Water/aluminate ratio

The principal cement and admixture factors that influence the rheological behavior in HPC, particularly at very low water-cement ratios, are the following (1) the quantity of tricalcium aluminate (C3A) and to a lesser degree... 

Figure 5.32. A. Al NMR spectra of (top) unhydrated alumina cement (principally monocalcium aluminate), and (bottom) product of full hydration with demineralised water at a cement water mass ratio of 1 1. Asterisks indicate spinning side bands. B. Change in the percentage of four-coordinated Al in alumina cement during hydration, as a function of time estimated by Al MAS NMR. Open symbols (a) hydration with demineralised water. Filled symbols (b) hydration with 0.5 mass percent Li2C03 solution. After Luong et al. (1989), by permission of the American Ceramic Society.

Dawsonite occurs in oil shale as a matrix mineral formed from the lake water as a chemical precipitate in the sediment. Bader and Esch (26) synthesized dawsonite by bubbling CO 2 into a sodium aluminate solution at a pH of 11. As the pH of their solution dropped, dawsonite precipitated. Dawsonite was obtained only when the sodium carbonate to aluminate ratios were higher... 

The rheology of calcium aluminate cement pastes is broadly similar to that of Portland cement pastes (Banfill and Gill, 1986). They show a Bingham-type behavior with slightly lower yield values and a plastic viscosity. If agitated, they exhibit a stmctural breakdown that depends on the intensity and duration of mixing. Their flow curves are characterized by a distinct hysteresis loop. Usually, concrete mixes made with calcium aluminate cement are somewhat more free flowing, which makes it possible to use a lower water/cement ratio. 

Owing to a higher water requirement for hydration, the porosity of hardened aluminous cement pastes is lower than that of Portland cement at the same water/cement ratio. This has a favorable effect on permeability and along with it on the corrosion resistance and freeze-thaw resistance of the hardened material. However, the conversion to aluminous cement is associated with a distinct increase of both porosity and permeability, which must be considered when this cement is used in practice. 

At low or medium water/cement ratios the porosity and permeability of hydrated non-converted aluminous cement pastes are sufficiently low to confine the corrosive action of any external chemical agents to the surface region of the concrete structure. However, as the porosity increases in the course of conversion, the susceptibility to chemical attack of concrete based on aluminous cement increases. An effective way to prevent this from happening is to use initial water/cement ratios that are too low for complete hydration. Under these conditions the water liberated in the conversion of the hexagonal calcium aluminate hydrate phases, formed initially, reacts with the non-hydrated fraction of the cement, thus preserving a low porosity of the hardened paste. Note that the permeability is the main factor determining the resistance of aluminous cement concrete to chemical agents, and this has to be kept in mind when calcium aluminate cement is used in practice. 

In conclusion, the resistance of aluminous cement based concrete to most chemical corrosive agents is outstanding, provided that the mix is produced with a sufficient cement content and with a low water/cement ratio, and is properly compacted and cured. Like other hydrated cementitious systems, hardened calcium alununate cement pastes decompose and undergo chemical reactions upon heating ... 

At low water/cement ratios, below w/c=0.35, the main ciystalline phase formed in the hydration of barium aluminate cement is monobarium aluminate heptahydrate (BaO.Al2O3.TH2O), whereas at higher water contents tribarium aluminate hexahydrate (3BaO.Al2O3.6H2O), together with hydrous alumina, is also formed. 

Barium aluminate cement liydrates quite rapidly, and at sufficiently high water/cement ratios it may hydrate completely within three days. The hardened cement paste exhibits only a small strength loss if heated to high temperatures. 

Concrete based on calcium aluminate cement performs rather well if exposed to sulfate solutions, especially if made with a low water/ cement ratio and high cement content. However, cases of expansion and cracking have also occasionally been reported (Scrivener and Capmas, 1998). The reasons for the good sulfate resistance of this type of cement are not obvious. It is mostly attributed to a surface densification of the hardened material, resulting in a very low permeability of the formed surface layer, and/or to the absence of calcium hydroxide in the system (Scrivener and Capmas, 1998). Unlike Portland cement and related binders, magnesium sulfate solutions are less aggressive to calcium aluminate cement than alkali sulfate solutions. This is due mainly to the absence of the C-S-H phase in the hardened calcium aluminate cement pastes, which is particularly sensitive to the action of magnesium sulfate. 

Ramachandran, V. S., and Feldman, R. F., Significance of Low Water/ Solid Ratio and Temperatnre on the Physico-Chemical-Mechanical Characteristics of Hydrates of Tricalcium Aluminate, J. Appl. Chem. Biotech., 23 625-633 (1973)... 

Figure 2.8 Comparison between measured and calculated heats of hydration of a calcium aluminate cement blended with hemihydrate (with a ratio of calcium alumi-nate to hemihydrate of 80 20 by mass). The sample was hydrated at 20°C using a water-solid ratio of 0.40. Heat flow was calculated from quantitative XRD analyses by using the dissolution enthalpies of the anhydrous phases and the precipitation enthalpies of the hydrate phases. CA calcium aluminate Ett ettringite HH hemihydrate Ms monosulfate. (From Bizzozero, J., Hydration and dimensional stability of calcium aluminate cement based systems , PhD Thesis no. 6336, Ecole Polytechnique Fed rale de Lausanne, Switzerland, 2014.)...

Type V (High Sulfate Resistance). Type V Pordand cement is used in concrete exposed to severe sulfate attack of 1,500 to 10,000 ppm. Low concentrations of tricalcium aluminate [12042-78-3] give Type V its sulfate resistance. The sulfate resistance is improved with air entrainment and low water to cement ratios in the wet concrete. U.S. production of Type V Pordand cement in 1989 was 0.9% of the total Pordand cement production. 

The oxidation of cobalt metal to inactive cobalt oxide by product water has long been postulated to be a major cause of deactivation of supported cobalt FTS catalysts.6 10 Recent work has shown that the oxidation of cobalt metal to the inactive cobalt oxide phase can be prevented by the correct tailoring of the ratio Ph2cJPh2 and the cobalt crystallite size.11 Using a combination of model systems, industrial catalyst, and thermodynamic calculations, it was concluded that Co crystallites > 6 nm will not undergo any oxidation during realistic FTS, i.e., Pi[,()/I)i,2 = 1-1.5.11-14 Deactivation may also result from the formation of inactive cobalt support compounds (e.g., aluminate). Cobalt aluminate formation, which likely proceeds via the reaction of CoO with the support, is thermodynamically favorable but kinetically restricted under typical FTS conditions.6... 

Addition of aluminum enhances most underwater expin effects. In conventional CHNO expls, A1 reacts to form A1203 with the liberation of a large amount of heat. This reaction is relatively slow and is rarely complete during the detonation regime in aluminized expls fired in air or under moderate confinement. In free water, however, sufficient confinement is available to enable the reaction to occur before appreciable expansion of the other detonation products. The enhancement of expin effects by A1 can be estimated from the curves shown in Fig 10 (from Ref 17). The abscissa of Fig 10 is the gram atom Al/O ratio of the expl compn... 

The sodium aluminate and appropriate amounts of surfactants/templates were dissolved in water using C(/Cl4 ratios of 85%/15% and 75%/25%, respectively, and the mixture was aged over night (16-20 hours) according to ref [6]. Subsequently, the usual gel preparation described earlier [4] were followed with no further changes, then the gels were transferred to PTFE-lined autoclaves and heated at 150-175°C for 6 days, quenched in cold water, washed 3-4 times in liberal amounts of water, decanted, filtered and dried overnight in ambient air. 

The above results show that post synthesis alumination of PSM with AlfNOjfi improves the hydrothermal stability of the resulting AMM material. Similar effect has been observed by Mokaya et al. [12], who reported that the hydrothermal stability of MCM-41 could be enhanced by reaction with chlorohydrate of aluminium. Moreover, from the study of high Si/Al ratio of Y zeolite, Lutz et al. [13] reported that the hydrothermal stability of Y zeolite was enhanced by an external introduction of non-structural aluminum species onto the surface of Y zeolite. The surface layer of Al-rich aluminosilicate or aluminum oxide was suggested to block the terminal OH groups and energy-rich =Si-0-Si= bonds on the surface of Y zeolite, hence minimizing the attack of water molecules on the framework. Due to these properties, the non-structural... 

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