A process for the manufacturing of a slurry containing nucleating agents which comprises reacting at least one source of a Ca containing compound with at least one source of a Si containing compound, in an aqueous media and in the presence of a doping agent selected from the group consisting of P, B, S, and mixtures thereof; wherein: (i) the reaction is carried out at a temperature comprised from 100 to 350° C.; (ii) the total molar ratio Ca/Si is 1.5-2.5, and (iii) the total molar ratio doping agent/Si is 0.01-2; provided that: (a) when the sole doping agent is P, the total molar ratio P/Si is 0.1-2; (b) when the sole doping agent is B, the total molar ratio B/Si is 0.01-2, and (c) when the sole doping agent is S, the total molar ratio S/Si is 0.1-2.