Phase aluminate

Figure 2 Model for the Bronsted sites in the supercage of a dealuminated HY, depending on the nature and location of the extraframework aluminic phase (the drawing of the extraframework phase is only schematic). In the center is the unperturbed Bronsted site.

U.S. imports for consumption, 4 528t Calcium(II), concentration formation constant of chelates, 5 717t 12-Calcium-7-aluminate, phase in Portland cement clinker, 5 472t Calcium absorption, 26 292 Calcium addition, in silicon production, 22 505-506... 

Although less compatible than Mg, Al apparently is still well retained by the residue during melting, which supports the presence of an aluminous phase in the residue as suggested by Johnson et al. (1990). If we knew the Al203/Mg0 ratio of the whole rock prior to melting, it would be possible to calculate that ratio in the last liquid extracted and compare its value with mid-ocean ridge basalt (MORB) values. <=... 

In the early part of stage II, reactions of the aluminate phase will predominate. It is at this stage that the SO4... 

C4AF. Aluminate phases and their hydration products therefore play an important role in the early hydration... 

Although gibbsite and kaolinite are important in quantity in some soils and hydrothermal deposits, they have diminishing importance in argillaceous sediments and sedimentary rocks because of their peripheral chemical position. They form the limits of any chemical framework of a clay mineral assemblage and thus rarely become functionally involved in critical clay mineral reactions. This is especially true of systems where most chemical components are inert or extensive variables of the system. More important or characteristic relations will be observed in minerals with more chemical variability which respond readily to minor changes in the thermodynamic parameters of the system in which they are found. However, as the number of chemical components which are intensive variables (perfectly mobile components) increases the aluminous phases become more important because alumina is poorly soluble in aqueous solution, and becomes the inert component and the only extensive variable. 

Two phase assemblages of any of these minerals are known. It should be noted that aluminous phases, such as kaolinite, have never been reported with corrensite neither are sedimentary phyllosilicates such as 7 8 chlorite or glauconite. Non-phyllosilicates in association with corrensite frequently include diagenetic quartz, albite and dolomite. Pelitic rocks, specially associated with those containing corrensite, contain allevardite and fully expanding montmorillonite (dioctahedral). 

When an expanding mineral is no longer stable, the iron content of the chlorite in equilibrium with illite will become more variable (Figure 49b). If chlorite is present due to a relatively high Fe/Fe + Mg content of a rock, it can occur with three other aluminous phases such as illite-montmorillonite and kaolinite. Thus the four-phase phyllosilicate assemblage common to argillaceous rocks can be accounted for by dividing the... 

Molybdenum oxide - alumina systems have been studied in detail (4-8). Several authors have pointed out that a molybdate surface layer is formed, due to an interaction between molybdenum oxide and the alumina support (9-11). Richardson (12) studied the structural form of cobalt in several oxidic cobalt-molybdenum-alumina catalysts. The presence of an active cobalt-molybdate complex was concluded from magnetic susceptibility measurements. Moreover cobalt aluminate and cobalt oxide were found. Only the active cobalt molybdate complex would contribute to the activity and be characterized by octahedrally coordinated cobalt. Lipsch and Schuit (10) studied a commercial oxidic hydrodesulfurization catalyst, containing 12 wt% M0O3 and 4 wt% CoO. They concluded that a cobalt aluminate phase was present and could not find indications for an active cobalt molybdate complex. Recent magnetic susceptibility studies of the same type of catalyst (13) confirmed the conclusion of Lipsch and Schuit. 

Visible Reflection Spectra. The final calcination temperature of MoCo-124 samples has been varied in order to study its influence on the coordination of the cobalt ions. The reflection spectra are shown in Figure 1. The spectra of MoCo-124, calcined at 400 and 500°C show a broad absorption band, covering the whole spectral region, with a weak superposition of the characteristic triplet of cobalt aluminate. This indicates that the cobalt ions are for the greater part still on the catalyst surface and not in the alumina lattice. The spectra of the MoCo-124 samples, calcined at 650-700 °C show a strong increase in intensity of the triplet band, while the broad absorption band has disappeared. This indicates the formation of a cobalt aluminate phase. 

Spectra of adsorbed pyridine have been recorded for the MoCo-124 catalysts, for which the final calcination temperature after the cobalt impregnation has been varied. It turns out that the 400 and 500°C calcined samples and the 650 and 700°C calcined samples show very similar spectra. Therefore we show only the spectra of the 400°C (low calcined) and the 650°C (high calcined) samples. Figure 4 shows spectra after desorption at 150 and 250°C. Few Brdnsted acid sites are observed in the low calcined MoCo-124 samples. The reflection spectra (Figure 1) indicate for these low calcined samples the presence of cobalt on the catalyst surface, because no cobalt aluminate phase could be detected. The high calcined samples do show the presence of Brdnsted acid sites the presence of a cobalt aluminate phase is concluded from the reflection spectra (Figure 1) for these samples. 

The molybdate surface layer in the molybdenum-alumina samples is characterized by the presence of BrGnsted acid sites ( 1545 cm- ) and one type of strong Lewis acid sites (1622 cm l). Cobalt or nickel ions are brought on this surface on impregnation of the promotor. The absence of BrtSnsted acid sites is observed for both cobalt and nickel impregnated catalysts, calcined at the lower temperatures (400-500°C). Also a second Lewis band is observed at 1612 cnrl.The reflection spectra of these catalysts indicate that no cobalt or nickel aluminate phase has been formed at these temperatures. This indicates that the cobalt and nickel ions are still present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate layer. These configurations will be called "cobalt molybdate" and "nickel molybdate" and are shown schematically in Figure 11a. 

For the high temperature calcined cobalt-molybdenum-alumina catalysts, the presence of a cobalt aluminate phase has been concluded from the reflection spectra. The BrtSnsted acid sites reappear in the spectrum of absorbed pyridine, indicating that the... 

The optimal activity for a cobalt-molybdenum-alumina catalyst is obtained by calcination at the higher temperatures. This means that the cobalt ions, present as a cobalt aluminate phase according to the reflectance spectra and the magnetic susceptibility measurements, still have a pronounced promoting action after this calcination. The assumption of cobalt present in the surface layer of the alumina lattice explains both the high activity due to the cobalt promotion as well as the presence of the second Lewis band. This configuration is shown schematically in Figure lib. 

The reappearance of Brdnsted acid sites has been observed for the high calcined nickel-molybdenum-alumina catalysts. The presence of a nickel aluminate phase has been concluded from the reflectance spectra. The second Lewis band (1612 cm l) has a very low intensity, in comparison with the cobalt containing catalysts of a same composition and after the same calcination conditions. 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

The aluminate phase constitutes 5-10% of most normal Portland cement clinkers. It is tricalcium aluminate (Ca3Al206), substantially modified in... 

Destructive expansion from reaction with sulphates can occur not only if the latter are present in excessive proportion in the cement, but also from attack on concrete by sulphate solutions. The reaction involves the Al,0,-containing phases in the hardened cement, and in sulphate-resisting Portland cements, its effects are reduced by decreasing the proportion of the aluminate phase, sometimes to zero. This is achieved by decreasing the ratio of AljOj to Fe203 in the raw materials. In the USA, sulphate-resisting Portland cements are called Type V cements. 

Tentative composition for aluminate phase in white cement clinkers. 

Production clinkers have been found to contain cubic or orthorhombic forms of the aluminate phase, alone or in combination. The monoclinic modification has not been observed. The orthorhombic modification is also known as the prismatic, dark interstitial material, and is sometimes pseudo-tetragonal. It can arise only if sufficient alkali is available, but its formation appears to be favoured also by rapid cooling and by bulk compositions potentially able to yield a relatively high proportion of aluminate phase (M12). 

The aluminate phase in clinkers can also be characterized by its composition, determined by X-ray microanalysis this is discussed in the next section. 

Fig. 1.7 Portions of XRD powder patterns of clinkers containing (A) cubic, (B) orthorhombic and (C) pseudotetragonal modifications of the aluminate phase. Peaks marked A and F are due to aluminate and ferrite phases, respectively, and arc rc-indexed, where necessary, to correspond to axes in the text and Table 1.7, and to calculated intensities. After Regourd and Guinier (Rl).

Because of the close admixture with other phases, which is often on a scale of lOpm or less. X-ray microanalysis of the aluminate phase in clinkers is frequently difficult or unreliable. Data have been reported for cubic, orthorhombic, pseudotetragonal or unspecified forms of the aluminate phases in ordinary clinkers (R1,K1,B2,H3) and for aluminate phase (G3,G4,S1,B3) and glass (B3) in white cement clinkers. Tables 1.2 and 1.3 include, respectively, average compositions based on these somewhat scanty data, and suggested site occupancies based on them. The values in both tables take into account both the experimental data and the requirement of reasonable site occupancies. 

As with the aluminate phase, the average compositions take into account the requirement that these site occupancies should be reasonable from the standpoint of crystal chemistry. There is no basis for allocating cations to octahedral and tetrahedral sites separately as the preferences of some of the eations, especially Mg, in this structure are unknown, as is the temperature at which equilibrium is attained. This temperature probably varies between clinkers, and may be expected to affect the distribution. 

Portland cement clinkers contain small amounts of alkalis and sulphates derived from the raw materials and fuel. Both alkalis and SO3 can be present in the major clinker phases, but tend to combine preferentially with each other to form alkali or potassium calcium sulphates, and it is necessary to consider these components together. In addition, silicate and aluminate phases containing sulphate can form either as intermediates or in undesirable deposits in eement making, and a calcium aluminate sulphate is a major constituent of some expansive and other speeial cements. 

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