CaO cements

In the relatively small Pordand cement 2one almost all modem cements fall in the high lime portion (about 65% CaO). Cements of lower lime content tend to be slow in hardening and may show trouble from dusting of the clinker by transformation of to especially if clinker cooling is... 

It dissolves in dilute acids (in rainwater that is used as public water supply, this causes temporary hardness). It is a raw material in the Solvay process and is also used in manufacture of lime (CaO), cement, and glass. 

Some carbonates are important industrial chemicals. Calcium carbonate occurs naturally in several forms, including limestone, and is used in the production of quicklime, calcium oxide CaO, slaked (or hydrated) lime, calcium hydroxide Ca(OH)2 and cement. 

Refined calcined alumina is commonly used in combination with high purity limestone [1317-65-3] to produce high purity calcium aluminate cement (CAC). The manufacture, properties, and appHcations of CAC from bauxite limestone, as weU as high purity CAC, has been described (104). High purity CAC sinters readily in gas-fired rotary kiln calcinations at 1600 —1700 K. CAC reactions are considered practically complete when content of free CaO is less than 0.15% andloss on ignition is less than 0.5% at 1373 K. 

Table 3 Hsts the characteristics of CAC mineral constituents that can occur in CAC of varying purities. The primary hydraulic setting cement phase in all CAC grades is CA [12042-68-1/, CaO AI2O2 the main secondary phase in 70% AI2O2 CAC is Ca2, [12004-88-5] CaO 2AI2O2. CA2 and C 2 ...

Low Cement, Ultra-Low Cement, and No-Cement Castables are classified on the basis of calcium oxide content. These are 1—2.5, 0.2—1.0, and 0.2% CaO maximum, respectively. In the latter case the lime content is not a result of a hydrauHc setting cement constituent but comes from aggregate impurities. The insulating class is also subdivided. This division is shown in Table 14. Refractories used in steel-pouring pits are classified under ASTM C435 (Table 15). 

Portland Cement. Portland cement is obtained by calcining a mixture of substances to produce an appropriate ratio of the oxides CaO, MgO, AI2O2, Fe202, and Si02 (9) (see Cement). 

The conventional cement chemists notation uses abbreviations for the most common constituents calcium oxide [1305-78-8], CaO, = C siUcon dioxide... 

Fig. 1. Phase equilibria ia the C—A—S (CaO—AI2O2—Si02) system (3,4) temperatures are ia °C. Shaded areas denote two Hquids compositional index marks on the triangle are indicated at 10% intervals B denotes cristobaUte [14464-46-17, and D denotes tridymite [15468-32-3] both of Si02 composition E is anorthite [1302-54-17, Al2CaSi20g G is muUite [55964-99-3]-, H, gehlenite [1302-56-3], Ca2Al2Si02 and J is the area of Pordand cement compositions.

The relations between the compositions of Pordand cements and some other common hydrauhc cements are shown in the CaO—Si02—AI2O2 phase diagram of Figure 2 (5). In this diagram, Fe202 has been combined with AI2O2 to yield the Al O content used. This is a commonly appHed approximation that permits a two-dimensional representation of the real systems. 

Fig. 2. Cement 2ones in the CaO—AI2O2—Si02 system (5) where B represents basic blast-furnace slag D, cement compositions which dust on cooling E, compositions showing no tendency to set G, aluminous cement and PC, Pordand cement.

Raw Material Proportions. The three main considerations in proportioning raw materials for cement clinker are the potential compound composition the percentage of Hquid phase at clinkering temperatures and the bumabiUty of the raw mix, ie, the relative ease, in terms of temperature, time, and fuel requirements, of combining the oxides into good quaUty clinker. The ratios of the oxides are related to clinker composition and bumabiUty. For example, as the CaO content of the mix is increased, more C S can be formed, but certain limits cannot be exceeded under normal burning conditions. The lime saturation factor (LSF) is a measure of the amount of CaO that can be combined (20) ... 

Other Phases in Portland and Special Cements. In cements free lime, CaO, and periclase, MgO, hydrate to the hydroxides. The in situ reactions of larger particles of these phases can be rather slow and may not occur until the cement has hardened. These reactions then can cause deleterious expansions and even dismption of the concrete and the quantities of free CaO and MgO have to be limited. The soundness of the cement can be tested by the autoclave expansion test of Portiand cement ASTM C151 (24). 

Cement and concrete are used in construction on an enormous scale, equalled only by structural steel, brick and wood. Cement is a mixture of a combination of lime (CaO), silica (SiOj) and alumina (AI2O3), which sets when mixed with water. Concrete is sand and stones (aggregate) held together by a cement. Table 15.4 summarises the most important facts. 

When the kiln material is cooled it forms into crystallized clinkers. These are rather large irregular pieces of the solidified cement material. These clinkers are ground and a small amount of gypsum is added (usually about 1.5 to 3%). The gypsum prevents flash setting of the cement and also controls free CaO. This final cement product is sampled, analyzed and stored. The actual commercial cement is usually a hlend of several different cements. This blending ensures a consistent product. 

C03-0081. Portland cement contains CaO, Si02, AI2 O3, and FC2 O3. Calculate the mass percent composition of each compound. 

Three other compounds of s-block elements—calcium oxide (CaO, known as lime ), sodium hydroxide (NaOH), and sodium carbonate (Na2 CO3)—are among the top 15 industrial chemicals in annual production. Lime is perennially in the top 10 because it is the key ingredient in construction materials such as concrete, cement, mortar, and plaster. Two other compounds, calcium chloride (CaCl2 ) and sodium sulfate (Na2 SO4 ), rank just below the top 50 in industrial importance. 

C15-0054. Calcium oxide, an important ingredient in cement, is produced by decomposing calcium carbonate at high temperature CaC03( ) CaO(5 ) + C02(g) In one reaction, 3.5 kg of calcium... 

Certain oxides of divalent metals, those of ZnO, CuO, SnO, HgO, and PbO, form cements that are hydrolytically stable in addition MgO, CaO, BaO and SrO form cements that are softened when exposed to water. Compressive strengths of these materials range from 26 to 83 MPa, the strongest being the copper(II) and zinc polyacrylate cements (Table 5.1). Crisp, Prosser Wilson (1976) found that for divalent oxides the rate of reaction increased in the order... 

Figure 5.6 Triangular composition diagram for SiO -Alj Oj-CaO glasses, showing that glasses with cement-forming ability fall within the gehlenite and anorthite composition region, and that only glasses with less than 61 to 62% by mass of silica have the potential to form a cement (Hill Wilson, 1988a).

Table 5.7. Properties of cements formed from glasses corresponding to the generic formula xSiO Alf). CaO...

Figure 5.7 Triangular composition diagram for SiOj-Al Oj-CaO glasses. Glasses in the gehlenite region yield stronger cements (95 to 104 MPa in compression) than those in the anorthite region (29 to 56 MPa) (Hill Wilson, 1988a).

The early history of the cement is obscure. Dreschfeld (1907) and Sanderson (1908) attributed its invention to Fletcher. Fletcher (1878,1879) certainly described cements formed from concentrated orthophosphoric acid solutions and sintered mixtures of oxides which included SiOj, AljOj, CaO and ZnO. One was reported by Fletcher (1879) as being slightly translucent. These cements were not successful in clinical use. 

The first period of development ended with the research of Wright (1919) who published the results of an extensive survey of cements prepared from experimental SiOj-AljOj-CaO glasses and orthophos-phoric acid solutions containing aluminium phosphate. By this time the main cement formulations had been established. Between 1919 and 1950 only minor improvements were attempted these were of a technological nature and unsuccessful. 

The composition of the leachates does not correspond to the composition of the cement at all (Wilson Batchelor, 1967a,b). The predominant species eluted are the soluble sodium salts of phosphate and fluorides, although sodium is only a minor constituent of the cement. For one example of cement examined, the leachate contained 0-28 % sodium and 0-20% phosphate (expressed as a percentage of the amount of the species contained in the cement). For the major constituents of the glass the figures were 0 07% fluoride, 0 02% Al Oj, 0 01 % SiOj and 0 003% CaO. 

Setting times and hydrolytic stability of these cements are given in Table 8.3. In some cases the speed of reaction was very high, and practical cements could not be formed from ZnO or CaO even when these oxides were deactivated by heating. All the faster-setting cements exhibited good hydrolytic stability. The stability of the complexes between divalent cations and PVPA was found by a titrametric procedure to follow the order Mg Ca < Cu Zn (Ellis Wilson, 1991). This result was... 

The low permittivity of these liquids compared with water inhibits dissociation of the acids so that cement formation demands much more reactive basic oxides. Oxides and hydroxides that are capable of cement formation are ZnO, CuO, MgO, CaO, Ca(OH)2, BaO, CdO, HgO, PbO and BiaOj (Brauer, White Moshonas, 1958 Nielsen, 1963). In practice these are confined to two calcium hydroxide and special reactive forms of zinc oxide. 

Eugenol, 4 allyl-2-methoxy phenol, is capable of forming cements with ZnO, CuO, MgO, CaO, CdO, PbO and HgO (Brauer, White Moshonas, 1958 Nielsen, 1963). Other 2-methoxy phenols are also capable of forming cements with metal oxides, provided the allyl group is not in a 3- or 6-position where it sterically hinders the reaction (Brauer, Argentar Durany, 1964). These include guaiacol, 2-methoxyphenol, and the allyl and propylene 2-methoxy phenols. 

Moshonas (1958) investigated the reactions between zinc oxide and a large number of chelating agents. Of these, EBA proved to be the most promising. They then examined cement formation between EBA and various metal oxides. Cement formation was found with MgO, CaO, BaO, ZnO, CdO, HgO and PbO. 

The commercial product is actually a blend of different cements. This blending leads to a more constant quality. The chemical composition of Portland cement is typically 60% to 69% CaO, 18% to 24% Si02, 4% to 8%... 

Expanding additives improve the filtering properties of a cement [21,1720]. An expanding additive is based on calcium oxide (CaO) fired at high... 

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