Calcium Carbonate Phase Research Articles

Reaction of pseudowollastonite with carbonate-bearing fluids: Implications for CO2 mineral sequestration

The kinetics of silicate carbonation in aqueous solutions are typically sluggish, especially at neutral to alkaline conditions. This hampers the complete understanding of the mechanisms and parameters that control mineral carbonation during carbon capture and storage (CCS). Here we study the hydrothermal dissolution and carbonation of pseudowollastonite (psw; α-CaSiO3), one of the most reactive silicates known, under a range of geochemical conditions ranging from acidic to strongly alkaline pH, presence/absence of different background alkali metal ions and carbonate sources (K2CO3 and Na2CO3, pH ~13, or NaHCO3 and KHCO3, pH ~9). We show that in addition to amorphous silica precipitation, the formation of secondary Na + Ca- or K + Ca-silicates in the presence of Na+ and K+ background ions, respectively, fosters the progress of psw carbonation. However, the formation of Ca-containing secondary crystalline silicates and Ca-containing amorphous silica is shown to be a strong handicap for a fully effective carbonation. In all cases a higher conversion into CaCO3 (up to ~70 mol%) is achieved when using bicarbonate salts (i.e., lower initial pH). By using a reactor with a pressurized CO2-solution, with and without Na+ or K+ background ions, rapid and nearly complete conversion of psw with a CaCO3 yield ~92 mol% is achieved because, in addition to the initial low pH (~3.7) that favored α-CaSiO3 dissolution, abundant Ca-free non-passivating amorphous silica formed along with calcite. These results imply that the presence (e.g., use of sea water during CO2 injection or mixing with saline formation solutions) or the release of different alkali metal ions (e.g., after feldspar and/or basaltic glass dissolution) in combination with a reaction-induced pH increase during in situ CCS scenarios may strongly limit carbonation due to the capture of alkaline-earth metals in secondary silicates and a reduction in reaction rates. In turn, our results show that the high conversion achieved in pure CO2-aqueous systems, while relevant for ex situ CCS, may not reflect the actual conversion in multicomponent natural systems following reactive transport during in situ CCS. Moreover, the precipitation of secondary silicate and calcium carbonate phases have a direct cementing effect, which could be detrimental for in situ CCS, as it would likely reduce host rock permeability, but would be relevant and beneficial for the setting of novel CaSiO3-based non-hydraulic cements with reduced CO2 footprint.

Elucidation of Calcite Structure of Calcium Carbonate Formation Based on Hydrated Cement Mixed with Graphene Oxide and Reduced Graphene Oxide.

In this study, the carbonation of Portland cement by direct chemical interaction with graphene oxide (GO) and reduced graphene oxide (rGO) at 7 and 28 days was examined. During the carbonation reaction, the calcium-bearing phases (calcium hydroxide, calcium silicate hydrate, and ettringite) formed calcium carbonate polymorphs, along with amorphous silica gel, gypsum, and alumina gel. These reaction products were examined using XRD (X-ray diffraction), XPS (energy-dispersive spectrometry), and FTIR (Fourier transform infrared). XRD patterns showed that the intensities of the calcium hydroxide and calcium carbonate peaks in the hydrated cement mixed with GO and/or rGO are higher than the corresponding peaks in the hydrated cement without any additives. The morphology of the reaction products was also characterized by SEM (scanning electron microscopy) measurements, which showed that a needle-like phase of calcium carbonate develops on the hydrated cement. The obtained microstructure parameters enabled the development of a more precise carbonation model.

Setting behavior and bioactivity assessment of calcium carbonate cements

AbstractCalcium carbonate cements have emerged in the last few years as an attractive candidate for biomedical applications. They can be easily prepared by mixing water with two metastable calcium carbonate phases––amorphous calcium carbonate (ACC) and vaterite––which (re)crystallize into calcite during setting reaction. The transformation kinetics (and therefore the final surface cement composition) strongly depends on the initial mixture design and is controlled by the dissolution of ACC, whereas calcite nucleation typically controls their recrystallization in fluid batch experiments. Novel compositions are presented in this paper by incorporating organic molecules as a proxy to test their capability to carry on other biomolecules like proteins or antibiotics. The hardened samples are microporous and show excellent bioactivity rates, although their mechanical properties still remain poor. However, this would not be a handicap for in‐vivo applications such as bone filling, especially in low mechanical stress locations.

Physical–chemical and microstructural study of shells of the Lucina Pectinata species

Lucina pectinata, popularly called Lambreta, is a delicacy very consumed in Bahian cuisine. The shell is composed of nacreous layers formed by keratin, collagen, and elastin, interspersed with layers of calcium carbonate, predominantly calcite, and aragonite phases, which provides protection to the mollusk. As these shells do not readily degrade, provoking environmental imbalance due to their accumulation, this work aims to evaluate the physicochemical and microstructural properties of shells of the species L. pectinata, focusing on the reuse of the solid residue generated by the consumption of this mollusk. The shells were washed, crushed, and subjected to chemical treatment and dried. Subsequently, the physical–chemical and microstructural properties of the samples LN—in nature and LB were treated with sodium hypochlorite. The samples were investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), zero load potential (pHpcz), pH determination, moisture content (%), ash content (%), specific surface area, and pore volume by the BET method, scanning electron microscopy (MEV), thermogravimetric/derivate thermogravimetry (TG/DTG), and differential thermal analysis (DTA). The results revealed that calcite and argonite phases are present in the two samples with high calcium carbonate content (95%) and that thermal decomposition occurs in the range of 575 to 810 °C, producing CaO and CO2. Treatment with sodium hypochlorite (sample LB) promoted leaching in the solid by dragging possible lamellae, which culminated in superficial erosion of solids, resulting in a better cavity and pore distribution profile on the surface, besides increasing the specific surface area in relation to sample LN.

Evaluating chemical-scale-inhibitor performance in external magnetic fields using a dynamic scale loop

Monoethylene glycol (MEG) is extensively used in oil and gas pipelines to prevent the formation of gas hydrates. However, the injection of MEG may inadvertently contribute to mineral-ion precipitation through reduced solubility of ionic species and operation under alkaline conditions for corrosion control. As a result, the injection of chemical scale inhibitors (CSI) to manage scaling is often required. Non-chemical inhibitors, such as external magnetic fields (MFs) have also been shown to inhibit scaling in aqueous solutions. With this in mind, we investigated the combined application of CSIs and MF treatment to study the formation of scale under conditions representative of a MEG regeneration system's reboiler, a system under a high risk of scale formation. Two commercial CSIs were examined in the presence and absence of a 0.650-T external MF using a dynamic-scale-loop device. The CSIs were injected into ionic solutions containing 1656.9 ppm calcium ions and 2628 ppm carbonate ions in aqueous MEG (80 vol%), and experiments were performed at 130 °C and pH 9.5. Despite the presence of a CSI, the MF promoted scale formation and afforded a stable calcium carbonate phase morphology at high MEG concentrations. Moreover, the zeta potential under the applied MF was lower than that in the absence of the MF, which led to greater scaling, implying that the magnetic field had an ionic interfacial effect. These results provide significant insight into the effect of a MF on scale formation, irrespective of the use of a CSI, and help to interpret the effects of various treatments for scale removal or the prevention of scale formation during the MEG regeneration process.

Control of crystalline phase and morphology of calcium carbonate by electrolysis: Effects of current and temperature

Abstract Calcium carbonate (CaCO3) can show various properties related to its different crystalline phases and is therefore a useful material for various applications. Wet processes are known to be suitable for preparing metastable CaCO3 polymorphs. Electrolysis has been proposed as a preparation method at ambient conditions. Although several electrolytic approaches have been reported, the effects of the applied current and temperature of the electrolyte on the crystalline phase and morphology of CaCO3 remain unclear. In the present study, we attempted the electrochemical preparation of CaCO3 particles under various electrolysis conditions and discuss the mechanism of CaCO3 particle formation. The crystalline phases and morphologies of the CaCO3 precipitates markedly changed depending on the applied current and method of cooling the electrolyte. We assume that these factors were governed by the degree of change in temperature, supersaturation, and pH of the electrolyte that were induced by differences in the electrolysis current.

Calcareous scales deposited in the organic coating defects during artificial seawater cathodic protection: Effect of zinc cations

In this work, the effect of Zn2+ cations on the calcareous scales during the cathodic protection of mild steel was studied. The Zn2+ cations were introduced into the epoxy and alkyd coatings in the form of zinc chloride. Also, the influence of Zn2+ cations was investigated in the zinc -rich primer coated mild steel. The study was conducted in the electrolyte and coating phases. In the first stage, pure calcareous scales deposited from artificial seawater (AS) were compared with zinc containing calcareous scales deposited from the zinc containing AS solution. For this purpose, scanning electron microscopy equipped with the energy dispersive spectroscopy and X-ray diffraction analyses were used to characterize the calcareous scales. Also, chronoamperometric, electrochemical impedance spectroscopy, and potentiostatic polarization were conducted to investigate the effect of Zn2+ cations on the calcareous scales. Results indicated that the Zn2+ cations have influenced the calcareous scales through altering calcareous deposited phases and phase proportions. Moreover, according to the electrochemical results, with increase in the Zn2+ cations concentration, an increase in calcareous scales inhibition was observed. In the coating phase, the effect of Zn2+ cations presented in the organic coatings and zinc-rich primer was studied. To evaluate this effect, the coated samples with an artificial defect were exposed to AS. Results showed that the presence of Zn2+ cations in the coatings led to a decrease in the brucite content of the scales and stabilization of the hydrated calcium carbonate phases over aragonite phases. Furthermore, presence of the Zn2+ cations in the form of zinc-rich primer leads to the brucite phase elimination in the calcareous scales.

Growth and regrowth of adult sea urchin spines involve hydrated and anhydrous amorphous calcium carbonate precursors

In various mineralizing marine organisms, calcite or aragonite crystals form through the initial deposition of amorphous calcium carbonate (ACC) phases with different hydration levels. Using X-ray PhotoEmission Electron spectroMicroscopy (X-PEEM), ACCs with varied spectroscopic signatures were previously identified. In particular, ACC type I and II were recognized in embryonic sea urchin spicules. ACC type I was assigned to hydrated ACC based on spectral similarity with synthetic hydrated ACC. However, the identity of ACC type II has never been unequivocally determined experimentally. In the present study we show that synthetic anhydrous ACC and ACC type II identified here in sea urchin spines, have similar Ca L2,3-edge spectra. Moreover, using X-PEEM chemical mapping, we revealed the presence of ACC-H2O and anhydrous ACC in growing stereom and septa regions of sea urchin spines, supporting their role as precursor phases in both structures. However, the distribution and the abundance of the two ACC phases differ substantially between the two growing structures, suggesting a variation in the crystal growth mechanism; in particular, ACC dehydration, in the two-step reaction ACC-H2O → ACC → calcite, presents different kinetics, which are proposed to be controlled biologically.

Phase-specific bioactivity and altered Ostwald ripening pathways of calcium carbonate polymorphs in simulated body fluid.

Calcium carbonate is an abundant biomineral, and already archeological records demonstrate its bioactivity and applicability for osseo-integrative implants. Its solubility, which is generally higher than those of calcium phosphates, depends on its polymorph turning calcium carbonate into a promising biomaterial with tunable bioresorption rate. However, the phase-dependent bioactivity of calcium carbonate, i.e., its osteoconductivity, is still insufficiently characterized. In this study, we address this issue by monitoring the behavior of the four most important calcium carbonate phases, i.e., calcite, aragonite, vaterite, and amorphous calcium carbonate, in simulated body fluid solution at 37 °C. Our results demonstrate that the thermodynamically stable calcite phase is essentially inert. In contrast, the metastable phases aragonite and vaterite are bioactive, thus promoting the formation of calcium phosphate. Amorphous calcium carbonate (ACC) shows prominent bioactivity accompanied by pronounced redissolution processes. Mg-stabilized ACC was additionally tested since its increased stability eases formulation and handling in future applications. It is highly bioactive and, moreover, the additional release of Mg promotes cell viability. Overall, our results demonstrate that bioactivity of calcium carbonate is phase-dependent, allowing tailored response and bioactivity of future calcareous biomaterials. Our results also reveal that phosphate ions strongly interfere with Ostwald–Lussac step ripening of calcium carbonate, kinetically stabilizing metastable polymorphs such as vaterite and aragonite; this is a distinctive feature of the calcium carbonate mineral system which clearly has to be considered in future applications of calcium carbonate as a bioceramic.

Biomimetic mineralization of the carbonates regulated by using hydroxypropyl methylcellulose macromolecules as organic templates

Control of the morphology of carbonate crystals plays an indispensable role in mineralization process. We investigated the relationship between HPMC concentration and different molar ratios of Mg/Ca on the carbonate crystallization by monitoring crystal polymorph, crystal size, and morphology. In this experiment, the total molars of [Mg2+] and [Ca2+] were 50 mM. The results demonstrated that different molar ratio of Ma/Ca primarily can help different morphology form in multidimensional spline space, for instance, lens-like calcite, spindle shaped aragonite and polyhedron nesquehonite. Moreover, the metastable aragonite phase of CaCO3 can come into being with the existence of HPMC. Meanwhile, the Mg2+ would inhibit the growth of calcite and facilitate the production of the aragonite when the Mg2+ concentration is lower than 25 mM in solution. With increasing the molar ratio of Mg/Ca, the nesquehonite precipitation would be induced when the magnesium concentration is sufficiently high. The nesquehonite grew along the same (4 0 0) orientation in that the HPMC can be controlled by the basic units during the aggregation and reorientation process. The results of polarizing microscope also provided forceful support for the crystal growth mechanism. The thermodynamical stable phase of anhydrous calcium carbonate (CaCO3) was nucleated preferentially after the combination between Ca2+ and CO32−, then, the Mg2+ can bond with the oxygen atoms in HPMC due to electrostatic adherence and combine with CO32− and H2O to form a nesquehonite crystal phase. The results illustrated that the extent of binding affinity during HPMC with Ca2+ or Mg2+ was suggested to be in connection with the reconstructuring process and the morphology of final carbonate crystals.

Potential of growth factor incorporated mesoporous bioactive glass for in vivo bone regeneration

Mesoporous bioactive glass (MBG) has drawn much attention due to its superior surface texture, porosity and bioactive characteristics. Aim of the present study is to synthesize MBG using different surfactants, viz., hexadecyltrimethylamonium(CTAB) (M1), poly-ethylene glycol (PEG) (M2) and pluronic P123 (M3); bioactivity study; and to understand their bone regeneration efficacy in combination with insulin-like growth factors (IGF-1) in animal bone defect model. SBF study revealed the formation of calcium carbonate (CaCO3) and hydroxyapatite (HAp) phase over 14 days. Formation of apatite layer was further confirmed by FTIR, FESEM and EDX analysis. M1 and M2 showed improved crystallinity, while M3 showed slightly decrease in crystalline peak of CaCO3 and enhanced HAp phase. More Ca-P layer formed in M1 and M2 supported the in vivo experiments subsequently. Degree of new bone formation for all MBGs were high, i.e., M1 (80.7 ± 2.9%), M2 (74.4 ± 2.4%) and M3 (70.1 ± 1.9%) compared to BG (66.9 ± 1.8%). In vivo results indicated that the materials were non-toxic, biodegradable, biocompatible, and is suitable as bone replacement materials. Thus, we concluded that growth factor loaded MBG is a promising candidate for bone tissue engineering application.

Fine fluorite nanoparticles synthesized from biomass ash

Fine fluorite (CaF2) nanoparticles were synthetised by reaction of ammonium fluoride with calcium carbonate phase obtained by processing of wood chips ash in sodium acetate trihydrate acting as transport medium and liquid environment at 120 °C on air. The synthesized nanoparticles were analysed by SEM-EDS, XRD, XPS and TEM measurements and nanocrystals of calcium fluoride with average size 12 nm were characterised in detail. The properties of the material as an anti-reflexive coating made from the synthesized nanoparticles were successfully tested. The suggested procedure represents a promising route of zero-waste technology of calcium-rich biomass ashes processing and may be utilised for production of large quantities of calcium fluoride nanoparticles.

Synthesis Mechanism and Material Characterization of C4AF Obtained by the Self-propagating Combustion Reaction Method

To investigate the influence of preparation process on the properties of synthesized C4AF, the powder was prepared via the self-propagating combustion reaction (SPCR) method using urea as fuel and metal nitrates as cation precursors. Synthesis mechanism of the SPCR method, calculation and adjusting principles of urea dosage were detailedly introduced. Material characterization of synthesized C4AF was performed with the aid of X-ray diffractometry, Fourier transform infrared spectrometry, scanning electron microscopy, energy-dispersive X-ray spectroscopy, 27Al nuclear magnetic resonance and isothermal microcalorimetric technique. Remaining content of transition phase of calcium carbonate in synthesized C4AF was determined by quantitative analysis of X-ray diffractometry. It was found that there was no difference in the hydration behavior of C4AF synthesized by the SPCR method and the traditional solid-state reaction (SSR) method. C3AH6 and amorphous iron (III) hydroxide (Fe(OH)3) would be formed during the hydration of C4AF while CH not. Crystallite size of synthesized C4AF was 16.1 A and the apparent activation energy was 36.2 kJ/mol. Coordinated condition of Al in C4AF can be detected by 27Al NMR technique, but the peaks were broadened and the intensities were relatively low, supporting the use of 27Al NMR for the quantitative analysis of C3A in Portland cements.

Response of Sea Urchin Fitness Traits to Environmental Gradients Across the Southern California Oxygen Minimum Zone

Marine calcifiers are considered to be among the most vulnerable taxa to climate-forced environmental changes occurring on continental margins with effects hypothesized to occur on microstructural, biomechanical, and geochemical properties of carbonate structures. Natural gradients in temperature, salinity, oxygen, and pH on an upwelling margin combined with the broad depth distribution (100-1100 m) of the pink fragile sea urchin, Strongylocentrotus (formerly Allocentrotus) fragilis, along the southern California shelf and slope provide an ideal system to evaluate potential effects of multiple climate variables on carbonate structures in situ. We measured for the first time trait variability across four distinct depth zones in a natural experiment to simulate species-specific implications of oxygen minimum zone (OMZ) expansion, deoxygenation and ocean acidification. Although S. fragilis may likely be tolerant of future oxygen and pH decreases predicted during the twenty-first century, we determine from adults collected across multiple depth zones that urchin size and potential reproductive fitness (gonad index) are drastically reduced in the OMZ core (450-900 m) compared to adjacent zones. Increases in porosity and mean pore size coupled with decreases in mechanical nanohardness and stiffness of the calcitic endoskeleton in individuals collected from lower pHTotal (7.57-7.59) and lower dissolved oxygen (13-42 µmol kg-1) environments suggest that S. fragilis may be potentially vulnerable to crushing predators if these conditions become more widespread in the future. In addition, elemental composition determined using inductively coupled plasma-mass spectrometry indicates that S. fragilis has a skeleton composed of the low Mg-calcite mineral phase of calcium carbonate (mean Mg/Ca= 0.02 mol mol-1), the lowest Mg/Ca values measured in sea urchins known to date. Together these findings suggest that ongoing declines in oxygen and pH will likely affect the ecology and fitness of a dominant echinoid on the California margin.

Intracellular carbonic anhydrase from Citrobacter freundii and its role in bio-sequestration

The aim of this work was to study the CO2 bio-sequestration application of indigenous Citrobacter species and its carbonic anhydrase (CA). Intracellular CA was purified from Citrobacter freundii (CF; accession no: MH283871) isolated from limestone rock site in Kumaun region of Indian Himalaya studied for the sequestration of carbon dioxide and the formation of calcite. CF showed maximum CA enzyme activity at 11.3 EU/ml at pH 7.0 and 37 °C. Hydration of CO2 into carbonate was characterized by calcite phase of calcium carbonate using absorption spectroscopy and imaging technique. Purified CA showed a significantly high CO2 sequestration capacity of 230 mg CaCO3/mg of purified as compared to crude enzyme (50 mg CaCO3/ml of enzyme).

Radiocarbon dating reveals the timing of formation and development of pedogenic calcium carbonate concretions in Central Sudan during the Holocene

Calcic soil horizons are common in arid and semi-arid lands and represent the result of the progressive accumulation of calcium carbonate in the soil profile over time. This process leads to the occurrence of several pedogenic phases of calcium carbonate dissolution/precipitation. For this reason, timing the formation and development of calcic horizons through radiometric dating is not straightforward as time-averaging effects, due to the superimposition of the same process over time, occur. On this basis, this study aims to define the timing and dynamics of formation and development of pedogenic calcium carbonate concretions from semi-arid Central Sudan, highlighting the relevance of a multi-disciplinary approach and the effectiveness of radiocarbon dating (coupled to an accurate sampling strategy) applied to pedogenic carbonates. Calcic soil horizons (Bk) were sampled during the archaeological excavation of site 16-D-4 at Al Khiday (Central Sudan) and studied by optical, cathodoluminescence and scanning electron microscopy, as well as by chemical-physical and stable isotopes (C and O) analyses. Micromorphological analyses reveal the occurrence of distinctive calcitic pedofeatures, here described as calcitic-cemented nodules and calcitic-rich matrix. Radiocarbon ages obtained for these pedofeatures mostly refer to the Early Holocene for calcitic-cemented nodules (11.5, 9.9 and 9.6 cal. ka) and to the Middle Holocene for powdery calcitic-rich matrix (7.9, 7.7, 6.3 and 6.1 cal. ka) samples. Additionally, δ13C and δ18O values indicate major shifts of environmental conditions from the Early to the Middle Holocene, in agreement with the available information from detailed palaeoenvironmental studies carried out in the region. In particular, Early Holocene pedogenic calcitic features can be related to short arid phases during general wetter climate conditions, whereas those formed in the Middle Holocene can be related to short humid phases. Results show that Bk horizons were characterized by alternate periods of calcite accumulation (precipitation) and dissolution, and periods of quiescence or extremely slow growth rates. Thus, the formation and development of Bk horizons have been a long-lasting process significantly influenced by climatic fluctuations. This study represents a further step in the comprehension of the development of calcium carbonate concretions and (at a wider perspective) calcrete; the paper also contributes to the definition of a reliable method to radiometrically date the formation of calcic pedofeatures. On a broader perspective, this work may offer a significant tool for archaeometric studies, where the interaction between secondary calcite and archaeological material is significant (e.g., radiocarbon dating of bioapatite in bones), and palaeoenvironmental studies in arid lands, where Bk horizon development is a function of past rainfall.

New method for the synthesis of Al2O3\u2013CaO and Al2O3\u2013CaO\u2013CaCO3 systems from a metallic precursor by the sol\u2013gel route

A series of binary Al2O3–CaO and Al2O3–CaO–CaCO3 systems with Ca/Al molar ratios of 0.05, 0.1, 0.25, 0.5 and 1.0 have been synthesised by the sol–gel technique from aluminium isopropoxide and metallic calcium powder. The rate of the metal reaction is used as a limiting factor to control the binary gel formation. The proposed modification of the traditional sol–gel method was used to examine the influence the effect of the metallic form of the second component as an oxide precursor on the form of the final product. By applying acetic acid instead of mineral acid, calcium acetate is formed and then decomposed to calcium carbonate upon thermal processing. During the synthesis of the binary systems, metallic calcium acts both as a precursor of calcium acetate and as a secondary pH modifier of the gel system. Calcination in air at 600 °C did not produce systems containing only oxides and the calcium carbonate phase was still present. Due to particle size reduction, the CaCO3 to CaO decomposition temperature was lowered. The systems were characterised by X-ray powder diffraction, low-temperature nitrogen adsorption, transmission and scanning electron microscopy (TEM, SEM and SEM/EDS), thermogravimetric analysis (TGA) and FTIR spectra.

Synthesis and Characterization of Some Alkaline-Earth-Oxide Nanoparticles

The present work reports the synthesis of MgO and CaO nanoparticles by using the sol-gel autocombustion method. The annealing of the precursor at 1200 °C was observed to lead the formation of MgO nanoparticles having average crystallite size of ~ 31 nm. Annealing the precursor at same temperature produced materials having a CaO phase with a minor impure phase of calcium carbonate (~ 3%). The crystallite size corresponding to the CaO phase was 38 nm. A change of thermal history in the precursor was observed not to result in an improvement of the CaO phase. The change of thermal history in the precursor gave rise to mixed phases of CaCO3 and Ca(OH)2 rather than the phase of CaO. Further, annealing at 1200 °C for 12 h resulted in the formation of the CaO phase along with almost 1 - 5% of calcium hydroxide as an impurity phase. X-ray absorption spectroscopic measurements carried out on these materials revealed that the local electronic/atomic structure of these oxides was not only affected by the impurity phases but also influenced by the carbaneous impurities attached to the crystallites.

The influence of CO2 accelerated carbonation on alkali‐activated fly ash cement under elevated temperature and pressure

AbstractAn alternative of wellbore cement system, formed through alkali‐activation of fly ashes also knows as fly ash geopolymers, provides attractive properties due to its high compressive strength and durability against acidic conditions. However, changes in microstructure and mineralogical during accelerated carbonation affected by CO2 alteration under elevated temperature and pressure have not been fully understood. This paper investigates and characterizes the alkali‐activated binders fly ash based in the acidic environment under elevated temperature and pressure. The results found that scales such a layer was formed in the gel phase at the spherical surface of fly ash after carbonation process. The effect is that the surface of the gel phase became rough and porous. Carbonate and/or bicarbonate salts were identified as major phase in the sample after immersion test. In the activated fly ash cement, both calcium aluminate silicate hydrate (C–A–S–H) and sodium aluminosilicate hydrate (N–A–S–H) gels were formed; C–A–S–H gel was decalcified, forming calcium carbonate phase in the form of calcite, aragonite or vaterite. These findings were confirmed by the analysis and identification of Fourier‐transformed infrared spectroscopy, X‐ray diffraction and scanning electron microscopy tests on the changes in the cement system.

Exploring the Influence of Reaction Parameters on the Preparation of Calcium Carbonate by Spontaneous Precipitation

AbstractDifferent polymorph and morphology of CaCO3 powder are prepared by spontaneous precipitation method in the presence of sodium citrate and NaOH at ambient temperature. The influence of spontaneous precipitation reaction parameters such as aging time, pH value, the concentration of complexing agent sodium citrate on the crystal phase, particle size and morphology of the precipitated calcium carbonate is studied in this paper. The prepared samples are characterized by scanning electron microscopy (SEM) and X‐ray diffraction (XRD). The results show that the aging time, the pH of the solution and the concentration of sodium citrate turned out to be important for the morphologies and crystal phase of CaCO3. Increasing the aging time, the content of vaterite decreases slightly, whereas the opposite effect is observed for increasing pH. When pH value is 13, pure calcite irregular nanoparticles can be prepared. In the presence of sodium citrate, the content of vaterite increases as the increasing of sodium citrate. When the concentration of sodium citrate is increased to 0.1 m, uniform vaterite microsphere with 5 µm size can be obtained.