The crystallization of ettringite at very early stage (similar to 10 s crystallization time) prepared by combining Ca(OH)(2) and Al-2(SO4)(3) solutions holding different polycarboxylate (PCE) comb polymers was investigated under microgravity (mu g) condition occurring on parabolic flights. The aim was to achieve a more controlled environment for the crystallization than under terrestric conditions. For all experiments, the crystal size, aspect ratio, morphology and amount of ettringite crystals formed under mu g were determined. It was observed that due to the absence of convection at mu g, crystal growth was generally decelerated as evidenced by the smaller size of the ettringite crystals. Highly anionic and thus strongly adsorbing PCE polymers show a strong impact on the crystal size of ettringite under both 1g and mu g conditions. Whereas less anionic, weakly adsorbing PCE polymers exhibit a pronounced effect on ettringite growth only under mu g conditions. For them, the diffusion-limited ion transport presents the parameter which determines the crystal size. Another remarkable observation is that under microgravity, some of the polymers change their affinity to specific crystal faces which leads to different aspect ratios compared to terrestric gravity. (C) 2016 Elsevier B.V. All rights reserved.