Chemical changes and phase analysis of OPC pastes carbonated at different CO<Subscript>2</Subscript> concentrations

Chemical changes and phase analysis of OPC pastes exposed to accelerated carbonation using different concentrations of CO₂ (3%, 10% and 100%) have been undertaken and compared with those of natural carbonation (≅0.03%). ²⁹Si Magic Angle Spinning-Nuclear Magnetic Resonance (²⁹Si M.A.S-N.M.R), Thermogravimetric analyses (TG) and X-Ray Diffraction (XRD) have been used for characterisation. The carbonation of the samples has resulted in a progressive polymerisation of CSH that leads to formation of a Ca-modified silica gel and calcium carbonate. The carbonation of CSH and portlandite occurs simultaneously and the polymerisation of the CSH after carbonation increases with the increase in concentration of CO₂. When ≅0.03% and 3% CO₂ are used_, CSH gel with a lower Ca/Si than that of the uncarbonated sample, and quite similar for both samples remained. When carbonating at 10% and 100% of CO₂, the CSH gel completely disappears. For every condition, a polymerised Ca-modified silica gel is formed, as a result of the decalcification of the CSH. From these results it can be deduced that among the different concentrations of CO₂ tested, carbonation up to a 3% of CO₂, (that is to say, by a factor of 100) results in a microstructure much more similar to those corresponding to natural carbonation at ≅0.03% CO₂ than those at the 10% and 100% concentrations.     

Hydration kinetics and microstructural development in the 3 CaO. Al2O3-CaSO4.2H2O-CaCO3-H2O system

Hydration and microstructural characteristics of mixtures containing C₃A and CaSO₄.2H₂O (0, 12.5, 25%) with or without addition of CaCO₃ (0, 12.5, 25%) are followed by X-ray diffraction, differential scanning calorimetry, differential thermogravimetry, scanning electron microscopy and conduction calorimetry. Depending on the length of hydration, products formed at different periods from 5 min to 3 days consisted of hexagonal calcium aluminate hydrate, cubic calcium aluminate hydrate, calcium monocarboaluminate hydrate, etrringite, calcium monosulfoaluminate hydrate and possibly a solid solution of hexagonal calcium aluminate hydrate with monocarbo-and sulfo-aluminate hydrates. Calcium carbonate retards or suppresses the formation of the cubic aluminate hydrate in the hydration of C₃A. It accelerates formation of ettringite and its conversion to the monosulfoaluminate phase when added to the C₃A+gypsum+H₂O mixture.

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Résumé On étudie l’hydratation et les caractéristiques de la microstructure de mélanges renfermant du C₃A et du CaSO₄.2H₂O (0, 12,5 et 25% avec ou sans addition de CaCO₃ (0, 12,5 et 25%) à l’aide de la diffraction X, de la calorimétrie par balayage différentiel, la thermogra-vimétrie différentielle, la microscopie électronique à balayage et la calorimétrie par conduction. Selon la durée de l’hydration, il se forme à différents intervalles allant de 5 minutes à 3 jours de l’hydrate d’aluminate de calcium hexagonal, de l’hydrate d’aluminate de calcium cubique, de l’hydrate de monocarboaluminate de calcium, de l’ettringite, de l’hydrate de monosulfoaluminate de calcium et parfois une solution solide d’hydrate d’aluminate de calcium hexagonal avec hydrate de monocarbo et sulfoaluminate. Le carbonate de calcium retarde ou empêche la formation d’hydrate d’aluminate cubique dans l’hydratation de C₃A. Il accélère la formation d’ettringite et sa conversion en phase monosulfoaluminate lorsqu’il s’ajoute au mélange C₃A+platre+H₂O.

     

Carbonate binders by “cold sintering” of calcium carbonate

The solubility of calcium carbonate (limestone) particles depends on the amount of CO₂ dissolved in the water, which is a function of temperature and the pressure of CO₂ that is in equilibrium with water. At a constant temperature, increasing CO₂ pressure increases the solubility of CaCO₃, and decreasing CO₂ pressure favours the crystallisation of CaCO₃. This dissolution–crystallisation behaviour of CaCO₃ can be used in the development of carbonate binders—a process called “cold sintering”—of limestone. This paper examines the effect of a range of parameters on the cold sintering process of limestone powder. The parameters studied are CO₂ gas pressure (atmospheric, 10 atm and 35 atm), exposure time (0–90 min), post-compaction pressure (10 and 15 MPa) and compact pressing time (10–60 min). The water/limestone powder ratio was kept constant at 0.2 (by weight). The compressive strength of the limestone compacts was used as a measure of the efficiency of the carbonate binder formation process, and scanning electron microscopy was used to study the microstructural developments. The results show that carbonate binders can be produced by cold sintering of limestone powder. Exposure of limestone compacts to high CO₂ pressure followed by post-compaction at high mechanical pressure, enhances the strength of the compact. From the microstructural data, it is evident that newly formed calcium carbonate crystal growths are responsible for the strength improvements observed. The amount of water that is used in the limestone powder mixture is one factor that controls the quantity of the cementing phase. Future work has to be focussed on the application of methods to further increase the solubility of calcium carbonate, as the amount of calcium carbonate available for recrystallisation is important in producing a strong binder.     

Carbonate binders: Reaction kinetics,strength and microstructure

This study focussed on the synthesis of calcium carbonate binders, in situ, from the reaction between hydrated lime and carbon dioxide (CO₂). The aim was to establish the characteristics of the calcium carbonate binders that are associated with its strength, which was considered as an indicator of binder performance. The role of the parameters that are known to play an important part in the kinetics of hydrated lime carbonation processes, in changing the strength of a binder was examined in detail.The parameters identified were CO₂ gas pressure, exposure time and the initial degree of compaction of raw material. All hydrated lime mixtures were prepared at a constant water/solid ratio of 0.25. The hydrated lime compacts made at a range of compaction water/solid ratio (W/S) of 0.25. The hydrated lime compacts made at a range of compaction pressures (0.65–6.0 MPa) were exposed to different CO₂ gas pressures (ambient to 2 MPa) for different periods of time. The resulting products were tested for the amount of Ca(OH)₂ that had converted to carbonate, and for compressive strength. A microstructural analysis of the products was carried out using scanning electron microscopy.The rate of Ca(OH)₂ conversion to carbonate seemed to be enhanced with increasing gas pressure, but it decreased with increasing compaction of the initial mixture. It was revealed that the crystalline state and the morphology of the carbonate formed, rather than the degree of conversion of calcium hydroxide into carbonate, is highly critical to the strength of the binder. The study concluded that in the development of calcium carbonate binder, it is important to meet the experimental conditions that favour the crystallisation of calcium carbonate.     

Effect of initial pH on formation of hollow calcium carbonate particles by continuous CO2 gas bubbling into CaCl2 aqueous solution

Hollow calcium carbonate (CaCO₃) particles are synthesized by continuous bubbling CO₂ gas into CaCl₂ aqueous solution. In order to study the formation kinetics of the hollow CaCO₃ particles, the transmitted light strength of the CaCl₂ solution during the reaction was measured automatically with an on-line monitoring. It was verified that the pH for the formation of the hollow particles depended on the reaction temperature. Higher initial pH of the CaCl₂ solution resulted in the higher ratio of hollow particles as compared to that obtained at lower pH.     

On the theory of formation of gas hydrate in partially water-saturated porous medium when injecting methane

We present a planar one-dimensional theoretical model and numerical solutions for the process of the formation of methane gas hydrate by injecting gas into a porous reservoir partially saturated with water. The case where the intensity of formation of the gas hydrate is limited by the diffusion of gas through a hydrate layer formed between water and gas in the pore channel core is considered. Within this process, the kinetics of hydrate formation is determined by empirical parameter D, having the dimension of a diffusion coefficient (m²/s). The effect of the value of this parameter on the characteristics of the hydrate formation process is studied depending on the parameters that determine the initial state of the porous reservoir and its porosity and permeability characteristics. The equilibrium mechanism of hydrate formation is considered, which is a limit adopted by the diffusion pattern that corresponds to the case of D → ∞.     

Formation and thermal decomposition of rare-earth carbonates

The preparation of carbonate-containing rare-earth compounds and their thermal decomposition in 0%–17% CO₂/N₂ gas streams has been studied. Three types of rare-earth carbonate or hydroxycarbonate were produced by precipitation; with ammonium bicarbonate as the precipitant, La₂ (CO₃)₃,CeOCO₃ and Ln (OH) _x (CO₃) _y (Sm, Tb, and Yb) were obtained, whereas with Na₂CO₃, only the normal carbonate, Ln₂(CO₃)₃ (La, Sm, Tb, Er and Yb) was found. Temperature programmed decomposition studies revealed that the normal carbonate decomposed stepwise via a dioxocarbonate, Ln₂O₂CO₃, to the oxide. In contrast, the hydroxycarbonates decomposed directly to the oxide. The presence of CO₂ during heating had minimal effect on the decomposition of Ln₂(CO₃)₃ to La₂O₂CO₃ but raised significantly the decomposition temperature of Ln₂O₂CO₃ to the oxide. As CO₂ is a major product of the rare-earth oxide catalysed oxidative coupling of methane, these observations indicate that the state of catalyst carbonation will be dependent on the reaction temperature, overall catalyst selectivity and preparative method.     

Ocean acidification alters the material properties of Mytilus edulis shells

Ocean acidification (OA) and the resultant changing carbonate saturation states is threatening the formation of calcium carbonate shells and exoskeletons of marine organisms. The production of biominerals in such organisms relies on the availability of carbonate and the ability of the organism to biomineralize in changing environments. To understand how biomineralizers will respond to OA the common blue mussel, Mytilus edulis, was cultured at projected levels of pCO₂ (380, 550, 750, 1000 µatm) and increased temperatures (ambient, ambient plus 2°C). Nanoindentation (a single mussel shell) and microhardness testing were used to assess the material properties of the shells. Young''s modulus (E), hardness (H) and toughness (K_IC) were measured in mussel shells grown in multiple stressor conditions. OA caused mussels to produce shell calcite that is stiffer (higher modulus of elasticity) and harder than shells grown in control conditions. The outer shell (calcite) is more brittle in OA conditions while the inner shell (aragonite) is softer and less stiff in shells grown under OA conditions. Combining increasing ocean pCO₂ and temperatures as projected for future global ocean appears to reduce the impact of increasing pCO₂ on the material properties of the mussel shell. OA may cause changes in shell material properties that could prove problematic under predation scenarios for the mussels; however, this may be partially mitigated by increasing temperature.     

Linear antenna microwave plasma CVD deposition of diamond films over large areas

Diamond thin films were grown by linear antenna microwave plasma CVD process over large areas (up to 20 × 10 cm²) from a hydrogen based gas mixture. The influence of the gas composition (H₂, CH₄, CO₂) and total gas pressure (0.1 and 2 mbar) on the film growth is presented. For CH₄/H₂ gas mixtures, the surface crystal size does not show dependence on the methane concentration and total pressure and remains below 50 nm as observed by SEM. Adding CO₂ (up to 10%) significantly improves the growth rate. However, still no significant change of morphology is observed on films grown at 2 mbar. The crucial improvement of the diamond film purity (as detected by Raman spectroscopy) and crystal size is found for deposition at 0.1 mbar. In this case, crystals are as large as 500 nm and the growth rate increases up to 38 nm/h.     

In situ preparation of calcium carbonate films

The in situ preparation of calcium carbonate films in an ultra high vacuum (UHV) is inhibited by the decomposition of CO₂ molecules at the surface and the absence of CO₂ bulk diffusion. Therefore, it is not possible to prepare such films simply by CO₂ exposure to a calcium layer.We investigated different approaches for the preparation of CaCO₃ films in an UHV. Among these, only the simultaneous evaporation of Ca atoms in a mixed O₂ and CO₂ atmosphere is able to produce well defined stoichiometric calcium carbonate films. Metastable Induced Electron Spectroscopy, Ultraviolet Photoelectron Spectroscopy and X-ray Photoelectron Spectroscopy are employed to verify quality and purity of the films.     

Effects of ionic surfactants on methane hydrate formation kinetics in a static system

This paper reports an experimental study of the kinetics of methane hydrate formation in the presence of ionic surfactants with equal carbon chain length, such as sodium dodecyl sulfate (SDS), dodecylamine hydrochloride (DAH), dodecyltrimethylammonium chloride (DTAC) and N-dodecylpropane-1,3-diamine hydrochloride (DN₂Cl). Methane hydrates were formed at 274 K with incipient gas pressure of 15 MPa in an unstirred isochoric/isothermal reactor containing aqueous solutions at different initial surfactant loadings. It was found that addition of DTAC had little effect on methane hydrate formation whereas SDS, DAH, and DN₂Cl had pronounced promoting effects. This result coincides with the fact that the Krafft point of DTAC is below 273 K and those of SDS, DAH, and DN₂Cl are near room temperature. At a given initial surfactant loading, the effectiveness of the surfactants for reducing the induction time of methane hydrate formation followed the order of SDS > DAH > DN₂Cl. SDS also gave higher hydrate growth rates than DAH and DN₂Cl. At 1000 and 2000 ppm, however, DN₂Cl gave slightly higher final methane uptake than SDS. It was also found that during the nucleation or induction period, addition of SDS, DAH and DN₂Cl instead of DTAC, considerably reduced methane mole fraction in the liquid phase. Possible promotion mechanisms of surfactants during the hydrate nucleation period were discussed.     

Electrochemical Behaviour of Ce0.9Gd0.1O2−δ SOFC‐Anodes

In order to compare the electrochemical performance of Ce_0.9Gd_0.1O_2−δ (CGO) in various fuels, impedance spectroscopy measurements were carried out in the atmospheres containing H₂, CO, CO₂, CH₄, N₂ at various compositions, in the temperature range 650°C to 850°C. Ohmic loss and polarization resistance were derived from impedance spectroscopy measurements. The stability at different temperatures of the anode was also investigated in 9%H₂/91%N₂ humidified with 3% H₂O. The microstructure of the anode before and after degradation test was analysed by SEM. These investigations indicated similarities in the impedance and the activation enthalpies in hydrogen/water, methane/water and CO/CO₂ atmospheres. No indications of methane cracking leading to carbon formation were found.     

Synthesis and characterization of cellulose acetate–calcium carbonate hybrid nanocomposite

A hybrid nanocomposite composed of calcium carbonate (CaCO₃) and cellulose acetate (CA) was fabricated by bubbling CO₂ gas into the mixture of CA and Ca(OH)₂ solution. Cellulose acetate–calcium carbonate (CA–CC) nanocomposite was characterized by spectral, thermal and optical methods. FTIR and XRD analysis confirmed the formation of the hybrid nanocomposite and XRD confirmed the formation of CaCO₃ with calcite polymorph. Thermal analysis showed CA–CC nanocomposite has better thermal stability than pristine CA. The CaCO₃ nanoparticles were in sphere shape with 100–1000 nm diameter.     

Hydroxyapatite-calcium carbonate ceramic composite materials

Hydroxyapatite/calcium carbonate (CC) composite powders containing up to 50 wt % CO₃²⁻, have been prepared via precipitation from aqueous solutions. According to chemical analysis data, the CO₃²⁻ content of the powders coincides with the intended one over the entire composition range studied. With increasing CO₃²⁻ content, the specific surface area of the powders decreases because of the formation and growth of large needlelike CC crystals up to 1.5 μm in size. The addition of low-melting-point alkali carbonates to the starting powder mixture reduces the sintering temperature of the powders to below 720°C, thereby preventing the thermal decomposition of CC. The highest bending strength, up to 76 MPa, is offered by the materials with the lowest CC content, about 20 wt %, and an average crystal size of 100 nm. The solubility of the composites gradually increases with CC content. In vitro experiments with a human fibroblast model (MTT test) demonstrate that the composites have zero cytotoxicity.     

Improved flow conditions in diamond hot filament CVD—Promising deposition results and gas phase characterization by laser absorption spectroscopy

A hot filament plant for chemical vapor deposition of crystalline diamond featuring new operating stages has been built. It allows (i) a separate methane feed locally at substrate position and (ii) supplying a forced gas flow towards the substrate. To understand the effect of these two features on diamond growth, the results of systematic diamond growth experiments are discussed. To reveal the effects of these features on the gas phase, infrared tunable diode laser absorption spectroscopy (IR-TDLAS) was employed. Using a forced gas flow showed a remarkable increase in the diamond growth rate of a factor >6 compared to standard coating setups. By lowering the methane content in the forced flow diamond quality factors >95% were achieved. IR-TDLAS showed an increase of all measured carbon-containing species CH₄, C₂H₂, CH₃ and CO when applying the forced flow. The mass transport dominated by diffusion in the standard setup shifts to a convective gas transport in the forced flow setup. The induced laminar flow causes a more effective transport of the growth species to the substrate and leads to higher growth rates. Application of feeding methane locally at substrate position leads to exceptionally high growth rates (0.68 μm/h) at correspondingly high diamond quality (91%). For this, the methane content has to be lowered, though, which at the same time leads to a more homogenous deposition lateral on the surface. From the IR-TDLAS gas phase measurements, a more effective precursor dissociation, a higher CH₃ density and a rise in the CH₃?C₂H₂ ratio above the substrate surface can be derived.     

Hydrous oxides of titanium: cation exchange properties and kinetics of exchange

The ion exchange properties of hydrous titania gels of different particle sizes, precipitated from titanous chloride through the agency of ammonium carbonate and hydroxide have been studied. Such studies were carried out under acidic and alkaline conditions with respect to Cu²⁺, Ni²⁺, Co²⁺ and Cr³⁺ ions.In the case of gels precipitated by ammonium carbonate, oxygen gas was used as the oxidizing agent whereas with ammonium hydroxide as precipitant, oxidation was performed with hydrogen peroxide.Ion exchange capacities were determined by visible spectrophotometry. Increasing the pH of preparation lead to an increase in exchange capacities of the hydroxide precipitated gels that are characterized to be mesoporous. Such an increase is not observed in the case of carbonate precipitated microporous gels. It is shown that in the latter case the NH ₄ ⁺ ions generated by the initial interaction of (NH₄)₂CO₃ with the acidic titanous chloride lead to the formation of titania exchangers that are predominantly in the ammonium form. The textural characteristics of the exchanger resulting from different conditions of preparation is a significant contributing parameter to the resulting data.Ageing of the microporous titania samples markedly reduces the exchanger capacity of the smaller Ni²⁺ ions but increases that of the bulkier Cr³⁺ as a result of the presence of some wide pores that appear upon agglomeration. The presence of Cr³⁺ ions in the hydroxo form in solution seems to inhibit its exchange with the appropriate surface species.Studies on the kinetics of exchange with respect to the Ni²⁺ ions seem to indicate that a particle diffusion mechanism is partly or completely responsible for the rate of exchange.     

Electron Paramagnetic Resonance Spectra of Carbonate Centers in Natural Apatites

The EPR spectra of carbonate centers in apatites of different compositions are measured in the range 77–290 K. The results indicate that the F^– : OH^– ratio on the 6₃ axis and the state of constitutional water (H₂O molecules) determine vibronic interactions in the CO₂ ^–, CO₃ ^–, and CO₃ ^3– centers and the parameters of their EPR spectra. Depending on the molecular water and OH^– contents, the spectra of the carbonate centers are isotropic or anisotropic, due to the dynamic or static Jahn-Teller effect. Computer simulation is used to estimate the contributions of the centers to the EPR spectrum.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 5, 2005, pp. 585–591.Original Russian Text Copyright © 2005 by Gilinskaya.     

Effect of Inorganic Anions on the Physicochemical State of Radionuclides in Process Freshwater Basins at the Mayak Production Association

Published data on the speciation of microcomponents and radionuclides (calcium, strontium, plutonium, and americium) in the water and bottom sediments of selected process water basins at the Mayak Production Association were discussed in terms of the model of the sorption behavior of a sorbate existing in several forms. It was shown that distribution of calcium, ⁹⁰Sr, and natural isotopes of strontium does not contradict the views on formation of Ca^{2 +}, CaSO₄ ⁰, Sr^{2 +}, SrSO₄ ⁰, Sr[SO₄]₂ ^{2 -} ionic and molecular species and of colloidal sulfate (CaSO₄·2H₂O) at selected sites of the water basin. By combining these factors it is possible to explain the variation in the distribition coefficients of Ca, Sr, Pu, and Am in the water-bottom sediments system in terms of the model of ion-exchange sorption in the presence of nonsorbable nonequilibrium colloid.     

Surfactant assisted synthesis of precipitated calcium carbonate nanoparticles using dolomite: Effect of pH on morphology and particle size

Synthesis of nanomaterials from readily available minerals for industrial applications is a growing research area. Understanding the causes of their properties becomes handy in utilization. In this study, an effective sucrose solution based method was employed for the extraction of calcium from dolomite to synthesize precipitated calcium carbonate nanostructures with different morphologies and sizes. It was found that 30% (w/v) sucrose solution extracted approximately 91% of calcium from dolomite forming a calcium-sucrate complex. Carbonation was achieved by CO₂ bubbling and aqueous sodium carbonate addition. Precipitation was performed under different pH values of 7.5, 10.5 and 12.5 in the absence of an anionic surfactant and in the template of sodium dodecyl sulfate (SDS)/calcium-sucrate at pH 12.5. It was found that CO₂ bubbling slightly promotes smaller particles. The anionic surfactant enables particle size and agglomeration reduction while introducing some hydrophobicity. The smallest particles were achieved at a range of 40–55 nm in the presence of SDS/sucrose template and were of spherical morphology. By changing the pH, a tendency to form different polymorphs and shapes of calcium carbonate was observed.     

Doped nanocrystalline calcium carbonate phosphates

The formation of nanocrystalline calcium carbonate phosphates doped with Fe²⁺, Mg², Zn²⁺, K⁺, Si⁴⁺, and Mn²⁺ has been studied by X-ray diffraction, IR spectroscopy, differential thermal analysis, and energy dispersive X-ray fluorescence analysis. The results indicate that the synthesis involves the formation of hydroxy carbonate complexes from the three calcium carbonate polymorphs (calcite, vaterite, and aragonite) in a solution of ammonium chloride and ammonium carbonate, followed by reaction with orthophosphoric acid. This ensures the preparation of a bioactive material based on chlorophosphates, octacalcium hydrogen phosphate, and calcium chloride hydroxide phosphates containing cation vacancies. Particle-size analysis data show that the materials contain nanoparticles down to 10 nm in size. Heat treatment of the doped calcium carbonate phosphates produces calcium hydroxyapatite containing cation vacancies, which can be used as a bioactive ceramic.