Rhombohedral Form Research Articles

Competition between homogeneous and heterogeneous crystallization of CaCO3 during water softening

Homogeneous and heterogeneous crystallization of CaCO3 simultaneously occur in seed-induced crystallization during water softening, while suppressing homogeneous crystallization is necessary due to the production of fine particulates that poorly precipitate. However, homogeneous crystallization is difficult to distinguish from heterogeneous crystallization. Consequently, a central focus in improving water softening is understanding their competing activities. In this study, a novel method for distinguishing homogeneous and heterogeneous calcium carbonate crystallization is described that utilizes magnetite as seed particles. Results showed that saturation index (SI) was the primary driver of both homogeneous and heterogeneous crystallizations. Heterogeneous crystallization was preferentially promoted at low SI, while homogeneous crystallization was promoted at high SI. The highest suppression effect to homogeneous crystallization occurred at SI of about 1.01. Seed dosage and mean particle size were the primary parameters related to the competition of the crystallization types. Higher seed dosage and smaller seed particle sizes promoted heterogeneous crystallization and suppressed homogeneous crystallization. Due to the good adaptability of heterogeneous crystallization at low SI, the absorption of CO2 from the air into the solutions also improved the efficiency of hardness removal. The introduction of seed particles did not change crystalline product phases, with calcite being the only observed phase and possessing rhombohedral forms with highly regular and smooth edges. Water softening pilot test results showed that SI of 1.5 was more favorite for CaCO3 layer formation on seed surface and hardness removal in comparison with SI of 1.0 and 2.0. Overall, the results from this study demonstrate that the introduction of seed particles is a promising approach to suppress the homogeneous crystallization of CaCO3. Moreover, these results can serve as a framework for improved seed-induced crystallization during water softening.

Effects of push–pull injection–suction spacing on sand biocementation treatment

The process of ureolysis-driven biocementation is used to improve granular soils. The precipitation of calcium carbonate (CaCO3) crystals results from the reactions of urease generated by ureolytic bacteria and chemical reagents, which strengthen or bind soil particles together. Using a lab-based scaled physical model, this study investigated the influence of selected spacing intervals (107, 214 and 321 mm) on the effectiveness of biocementation through the injection–suction or ‘push–pull’ approach. Polystyrene moulds were used to create soil specimens. These were then injected with six cycles of treatment solutions at the intervals stated. The compressive strengths and calcium carbonate contents of the biocemented soil specimens were measured after curing. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy and effluent analysis (pH and ammonium measurements) were also performed. The biocemented soil specimens with different spacing intervals obtained compressive strengths of 2.53 ± 1.06 to 4.2 ± 2.3 MPa, while the calcium carbonate contents were from 2.78 ± 0.3 to 11.16 ± 1.5%. The elemental compositions and bonding of calcium carbonate precipitates in the biocemented soil were confirmed by EDS and FTIR spectra, while SEM micrographs revealed chip-like and irregular rhombohedral crystal forms. The results demonstrated that injection spacing had an effect on biocemented soil treated by microbially induced carbonate precipitation.

Oxidation behavior of a cathodic arc evaporated Cr0.69Ta0.20B0.11N coating

CrTaBN hard coatings deposited by cathodic arc evaporation are a promising new material class for use in demanding applications, due to their high hardness and good thermal stability in protective atmosphere. Up to now however, studies on the detailed oxidation mechanism of quaternary CrTaBN coatings are lacking in the literature. Thus, within this work, the oxidation behavior of a Cr0.69Ta0.20B0.11N coating grown by cathodic arc evaporation was studied in a combinatorial approach of advanced characterization techniques. In situ high-energy x-ray diffraction at a synchrotron radiation facility showed that up to ∼1100 °C, only the face-centered cubic (fcc) CrxTayB1−x−yN solid solution of powdered CrTaBN contributes to the crystalline phase composition. As the temperature is further increased, tetragonal CrTaO4 and rhombohedral Cr2O3 form. In situ high-temperature Raman spectroscopy evidenced that B2O3 contributes to the phase composition of the material in the temperature regime from ∼600 to 1000 °C. Applying high-resolution transmission electron microscopy allowed to identify the presence of four discrete zones in a partly oxidized CrTaBN coating on sapphire: intact fcc-CrTaBN at the interface to the substrate, followed by a Cr-deficient and Cr-enriched layer, respectively, and a porous layer with small grains at the surface.

Phase Relationships of the Gd-Zn System

Abstract The Gd-Zn system has been studied by performing differential thermal analysis (DTA) in closed crucibles, optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray examination (XRD). Eight intermetallic phases were found: GdZn, cubic, cP2-CsCl type, congruent melting at 1020 °C, GdZn2, orthorhombic, oI12-CeCu2 type, congruent melting at 1010 °C, GdZn3, orthorhombic, oP16-YZn3 type, peritectic decomposition at 905 °C, Gd3Zn11, orthorhombic, oI28-La3Al11 type, peritectic decomposition at 880 °C, Gd13Zn58, hexagonal, hP142-Gd13Zn58 type, melting point at 920 °C, Gd3Zn22, tetragonal, tI100-Pu3Zn22 type, peritectic decomposition at about 910 °C, Gd2Zn17, high temperature (H.T.) form, rhombohedral, hR57-Th2Zn17 type and low temperature (L.T.) form, hexagonal, hP38-Th2Ni17 type, congruent melting at 935 °C and GdZn12 tetragonal, tI26-ThMn12 type, peritectic decomposition at 660 °C. Five eutectic reactions occur at 780 °C and 27 at% Zn ((αGd) + GdZn), at 945 °C and 59 at% Zn (GdZn + GdZn2), at 875 °C and 77 at% Zn (GdZn3 + Gd3Zn11), at 885 °C and 84 at% Zn (Gd13Zn58 + Gd3Zn22) and finally at 400 °C and 99.5 at% Zn (GdZn12 + (Zn)). A catatectic equilibrium was found in the Gd-rich end. The results are discussed and compared with those for the other rare earthzinc (R-Zn) systems. A brief outline of the general alloying behaviour of the 12th group elements (Zn, Cd and Hg) with the rare earth metals is also presented.

Texture evolution in rhombohedral boron carbide films grown on 4H-SiC(0001̄) and 4H-SiC(0001) substrates by chemical vapor deposition.

Boron carbide in its rhombohedral form (r-BxC), commonly denoted B4C or B13C2, is a well-known hard material, but it is also a potential semiconductor material. We deposited r-BxC by chemical vapor deposition between 1100 °C and 1500 °C from triethylboron in H2 on 4H-SiC(0001) and 4H-SiC(0001̄). We show, using ToF-ERDA, that pure B4C was grown at 1300 °C, furthermore, using XRD that graphite forms above 1400 °C. The films deposited above 1300 °C on 4H-SiC(0001̄) were found to be epitaxial, with the epitaxial relationships B4C(0001)[101̄0]‖4H-SiC(0001̄)[101̄0] obtained from pole figure measurements. In contrast, the films deposited on 4H-SiC(0001) were polycrystalline. We suggest that the difference in growth mode is explained by the difference in the ability of the different surfaces of 4H-SiC to act as carbon sources in the initial stages of the film growth.

Structures of additional crystal forms of Satellite tobacco mosaic virus grown from a variety of salts.

The structures of new crystal forms of Satellite tobacco mosaic virus (STMV) are described. These belong to space groups I2, P21212 (a low-resolution form), R3 (H3) and P23. The R3 crystals are 50%/50% twinned, as are two instances of the P23 crystals. The I2 and P21212 crystals were grown from ammonium sulfate solutions, as was one crystal in space group P23, while the R3 and the other P23 crystals were grown from sodium chloride, sodium bromide and sodium nitrate. The monoclinic and orthorhombic crystals have half a virus particle as the asymmetric unit, while the rhombohedral and cubic crystals have one third of a virus particle. RNA segments organized about the icosahedral twofold axes were present in crystals grown from ammonium sulfate and sodium chloride, as in the canonical I222 crystals (PDB entry 4oq8), but were not observed in crystals grown from sodium bromide and sodium nitrate. Bromide and nitrate ions generally replaced the RNA phosphates present in the I222 crystals, including the phosphates seen on fivefold axes, and were also found at threefold vertices in both the rhombohedral and cubic forms. An additional anion was also found on the fivefold axis 5 Å from the first anion, and slightly outside the capsid in crystals grown from sodium chloride, sodium bromide and sodium nitrate, suggesting that the path along the symmetry axis might be an ion channel. The electron densities for RNA strands at individual icosahedral dyads, as well as at the amino-terminal peptides of protein subunits, exhibited a diversity of orientations, in particular the residues at the ends.

Valorization of Crocus Sativus L waste extracts as efficient, eco-friendly and economical inhibitors of scaling: Experimental and computational investigations

The valorization of biomass and the use of plant extracts as new green inhibitors of scaling are amongthe most innovative research topics. In this light, this study aims to evaluate the effectiveness of two extracts of Crocus Sativus L (CSL) wastes towards the formation of calcareous scale, namely the Aqueous Extract of Leaf (ALE) and the Aqueous Extract of Petal and Stamen (AEPS). Folin-Ciocalteau and colorimetric methods are used to determine thetotal polyphenols and flavonoids contents, respectively. Then, the anti-scaling property of the investigated extracts is determined using a conductivity test and the Laboratory of Chemistry and Genie of the Environment (LCGE) technique at a temperature of 25 °C. After that, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analysesare employed to examine the formed scale during the LCGE tests. To support the experimental findings, Molecular Dynamic (MD) simulations and Density Functional Theory (DFT) calculations are carried out. The data obtained revealed that AEPS has a higher concentration of total polyphenol and flavonoids than ALE. Total inhibition occurs after the addition of 60 mg‧L-1 of AEPS to the calco-carbonic solution with a hardness of 40°F. Whereas an amount of 82 mg‧L-1 of ALE was the optimal concentration to completely inhibit CaCO3 formation. This allows us to conclude that the anti-scaling effectiveness is correlated with polyphenols and flavonoids contents in CSL extracts. SEM and FTIR analyses reveal that the crystallization of CaCO3 without inhibitor was a stable variety of calcite with a rhombohedral form. However, the presence of a low concentration of AEPS (20 mg‧L-1) resulted in a significant modification on the morphology of CaCO3 and a mixture of vaterite with calcite phases. The theoretical outcomes agree with the anti-scaling efficiencies reported experimentally. Based on these results, the CSL biomass could be an interesting renewable source of eco-friendly and inexpensive inhibitors of scaling.

Crystal chemistry rationale and ab initio investigation of ultra-hard dense rhombohedral carbon and boron nitride

Rhombohedral dense forms of carbon, rh-C2 (or hexagonal h-C6), and boron nitride, rh-BN (or hexagonal h-B3N3), are derived from rhombohedral 3R graphite based on original crystal chemistry scheme backed with full cell geometry optimization to minimal energy ground state computations within the quantum density functional theory. Considering throughout hexagonal settings featuring extended lattices, the calculation of the hexagonal set of elastic constants provide results of large bulk moduli with B0 (rh-C2) = 438 GPa close to that of diamond, and B0 (rh-BN) = 369 GPa close to that of cubic BN. The hardness assessment in the framework of three contemporary models enables both phases to be considered as ultra-hard. From the electronic band structures calculated in the hexagonal Brillouin zones, 3R graphite is a small-gap semiconductor, oppositely to rh-C2 that is characterized by a large band gap of 6 eV, and rh-BN is a large band gap insulator with Egap = 5 eV.

Experimental investigation and molecular dynamic simulation of Tannic acid as an eco-friendly inhibitor for calcium carbonate scale

The present study aims to assess the performance of Tannic acid (TA), a natural polyphenol, as a new eco-friendly inhibitor on the deposition scale of calcium carbonate. The conductivity test and the Laboratory of Chemistry and Genie of the Environment (LCGE) technique are used to evaluate the anti-scaling property of TA. The dispersion property of CaCO3 is also investigated using UV–visible method. To characterize the morphology and crystal structure of the formed scale during the LCGE tests, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses are utilized. Then, Molecular dynamic simulations are carried out to prove theoretically the ability of TA molecule to inhibit CaCO3 scaling. Results show that the addition of a low concentration of TA (7.8 mg L −1) inhibits completely the precipitation of calcium carbonate of calco-carbonic solution with a hardness of TH = 40 ° F (1 ° F = 4 mg L −1 of calcium ions) at 25 °C. We recorded also that TA has a good dispersion property of CaCO3. It was observed that the scales formed in the absence of TA are regular rhombohedral forms. However, in the presence of a low concentration of TA (2 mg L −1) the significant modification in the crystal morphology and the crystal sizes are noted. XRD diffractogram reveals that CaCO3-calcite is the main polymorph formed during the tests without inhibitor. Nevertheless, in the presence of inhibitor, a mixture of vaterite and calcite was obtained. The geometrical and energetic analyses of the interfacial interactions of TA inhibitor with calcite (110) and (104) surfaces proved its enhanced capacity to interact with calcite crystals. These findings explain the good obtained scale inhibition efficiency of TA. Consequently, we suggest the use of TA as a new eco-friendly inhibitor for calcareous scaling.

Enhanced electrical magnetochiral effect by spin-hedgehog lattice structural transition

Nonreciprocal resistance, depending on both directions of current ($j$) and magnetic-field ($H$) or magnetization ($M$), is generally expected to emerge in a chiral conductor, and be maximized for $j$ $\parallel$ $H$($M$). This phenomenon, electrical magnetochiral effect (eMChE), is empirically known to increase with $H$ in a paramagnetic or fully ferromagnetic state on chiral lattice or to depend on fluctuation properties of a helimagnetic state. We report here the eMChE over a wide temperature range in the chiral-lattice magnet MnGe in which the spin hedgehog lattice (HL) forms with the triple spin-helix modulation vectors. The magnitude of nonreciprocal resistivity is sharply enhanced in the course of the field-induced structural transition of HL from cubic to rhombohedral form. This is attributed to the enhanced asymmetric electron scatterings by vector spin chirality in association with the large thermal fluctuations of spin hedgehogs.

Influence of synthesis methodology and excess Na on the ionic transport properties of natrium super ionic conductor, Na3Zr2Si2PO12

Na3Zr2Si2PO12 is a well-known solid electrolyte because of its potential applications in Na-ion batteries and gas sensors. In this work, the structural and transport properties of Na3Zr2Si2PO12 synthesized by solid-state reaction and sol–gel techniques are compared. In both methods, a monoclinic phase for Na3Zr2Si2PO12 is confirmed by X-ray Diffraction and FTIR spectroscopy techniques. The impedance measurements show higher conductivity in Na3Zr2Si2PO12 prepared by solid-state reaction. The role of sintering temperatures on the transport results of Na3Zr2Si2PO12 has been investigated. The temperature dependent conductivity plots of Na3Zr2Si2PO12 show a change in the slope at 163 °C corresponding to its structural phase transition from monoclinic to rhombohedral form, which is further supported by its DSC result. Furthermore, the role of excess Na on the ionic conductivity of Na3Zr2Si2PO12 using two different Na precursors Na2CO3 and Na3PO4 keeping same Na-stoichiometry has been investigated.

Towards a morphology diagram for terrestrial carbonates: Evaluating the impact of carbonate supersaturation and alginic acid in calcite precipitate morphology

Ancient and recent terrestrial carbonate-precipitating systems are characterised by a heterogeneous array of deposits volumetrically dominated by calcite. In these environments, calcite precipitates display an extraordinary morphological diversity, from single crystal rhombohedral prisms, to blocky crystalline encrustations, or spherulitic to dendritic aggregates. Despite many decades of thorough descriptive and interpretative work on these fabrics, relating calcite micro-morphology with sedimentary hydrogeochemical conditions remains a challenge. Environmental interpretations have been hampered by the fact that calcite morphogenesis results from the complex interaction between different physico-chemical parameters which often act simultaneously (e.g., carbonate mineral supersaturation, Mg/Ca ratio of the parental fluid, organic and inorganic additives). To try to experimentally address the sedimentological causes of calcite morphogenesis, an experimental approach yielding a first attempt at a calcite growth-form phase diagram is presented here. The initial aim was to account for the carbonate products experimentally nucleated in alkaline, saline lake settings. These are the result of at least two competing calcite precipitation ‘driving forces’ that affect morphogenesis: the calcite supersaturation level of the parental fluid, and the concentration of microbial-derived organic molecules (alginic acid). A key finding of this study is that common naturally-occurring calcite products such as calcite floating rafts, rhombohedral prismatic forms, di-pyramid calcite crystals, spherulitic calcite grains, or vertically stacked spheroidal calcite aggregates, can be related to specific hydrogeochemical contexts, and their physical transitions pinpointed in a phase diagram. By exploring binary or ternary responses to forcing in morphological phase-space, links between calcite growth forms and (palaeo)environmental conditions can be determined. This provides a truly process-oriented means of navigating questions around carbonate precipitate morphogenesis for the future.

HI-Light: A Glass-Waveguide-Based "Shell-and-Tube" Photothermal Reactor Platform for Converting CO2 to Fuels.

SummaryIn this work, we introduce HI-Light, a surface-engineered glass-waveguide-based “shell-and-tube” type photothermal reactor which is both scalable in diameter and length. We examine the effect of temperature, light irradiation, and residence time on its photo-thermocatalytic performance for CO2 hydrogenation to form CO, with a cubic phase defect-laden indium oxide, In2O3-x(OH)y, catalyst. We demonstrate the light enhancement effect under a variety of reaction conditions. Notably, the light-on performance for the cubic nanocrystal photocatalyst exhibits a CO evolution rate at 15.40 mmol gcat−1 hr−1 at 300°C and atmospheric pressure. This is 20 times higher conversion rate per unit catalyst mass per unit time beyond previously reported In2O3-x(OH)y catalyst in the cubic form under comparable operation conditions and more than 5 times higher than that of its rhombohedral polymorph. This result underscores that improvement in photo-thermocatalytic reactor design enables uniform light distribution and better reactant/catalyst mixing, thus significantly improving catalyst utilization.

Mineralogical and Leaching Characteristics of Altered Ilmenite Beach Placer Sands

In order to have a good understanding of the treatment process and improvement on the market value of ilmenite beach placer sands, knowledge of its mineral composition and phase distribution is fundamental. In this study, a combination of characterization techniques including high-resolution scanning electron microscopy (HR-SEM), high-resolution transmission electron (HR-TEM) microscope, and X-ray diffraction (XRD) techniques was used to understand the mineralogical characteristics of ilmenite beach placer sands obtained from Richards Bay, South Africa. The mineral phase constituents of the ilmenite beach placer sands were studied before pre-oxidation and additive leaching in a chloride environment. During observations using XRD and HR-SEM, the ilmenite beach placer sands exhibited signature rhombohedral crystal form and crescentic pits with evidence of alteration phases. The characterized pre-oxidized ilmenite showed the presence of a ferric oxide film deposit of the particles. The leaching characteristics of both raw and pre-oxidized ilmenite was studied in the presence of additives. The leaching efficiency of the pre-oxidized ilmenite in the presence of additives increased by 20% at atmospheric conditions. The characterized residues show the improved amenability of pre-oxidized leach in chloride media. The formation of new phases containing pseudo-rutile indicated crystallographic disintegration by the movement of atoms during dissolution. Some particles retained the crescentic pit and the subangular grain structure; however, the phase changes were observed at the grain boundaries and grain edges. The leached residue’s EDS results still indicated the presence of pseudo-rutile and some minor unreacted oxides such as SiO2, Al2O3, and other trace metals. The trace metals impurities present in the as-received ilmenite were reduced by 80% in the final residue after the leaching.

Maximization of ferromagnetism in LaCoO3 films by competing symmetry

Spin state transition in perovskite cobaltite is delicately controlled by the competition between exchange interaction energy and crystal-field energy. The latter is mainly governed by the change of bond length and bonding angle. Previous work has revealed that the electronic configuration associated with spin state transition in $\mathrm{LaCo}{\mathrm{O}}_{3}$ (LCO) thin films can be effectively modulated by epitaxial strain. However, a systematic study on the spin state transition of $\mathrm{C}{\mathrm{o}}^{3+}$ ions in LCO films with different crystallographic symmetry is still missing. Here, keeping the in-plane strain unchanged, we report that the magnetization of LCO films can be manipulated with crystallographic symmetry. The ultrathin LCO layers, constrained by the cubic substrate, have pseudotetragonal structure and small magnetization. Upon increasing the layer thickness, the monoclinic structure dominates the LCO film and maximizes its ferromagnetism. For the LCO films with a thickness beyond 35 unit cells, the symmetry relaxes gradually towards its rhombohedral bulk form, and meanwhile the magnetization reduces. These results highlight the importance of spin-lattice entanglement in a ferroelastic material and provide a concise way to maximize its functionality using symmetry engineering.

Synthesis and Characterization of Bi2Te3 Thermoelectric Material Using the Cold Pressing and Sintering Method

Research on thermoelectric material that could convert heat into electricity is expanding rapidly in the effort to improve efficiency. Bismuth telluride (Bi2Te3 or BT) is one of the most promising thermoelectric materials in converting waste heat into electrical energy; however, oxidation that occurs in the sintering process often decreases efficiency. In this study, we propose the use of activated carbon in the sintering of Bi2Te3, which is prepared using cold pressing to reduce the oxidation effect. Our results show Bi2Te3 possessing a polycrystalline single-phase rhombohedral form with lattice parameters of a = 4.3841 Å and c = 30.4816 Å. Covering the BT samples with activated carbon during the sintering process improved the power factor of Bi2Te3 from 0.775 × 10−3W.m−1K−2 to 1.0125 × 10−3W.m−1K−2. The improved thermoelectric properties exhibited in the carbon-covered BT samples may be attributed to the absence of an oxidation process occurring during sintering, as indicated by the energy dispersive spectrometry (EDS).

Preparation, Structural and Electrophysical Studies of Ferroelectric Ceramic Samples of the System (1-2x)BiScO3-xPbTiO3-xPbMg1/3Nb2/3O3, 0 ≤ x ≤ 0.50

Ceramic samples of the (1-2x)BiScO 3 -xPbTiO 3 -xPbMg 1/3 Nb 2/3 O 3 system in the whole compositions region x = 0-0.50 were synthesized. According to the X-ray diffraction studies (XRD), the region of solid solutions with perovskite structure formation in the system was determined (x = 0.23-0.50). The presence of a morphotropic phase boundary between tetragonal and rhombohedral forms of solid solutions was established at x = 0.40-0.42. Refined XRD with profile peak analysis established the presence of the additional cubic phase with broadened X-ray reflexes in the solid solutions along with the main phase. It was concluded that the main and additional phases are located in the volume and in the surface layer of ceramic crystallites, respectively. The crystal structures of phases in the polarized and depolarized sample with x = 0.42 were determined by the Rietveld method. It was found that the monoclinic perovskite phase is present in the polarized sample. Temperature-frequency dependences of dielectric permittivity and losses of solid solutions were studied. It was found that the increasing content of BiScO 3 in samples causes a change in their dielectric properties from the usual ferroelectric (FE) in the region (1-2x) = 0-0.08 to ferroelectric-relaxor (FE-R) in the region (1-2x) = 0.08-0.40, and then to the properties of dipole glass with weak correlations in the region (1-2x) > 0.40. It was found that the samples with x = 0.434 and 0.446 below T c = 414 and 445 K spontaneously pass to the FE state. In the samples with x = 0.42 the application of an electric field of 2.0 kV/ cm induces a transition from FE-R to FE state with T c = 350 K, which remains after the field is removed.

Polymorph selection towards photocatalytic gaseous CO2 hydrogenation

Titanium dioxide is the only known material that can enable gas-phase CO2 photocatalysis in its anatase and rutile polymorphic forms. Materials engineering of polymorphism provides a useful strategy for optimizing the performance metrics of a photocatalyst. In this paper, it is shown that the less well known rhombohedral polymorph of indium sesquioxide, like its well-documented cubic polymorph, is a CO2 hydrogenation photocatalyst for the production of CH3OH and CO. Significantly, the rhombohedral polymorph exhibits higher activity, superior stability and improved selectivity towards CH3OH over CO. These gains in catalyst performance originate in the enhanced acidity and basicity of surface frustrated Lewis pairs in the rhombohedral form.

Effect of the thermal annealing on the phase transitions of biogenic CaCO3 nanostructures

The issue of the present research lays its foundation on the proposal of the Crassostrea virginica waste oyster shells (WOS) reuse to obtain calcium carbonate powder (CaCO3) and calcium hydroxide (Ca(OH)2) nanostructured, using thermal annealing treatments. The oysters shells were subjected to a previous physical grinding process to decrease their size (smaller sizes 0.074 mm). The parameter studied was the effect of annealing temperature (500, 700 and 900 °C in air atmosphere) on the structural properties and morphology of the powders by FTIR, XRD, SEM and HRTEM. The X-ray diffraction results indicate that the WOS in their natural state and thermally annealed at 500 °C have two phases of CaCO3 the rhombohedral form for calcite with crystallite size around 24 nm and aragonite traces in orthorhombic phase. At 700 °C, the WOS powder is transformed into calcium hydroxide, also known as portlandite (Ca(OH)2), attributed to the absorption of water released during the thermal decomposition of CaCO3. This crystalline phase does not change when the temperature increases to 900 °C. The SEM and HRTEM analysis of WOS powders reveals that with a thermal annealing treatment it is possible to obtain nanostructured CaCO3. FTIR analysis demonstrates the biogenic origin of CaCO3, due to amide groups. The nanostructured CaCO3 obtained by grinding and thermal annealing of WOS, can be used as drying agent, or as additive in ceramic and glass.
 The issue of the present research lays its foundation on the proposal of the Crassostrea virginica waste oyster shells (WOS) reuse to obtain calcium carbonate powder (CaCO3) and calcium hydroxide (Ca(OH)2) nanostructured, using thermal annealing treatments. The oysters shells were subjected to a previous physical grinding process to decrease their size (smaller sizes 0.074 mm). The parameter studied was the effect of annealing temperature (500, 700 and 900 °C in air atmosphere) on the structural properties and morphology of the powders by FTIR, XRD, SEM and HRTEM. The X-ray diffraction results indicate that the WOS in their natural state and thermally annealed at 500 °C have two phases of CaCO3 the rhombohedral form for calcite with crystallite size around 24 nm and aragonite traces in orthorhombic phase. At 700 °C, the WOS powder is transformed into calcium hydroxide, also known as portlandite (Ca(OH)2), attributed to the absorption of water released during the thermal decomposition of CaCO3. This crystalline phase does not change when the temperature increases to 900 °C. The SEM and HRTEM analysis of WOS powders reveals that with a thermal annealing treatment it is possible to obtain nanostructured CaCO3. FTIR analysis demonstrates the biogenic origin of CaCO3, due to amide groups. The nanostructured CaCO3 obtained by grinding and thermal annealing of WOS, can be used as drying agent, or as additive in ceramic and glass.

Changes in the physicomechanical properties of materials of carbonate hardening on the basis of lime and lime stone systems with the course of time

Abstract The paper presents studies of changes in the physicomechanical properties of test samples based on lime and limestone compositions hardening in an environment with a high concentration of carbon dioxide, after their storage in natural conditions for 24 months. The studies have shown that there is an increase in the strength of the samples, both as a result of the transition of portlandite to calcite during forced carbonization, and as a result of recrystallization of the scalerocalcite into rhombohedral form during storage of the samples in natural conditions.