Cement aluminosilicates

The potassium carbonate is soluble and washes away, but the aluminosilicate remains as the clay. Cements are obtained when aluminosilicates are roasted with limestone and other minerals and then allowed to solidify (see Section 14.10). 

In the 1870s more effective liquid cement-formers were found ortho-phosphoric acid and eugenol (Wilson, 1978). It was also found that an aluminosilicate glass could replace zinc oxide, a discovery which led to the first translucent cement. Thereafter the subject stagnated until the late 1960s when the polyelectrolyte cements were discovered by Smith (1968) and Wilson Kent (1971). 

Aluminosilicate glasses are used in certain AB cement formulations, and the acid-base balance in them is important. The Bronsted-Lowry theory cannot be applied to these aluminosilicate glasses it does not recognize silica as an acid, because silica is an aprotic acid. However, for most purposes the Bronsted-Lowry theory is a suitable conceptual framework although not of universal application in AB cement theory. 

From this discussion it can be seen that there is no ideal acid-base theory for AB cements and a pragmatic approach has to be adopted. Since the matrix is a salt, an AB cement can be defined quite simply as the product of the reaction of a powder and liquid component to yield a salt-like gel. The Bronsted-Lowry theory suffices to define all the bases and the protonic acids, and the Lewis theory to define the aprotic acids. The subject of acid-base balance in aluminosilicate glasses is covered by the Lux-Flood theory. 

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. 

On mixing the cement paste, the calcium aluminosilicate glass is attacked by hydrogen ions from the poly(alkenoic acid) and decomposes with liberation of metal ions (aluminium and calcium), fluoride (if present) and silicic acid (which later condenses to form a silica gel). 

Ellison, S. Warrens, C. (1987). Solid-state nmr study of aluminosilicate glasses and derived dental cements. Report of the Laboratory of the Government Chemist. 

Wilson, A. D., Crisp, S., Prosser, H. J., Lewis, B. G. Merson, S. A. (1980). Aluminosilicate glasses for polyelectrolyte cements. Industrial Engineering Chemistry Product Research Development, 19, 263-70. 

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. 

In vivo studies have indicated that zinc phosphate cements erode under oral conditions. Also, cements based on zinc oxide, including the zinc phosphate cement, are less durable in the mouth than those based on aluminosilicate glasses, the dental silicate and glass-ionomer (Norman et al., 1969 Ritcher Ueno, 1975 Mitchem Gronas, 1978,1981 Osborne et al., 1978 Pluim Arends, 1981, 1987 Sidler Strub, 1983 Mesu Reedijk, 1983 Theuniers, 1984 Pluim et al., 1984, Arends Havinga, 1985). However, there is some disagreement on whether the zinc phosphate cement is more durable than the zinc polycarboxylate cement. 

Dental silicate cement was also variously known in the past as a translucent, porcelain or vitreous cement. The present name is to some extent a misnomer, probably attached to the cement in the mistaken belief that it was a silicate cement, whereas we now know that it is a phosphate-bonded cement. It is formed by mixing an aluminosilicate glass with an aqueous solution of orthophosphoric acid. After preparation the cement paste sets within a few minutes in the mouth. It is, perhaps, the strongest of the purely inorganic cements when prepared by conventional methods, with a compressive strength that can reach 300 MPa after 24 hours (Wilson et al, 1972). 

The silicophosphate cement originated with the dental silicate cement, for there is no doubt that early investigators experimented with mixtures of aluminosilicate glass and zinc oxide (Fletcher, 1878,1879 Eberly, 1928). It appears to have no particular advantages. As is often the case with hybrids, it can combine the worst features as well as the best of the parents, and often properties have intermediate values. Nevertheless, it continues to have a small but persistent usage arising from its one advantage over the... 

In 1968 Wilson published an account of his early search for alternatives to orthophosphoric acid as a cement-former with aluminosilicate glasses. Aluminosilicate glasses of the type used in dental silicate cements were used in the study and were reacted with concentrated solutions of various organic and inorganic adds. Wilson (1968) made certain general observations on the nature of cement formation which apply to all cements based on aluminosilicate glasses. 

Table 8.1. Properties of aluminosilicate glass cements prepared with various acids in aqueous solution Wilson, 1968)...

Ellis and Wilson also examined cement formation from aluminosilicate glasses and concentrated solutions of PVPA (Wilson Ellis, 1989 Ellis, 1989 Ellis Wilson, 1990). These cements, like the glass polyalkenoate cements, are a type of glass-ionomer cement. 

An equally simple chemical study was carried out on phytic acid-aluminosilicate cements (Prosser et al., 1983). Phytic acid, myo-inositol hexakis(dihydrogen phosphate), is a naturally occurring substance found in seeds, and it is a stronger acid than phosphoric acid. Cements were prepared using aqueous solutions of phytic acid, concentrated to 50 wt%, and with 5 wt % zinc dissolved in the acid to moderate the rate of reaction with the glass powder. Discs of cement were prepared and these were... 

NMR spectroscopy of various nuclei has been used in the study of AB cements derived from various acids, including phosphoric acid and poly(acrylic acid). For example, NMR has been used in studies of dental silicate cement, i.e. the AB cement made from aqueous phosphoric add and powdered aluminosilicate glass (Wilson, 1978). In this cement, the... 

In a study of dental silicate cements, Kent, Fletcher Wilson (1970) used electron probe analysis to study the fully set material. Their method of sample preparation varied slightly from the general one described above, in that they embedded their set cement in epoxy resin, polished the surface to flatness, and then coated it with a 2-nm carbon layer to provide electrical conductivity. They analysed the various areas of the cement for calcium, silicon, aluminium and phosphorus, and found that the cement comprised a matrix containing phosphorus, aluminium and calcium, but not silicon. The aluminosilicate glass was assumed to develop into a gel which was relatively depleted in calcium. 

Materials formed by acid-base reactions between calcium aluminate compounds and phosphate-containing solutions yield high-strength, low-permeability, C02-resistant cements when cured in hydrothermal environments. The addition of hollow aluminosilicate microspheres to the uncured matrix constituents yields slurries with densities as low as approximately 1200 kg/m, which cure to produce materials with properties meeting the criteria for well cementing. These formulations also exhibit low rates of carbona-tion. The cementing formulations are pumpable at temperatures up to 150° C. 

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