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The long-term expansion of calcium aluminate cement and calcium sulfoaluminate cement in the presence of added gypsum has been studied for samples cured under water. Progressively higher amounts of gypsum were added to the CAC or CSA and it was found that there is a critical amount of gypsum leading to unstable expansion and failure of the samples. The microstructures of systems with gypsum additions just below and above the threshold were similar. Pore solution analyses showed that supersaturation with respect to ettringite increases with the calcium sulfate content, which results in an increase of the crystallization pressure. The supersaturation determines the minimum pore size in which crystals can grow. Therefore with higher supersaturation a larger pore volume is accessible to growing ettringite crystals exerting pressure in the porous skeleton. This could explain the critical amount of gypsum leading to high unstable expansion. (C) 2013 Elsevier Ltd. All rights reserved.