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

Abstract

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.

Local structure and Ca/Si ratio in C-S-H gels from hydration of blends of tricalcium silicate and silica fume

“…Additionally, crystalline portlandite is known to have defects in the packing along the c-axis which results in anisotropic peak broadening of its X-ray powder diffraction pattern [24]. Furthermore, and very interestingly, it has been very recently reported that under very high supersaturation conditions amorphous Ca(OH) 2 and/or a metastable nanocrystalline Ca(OH) 2 precede the formation of stable crystalline portlandite [25]. This process takes place through a nonclassical aggregation mechanism, called as "oriented attachment".…”

Amorphous phase-mediated crystallization of calcium pyrophosphate tetrahydrate: the role of alkaline earth metal ions

“…An increasing body of evidence has shown the presence of a metastable disordered phase involved in the formation of calcium-containing crystals, such as calcium carbonates, calcium phosphates, calcium sulfates, and calcium hydroxides. [16][17][18][19] This amorphous phase-mediated crystallization strategy is essential for physiological mineralization of bones and teeth and pathological mineralization. 20 Amorphous intermediates are considered to be potential targets to prevent or delay the formation of pathological crystals and treat diseases.…”

Crystallization via Oriented Attachment of Nanoclusters with Short-Range Order in Solution

“…In fact, short-range order similar to what is observed in their larger, crystallized counterparts is often detected in these clusters, 3,4,10 and it is still under debate whether these clusters should be regarded as "amorphous clusters" or "extremely small crystals". 5,9 In aqueous solution, these clusters can spontaneously transform into larger crystals, 2,4 with the underlying transformation mechanisms far from being understood. One possible pathway is that these clusters dissolve into ions and then recrystallize, which has been shown for amorphous phases such as amorphous calcium carbonate (ACC).…”

“…Many mineral crystallization processes in solution involve the formation of metastable intermediates consisting of 1–2 nm sized clusters. − These clusters have been observed in a range of systems including calcium carbonate, , calcium phosphate, calcium hydroxide, calcium sulfate, zinc oxide, and iron oxide, and they are usually considered to be “amorphous” as they only display very broad signals in X-ray or electron diffraction analyses. However, the broadening of diffraction signals can also be attributed to the extremely small sizes of the clusters, and the internal structures of the clusters are not necessarily disordered.…”

“…However, the broadening of diffraction signals can also be attributed to the extremely small sizes of the clusters, and the internal structures of the clusters are not necessarily disordered. In fact, short-range order similar to what is observed in their larger, crystallized counterparts is often detected in these clusters, ,, and it is still under debate whether these clusters should be regarded as “amorphous clusters” or “extremely small crystals”. , In aqueous solution, these clusters can spontaneously transform into larger crystals, , with the underlying transformation mechanisms far from being understood. One possible pathway is that these clusters dissolve into ions and then recrystallize, which has been shown for amorphous phases such as amorphous calcium carbonate (ACC). , Alternatively, clusters with short-range orders may directly attach onto larger crystals, similar to the “oriented-attachment” pathway observed for crystalline building blocks. ,− Up to now, however, direct imaging of this transformation process for the nanoclusters remains unavailable.…”