On parabolic flights, the growth of ettringite, [Ca3Al(OH)6?12H2O]2?(SO4)3?2H2O, a major reaction product of cement with water which forms instantaneously, was crystallized under microgravity conditions and studied. In the experiments, Ca(OH)2/Al2(SO4)3 solutions were combined and reacted for 10? s, followed by immediate filtration of the suspension and subsequent quenching with acetone. For the ettringite crystals, the size, aspect ratios, quantity and morphology were determined and the results were compared with those from identical experiments performed under terrestric gravity. Under microgravity, generally smaller crystals (l-2.9 μm) precipitated in larger amount than under normal gravity (l-3.5 μm). The aspect ratios of the crystals grown under terrestric or microgravity condition were comparable at about 5.6. It is assumed that the reason for the smaller ettringite crystals is the absence of convection leading to more initial nuclei, but slower crystal growth which is diffusion limited. Apparently, no preference relative to the ion transport to the different faces of the crystals exists. The results contribute to the understanding of the mineralization of inorganic salts under microgravity conditions for which hitherto only a handful of examples were reported.