Hydration and dimensional stability of calcium aluminate cement based systems

TLDR: In this paper, the authors studied the properties of CACs with and without supplementary cementitious materials (SCM) from the first hours up to two years of hydration and showed that the calcium sulfate reacts with CAC and only when sulfate is exhausted does slag or limestone react with aluminate phases.

Abstract: Calcium aluminate cements (CAC) are often used in combination with calcium sulfate and Portland cement for special applications where rapid setting, rapid drying and shrinkage compensation are required. A growing fraction of the clinker, which has the highest CO2 intensity, is being replaced by supplementary cementitious materials (SCM). These SCM are usually from industrial by-products such as slag, fly ashes or can simply be raw materials such as limestone. This project aims at replacing the Portland cement and fraction of the CAC in ternary binders with such SCM. Indeed, the understanding of their interactions, both physical and chemical, with calcium aluminate cements and calcium sulfate is fundamental for the development of innovative products. This thesis focuses first on the hydration of systems with and without SCM, slag or limestone, from the first hours up to two years of hydration. Second, this study focuses on the expansion of these systems and on the mechanisms underlying the dimensional stability. The influence of the ratio between calcium sulfate and calcium aluminate on the hydration of the cement, on the reaction of SCM, and on the expansion is highlighted. This study indicated that the calcium sulfate reacts with CAC from the first hours of hydration and, only when sulfate is exhausted, does slag or limestone react with aluminate phases. Moreover, this research brings new insights on the main parameters influencing the dimensional stability of the studied systems and on the mechanisms governing expansion. 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 increasing supersaturation a larger pore volume can be accessed by ettringite crystals exerting pressure. This could explain the critical amount of calcium sulfate leading to high expansion. Both the confinement in nanometer size pores and the supersaturation of ettringite are necessary for expansion to occur.