The formation of calcium complexes was further investigated by plotting the data only up to an added calcium concentration of 40 mmol L -1 (Figure ). It can be seen that the local minimum of the calcium curve is located at the equivalence point of the pH curve, where the acid molecules get deprotonated and subsequently form complexes with the calcium ions in the solution. The grey dashed line shows the calculated location of the equivalence point exemplarily for an additive concentration of 10 mmol L -1 . Complete deprotonation of 10 mmol L -1 additive should therefore happen after the addition of 20 mmol L -1 hydroxide in the case of dibasic acids, i.e. tartaric and succinic acid, and 30 mmol L -1 hydroxide for the tribasic molecule citric acid. As calcium and hydroxide ions are added in parallel at a stoichiometric ratio of 1:2, the x-axis representing the added concentration of calcium can easily be transformed to the added concentration of hydroxide ions by multiplying by a factor of two, i.e. the equivalence points for 10 mmol L -1 citric acid should be located at 15 mmol L -1 dosed calcium and 10 mmol L -1 for the other two acids. It can be seen that the equivalence points of almost all of the curves fit the expected values, only for the tartaric acid concentration 10 mmol L -1 , the minimum of the calcium curve, as well as the equivalence point of the pH curve are shifted towards higher dosages of calcium and hydroxide ions. This shift indicates the additional (but incomplete) deprotonation of the hydroxy groups. Calcium coordination has been reported to induce tartrate deprotonation at a highly alkaline pH of above 12 . The experimental amount of calcium complexed by the acids at the equivalence point of the pH curve is calculated using equation 6. The resulting values are consistent with the expected amount of calcium complexed calculated using PHREEQC (Table ). All three acids show a delaying effect in that more calcium needs to be introduced in the solution compared to the reference system before reaching the onset of nucleation (Table ). This delaying effect is concentration-dependent and increases with increasing acid concentration. The efficiency of the additives to inhibit the nucleation of calcium hydroxide can be quantified by calculating the saturation index required for the nucleation of portlandite, which corresponds to the maximum of the curve (Table ). A supersaturation index greater than zero shows a supersaturation of the system for the corresponding phase. The increase of the saturation index of each additive and concentration is shown as ∆SI. Citric and tartaric acid significantly increase the required supersaturation at nucleation by about 0.3 to 0.5, while succinic acid shows only slight effects with an increase of the SI of about 0.1. As the maxima of the individual curves of various additive concentrations in Figure are shifted towards higher dosed calcium concentrations but not to higher supersaturation (Table ), it can be concluded that, while the delaying effect of the additive is concentration dependant, the inhibiting effect is not. In addition, the amount of calcium complexed in the acid systems is much lower than the excess missing calcium compared to the reference (Table ). This means that the complexation of calcium is not the only factor explaining the delaying and inhibiting effects of the acids. Rather, as suggested in the literature , all three species may interact with the PNC droplets, thus delaying and/or inhibiting the nucleation/crystallization process. Some additives can form weak bounds with the surface of PNCs, which prevents the oligomerization process via steric stabilization , primarily resulting in a delay of the nucleation . Other additives with stronger affinity with the PNC droplets can form bonds with the surface of the PNCs, but can also incorporate inside the structure of the nano-droplets , which induces both retardation and inhibition of the nucleation, and may potentially affect the shape of the crystals formed after nucleation . The curve in figure of tartaric acid shows an interesting decrease of the calcium concentration after the nucleation event (maximum of the curve). Instead of a monotonous decrease to the equilibrium concentration, a very small local minimum is observed, followed by a second (small) maximum in free calcium concentration. This behavior indicates hindering effects on the growth of the previously formed nuclei. Scheck et al. found a similar pattern for the growth of Mg(OH) 2 particles. They proposed that the additives block crystal faces via adsorption, preventing further growth until the energy barrier is surpassed again and growth continues . The additives, particularly tartaric acid, also influence the equilibrium calcium concentration in the solution, as the final plateau of the calcium concentration is significantly higher than that for the reference curve. Bouzouadid et al. found that, for solubility measurements of portlandite with gluconate and hexitols, the formation of calcium-ligand complexes influences the equilibrium of [Ca total ] = [Ca 2+ ] + [Ca(OH) + ] + [Ca-organic] leading to an increased calcium concentration in solution . The SEM analysis of the precipitates obtained in the following section confirms the additives' influence on crystal growth.