Calcium aluminate hydroxide

Castable Monolithic Refractories Standard portland cement is made of calcium hydroxide. In exposures above 427°C (800°F) the hydroxyl ion is removed from portland (water removed) below 427°C (800°F), water is added. This cyclic exposure results in spalling. Castables are made of calcium aluminate (rather than portland) without the hydroxide they are not subject to that cychc spaJhng failure. 

Aluininatcs Sodium aluminate NaAlOi, white solid, soluble, (1) by reaction of aluminum hydroxide and NaOH solution. (2) by fusion of aluminum oxide and sodium carbonate the solution of sodium aluminate is reactive with CO2 to form aluminum hydroxide. Used as a mordant in the textile industry, in the manufacture of artificial zeolites, and in the hardening of budding stones. See silicates below and calcium aluminates. 

The primary reaction of any pozzolanic material is an attack on the SiOj or AljOj-SiOj framework by OH ions. It may be supposed that the OH ions attach themselves to silicon and other network-forming atoms, with consequent breaking of bonds between the latter and oxygen atoms. After this has occurred several times, the silicate or other oxy anion is detached from the framework. It may either remain in situ or pass into the solution. The charges of those that remain are balanced, partly by H, and partly by metal cations. Since a cement pore solution is essentially one of potassium and sodium hydroxides, the immediate product is likely to be an amorphous material with and Na as the dominant cations, but the more abundant supply of Ca and the lower solubility of C-S-H and hydrated calcium aluminate or silicoaluminate phases will ensure that this is only an intermediate product. Its presence is indicated by the relatively high potassium contents observed in or near to the reacting pfa particles. 

In contrast, resistance to attack by alkali hydroxide solutions is low, probably because the hydrous alumina is dissolved. Attack is accentuated if CO2 is also present, as can occur if the calcium aluminate concrete is in contact with Portland cement concrete and exposed to atmospheric CO2 (R48). For CAHjq, the following reactions occur ... 

Portland cement before treatment with water consists of a mixture of calcium silicates, mainly Ca2Si04 and CagSiO, and calcium alumi-nate, Ca3Al20. . When treated with water the calcium aluminate hydrolyzes, forming calcium hydroxide and aluminum hydroxide, and these substances react further with the calcium silicates to produce calcium aluminosilicates, in the form of intermeshed crystals. 

The calcium aluminate still present reacts with the calcium hydroxide formed to tetracalcium aluminate hydrate. Calcium aluminum ferrite forms the analogous hydration product, which reacts more slowly. Part of the aluminum is replaced by iron ... 

Portland cement is typically composed of about 25% P-dicalcium silicate (lamite), and 50% tricalcium silicate with the balance made up of various calcium aluminates and calcium iron aluminate (brownmillerite). Setting occurs when the cement is hydrated all the components show varying degrees of reactivity with water, but the most significant hydraulic activity is associated with the tricalcium silicate, which forms a cohesive mixture of calcium hydroxide and calcium silicate hydrate (C-S-H)... 

Western-world bauxite production in 1988 totaled about 90 x 10 t, approximately 90% of which was refined to aluminum hydroxide by the Bayer process. Most of the hydroxide was then calcined to alumina and consumed in making aluminum metal. The balance, which constituted about 2.3 x 10 t in 1988 (Table 2), was consumed in production of abrasives (qv) adhesives (qv) calcium aluminate cement used in binding ceramics (qv) and refractories (qv) catalysts used in petrochemical processes and automobile catalytic converter systems (see Petroleum Exhaust control, automotive) ceramics that insulate electronic components such as semiconductors and spark plugs chemicals such as alum, aluminum halides, and zeoHte countertop materials for kitchens and baths cultured marble fire-retardant filler for acryhc and plastic materials used in automobile seats, carpet backing, and insulation wrap for wire and cable (see Flame retardants) paper (qv) cosmetics (qv) toothpaste manufacture refractory linings for furnaces and kilns and separation systems that remove impurities from Hquids and gases. 

Artificial hydraulic limes consist mainly of calcium hydroxide, calcium silicates and calcium aluminates. They are produced by blending suitable powdered materials, such as natural hydraulic limes, fully hydrated air limes and dolom-itic limes, pulverised fuel ash, volcanic ash, trass, ordinary Portland cement and blast furnace slag. 

Natural hydraulic limes are limes produced by burning, at below 1250 °C, of more or less argillaceous or siliceous limestones, with reduction to powder by slaking with or without grinding. They consist of calcium silicates, calcium aluminates and calcium hydroxide. 

The sensitivity of concrete towards the action of seawater is above all due to the presence of calcium hydroxide and hydrated calcium aluminates, which are both susceptible to reactions with sulfate ions from the seawater. Expansive effects decrease, therefore, as the percentage of CjA in the cement or the content of Ca(OH)2 in the concrete diminishes. The traditional approach to avoid these reac-... 

Using ordinary and sulfate-resistant Portland cement to represent differing chloride environments, short-term electrochemical monitoring and SEM were used to characterize corrosion behavior [34]. Steel electrodes attained passivity in mortar with high levels of calcium aluminate, up to 1% wt. chloride. At 1.75% wt. chloride, steel electrodes corrode. All chloride levels resulted in steel corrosion for low levels of calcium aluminate. Pore solution was also impacted by mortar exposure conditions. Atmosphere exposure had a high influence on hydroxide concentration in pore solution but no impact on chloride concentration. Carbonation was also investigated samples in a sealed container had a chloride/hydroxide ratio half that of unsealed samples. 

There is a cormnon opinion that corrosion of concrete needs the liquid environment or at least an atmosphere of high hmnidity. The transport of liquid through the concrete causes the sequence of processes, includiug at first the concrete components of the highest reactivity calcium hydroxide and calcium aluminate hydrates. One can thus conclude that the phase composition of cement has a great impact on the behavior of concrete in any aggressive environment. 

The role of the ferrite phase, generally identified as brownmillerite, should be mentioned too. In the case of sulphate attack this phase can be the source of almninate ions [237] moreover the ferrite ions can form the analogue of ettringite or to substitute the aluminate ions in all calcium aluminate phases [222]. The latter case is undoubtedly the most common one in the Portland cement paste. However, the reaction of sulphate ions with ferrites is slower. There is a view that the F/Al ratio in the hydrated phases is lower than in brownmillerite hence, some amount of iron(in) hydroxide is always present [222] (see also Sect. 4.1.1.). This hydroxide occurs in the gel-like form and therefore the diffusion of ions through the gel layer is slowed down. Therefore, the corrosion process is hindered. The other phases containing the Fe ions can be produced too, it is discussed in Chap. 3. 

Usually in the process of CA hydration the mixture of the two hexagonal hydrates together with the colloidal aluminum hydroxide are formed. It is all the more probable that calcium aluminate cement contains always some amount of C,2A2 phase, which hydrating gives at once these two hexagonal phases ... 

The calcium aluminate cements are not resistant to the attack of alkalis, because the aluminum hydroxide is soluble in the water solutions of sodium and potassium hydroxides, with which the calcium aluminates are also reacting with. The alkali metals carbonates react with calcium aluminate hydrates and these reactions cause destmction of concrete ... 

As it was shown by Mehta [46], the rate of ettringite formation in the mixture of anhydrous calcium aluminates, calcium hydroxide and gypsum was the highest in the case of CA. The reaction in the mixture with C4 AjS was slower, while the reaction of C3A was hampered, because of the impermeable ettringite layer formation (see Sect. 3.3). The C,2A.7 is also not a good source of ettringite, because it reacts... 

D. Boiling sodium hydroxide solution (10% by mass) will turn calcium aluminate blue or brown in 20 seconds in a high-alumina cement. Etching 30 seconds with a 1% borax solution turns Cj2A gray (Long, 1983). 

Alumina Hydrate RH-10W, Alumina Hydrate RH-20, Alumina Hydrate RH-30, Alumina hydrate Alumina hydrated. See Aluminum hydroxide Aluminasol 100, Aluminasol 200, Alumina, Tabular 635, Alumina, tabular. See Alumina Aluminate, calcium. See Calcium aluminate Aluminate, sodium. See Sodium aluminate Alumina Trihydrate 617. See Alumina Alumina trihydrate. See Aluminum hydroxide Aluminex-1. See Polyaluminum chloride Aluminic acid. See Aluminum hydroxide Aluminium. See Aluminum Aluminium oxide alumite Aluminium oxydes C. See Alumina... 

Aluminum calcium silicate Ammonium polyphosphate Calcium aluminate Calcium hydroxide Calcium sulfite Candelilla synthetic Collodion Copal resin Elemi gum Epoxy resin Feldspar Gelatin Magnesium chloride Potassium carbonate Shellac Zirconium silicate cement additive... 

The products formed in the hydration of the ferrite phase are similar to those formed in the hydration of C3A the Fe replaces to a limited degree in the crystalline lattice. The A/F ratio in the hydrates formed is usnally higher than that in the original calcium aluminate ferrite, and the fraction of iron that has not been incorporated into any of the hydrate phases remains in the hardened cement paste in the form of amorphous iron oxide, hydroxide or another trrtspecified iron-containing phase. 

In addition to calcium aluminate hydrates, crystalline aluminum hydroxide [Al(OH)3 abbreviation AH3, gibbsite] or amorphous aluminate hydrate (AH ) is also formed in the hydration of some calcium aluminates. 

Dicalcium silicate in calcium aluminate cements does not yield a C-S-H phase and free calcium hydroxide, as occurs in Portland cement. Instead, stratlingite (also called gehlenite hydrate, C2AHg) is formed as the product of hydration. 

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