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A theoretical model of the nucleation of portlandite is proposed, and the critical size of a portlandite cluster and the energy barrier of nucleation are determined. The steady state nucleation rate and the time lag of the nucleation of portlandite are estimated for a pure solution of Ca(OH)₂ in water. Possible connections with the corresponding properties for cement paste are discussed. A new method is developed for experimentally determining the concentration of Ca²⁺ ions during the initial stage of hydration of a cement paste. The time dependence of Ca²⁺ ions is measured for various water-to-cement ratio values. The results are discussed from the point of view of existing models of the induction period.