Portlandite (Ca(OH)(2), CH) is the second most abundant hydrate formed in the reaction of Portland cement with water, making it an important component in the built environment. Formation of CH is closely linked to the growth of the main hydrate phase, calcium silicate hydrate (C-S-H), affecting the microstructure and properties of cement. Understanding the interplay between the growth of CH and C-S-H is the key to comprehend the hydration reaction kinetics. This interplay mainly happens via the interaction of the different species present in the pore solution with the hydrates formed. It has been speculated that silicate species poison the growth of portlandite. In this work, we give evidence and propose a mechanism toward this experimentally observed effect using atomistic simulations. We also study the stability of a Ca-Si complex (CaSiO2(OH)(2)) expected to exist in pore solution using metadynamics calculations. We find that the adsorption of this stable Ca-Si complex at the (0001) portlandite surface is energetically favorable, contrary to the previously observed adsorption of the CH growth species Ca2+ and OH-. Additionally, the adsorbed complex retains a certain mobility at the surface. Growth poisoning is thus likely to happen by preferential adsorption of Ca-Si complexes. The interplay of CH and C-S-H growth is likely to be enhanced by the easier polymerization of calcium-silicate species adsorbed at portlandite surfaces.