Portlandite is an important constituent of cementitious materials. Consequently the growth of portlandite is of great interest to fully understand the hydration of cement, a process still posing many scientific challenges. In this paper the growth of portlandite in aqueous systems is studied by simulating the adsorption of Ca2 + and OH− at different portlandite surfaces. For the simulation an adapted version of the Freeman (Freeman et al., 2007) in combination with the TIP4P/2005 (Abascal and Vega, 2005) force field was used for both molecular dynamics, conventional and well-tempered metadynamics calculations. Depending on the structure of the portlandite–water interface, different adsorption sites were observed. Based on these we were able to propose an atomistic mechanism of portlandite growth in different crystallographic directions. The proposed mechanism indicates that different species control the growth in different directions, consistent with experimental observations reported in literature (Arnold, 2004).