Although calcium oxalates are relevant biominerals, their formation mechanisms remain largely unresolved. Here, we investigate the early stages of calcium oxalate formation in pure and citrate-bearing solutions. Citrate is used as a well-known oxalate precipitation inhibitor; moreover, it resembles the functional domains of the biomolecules that modulate biomineralization. Our data suggest that calcium oxalate forms after Ca2+ and C2O4 2− association into polynuclear stable complexes that aggregate into larger assemblies, from which amorphous calcium oxalate nucleates. Previous work has explained citrate inhibitory effects according to classical theories. Here we show that citrate interacts with all early stage CaC2O4 species (polynuclear stable complexes and amorphous precursors), inhibiting calcium oxalate nucleation by colloidal stabilization of polynuclear stable complexes and amorphous calcium oxalate. The control that citrate exerts on calcium oxalate biomineralization may thus begin earlier than previously thought. These insights provide information regarding the mechanisms governing biomineralization, including pathological processes (e.g., kidney stone formation).

The formation of calcite (CaCO3), the most abundant carbonate mineral on Earth and a common biomineral, has been the focus of numerous studies. While recent research underlines the importance of nonclassical crystallization pathways involving amorphous precursors, direct evidence is lacking regarding the actual mechanism of calcite growth via an amorphous phase. Here we show, using in situ atomic force microscopy and complementary techniques, that faceted calcite can grow via a nonclassical particle-mediated colloidal crystal growth mechanism that at the nanoscale mirrors classical ion-mediated growth, and involves a layer-by-layer attachment of amorphous calcium carbonate (ACC) nanoparticles, followed by their restructuring and fusion with the calcite substrate in perfect crystallographic registry. The ACC-to-calcite transformation occurs by an interface-coupled dissolution–reprecipitation mechanism and obliterates or preserves the nanogranular texture of the colloidal growth layer in the absence or presence of organic (macro)­molecules, respectively. These results show that, in addition to classical ion-mediated crystal growth, a particle-mediated growth mechanism involving colloidal epitaxy may operate in the case of an inorganic crystal such as calcite. The gained knowledge may shed light on the mechanism of CaCO3 biomineralization, and should open new ways for the rational design of novel biomimetic functional nanomaterials.

Hydrated lime (Ca(OH)) is a vernacular art and building material produced following slaking of CaO in water. If excess water is used, a slurry, called lime putty, forms, which has been the preferred craftsman selection for formulating lime mortars since Roman times. A variety of natural additives were traditionally added to the lime putty to improve its quality. The mucilaginous juice extracted from nopal cladodes has been and still is used as additive incorporated in the slaking water for formulation of lime mortars and plasters, both in ancient Mesoamerica and in the USA Southwest. Little is known on the ultimate effects of this additive on the crystallization and microstructure of hydrated lime. Here, we show that significant changes in habit and size of portlandite crystals occur following slaking in the presence of nopal juice as well as compositionally similar citrus pectin. Both additives contain polysaccharides made up of galacturonic acid and neutral sugar residues. The carboxyl (and hydroxyl) functional groups present in these residues and in their alkaline degradation byproducts, which are deprotonated at the high pH (12.4) produced during lime slaking, strongly interact with newly formed Ca(OH) crystals acting in two ways: (a) as nucleation inhibitors, promoting the formation of nanosized crystals, and (b) as habit modifiers, favoring the development of planar habit following their adsorption onto positively charged (0001) faces. Adsorption of polysaccharides on Ca(OH) crystals prevents the development of large particles, resulting in a very reactive, nanosized portlandite slurry. It also promotes steric stabilization, which limits aggregation, thus enhancing the colloidal nature of the lime putty. Overall, these effects are very favorable for the preparation of highly plastic lime mortars with enhanced properties.

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Barium sulphate (BaSO 4 ) precipitation has been suggested to occur by non-classical pathways that include the formation of a dense liquid precursor phase, nucleation of primary nanoparticles and two levels of oriented aggregation resulting in micron-sized barite single crystals. In this study we build from this previous knowledge and explore how Ba 2+ and SO 4 2− ions associate in solution prior to nucleation of a solid phase and the effects of poly(acrylic acid) (PAA) in these pre-nucleation and post-nucleation stages. With this aim, in situ potentiometric experiments and transmission electron microscopy (TEM) observations of timeresolved quenched samples were carried out. Additional bulk precipitation experiments in which supersaturation was achieved by rapid mixing of Ba-and SO 4 -bearing solutions were performed. The resultant precipitates were characterized by scanning electron microscopy (FESEM and ESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric (TG) analyses. Our study provides evidence that barium sulphate precipitation occurs via the formation of ion associates in solution (ion pairs and/or clusters), that are significantly destabilized in the presence of PAA. This is associated with a noticeable delay in nucleation in the presence of PAA. Thus, our results provide indirect evidence that suggests that prenucleation ion associates must form prior to solid BaSO 4 nucleation. Alternatively, BaSO 4 mineralization in the presence of PAA seems to occur by a different route that consists in the formation of Ba-PAA globules in solution followed by an amorphous hydrated BaSO 4 phase that transform into crystalline barite nanoparticles. PAA seems to stabilize this amorphous phase, which nevertheless also forms in the absence of PAA. Finally, single micron-sized crystals are formed by the oriented attachment of distinguishable smaller subunits, thus forming mesocrystals in the presence and absence of PAA.

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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.

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