Cements of various types have been in more or less constant use since at least Ancient Roman times. However, the ways in which cements set (via a complex series of chemical hydration reactions) have still not been fully described.

The hydration of cement is an extremely complex reaction involving chemo-physical phenomena spanning from a nanometer to micrometers in length and from seconds to decades in time."

Source : Journal of the Mechanics and Physics of Solids, (paywalled) Volume 99, February 2017, Pages 211–224

Although several chemical models give a good approximation of how a cement will set, they have limitations, particularly in view of the extremely long setting time of some cements which continue to harden over several decades.

[…] the molecular compositions, structures and changes that occur as a result of hydration are poorly understood, because cement has a composition that is partially crystalline, partially disordered, and holds multiple components that also change with time"

Source : Scientific American