Stable Hydrate Phase Research Articles

Wet carbonation of C3A and pre-hydrated C3A

The carbonation of C3A and pre-hydrated C3A was studied at different initial pH values, temperatures, pre-hydration and carbonation times. Based on these investigations, the reaction kinetics of C3A under humid conditions were considered in more detail. The reaction products were characterized by quantitative X-ray diffraction (QXRD) and thermogravimetric analysis (TGA). C3A shows no significant reaction when carbonated directly in suspension, however, it has been observed that pre-hydrated C3A can be carbonated. A higher reaction degree was obtained with increasing hydration time. Consistent with reaching a stable pH, no further reaction of the hydrate phases was detected after ~20 min, indicating fast kinetics of the carbonation reaction. At temperatures ≤21 °C, the CaCO3 polymorph vaterite was found to be the main phase. At a pre-hydration temperature of 40 °C, C3AH6 appeared as a stable hydrate phase, but showed no reaction with CO2 under our experimental conditions.

Ammonium Metatungstate, (NH4)6[H2W12O40]: Crystallization and Thermal Behavior of Various Hydrous Species

Various hydrous ammonium metatungstate phases (NH4)6[H2W12O40]·XH2O (AMT-X) have been obtained in the form of single crystals, using supersaturated solutions, antisolvent crystallization, and partial dehydration as strategies for crystal growth. On the basis of laboratory X-ray diffraction data, the crystal structures of the hydrous phases with X = 22, 12.5, 9.5, 8.5, 6, 4, and 2 as well of the cocrystals with X = 12-ethanol and X = 4-2acetone have been determined for the first time. AMT-22 forms the solid solution series (NH4)6–xKx[H2W12O40]·22H2O with the potassium salt (PMT-22) without a miscibility gap. Its tetragonal crystal structure shows a distorted cubic close-packed arrangement of the spherical α-Keggin-type [H2W12O40]6– anions with disordered ammonium N and water O atoms in the voids. The crystal structures of the other, less-hydrated AMT phases can be derived from distorted hexagonal rod packings of the α-Keggin anions, likewise with the cations and crystal water molecules in the voids. The thermal behavior of AMT-22 crystals reveals a quick dehydration, with AMT-9.5 as an intermediate and AMT-4 as a stable hydrate phase under ambient conditions. Upon further heating, the material decomposes with stepwise formation of AMT-2, AMT-1, AMT-0, and an amorphous phase, before orthorhombic WO3 forms above 400 °C. One of the commercially available AMT phases, “(NH4)6[H2W12O40]·XH2O” (with X = unspecified) has a water content of X = 4 but crystallizes in a structure other than AMT-4. Consequently, the 4-hydrate shows polymorphic behavior.

Technological solutions for well construction in high-viscous shale hydrocarbon fields

The article discusses the main technological processes of well construction for the production of high-viscosity hydrocarbons from productive lowporosity reservoirs with high temperature and pressure conditions, which include shale deposits of Bazhenov formation. According to the results of the review and analysis of existing solutions in the development of this deposits, the following measures were justified and proposed: construction of branched multi-hole azimuth horizontal wells, implementation of selective multi-stage hydraulic fracturing in the productive formation; the use of oil-based process fluids when opening the reservoir, the use of plugging materials for isolation of the reservoir, the hardening product of which is represented by thermally stable hydrate phases (hydrobasic hydrosilicates). Вranched wells have a long horizontal end (about 1 000 meters or more). Only a part of the horizontal section works effectively, which is the basis for the development and application of the staged, both in time and along the strike, hydraulic fracturing method. At the level of the invention, a method and apparatus for carrying out multistage selective hydraulic fracturing in wells with horizontal completion have been developed. The article describes a method for implementing multistage selective hydraulic fracturing, comparing this method with the existing ones. Much attention is given to the need to use hydrocarbon-based solutions for the initial opening the reservoir, to use cement slurries from composite materials to separate the reservoir, the hardening product of which is a stone formed by low-basic calcium hydrosilicate.

Application of thermodynamic modeling to predict the stable hydrate phase assemblages in ternary CSA-OPC-anhydrite systems and quantitative verification by QXRD

Thermodynamic modeling was used to predict the stable hydrate phase assemblages in a ternary CSA-OPC-anhydrite system. On the basis of this modeling, five systems with diverging expected hydrate phases were chosen for experimental examination via QXRD and TGA. The aim of this study was to examine whether and to which extent the predicted equilibrium states are reached after 28 days of hydration. Qualitatively the predicted hydrate phase assemblages were reached for all five systems after 28 d of hydration. When the reaction degrees of the anhydrous phases are taken into account the experimental data fits remarkably well to the thermodynamic model. Additionally, C2S dissolution was found to be the most rapid in straetlingite forming environments.

Assessment on the performances of air lime-ceramic mortars with nano-Ca(OH)2 and nano-SiO2 additions

This research presents a novel approach based on the combination of nanotechnology and Roman technology by investigating how adding nanoCa(OH)2 and nanoSiO2 modify the performance of air lime mortars containing Roman ceramics. Microstructural, physico-mechanical properties were periodically controlled until 120 days of curing. XRD and TGA analyses showed that adding nanoSiO2 either alone or with nanoCa(OH)2 were more beneficial to improve the pozzolanic activity in the mortars. The less stable hydrated phases generated led to microcracks which eventually impaired compressive strength but enhanced deformability capacity. These results provide insight into the development of highly compatible mortars for cultural heritage.

Unraveling the metastability of the SI and SII carbon monoxide hydrate with a combined DFT-neutron diffraction investigation.

Clathrate hydrates are crystalline compounds consisting of water molecules forming cages (so-called "host") inside of which "guest" molecules are encapsulated depending on the thermodynamic conditions of formation (systems stable at low temperature and high pressure). These icelike systems are naturally abundant on Earth and are generally expected to exist on icy celestial bodies. Carbon monoxide hydrate might be considered an important component of the carbon cycle in the solar system since CO gas is one of the predominant forms of carbon. Intriguing fundamental properties have also been reported: the CO hydrate initially forms in the sI structure (kinetically favored) and transforms into the sII structure (thermodynamically stable). Understanding and predicting the gas hydrate structural stability then become essential. The aim of this work is, thereby, to study the structural and energetic properties of the CO hydrate using density functional theory (DFT) calculations together with neutron diffraction measurements. In addition to the comparison of DFT-derived structural properties with those from experimental neutron diffraction, the originality of this work lies in the DFT-derived energy calculations performed on a complete unit cell (sI and sII) and not only by considering guest molecules confined in an isolated water cage (as usually performed for extracting the binding energies). Interestingly, an excellent agreement (within less than 1% error) is found between the measured and DFT-derived unit cell parameters by considering the Perdew-Burke-Ernzerhof (denoted PBE) functional. Moreover, a strategy is proposed for evaluating the hydrate structural stability on the basis of potential energy analysis of the total nonbonding energies (i.e., binding energy and water substructure nonbonding energy). It is found that the sII structure is the thermodynamically stable hydrate phase. In addition, increasing the CO content in the large cages has a stabilizing effect on the sII structure, while it destabilizes the sI structure. Such findings are in agreement with the recent experimental results evidencing the structural metastability of the CO hydrate.

Physico-Mechanical Features of Portland Cement Mortars Replaced Partially with Aluminium Cement

The purpose of the research was to recognize the effect of the addition of aluminium cement on the properties of cement mortar made of Portland cement CEM I 42.5R. Alumina cement is a hydraulic binder. It is a finely ground organic material that when mixed with water binds as a result of reactions and hydration processes. After hydration, stable hydrate phases form and, as a consequence, the material strength parameter is obtained. The main constituent is calcium aluminate (CaO⋅Al2O3), in smaller amounts there are: calcium aluminate, dicalcium silicate and calcium aluminium silicate or gelenite. Hydraulic hardening of aluminium cement occurs mainly through calcium hydration but other chemicals may also take part in the hardening process. The specific chemical composition of aluminium cement and the fact that calcium hydroxide is not released during hydration allows the leaven to resist many aggressive factors. Therefore, in the literature, we can meet guidelines so that the w / c ratio is not greater than 0.40 for applications to structural elements. The article discusses the results of laboratory tests on the impact of replacing Portland cement with aluminium cement in the amount of 10%, 20%, 50%, 100% on the physical and mechanical properties of mortars. The rheological properties of mortars, i.e. consistency, setting time and parameters of hardened mortars were examined, among others compressive and bending strength, capillary rise and absorbability. Compressive strength and bending strength were tested after 1, 7 and 28 days on samples with dimensions 4 × 4 × 16 cm. Absorption and capillary rupture were tested 28 days after being formed on standard bar beams 4×4×16cm. The mortar was made with a constant coefficient w/c = 0.38.

The clays after heat treatment as the concrete active mineral additive

Active mineral additives are one of the most common components of cement systems now. They are entered cements of increase in extent of hydration, the directed formation of structure of a cement stone from more stable hydrate phases of the lowered basicity, for the purpose of improvement of construction and technical properties of cements, by cutting of costs of fuel raw material resources for their production, giving to cements of some specific properties. In work the possibility of use as active mineral additives not only the granulated slags, but also local clays which industrially can give certain puzzolan properties are considered. It is proved that heat treatment of clay breeds significantly increases their puzzolan activity that does them suitable for use as active mineral additive instead of the domain granulated slag by production of the portland cement.

Stability of single phase C3A hydrates against pressurized CO2

The stability of single phase C3A hydrates including ettringite, monosulfoaluminate, C2AH8, C4AH13 and katoite (C3AH6) against pressurized CO2 was investigated performed at 20°C or 50°C under 7bar CO2 for 1–30days. Raman microspectroscopy, X-ray diffraction, infrared spectroscopy and SEM were utilized to analyze the carbonation products. Ettringite rapidly reacted with CO2 into gypsum, CaCO3 and Al(OH)3 while monosulfoaluminate started to decompose after 15days into [Ca2Al(OH6)]2(SO4)1−x(CO3)x-LDH. C2AH8, and C4AH13 were less stable than monosulfoaluminate whereas katoite remained unchanged even after 30days of CO2 exposure and presented the most stable hydrate phase of C3A.

Physics-chemical study of hydration process of three calcium aluminate phase and Metakaolin

This research based on X-Ray Diffraction, SEM and chemical analyses were revealed that decreasing of Ca(OH)2 phase content and increasing of C3AH6 and trisulfate aluminate calcium-3 as the stable phases for the hydration period of first 30 to 60 minutes when comparing K-crent doping to the phase to without any doping. Also results have been proposed in case of doping metakaoline to draw structural analogies as formation of stable hydrated phases and the decrease of new formed unstable crystalline metaphases in the early stage of 3CAO·Al2O3 hydration process.DOI: http://dx.doi.org/10.5564/mjc.v12i0.183 Mongolian Journal of Chemistry Vol.12 2011: 107-112

Theoretical study of hydrogen storage in binary hydrogen-methane clathrate hydrates

The properties of binary H2 + CH4 clathrate hydrates have been estimated using the extended van der Waals and Platteeuw statistical thermodynamic model that takes into account the lattice relaxation, host-guest, and guest-guest interactions as well as the quantum nature of guest behavior in the clathrate cavities. It has been found that at a small methane concentration in the gas phase the stable hydrate phase has cubic structure II (CS-II) and at a methane concentration of 6% stabilizes cubic structure I, which is metastable in the case of the pure hydrogen hydrate. This is in agreement with recent experimental data. The amount of hydrogen storage depends on the methane concentration in the gas phase as well as the thermodynamic conditions of hydrate formation. Hydrogen storage up to 2.6 wt. % can be achieved in the binary H2 + CH4 CS-II hydrate at T = 250 K.

Identification and characterisation of stable phases of silicotungstic acid, H4SiW12O40·nH2O

Thermal analysis data recorded from silicotungstic acid show the existence of stable phases of composition H4SiW12O40·nH2O where n=24, 14, 6 and 0. FT-IR data confirms the presence of the Keggin anion, SiW12O404−, in each of these phases. In addition, dehydration of the higher hydrates (n>6) is shown to occur via the loss of zeolitic type water followed by (n<6) the removal of water associated with the acidic protons. Each of the hydrated phases, as well as the anhydrous phase, has been characterised by X-ray powder diffraction and the unit cell parameters are reported. 29Si MAS NMR spectroscopy is consistent with a single silicon site in each of the four single phases and deshielding of the silicon nucleus in the range n=6 to n=0 is associated with the transfer of acidic protons from the H5O2+ ions to random locations on oxygen atoms in the Keggin unit. The observed increase in magnitude of T1 (29Si) for the hexahydrate is interpreted in terms of a reduction in motional freedom of the water molecules in this hydrate compared to those in the higher hydrates. Nominal hydrated states between n=6 and 14, as well as those between n=14 and 24, are shown by X-ray powder diffraction to be composed of the stable hydrated phases in the given ranges.

Raman Spectroscopy of Anhydrous and Hydrated Calcium Aluminates and Sulfoaluminates

Recent investigations have revealed the great potential of Raman spectroscopy for the characterization of clinker minerals and commercial Portland cements. The usefulness of this technique for the identification of anhydrous, hydrated, and carbonated phases in cement‐based materials has been demonstrated. In the present work, the application of micro‐Raman spectroscopy for the characterization of the main clinker phases of calcium aluminate cements and calcium sulfoaluminate cement is explored. The main stable hydrated phases as well as several important carbonated phases are investigated. Raman measurements on the following phases are reported: (i) pure, unhydrated phases: CA, C12A7, CA2, C2AS, cubic‐C3A, C4AF, and C4A3; (ii) hydrated phases: ettringite, monosulfoaluminate, and hydrogarnet (C3AH6); (iii) carboaluminate phases: hemicarboaluminate and monocarboaluminate. The present results, which are discussed in terms of the internal vibrational modes of the aluminate, carbonate, and sulfate molecular groups as well as stretching O–H vibrations, show the ability of Raman spectroscopy to identify the main hydrated and unhydrated phases in the aluminate and sulfoaluminate cements. The Raman spectra obtained in this work provide an extended database to the existing data published in the literature.

Differential scanning calorimetry investigation of formation of poly(ethylene glycol) hydrate with controlled freeze–thawing of aqueous protein solution

AbstractThe formation of poly(ethylene glycol) (PEG) hydrate during freeze–thawing of dilute lactate dehydrogenase solutions with the addition of 0.05–160 mg/mL PEG 6000 is investigated by differential scanning calorimetry and modulated temperature differential scanning calorimetry. The freeze–thawing process is performed with a controlled temperature history. A moderate cooling rate to a low freezing temperature in combination with a low heating rate seems to create the most stable PEG hydrate. The maximum amount and the most stable hydrate phase are obtained when the freezing temperature is at or below −60°C. The enthalpy of melting for the hydrate at −15°C is dependent on the heating rate but not on the cooling rate if the freezing temperature is −60°C. The effect of the addition of reduced form nicotinamide adenine dinucleotide to the PEG and protein solution indicates that competing interactions with the protein can increase the stability of the PEG hydrate. The amount of bound water in the PEG hydrate can be calculated directly from the melting enthalpy of the hydrate if an adequate temperature history is used. For solutions with &gt;10 mg/mL PEG there are 1.7–2.7 water molecules bound per PEG unit. The PEG protection of the protein at freeze–thawing can be an effect of the amount of available PEG hydrate in relation to the amount of ice surface. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1626–1634, 2004

Freezing nucleation of levitated single sulfuric acid/H2O micro-droplets. A combined Raman- and Mie spectroscopic study

The phase behaviour of single sulfuric acid/H2O micro-droplets upon supercooling has been studied using optical levitation of such droplets in combination with Raman- and Mie spectroscopy for chemical and size analysis. It is found that the freezing nucleation behaviour depends on the sulfuric acid concentration of the droplets and hence the region of the phase diagram which is reached upon cooling. Whereas for diluted droplets (cH2SO4<30wt%) instantaneous ice nucleation is observed, more concentrated droplets tend to supercool with no tendency to nucleate the thermodynamically stable hydrate phase sulfuric acid tetrahydrate (SAT) or sulfuric acid monohydrate (SAM). The upper limits of the nucleation rate in this regime were determined to be J≤5×105 cm−3 s−1.

Stable methane hydrate above 2 GPa and the source of Titan's atmospheric methane.

Methane hydrate is thought to have been the dominant methane-containing phase in the nebula from which Saturn, Uranus, Neptune and their major moons formed. It accordingly plays an important role in formation models of Titan, Saturn's largest moon. Current understanding assumes that methane hydrate dissociates into ice and free methane in the pressure range 1-2 GPa (10-20 kbar), consistent with some theoretical and experimental studies. But such pressure-induced dissociation would have led to the early loss of methane from Titan's interior to its atmosphere, where it would rapidly have been destroyed by photochemical processes. This is difficult to reconcile with the observed presence of significant amounts of methane in Titan's present atmosphere. Here we report neutron and synchrotron X-ray diffraction studies that determine the thermodynamic behaviour of methane hydrate at pressures up to 10 GPa. We find structural transitions at about 1 and 2 GPa to new hydrate phases which remain stable to at least 10 GPa. This implies that the methane in the primordial core of Titan remained in stable hydrate phases throughout differentiation, eventually forming a layer of methane clathrate approximately 100 km thick within the ice mantle. This layer is a plausible source for the continuing replenishment of Titan's atmospheric methane.

Thermal studies on sodium silicate hydrates. I. Trisodium hydrogensilicate pentahydrate, Na 3HSiO 4 · 5 H 2O; Thermal stability and thermal decomposition reactions

Presented in this paper is a thermoanalytical study on Na 3HSiO 4 · 5 H 2O under a dry, inert nitrogen atmosphere, demonstrating the following thermal decomposition reactions and thermodynamically stable hydrate phases ▪ ▪ This reaction behaviour was reproducibly observed by means of simultaneous thermal analysis (TG, DTG, DTA), differential scanning calorimetry (DSC) and X-ray heating experiments. Annealing of Na 3HSiO 4 · 5 H 2O at the above stability ranges yielded the subsequent hydrate phases as well as the dry silicate. The decomposition products were characterized by X-ray diffraction methods.