering the different nucleation mechanisms, a central role for secondary nucleation is proposed to explain the present findings. Collisions between sand grains, on which initial crystals have formed, eject some or all of them into the solution to continue secondary nucleation and crystal growth. In seeded growth experiments with gypsum, it is confirmed that crystal growth rates were controlled by a surface reaction. For some of the experiments conducted in the presence of sand, the crystallization rate was higher than with gypsum seed crystals, over a significant range of concentrations. Moreover, the use of recycled sand decreased the induction period significantly compared to new sand.