Nonclassical Crystallization of Calcium Hydroxide via Amorphous Precursors and the Role of Additives

In many systems, prenucleation clusters (PNC), dense liquids, and solid amorphous phases precede the formation of crystalline phases, which can grow via a nanoparticle aggregation mechanism. Despite intensive efforts, the current understanding of the mechanisms of such nonclassical crystallization processes is far from complete. Here by means of calcium potentiometric titration tests complemented by X-ray diffraction, dynamic light scattering, and electron microscopy analyses, we show that in the case of calcium hydroxide (CH), one of the main components of set Portland cement, PNCs and dense liquid precursors (prenucleation stage), and amorphous CH (ACH) and a metastable nanocrystalline CH (postnucleation stage) precede the formation of stable CH crystals. Such a phase sequence is also observed in the presence of additives commonly used as cement set-retarders and plasticizers (polysaccharides, lignosulfonate, and polyacrylate). We show that the main action of additives occurs during the prenucleation stage via destabilization/stabilization of PNCs, and the promotion/stabilization of dense liquid precursors leading to a significant delay in the onset of ACH nucleation at high supersaturations. Additives also stabilize amorphous and metastable crystalline CH phases and modify the number, size, and morphology of stable CH crystals. In contrast to classical crystallization theory, an inverse relationship between supersaturation at the onset of nucleation and the final number and size of CH crystals is observed. This unexpected result is explained by the fact that CH crystals nucleate after dissolution of ACH, whose solubility marks the maximum supersaturation in the system with respect to primary CH nanoparticles, which subsequently undergo oriented attachment to form large CH particles that further grow via aggregation of ACH nanoparticles. These results help to understand how CH forms, show that nonclassical crystallization can take place in cement systems, and shed light on how cement admixtures work.