Optimising the hydration of cementitious materials is crucial to leverage their full potential and avoid wasting resources, embodied energy and CO2. Understanding the fundamental mechanisms is key to reach these objectives. In this paper, we review progress in understanding hydration kinetics. We highlight how our practical understanding of cementitious materials is linked to the more fundamental field of aqueous solution thermodynamics. We concentrate on the knowledge gaps related to the aqueous speciation, the composition and structure of complex solid solutions (C-A-S-H, AFm, AFt), the solubility of anhydrous species and hydrates, and the water activities. We illustrate how these challenges are related to the development of meaningful models of early- and late-age hydration.