State of water in calcium sulfoaluminate cement paste modified by hydroxyethyl methyl cellulose ether

To obtain satisfying water retention and improved viscosity, cellulose ether (CE) has been widely used in fields such as tile adhesives, repair mortars and self-levelling mortars. But the impacts of CE on the hydration and microstructure of calcium sulfoaluminate (CSA) cements, especially on the hydration and water evolution are still missing some basic insights. This paper mainly focusses on the influence of two hydroxyethyl methyl cellulose ether (HEMC) on the water evolution and hydration properties of commercial CSA cements with variable composition. The properties at fresh state such as setting times, air void content and water retention were recorded, while the interaction between HEMC and CSA cements was investigated by the combination of calorimetry, 1H NMR and DSC-TG. Results indicate that regardless of the composition of CSA cements, HEMC greatly increases the air void content and water retention. Setting time and hydration heat evolution within 1 d as well as the formation of cement hydrates (ettringite, monosulfate and alumina gel) are only slightly or unaffected by the addition of HEMC. As shown in the T2 distribution of 1H NMR pattern, it is interesting to notice that the addition of HEMC leads to the appearance of a new peak around 100 ms–1000 ms within 1 h and it disappears after 150 min. This finding is assigned to the evolution of a special water state in cement pastes containing HEMC. Additionally, variations in the polymer structure of HEMC have comparatively weaker impact than CSA cement type on the properties of prepared cementitious materials.