Microstructural development of calcium aluminate cement based systems with and without supplementary cementitious materials

Calcium Aluminate Cements (CAC) are renowned in the field of construction for specific applications requiring rapid hardening and chemical resistance. These properties result from rapid exothermic reactions occurring immediately after setting. Due to the much lower use of CACs compared to Portland cements based materials, there is a lack of knowledge on the hydration of this cement and especially on its microstructural development. This study aimed to understand the hydrated phase assemblages in CACs systems and in particularly blended systems using Supplementary Cementitious Materials (SCMs). A detailed multi-techniques approach was developed to follow the microstructural development during hydration with strong emphasis on phase quantification (using isothermal calorimetry, XRD, TGA, BSE-IA, MIP). The cement pastes and mortars were cured in realistic self heating conditions to simulate the thermal effects occurring in practical sections of concrete. The effects of SCMs (nucleation, dilution and chemical reaction) were investigated to understand the microstructure property relationship of CACs systems. It was shown that the formation of hydration products rapidly levelled off. It appears that this levelling off is mainly determined by the space availability even though porosity was not completely filled and further hydration of reactants continued over several months of water curing. Subsequent densification of the matrix was partly attributed to the hydration of C2AS to C2ASH8. The addition of lithium sulphate allows the setting time to be controlled and contributes to the formation of stable hydrates (C3AH6 in the plain system and C2ASH8 in the blend). However there was also a noticeably inhibition of the hydration of CA. Self heating contributed to formation of distinct polymorphs of AH3 and their subsequent space filling. Microstructure property relationships depend on the density of hydrates but also their distribution. Detailed elucidation of the exact relationships is rendered difficult by the impact of drying methods on the measurement of porosity.