Calcium silicates/aluminates

Several different possible zeolite structures may result, and if the sodium content is too high, calcium and magnesium are excluded and a hard zeolite scale of sodium-aluminum silicate preferentially forms. If only calcium is present, calcium-aluminum silicate zeolite forms, also as a hard scale. If only magnesium is present in solution, it forms the flocculant magnesium aluminate, MgAl204. 

Small amounts of other compounds can be added to Ni-based catalysts to improve the functional characteristics of the final catalyst. Typically, they are calcium aluminate to enhance the mechanical resistance of the catalyst pellets, potassium oxide to improve the resistance to coke formation and silica to form a stable silicate with potassium oxide [34]. Promotion with rare earth oxides such as La2C>3 also results in enhanced resistance to coking. 

Another field with a large potential for improvements concerns aluminosilicate minerals, which are of great importance in determining the chemistry of water in many types of rock. In backfill clays, aluminosilicates are responsible for the retention (sorption, incorporation) of trace elements and may affect both oxidation potential (incorporation of Fe(II)/Fe(III)) and pH (hydrolysis of silicate and/or exchange of H+). Related classes of compounds (i.e., calcium silicates and calcium aluminates) form the chemical backbone of cementitious materials. The thermodynamic properties of these substances are still largely unexplored. 

A shorthand notation is often nsed to designate the oxides (e.g., C for CaO and A for AI2O3 see Table 5.7), and it is also used to designate the compounds formed between the components during heating, such as calcium aluminate (CA) and tricalcium silicate (C3S) ... 

An alternative to silicate-based Portland cement is the calcium aluminate cement, ciment fondu, which originated with the Lafarge company in France in 1908. Ciment fondu is typically made by heating limestone with bauxite, which is mainly AIO(OH) but contains much iron oxide (see Section 17.2). As noted above, calcium aluminate hydrates and hardens much more rapidly than alite, and so ciment fondu, either as such or mixed with Portland cement, can be used whenever a rapidly setting cement is required, for example, for construction at low temperatures. Concretes made from aluminate cements remain serviceable at higher temperatures than Portland cements and so are used to make cast refractories for pyrometal-lurgical applications. 

Portland cement is classified as a hydraulic cement, ie, it sets or cures in the presence of water. The term Portland comes from its inventor, Joseph Aspdin, who in 1824 obtained a patent for the combination of materials referred to today as Portland cement. He named it after a grayish colored, natural limestone quarried on the Isle of Portland, which his cured mixture resembled. Other types of hydraulic cements based on calcium materials were known for many centuries before this, going back to Roman times. Portland cement is not an exact composition but rather a range of compositions, which obtain the desired final properties. The compounds that make up Portland cements are calcium silicates, calcium aluminates, and calcium aluminoferrites (see ). 

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 dry product is ground to a powder and then a little calcium sulfate (CaS04) is added to slow down the setting rate of the cement. When water is added to the mixture, slow complex chemical changes occur, resulting in the formation of a hard interlocking mass of crystals of hydrated calcium aluminate and silicate. 

Type IV. Low-heat portland cements contain a lower percentage of calcium silicate and calcium aluminate, thus lowering the heat evolution. Type V. Sulfate-resisting portland cements are those that, by their composition or processing, resist sulfates better than the other four types. Type V is used when high sulfate resistance is required. 

This is by far the most frequently encountered interference in AAS. Basically, a chemical interference can be defined as anything that prevents or suppresses the formation of ground state atoms in the flame. A common example is the interference produced by aluminium, silicon and phosphorus in the determination of magnesium, calcium, strontium, barium and many other metals. This is due to the formation of aluminates, silicates and phosphates which, in many instances, are refractory in the analytical flame being used. 

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. 

Mineral additions may be broadly categorized as pozzolanic materials or latent hydraulic cements. Neither type reacts significantly with water at ordinary temperatures in the absence of other substances. Pozzolanic materials are high in Si02 and often also in AI2O3, and low in CaO they are sufficiently reactive that mixtures of them with water and CaO produce C-S-H at ordinary temperatures and thereby act as hydraulic cements. If they contain AI2O3, calcium aluminate or aluminate silicate hydrates are also formed. Because they are low in CaO, this component must be supplied in stoichiometric quantity. In a composite cement, it is provided by the Portland cement through decreased formation of CH and decreased Ca/Si... 

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. 

Since calcium oxide is more than sparsely soluble and its reaction with phosphoric acid or a soluble phosphate is highly exothermic, researchers have used less soluble salts of calcium to react with the phosphates and form a phosphate ceramic [4-12]. In the acidic medium of the phosphate solutions, the salts of calcium dissolve slowly and release Ca (aq) into the solution, which subsequently reacts with phosphate anions and forms calcium phosphates. The best calcium minerals for forming CBPCs are combination of oxides of calcium and insoluble oxides such as silica or alumina, e.g., calcium silicate (CaSi03) and calcium aluminate (CaAl204), or even a phosphate of calcium such as tetracalcium phosphate (Ca4(P04)2 0). These minerals are reacted with acid phosphate salts to form phosphate cements. 

With the procedure given above, one may also calculate the pH dependence of (Ca (aq)) from dissolution of calcium aluminates. The results of such calculations are summarized in Table 13.1. Figure 13.2 shows the solubility diagram for (Ca (aq)) as a function of pH for both silicates and aluminates drawn from the equations given in Table 13.1. 

The authors [34] proposed to use perovskites ABO3, where A are calcium cations, or a mixture of calcium and lanthanum, and B are iron, cobalt, nickel or manganese cations, or their mixtures. Besides, aluminates, silicates, aluminium sihcates, zirconates and chromates of different types are added as structure-forming components providing strength and stability to thermal shocks [34]. 

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. 

Inorganic monolithics like calcium aluminate, calcium silicate, sodium silicate and potassium silicate gunites are able to withstand the dry flue gases at high temperatures. Because they are porous and tend to crack, however, they allow acid vapors to reach the substrate and condense. 

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)... 

Portland cement and high-alumina cements contain, in addition to calcium silicate phases, calcium monoaluminate, CaAl204 (or CA in cement chemist s shorthand, where C = CaO and A = AI2O3). The Al NMR spectra of this compound, in which the Al is exclusively in tetrahedral coordination, and a number of other calcium alu-minates have been determined (Muller et al. 1986), and more recently, using satellite transition spectroscopy (SATRAS) which has allowed the multiple tetrahedral sites in the various calcium aluminates to be distinguished (Skibsted et al. 1993). The NMR parameters for the synthetic aluminates and a number of their hydration products are shown in Table 5.4. 

Carlson, K. and Hall, M.. Streaming potential measurements performed on silicate and calcium aluminate glass microspheres, Colloids Surf. A, 325, 101. 2008. 

The mixture of calcium silicate and calcium aluminate that remains molten at the furnace temperature is known as slag. 

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. 

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