  The present paper investigates the influence of gluconate and hexitol
additives on the precipitation mechanism and kinetics of C-S-H. To this end,
wet chemistry C-S-H precipitation experiments were performed under controlled
conditions of solution supersaturation, under varying silicate concentration,
while the transmittance of the solution was followed. This allowed determining
induction times for the formation of C-S-H precursors in the presence and
absence of gluconate and three hexitol molecules. Characterization of the
precipitates was performed via small angle X-ray scattering and
cryo-transmission electron microscopy experiments, which allowed the
identification of a multi-step nucleation pathway also in the presence of the
organics. Analysis of the induction time data in the framework of the classical
nucleation theory revealed a significant increase of the kinetic pre-factor,
which is associated to physical rates of cluster collision and aggregation
during the nucleation process. Values of the kinetic pre-factor increase in the
same order as the complexation constants of calcium and silicate with the each
of the organics. This points to a retarding mechanism of crystallization
related to steric hindrance of the aggregation of the early formed clusters via
adsorption of the organics at their surfaces.
