Other hydrated phases

CjAHg is the only stable ternary phase in the CaO-AUOj H,0 system at ordinary temperatures, but neither it nor any other hydrogarnet phase is formed as a major hydration product of typical, modern Portland cements under those conditions. Minor quantities are formed from some composite cements and, in a poorly crystalline state, from Portland cements. Larger quantities were given by some older Portland cements, and are also among the normal hydration products of autoclaved cement-based materials. CjAHg is formed in the conversion reaction of hydrated calcium aluminate cements (Section 10.1). 

Passaglia and Rinaldi (P23) discussed IR spectra and TG curves for C3AHg and other hydrogarnet phases. The TG curve of C3AHg shows major loss at 250-310°C and further loss at 450-550 C, but in that of a katoite specimen the two steps were barely distinguishable. Majumdar and Roy (M63) reported DTA and IR data for CjAH. The refractive indices of C3AS3 and CjAHg are 1.734 and 1.604, respectively those of the solid solutions are linearly related to the composition (P23). 

DTA curves at 10 deg C min show a large endotherm at 130-150 C and a smaller one at about 290°C. Since the first of these is due to loss of molecular water, its height is affected by any preliminary drying that the specimen may have undergone. DTA has been used to determine the relative amounts of CAHk, and AHj, and thus indirectly also of CjAH in calcium aluminate cement concretes, but caution is needed because the CAHk, may have undergone partial dehydration and also because its thermal decomposition can itself yield AHj. To some extent, this caution also applies to the determination of CAH,o by QXDA. 

A Al NMR study (G54) confirmed the octahedral coordination of the aluminium atom. Henning (H34) reported IR spectra for CAH,o. CAH, and CAH,. 

Brucite [magnesium hydroxide Mg(OH)2] is isostructural with CH. It is formed in Portland cement concrete that has been attacked by magnesium salts, and on hydration of Portland cements high in MgO and possibly of Portland cements in general. It has a = 0.3147 nm, c = 0.4769 nm, Z = 1, , = 2368kgm , oi = 1.561, e = 1.581 (S63). Three polytypes of aluminium hydroxide [Al(OH)3], gibbsite, bayerite and nordstrandite. contain layers essentially similar to those in brucite, but with an ordered pattern of 

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At very high pressures (0.3-2.1 GPa), gas hydrates undergo structural transitions to other hydrate phases and filled ice phases. Guests can multiply occupy the large cages of these high-pressure hydrate phases. 

The C-S-H present in a saturated paste is assumed to include that part of the porosity, and its content of water, without which it cannot be formed. The H20/Si02 ratio is approximately 4.0. Assuming the water in excess of that present at 11% RH to have a density of 1000 kg m", Hansen (H28) calculated the density of the C-S-H to be 1850-1900 kg m . This value is near those of 1900-2100kgm" for cement pastes under saturated conditions (P20), if allowance is made for the CH and other hydrated phases which these contain. 

To form Mg3(P04)2-4H20, ionization of (NH4)2HP04 will yield PO , which does not occur in the pH range of 3.8-8 in which these ceramics are formed. However, other hydrated phases such as dittmarite or schertelite could be formed by the reactions 9.8 and 9.17, which follow from the dissolution reactions... 

Other reactions taking place throughout the hardening period are substitution and addition reactions (29). Ferrite and sulfoferrite analogues of calcium monosulfoaluminate and ettringite form soHd solutions in which iron oxide substitutes continuously for the alumina. Reactions with the calcium sihcate hydrate result in the formation of additional substituted C—S—H gel at the expense of the crystalline aluminate, sulfate, and ferrite hydrate phases. 

Many salts crystallize from aqueous solution not as the anhydrous compound but as a well-defined hydrate. Still other solid phases have variable quantities of water associated with them, and there is an almost continuous gradation in the degree of association or bonding between the molecules of water and the other components of the crystal. It is convenient to recognise five limiting types of interaction though the boundaries between them are vague... 

Rawlings and Lingafelter [69] studied the hydrated phases of sodium alcohol sulfates ranging from C6 to C20 and their crystal structures by X rays. The a phase is almost identical to that of the alkylsulfonates but all other phases are different. The crystals of all phases are monoclinic. This work was completed by Prins and Prins [70] who gave more precise details of the polymorphism of sodium alcohol sulfates. 

Defect clusters are similarly prominent in hydrated phases. For example, anatase nanocrystals prepared by sol-gel methods contain high numbers of vacancies on titanium sites, counterbalanced by four protons surrounding the vacancy, making a (Vxi 4H ) cluster. In effect the protons are associated with oxygen ions to form OH- ions, and a vacancy-hydroxyl cluster is an equally valid description. Similar clusters are known in other hydrated systems, the best characterized being Mn4+ vacancies plus 4H in y-Mn02, known as Reutschi defects. 

The combination of C02 injection and methane production over specific PT regimes allows the heat effects of C02 hydrate formation and methane hydrate decomposition to nullify each other resulting in a sustainable delivery process which both reduces C02 emissions to combat global warming and recovers methane to supplement the declining reserves of conventional natural gas (Fig. 4). This gas hydrate phase-behaviour in response to the dissociation and formation processes clearly demonstrates the potential of C02 enhanced CH4 recovery from the Mallik gas hydrate deposit. 

The gas phase enthalpy of reaction 6 for bis(hydroxymethyl) peroxide is — 192 kJ mol , which deviates from the other hydrate-producing peroxides by nearly 89 kJ mol . The enthalpy of reaction 8, 145 kJmol, is likewise discrepant by some 120 kJmol from that for diethyl peroxide, ca 26 kJ mol. From the high-level calculations reported in Reference 28, the reaction enthalpy for the addition of H2O2 to formaldehyde is —59 kJ mol. A similar reaction is equation 10 for the gas phase addition of tert-butyl hydroperoxide to a carbonyl group. 

It would appear from the above summary of natural occurrences that quartz is the most stable form of silica at near-surface conditions but that other metastable phases, representing initially poorly organized material, predominate in the natural occurrences or newly formed silica. Experiments demonstrate the persistence of metastable amorphous or cryptocrystalline hydrated Si02 at low temperature (Kittrick, 1969 Krauskopf, 1956, 1959) and slow conversion at higher temperatures (above 100 bars) (Frondel, 1962 Heydemann, 1964 Carr and Fyfe, 1958 Mlzutanl, 1970). 

While si, sll, and sH are the most common clathrate hydrates, a few other clathrate hydrate phases have been identified. These other clathrate hydrates include new phases found at very high pressure conditions (i.e., at pressures of around 1 GPa and higher at ambient temperature conditions). Dyadin et al. (1997) first reported the existence of a new methane hydrate phase at very high pressures (500 MPa). This discovery was followed by a proliferation in molecular-level studies to identify the structure of the high pressure phases of methane hydrate (Chou et al., 2000 Hirai et al., 2001 Kurnosov et al., 2001 Loveday et al., 2001, 2003). 

Less common clathrate hydrates formed by compounds other than natural gas guests (such as Jeffrey s structures III-VII, structure T, complex layer structures) and high pressure hydrate phases are also briefly described to provide a comprehensive account of clathrate hydrate structural properties. 

The calculation of two-phase (hydrate and one other fluid phase) equilibrium is discussed in Section 4.5. The question, To what degree should hydrocarbon gas or liquid be dried in order to prevent hydrate formation is addressed through these equilibria. Another question addressed in Section 4.5 is, What mixture solubility in water is needed to form hydrates ... 

Finally, Section 4.6 concerns the relationship of phase equilibrium to other hydrate properties. The hydrate application of the Clapeyron equation is discussed... 

The phase behavior of hydrocarbon + water mixtures differs significantly from that of normal hydrocarbon mixtures. Differences arise from two effects, both of which have their basis in hydrogen bonding. First, the hydrate phase is a significant part of all hydrocarbon + water phase diagrams for hydrocarbons with a molecular size lower than 9 A. Second, water and hydrocarbon molecules are so different that, in the condensed state, two distinct liquid phases form, each with a very low solubility in the other. 

In this section two prediction techniques are discussed, namely, the gas gravity method and the Kvsi method. While both techniques enable the user to determine the pressure and temperature of hydrate formation from a gas, only the KVSI method allows the hydrate composition calculation. Calculations via the statistical thermodynamics method combined with Gibbs energy minimization (Chapter 5) provide access to the hydrate composition and other hydrate properties, such as the fraction of each cavity filled by various molecule types and the phase amounts. 

It should be noted that the use of the KVSj charts implies that both the gas phase and the hydrate phase can be represented as ideal solutions. This means that the Kvsi of a given component is independent of the other components present, with no interaction between molecules. While the ideal solution model is approximately acceptable for hydrocarbons in the hydrate phase (perhaps because of a shielding effect by the host water cages), the ideal solution assumption is not accurate for a dense gas phase. Mann et al. (1989) indicated that gas gravity may be a viable way of including gas nonidealities as a composition variable. 

The activity of the hydrate phase is constant at a given temperature regardless of the other phases present. 

Encapsulation of the gas decreases the pressure to the three-phase (Lw-H-V) condition. The system pressure may be controlled by an external reservoir for addition or withdrawal of gas, aqueous liquid, or some other fluid such as mercury. After hydrate formation, the pressure is reduced gradually, the equilibrium pressure is observed by the visual observation of hydrate crystal disappearance. Upon isothermal dissociation, the pressure will remain constant for a simple hydrate former until the hydrate phase is depleted. 

The butyl-ammonium end of the AA is very attractive to water and to hydrates, so that it remains firmly attached either to the water droplet, or to the hydrate phase after the water droplet conversion. The other, long carbon end of the AA has the... 

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