The concentrations of the acids were set to 2, 6, and 10 mmol L -1 respectively. The nucleation curve for portlandite without an additive is shown in Non-classical crystallization pathways are also relevant for Ca(OH) 2 , as recently shown by Madeja et al. . Under non-classical crystallization conditions, prenucleation clusters (PNC) are initially formed, even at undersaturated conditions . These prenucleation clusters are liquid-like droplets of aggregated ions that lack the long-range order characteristic of crystalline materials. The ion-selective electrode can no longer detect the calcium ions consumed by the formation of PNCs, explaining the lower measured free calcium concentration compared to the calculated curve. Consequently, the equilibrium between free calcium ions in solution and those within the prenucleation clusters controls the slope of the calcium curve depicted in Figure . The stabilizing or destabilizing effects of additives on PNCs in solution are then observable in changes in the slope (section I in Figure ) before the formation of macroscopic precipitates. While a flatter slope shows stabilizing effects on PNCs, a steeper rise in calcium concentration indicates a destabilization of the PNCs . According to previous studies for CaCO 3 , the PNCs can be described as liquid-like droplets and coagulation of these clusters leads to the transformation to macroscopic solid particles . Adsorption of additives onto the surface of these droplets can prevent the agglomeration of these PNCs, similar to colloidal stabilization . Furthermore, the adsorbed additives can prevent the re-organization of the ions into a crystalline order inside the initial clusters and, therefore, inhibit nucleation.