Cement-forming metal oxides

Concentrated solutions of orthophosphoric acid, often containing metal salts, are used to form cements with metal oxides and aluminosilicate glasses. Orthophosphoric acid, often referred to simply as phosphoric acid, is a white crystalline solid (m.p. 42-35 °C) and there is a crystalline hemihydrate, 2H3PO4.H2O, which melts at 29-35 °C. The acid is tribasic and in aqueous solution has three ionization constants (pA J 2-15,7-1 and 12-4. 

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. 

Nielsen, T. H. (1963). The ability of 39 chelating agents to form cements with metal oxides, respecting their usability as root-filling materials. Acta Odontologica Scandinavica, 21, 159-74. 

The polyelectrolyte cements are modern materials that have adhesive properties and are formed by the cement-forming reaction between a poly(alkenoic acid), typically poly(acrylic acid), PAA, in concentrated aqueous solution, and a cation-releasing base. The base may be a metal oxide, in particular zinc oxide, a silicate mineral or an aluminosilicate glass. The presence of a polyacid in these cements gives them the valuable property of adhesion. The structures of some poly(alkenoic acid)s are shown in Figure 5.1. 

Oxysalt bonded cements are formed by acid-base reactions between a metal oxide in powdered solid form and aqueous solutions of metal chloride or sulphate. These reactions typically give rise to non-homo-geneous materials containing a number of phases, some of which are crystalline and have been well-characterized by the technique of X-ray diffraction. The structures of the components of these cements and the phase relationships which exist between them are complex. However, as will be described in the succeeding parts of this chapter, in many cases there is enough knowledge about these cements to enable their properties and limitations to be generally understood. 

Ellis and Wilson studied cements formed from concentrated solutions of poly(vinylphosphonic acid) (PVPA) and oxides and silicate glasses, which they termed metal oxide and glass polyphosphonate cements (Wilson ... 

Ellis Wilson (1991, 1992) examined cement formation between a large number of metal oxides and PVPA solutions. They concluded that setting behaviour was to be explained mainly in terms of basicity and reactivity, noting that cements were formed by reactive basic or amphoteric oxides and not by inert or acidic ones (Table 8.3). Using infrared spectroscopy they found that, with one exception, cement formation was associated with salt formation the phosphonic add band at 990 cm diminished as the phosphonate band at 1060 cm" developed. The anomalous result was that the acidic boric oxide formed a cement which, however, was soluble in water. This was the result, not of an add-base readion, but of complex formation. Infrared spectroscopy showed a shift in the P=0 band from 1160 cm" to 1130 cm", indicative of an interaction of the type... 

Ellis, J. Wilson, A. D. (1991). A study of cements formed between metal oxides and polyvinylphosphonic acid. Polymer International, 24, 221-8. 

For example, calcium oxide is a metal oxide that is important in the construction industry as an ingredient of cement. Calcium oxide reacts with water to form a basic solution of calcium hydroxide. 

Oxychloride and oxysulfate cements are another class of acid-base cements. These are formed by reaction of a metal oxide such as that of magnesium oxide with a chloride or sulfate of a metal in the presence of water. Magnesium and zinc based oxychloride cements have been developed fully. 

Numerous important mixed oxide phases are formed by the combination of aluminum oxide and one or more other metal oxides. The foremost of these from an industrial viewpoint are spinels, jS-aluminas, and tricalcium aluminate, an important constituent of Portland cement. 

Within clinical dentistry, there are several types of cement available, including the zinc phosphate and the zinc polycarboxylate. They share with glass-ionomers the feature of being acid-base cements and setting as the result of a neutralization reaction, and consequently they are hydrophilic by nature [7]. These cements differ from each other in that they have different acid and base components, but they resemble each other in that the acid is always an aqueous solution and the base is a water-insoluble soUd metal oxide powder. The setting reaction, which begins immediately when the components are mixed, involves acid attack on the solid powdered base, and leads to the release of metal ions into the aqueous phase. In this phase, the metal ions interact with the acid (or its anion) to form metal salts, and these are rigid and insoluble. As these salts form, so the overall cement hardens and becomes insoluble in saliva and other aqueous media [7]. 

The alkaline condition leads to a passive layer forming on the steel surface. The passive layer is a dense, impenetrable film, which, if fully established and maintained, prevents further corrosion of the steel. The layer formed on steel in concrete is probably part metal oxide/hydroxide and part mineral from the cement. A true passive layer is a very dense, thin layer of oxide that leads to a very slow rate of oxidation (corrosion). There is some discussion as to whether or not the layer on the steel is a true passive layer as it seems to be thick compared with other passive layers and it consists of more than just metal oxides but as it behaves like a passive layer it is generally referred to as such. 

Many salts occur in nature, and some are used as industrial raw materials. Examples are sodium chloride, NaCl (a source of CI2 and NaOH) calcium carbonate or limestone, CaC03 (a source of cement and building stone) and calcium phosphate or rock phosphate, Ca3(P04)2 (a source of fertilizer). In the laboratory, salts can be prepared by reacting a solution of an appropriate acid with a metal, a metal oxide, a metal hydroxide, a metal carbonate, or a metal bicarbonate. These reactions, given earlier as examples of acid properties (Equations 9.19, 9.13, 9.14,9.15, 9.16), are given below in a general form ... 

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