Calcium Aluminates: Proceedings of International Conference, Avignon, 18 –21 May 2014. Fentiman CH, Mangabhai RJ and Scrivener KL (Editors). IHS BRE Press, 2014, EP104. ISBN 978-1-84806-316-7. VOLUME STABILITY OF CAC-CaSO 4 - CaCO 3 -H 2 O SYSTEMS DURING DRYING L. BAQUERIZO and T. MATSCHEI Innovation, Holcim Technology Ltd, Im Schachen, 5113 Holderbank, Switzerland K. SCRIVENER EPFL STI CCMX-GE MXG 232 (Bâtiment MXG) Station 12, CH-1015 Lausanne, Switzerland SUMMARY: Cement hydrates and their chemically bound water content are sensitive against changes in relative humidity and temperature. This may cause specific solid volume changes affecting dimensional properties of hydrated cement paste such as shrinkage, swelling and expansion, and therefore impact the performance and also the transport properties of cementitious materials. In the present paper we present preliminary results on the study of hydration states of the main crystalline cement hydrates found in hydrated Calcium Aluminate Cements (CAC) – CaSO 4 – CaCO 3 systems when exposed to different drying conditions. The techniques used to measure water content and volume of crystalline cement hydrates as well as thermodynamic properties associated to these changes are described. A first attempt to thermodynamically model volume changes of a cementitious system during drying is shown. Keywords: AFm and AFt phases, hydration states, thermodynamic modelling, volume stability INTRODUCTION One important application of calcium aluminate cements nowadays is its use as a component in mixed binder systems containing different amounts of Portland cement and calcium sulphate. These blended systems are used for special applications such as self-levelling compounds floors, tile adhesives or fixing mortars [1] . These mixtures can be fluid for a few hours and then harden quickly mainly due to the formation of ettringite [2] according to the reaction [1] : 3CA + 3CŜH x + (38-x)H → C3A∙(CŜ) 3 ∙32H + 2AH 3 (1) ettringite gibbsite This reaction consumes considerable amounts of water and decreases rapidly the Relative Humidity (RH) during drying, which make these mixtures very good as floor- levelling compounds [1] . But an often neglected phenomenon which can influence