CaCO3 Polymorphs Research Articles

Characterisation of CaCO3 phases during strain-specific ureolytic precipitation

Numerous microbial species can selectively precipitate mineral carbonates with enhanced mechanical properties, however, understanding exactly how they achieve this control represents a major challenge in the field of biomineralisation. We have studied microbial induced calcium carbonate (CaCO3) precipitation (MICP) in three ureolytic bacterial strains from the Sporosarcina family, including S. newyorkensis, a newly isolated microbe from the deep sea. We find that the interplay between structural water and strain-specific amino acid groups is fundamental to the stabilisation of vaterite and that, under the same conditions, different isolates yield distinctly different polymorphs. The latter is found to be associated with different urease activities and, consequently, precipitation kinetics, which change depending on pressure-temperature conditions. Further, CaCO3 polymorph selection also depends on the coupled effect of chemical treatment and initial bacterial concentrations. Our findings provide new insights into strain-specific CaCO3 polymorphic selection and stabilisation, and open up promising avenues for designing bio-reinforced geo-materials that capitalise on the different particle bond mechanical properties offered by different polymorphs.

Calcium carbonate polymorph dictates in vitro osteoblast proliferation

Calcium carbonate (CaCO3) is a widely used material in biomedical applications owing to its biocompatibility and widespread occurrence in nature, especially in the exoskeletons of marine organisms. The ability to undergo polymorphic transformation between CaCO3 polymorphs in aqueous environments provides a wide range of applications in the biomedical field, i.e. vaterite can gradually dissolve to release drugs or calcite can provide rigidity as a bone regeneration material. In this study, CaCO3 particles including ellipsoidal vaterite, bowknot-like aragonite, and rhombohedral calcite were synthesized via solution mixing method by altering precursor ratio, pH, temperature, and solvent concentration. Afterwards, CaCO3 particles were interacted with human bone cells (hFOB) to assess cellular proliferation for orthopedic applications. Results showed that hFOB cell viability can be altered by the polymorph and concentration of CaCO3 particles. As an important finding, while proliferation of hFOB cells was promoted up to 5 days in vitro upon the interaction with vaterite and calcite particles, cellular proliferation was inhibited by aragonite particles at the highest particle concentration. Therefore, this work systematically showed the potential use of different CaCO3 polymorphs in orthopedic applications.

Marine-Inspired Enzymatic Mineralization of Dairy-Derived Whey Protein Isolate (WPI) Hydrogels for Bone Tissue Regeneration.

Whey protein isolate (WPI) is a by-product from the production of cheese and Greek yoghurt comprising β-lactoglobulin (β-lg) (75%). Hydrogels can be produced from WPI solutions through heating; hydrogels can be sterilized by autoclaving. WPI hydrogels have shown cytocompatibility and ability to enhance proliferation and osteogenic differentiation of bone-forming cells. Hence, they have promise in the area of bone tissue regeneration. In contrast to commonly used ceramic minerals for bone regeneration, a major advantage of hydrogels is the ease of their modification by incorporating biologically active substances such as enzymes. Calcium carbonate (CaCO3) is the main inorganic component of the exoskeletons of marine invertebrates. Two polymorphs of CaCO3, calcite and aragonite, have shown the ability to promote bone regeneration. Other authors have reported that the addition of magnesium to inorganic phases has a beneficial effect on bone-forming cell growth. In this study, we employed a biomimetic, marine-inspired approach to mineralize WPI hydrogels with an inorganic phase consisting of CaCO3 (mainly calcite) and CaCO3 enriched with magnesium using the calcifying enzyme urease. The novelty of this study lies in both the enzymatic mineralization of WPI hydrogels and enrichment of the mineral with magnesium. Calcium was incorporated into the mineral formed to a greater extent than magnesium. Increasing the concentration of magnesium in the mineralization medium led to a reduction in the amount and crystallinity of the mineral formed. Biological studies revealed that mineralized and unmineralized hydrogels were not cytotoxic and promoted cell viability to comparable extents (approximately 74% of standard tissue culture polystyrene). The presence of magnesium in the mineral formed had no adverse effect on cell viability. In short, WPI hydrogels, both unmineralized and mineralized with CaCO3 and magnesium-enriched CaCO3, show potential as biomaterials for bone regeneration.

Comparison of CH4 and CO2 Adsorptions onto Calcite(10.4), Aragonite(011)Ca, and Vaterite(010)CO3 Surfaces: An MD and DFT Investigation.

The interaction between greenhouse gases (such as CH4 and CO2) and carbonate rocks has a significant impact on carbon transfer among different geochemical reservoirs. Moreover, CH4 and CO2 gases usually associate with oil and natural gas reserves, and their adsorption onto sedimentary rocks may influence the exploitation of fossil fuels. By employing the molecular dynamics (MD) and density functional theory (DFT) methods, the adsorptions of CH4 and CO2 onto three different CaCO3 polymorphs (i.e., calcite(10.4), aragonite(011)Ca, and vaterite(010)CO3) are compared in the present work. The calculated adsorption energies (Ead) are always negative for the three substrates, which indicates that their adsorptions are exothermic processes and spontaneous in thermodynamics. The Ead of CO2 is much more negative, which suggests that the CO2 adsorption will form stronger interfacial binding compared with the CH4 adsorption. The adsorption precedence of CH4 on the three surfaces is aragonite(011)Ca > vaterite(010)CO3 > calcite(10.4), while for CO2, the sequence is vaterite(010)CO3 > aragonite(011)Ca > calcite(10.4). Combining with the interfacial atomic configuration analysis, the Mulliken atomic charge distribution and overlap bond population are discussed. The results demonstrate that the adsorption of CH4 is physisorption and that its interfacial interaction mainly comes from the electrostatic effects between H in CH4 and O in CO32–, while the CO2 adsorption is chemisorption and the interfacial binding effect is mainly contributed by the bonds between O in CO2 and Ca2+ and the electrostatic interaction between C in CO2 and O in CO32–.

Thermodynamic and Kinetic Parameters for Calcite Nucleation on Peptoid and Model Scaffolds: A Step toward Nacre Mimicry.

The production of novel composite materials, assembled using biomimetic polymers known as peptoids (N-substituted glycines) to nucleate CaCO3, can open new pathways for advanced material design. However, a better understanding of the heterogeneous CaCO3 nucleation process is a necessary first step. We determined the thermodynamic and kinetic parameters for calcite nucleation on self-assembled monolayers (SAMs) of nanosheet-forming peptoid polymers and simpler, alkanethiol analogues. We used nucleation rate studies to determine the net interfacial free energy (γnet) for the peptoid–calcite interface and for SAMs terminated with carboxyl headgroups, amine headgroups, or a mix of the two. We compared the results with γnet determined from dynamic force spectroscopy (DFS) and from density functional theory (DFT), using COSMO-RS simulations. Calcite nucleation has a lower thermodynamic barrier on the peptoid surface than on carboxyl and amine SAMs. From the relationship between nucleation rate (J0) and saturation state, we found that under low-saturation conditions, i.e. <3.3 (pH 9.0), nucleation on the peptoid substrate was faster than that on all of the model surfaces, indicating a thermodynamic drive toward heterogeneous nucleation. When they are taken together, our results indicate that nanosheet-forming peptoid monolayers can serve as an organic template for CaCO3 polymorph growth.

Effect of barium and strontium ions on the morphology and polymorphism of CaCO3

Industrial internal water treatment programs provide a robust solution for many water related problems such as scale inhibition, corrosion control, microbial growth etc. in heat transfer equipment. In order to design a good program it is essential to study the individual effect of each constituent in the program and how it affect the parameters for which it is added. Ethylenediaminetetraacetic acid (EDTA) is one such constituent widely used in internal treatment for scale inhibition. Calcium carbonate (CaCO3) is one of the major constituents in scale. Polymorphism and morphology of the crystallites have significant effect on the nature of scale. In this work, we report the effect of EDTA on the morphology and polymorphic composition of CaCO3 in the presence of barium and strontium ions and their effect on the scale inhibition.

Ultrasonic-assisted eggshell extract-mediated polymorphic transformation of calcium carbonate.

This study aimed to evaluate the combined effects of eggshell extract and ultrasonic irradiation on the polymorphic transformation of calcium carbonate (CaCO3). In this context, XRD, Raman spectroscopy, SEM, AFM, TGA-FTIR, BET, and zeta potential analysis were used to identify and characterize the different polymorphs of CaCO3 obtained in the absence and presence of eggshell extract in the media with and without ultrasonic irradiation. The morphology and polymorphic nature of the CaCO3 crystals were observed to change, which indicated that the eggshell extract and ultrasonication influenced the structure and crystallization of CaCO3. The structural analysis results indicated that the addition of eggshell extract to the media resulted in the full transformation of calcite to the vaterite polymorph. The results also showed that ultrasonic irradiation had a more significant influence on the BET specific surface area of the crystals compared to the eggshell extract media. Furthermore, a Box-Behnken design with response surface methodology was employed to determine the optimal operating conditions for CaCO3 crystallization. The effects of stirring rate, extract concentration, and ultrasonic power on the BET surface area were investigated. The results show that the data sufficiently fit the second-order polynomial model. Understanding the eggshell extract-mediated polymorphic transformation with ultrasonic irradiation obtained in this study makes it possible to control the polymorphic formation and modify the product characteristics.

Ps19, a novel chitin binding protein from Pteria sterna capable to mineralize aragonite plates in vitro.

Mollusk shell is composed of two CaCO3 polymorphs (calcite and aragonite) and an organic matrix that consists of acetic acid- or ethylenediaminetetraacetic acid (EDTA)-soluble and insoluble proteins and other biomolecules (polysaccharides, β-chitin). However, the shell matrix proteins involved in nacre formation are not fully known. Thus, the aim of this study was to identify and characterize a novel protein from the acetic acid-insoluble fraction from the shell of Pteria sterna, named in this study as Ps19, to have a better understanding of the biomineralization process. Ps19 biochemical characterization showed that it is a glycoprotein that exhibits calcium- and chitin-binding capabilities. Additionally, it is capable of inducing aragonite plate crystallization in vitro. Ps19 partial peptide sequence showed similarity with other known shell matrix proteins, but it displayed similarity with proteins from Crassostrea gigas, Mizuhopecten yessoensis, Biomphalaria glabrata, Alpysia californica, Lottia gigantea and Elysia chlorotica. The results obtained indicated that Ps19 might play an important role in nacre growth of mollusk shells.

Impurity Effects on Habit Change and Polymorphic Transitions in the System: Aragonite–Calcite–Vaterite

Crystal growth experiments carried out in aqueous solutions, supersaturated with respect to calcite (CaCO3) at room temperature and pressure, show that the lithium ion added to growth solutions not only acts as a habit modifier of calcite, but promotes the nucleation and growth of the two other CaCO3 polymorphs: aragonite and vaterite. This behavior is interpreted on the grounds of foreign adsorption going beyond Langmuir’s isotherm model: two-dimensional (2D)-adsorbed nuclei of Li2CO3 (mineral zabuyelite) can change, under varying Li+ concentrations in solution, the growth habit of calcite, promoting as well both nucleation and growth of aragonite and vaterite, through the mechanism of 2D-epitaxy. Calculation proves how the epitaxy aragonite/2D-zabuyelite reduces the average value of the specific surface energy of aragonite, allowing the nucleation of aragonite and then the coexistence of calcite and aragonite in the same batch. Finally, it is outlined that the 2D lattices fulfilling the epitaxy constraints for the couples calcite/zabuyelite and aragonite/zabuyelite show, as a common feature, a remarkable pseudohexagonality.

Triple oxygen isotope fractionation between CaCO3 and H2O in inorganically precipitated calcite and aragonite

Carbonate bearing materials, such as foraminifera, mollusks shells, or speleothems, have the potential to preserve geochemical and isotopic signatures reflecting the environmental conditions at the time they formed. Beyond the conventional δ18O, information from triple oxygen isotopes (reported as 17Oexcess, which is defined as (ln (δ17O/1000 + 1) − λref ln (δ18O/1000 + 1)) × 106) in these archives promises to be a valuable tool to reconstruct past hydrological processes. The goal of this study is to determine the triple oxygen isotope fractionation between CaCO3 and H2O under well-constrained laboratory conditions. We performed laboratory experiments to precipitate CaCO3 polymorphs, either calcite or aragonite, at temperatures between 10 and 35 °C. We then evaluated the effect of polymorphism, temperature, and solution concentration on the 17Oexcess of CO2 extracted from these carbonates and the 17O isotopic fractionation (17α) between water and CaCO3. The obtained values of 18α and 17α between CO2 extracted from CaCO3 and parent water allow us to calculate the fractionation slope θ (=ln17α/ln18α). Our observations suggest that θ is indistinguishable at temperatures of 10 and 27 °C, but is slightly lower at 35 °C. The lower value at 35 °C may be related to disequilibrium during these experiments. We found that θ is independent of polymorph and of solution concentration, indicating that 17Oexcess is less sensitive than δ18O to these geochemical parameters and can thus be a robust proxy for reconstructing 17Oexcess of parent water.

Carbon utilization based on post-treatment of desalinated reject brine and effect of structural properties of amines for CaCO3 polymorphs control

This study proposed simultaneous disposal and utilization process of desalinated reject brine and CO2, or brine-based carbon capture and utilization (CCU). Using metal cations in desalinated reject brine, the feasibility of amines with different structural properties as crystallization inhibitors, morphology controller of calcium carbonate formation was investigated in this study. Ca2+ ions were separated from desalinated reject brine in a salt refining plant by using a pH swing method and used for CaCO3 formation. Through this process, disposal of the large amount of metal cations in the desalinated reject brine was achieved. CO2 was selectively captured by alkanolamine, alkylamine, and multi-amine CO2 carriers with numerous amino groups and carried into the separated Ca(OH)2 to generate CaCO3. X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FT-IR) were used to investigate crystal shape and polymorph of the product. Additionally, thermogravimetric/derivative thermogravimetric analysis (TG/DTG) was used for purity analysis. The results demonstrated that crystallization inhibition depending on structural properties of amines was possible, leading to successful CaCO3 polymorph control. The results of this research can be applied to process where the industrial production of metal carbonates based on inorganic CCU and disposal of the desalinated reject brine were involved.

Single-crystal diffraction at the high-pressure Indo-Italian beamline Xpress at Elettra, Trieste.

In this study the first in situ high-pressure single-crystal X-ray diffraction experiments at Xpress, the Indo-Italian beamline of the Elettra synchrotron, Trieste (Italy), are reported. A description of the beamline experimental setup and of the procedures for single-crystal centring, data collection and processing, using diamond anvil cells, are provided. High-pressure experiments on a synthetic crystal of clinoenstatite (MgSiO3), CaCO3 polymorphs and a natural sample of leucophoenicite [Mn7Si3O12(OH)2] validated the suitability of the beamline experimental setup to: (i) locate and characterize pressure-induced phase transitions; (ii) solve ab initio the crystal structure of high-pressure polymorphs; (iii)perform fine structural analyses at the atomic scale as a function of pressure; (iv) disclose complex symmetry and structural features undetected using conventional X-ray sources.

Sunscreen Properties of Marl.

The physicochemical and sunscreen properties of marl as a function of particle size were investigated. The research findings established that the marl under investigation consisted of more than 95% calcium carbonate (CaCO3). The particles of marl inspected under a scanning electron microscope were calcite, which is the stable polymorph of CaCO3, with a rhombohedral structure. The particle size classification by the sieving method showed that grinding using a ball mill could downsize the marl particles by 2 to 3 times, reaching below 15 µm on average. Marl particles showed a tendency to reflect ultraviolet A (UVA) rays rather than UVB rays and a possibility to steadily absorb both UVAII and UVAI. Finer particles obtained after a longer grinding process demonstrated higher efficacy regarding UV reflection and absorption properties. The 3 wt.% marl displayed a sun protection factor (SPF) value of 1 to 2. However, marl demonstrated a good ability to protect against radiation over a broad spectrum range with a critical wavelength above 370 nm. The addition of marl in the formulation containing avobenzone and octinoxate had a positive synergistic effect because the marl was able to increase the UV absorbance efficacy (based on the area under the curve (AUC) value) and SPF value of the cream. Furthermore, it was also discovered that the added marl powder could slow the decrease in UV protection efficacy of the products in terms of the AUC calculated from the absorbance profile after exposure to simulated UV rays with an amplitude range of 10 J/cm2 to 40 J/cm2 for 30 min, which was similar to the results obtained from octocrylene and bemotrizinol.

A New Bacterial Strain Alcaligenes aquatilis MAG 1 Associated with the Mediterranean Sea Lithophaga (Bivalvia) Precipitating in Nutrient Free Medium Shell-Like Calcium Carbonate Polymorphs

Mediterranean Sea Lithophaga bivalve associated bacteria and their capability in calcium carbonate polymorphs formation have been investigated in the present study. Isolated bacteria were a single strain of Gram-negative, rod-shaped, and flagellated motile cells. Phylogenetic analysis based on 16S rDNA sequences consigned the isolated bacterium to Alcaligenes aquatilis species as a distinct strain assigned as Alcaligenes aquatilis strain MAG 1. Phenotypic and biochemical characteristics of the strain recorded no acids produced from different carbohydrates under aerobic conditions except for D-mannose, amygdalin, glucose, L-rhamnose, L- arabinose and D- melibiose. Isolates showed negative results for nitrate reduction, urea degradation, sulphate reduction (H2S production) and tryptophan deaminase (TDA). Alcaligenes aquatilis strain MAG 1 precipitated in nutrient-free medium different polymorphs of anhydrous CaCO3, initially, as a mixture of aragonite and calcite, and well-organized rhombohedral calcite crystals dominated by the fourth day. Conclusively, calcium carbonate biomineralization away from any added nutrients by this new strain magnifies its role as nucleation sites and retracts the effect of bacterial metabolic pathways. Besides, various CaCO3 crystal polymorphs formed by the isolated strain advocate a potential complementary role in Lithophaga shell assembly.

Amorphous calcium carbonate (ACC) in fresco mural paintings

A fresco painting has a calcium carbonate binder produced as result of the carbonation process of a lime paste. The reaction environmental conditions are similar to those of bio-mineral formation where an amorphous calcium carbonate (ACC) phase has been found to be the precursor of other CaCO3 polymorphs following the sequence: hydrated ACC, anhydrous ACC, vaterite, aragonite, and, finally, stable calcite. In order to determine whether ACC is also formed during the fresco drying process, as well as, the final surface calcium carbonate phases present, a series of laboratory mock-ups, replicating as close as possible each of the paint strata and submitted to the same atmospheric conditions have been designed and studied. The results indicate that the continuous supply of water and reagents by the lime and sand mortar promote the ACC formation even at the completion of water evaporation and that the high pH of the medium does not favour the formation of calcium carbonate polymorphs other than calcite. In fact, the results demonstrate that the presence of an ACC layer in the mural painting surface confirms that the original painting technique used was fresco. Finally, the presence of an ACC layer is important for stablishing adequate cleaning (the solubility of hydrated ACC is superior to those of calcite) and conservation protocols (ACC is susceptible to suffer dehydration) for the fresco paintings.

Coral acid rich protein selects vaterite polymorph in vitro

Corals and other biomineralizing organisms use proteins and other molecules to form different crystalline polymorphs and biomineral structures. In corals, it’s been suggested that proteins such as Coral Acid Rich Proteins (CARPs) play a major role in the polymorph selection of their calcium carbonate (CaCO3) aragonite exoskeleton. To date, four CARPs (1–4) have been characterized: each with a different amino acid composition and different temporal and spatial expression patterns during coral developmental stages. Interestingly, CARP3 is able to alter crystallization pathways in vitro, yet its function in this process remains enigmatic. To better understand the CARP3 function, we performed two independent in vitro CaCO3 polymorph selection experiments using purified recombinant CARP3 at different concentrations and at low or zero Mg2+ concentration. Our results show that, in the absence of Mg2+, CARP3 selects for the vaterite polymorph and inhibits calcite. However, in the presence of a low concentration of Mg2+ and CARP3 both Mg-calcite and vaterite are formed, with the relative amount of Mg-calcite increasing with CARP3 concentration. In all conditions, CARP3 did not select for the aragonite polymorph, which is the polymorph associated to CARP3 in vivo, even in the presence of Mg2+ (Mg:Ca molar ratio equal to 1). These results further emphasize the importance of Mg:Ca molar ratios similar to that in seawater (Mg:Ca equal to 5) and the activity of the biological system in a aragonite polymorph selection in coral skeleton formation.

The role of relative humidity on crystallization of calcium carbonate from calcium acetoacetate precursor

Calcium acetoacetate, Ca(OAcAc)2, was exposed 7, 30 and 365 days to different values of relative humidity (33%, 48%, 75% and 96%) at 40 °C in order to study its transformation to CaCO3. The resulting Ca(OAcAc)2 decomposition and the time dependence of the phase transformations were monitored and critically evaluated by Fourier transform infrared spectroscopy, field emission scanning electron microscopy and X-ray powder diffraction. The impact of relative humidity on CaCO3 polymorph formation was thoroughly assessed. In all of the conditions used and for all ageing periods, the formed crystal structure is found to be vaterite. At the lowest relative humidity (33%), the amorphous CaCO3 remains more or less almost untransformed even after one year of exposure. It is proposed that the reason for the stability of amorphous CaCO3 is due to the limited amount of physisorbed water on the surfaces of the particles, which is considered the driving force for its transformation. However, the carbonation process is faster in the case of the highest humidity (96%). The findings are not only important for better solutions in the field of cultural heritage, but also shed new light on the fundamental mechanism of CaCO3 crystallization.

Geochemistry and mineralogy of serpentinization-driven hyperalkaline springs in the Ronda peridotites

We present a detailed study of the water geochemistry, mineralogy and textures in serpentinization-related hyperalkaline springs in the Ronda peridotites. Ronda waters can be classified into hyperalkaline fluids and river waters that are broadly similar to Ca2+–OH- and Mg2+–HCO3- water types described in serpentinite-hosted alkaline springs elsewhere. At the discharge sites of the fluids (fractures or human made outlets) and ponds along the fluid flow paths, the fluids are hyperalkaline (10.9 < pH < 12) and characterized by low Mg and high Na, K, Ca, and Cl- concentrations. River waters, occurring near the spring sites, are mildly alkaline (8.5 < pH < 8.9) and enriched in Mg and DIC compared to Na, K, Ca and Cl-. The chemistry of Ronda Mg–HCO3 river waters is likely due to the hydrolysis of ferromagnesian peridotite minerals in equilibrium with the atmosphere by infiltrated meteoric water and shallow groundwater in the serpentinized peridotite. The Ronda Ca–OH hyperalkaline fluids are generated by the combination of low temperature serpentinization reactions from infiltrated surface Mg–HCO3 river waters —or Ca–HCO3 waters from near karst aquifers— and deep carbonate precipitation isolated from atmospheric CO2. Mass balance calculations indicate that the weathering of Ca-bearing peridotite silicates, such as diopside, is a feasible source of Ca in Ronda Ca–OH hyperalkaline fluids; however, it requires steady-state dissolution rates substantially greater than those determined experimentally.Travertine, crystalline crusts and sediment deposits comprise the types of solid precipitates observed in Ronda hyperalkaline spring sites. Calcite, aragonite, dolomite and Mg-Al-rich clays are the main mineral phases identified in the spring sites. As illustrated in the Baños del Puerto spring site, (i) calcite-dominated precipitation is due to uptake of atmospheric CO2 by Ca–OH hyperalkaline fluids during discharge, and (ii) aragonite-dominated precipitation is due to mixing of Ca–OH hyperalkaline fluids with Mg–HCO3 river waters. Aragonite and dolomite contents increase away from the springs and toward the river waters that uniquely reflects the effect of Mg ions on the precipitation of aragonite versus calcite. Other potential factors controlling the precipitation of these CaCO3 polymorphs are the Mg/Ca ratio, the CO2 content, and the temperature of the fluids. Dolomite forms during lithification of travertine due to periodic flooding of river water combined with subsequent evaporation.

PO4 adsorption on the calcite surface modulates calcite formation and crystal size

Abstract Calcium carbonate (CaCO3) and particularly its stable phase, calcite, is of great geological significance in the deep carbon cycle since CaCO3 from biomineralized shells and corals form sedimentary rocks. Calcite also attracts attention in medical science and pharmacy as a primary or intermediate component in biomaterials because it possesses excellent biocompatibility along with suitable physicochemical properties. Calcite blocks have already been used during surgical procedures as a bone substitute for reconstructing bone defects formed by diseases and injury. When producing CaCO3 biomaterials and bioceramics, in particular, in vivo control of the size and polymorphic nature of CaCO3 is required. In this study, we investigated the effects of PO4 on calcite formation during the phase conversion of calcium sulfate anhydrate (CaSO4, CSA), which is sometimes used as a starting material for bone substitutes because of its suitable setting ability. CSA powder was immersed in 2 mol/L Na2CO3 solution containing a range of PO4 concentrations (0–60 mmol/L) at 40 °C for 3 days. The treated samples were investigated by X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray fluorescence spectroscopy, and thermal analysis. In addition, the fine structures of the treated samples were observed by field-emission scanning electron microscopy, and the specific surface area was measured. We found that PO4, which is universally present in vivo, can modulate the calcite crystal size during calcite formation. A fluorescence study and calcite crystal growth experiments indicated that PO4 adsorbs tightly onto the surface of calcite, inhibiting crystal growth. In the presence of high PO4 concentrations, vaterite is formed along with calcite, and the appearance and stability of the CaCO3 polymorphs can be controlled by adjusting the PO4 concentration. These findings have implications for medical science and pharmacology, along with mineralogy and geochemistry.

Local scaling of CaCO3 on carbon steel surface with different corrosion types

CaCO3 scaling on metal is closely related to its electrochemical corrosion. However, little attention has been paid to the influences of interfacial corrosion on scaling behavior. In this work, the local deposition behavior of CaCO3 on carbon steel with different corrosion types was investigated. Results reveal that the interfacial distribution of anodically released Fe2+ plays important roles in CaCO3 polymorph. For general corrosion, anodically released Fe2+ easily diffuses to all micro cathodic regions and suppresses pH increase by preferentially consuming cathodically produced OH– and interfacial O2, and its incomplete hydrolysis. This lowers CaCO3 nucleation driving force and favors aragonite deposition. In contrast, localized corrosion results in an uneven distribution of Fe2+ at carbon steel/water interface. Aragonite tends to deposit in the micro cathodic regions where Fe2+ ions easily arrive from the adjacent anodic sites, while this effect diminishes in the cathodic regions where fewer Fe2+ ions arrive.