Cement-forming acids orthophosphoric acid

Park, C.K., Silsbee, M.R., and Roy, D.M. (1998) Setting reaction and resultant structure of zinc phosphate cement in various orthophosphoric acid cement-forming liquids. Cement and Concrete Research 28,141-150. 

Although this account of gelation is made with reference to organic polyelectrolytes, it is of wider application and may be applied to phosphoric acid cements. Orthophosphoric acid solutions used in these cements contain aluminium, and soluble aluminophosphate complexes are formed. Some appear to be multinuclear and there is evidence for polymers based on the bridging Al-O-P unit. These could be termed polyelectrolytes (Akitt, Greenwood Lester, 1971 Wilson et al., 1972 O Neill et al., 1982). 

The phosphate bonded cements described in this chapter are the products of the simple acid-base reaction between an aqueous solution of orthophosphoric acid and a basic oxide or silicate. Such reactions take place at room temperature. Excluded from this chapter are the cementitious substances that are formed by the heat treatment of aqueous solutions of acid metal phosphates. 

Here we are concerned with the cement-forming reaction between orthophosphoric acid solutions and basic oxides and silicates where the reaction is much simpler. Polymeric phosphates are not involved, there are no P-O-P bonds, and the structural unit is the simple [POJ tetrahedron. 

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. 

Early workers, and some later ones, ignored the fact that aluminium is always found in the orthophosphoric acid liquid of the practical cement its presence profoundly affects the course of the cement-forming reaction. It affects crystallinity and phase composition, and renders deductions based on phase diagrams inappropriate. Nevertheless we first describe the simple reaction between zinc oxide and pure orthophosphoric acid solution, which was the system studied by the earliest workers. 

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 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. 

A deficiency of water in the cement liquid has the same effect and this occurs when the H3PO4 content exceeds 60%. Wilson Mesley (1968) noted that in a cement formed from a solution of 65 % H3PO4 there was evidence of incomplete reaction even after 6 hours. We have noted in Section 6.5.3 that there is a sharp decline in the rate of reaction when the orthophosphoric acid concentration exceeds 65% H3PO4 (Figure 6.14). The avidity of cements to absorb water from humid surroundings also increases sharply when the phosphoric acid in the cement-forming liquid exceeds 60%. It is difficult to avoid the conclusion that these two phenomena are related and that a deficiency of water retards the cementforming reaction. 

The development of the dental silicate cement in its final, satisfactory form occurred slightly later than the development of the zinc phosphate, though it, too, traces its earliest history back to the later 1870s. The pioneering woik on this material was reported in 1878 and 1879 by Fletcher [15,16], and involved canents prepared from concentrated solutions of orthophosphoric acid and sintered mixtures of oxides, including silica and alumina, with inclusions of calcium oxide and zinc oxide. One of these cements showed slight translucency when set [16], but overall the cements were not a success clinically [7]. 

The aluminum phosphate binder consists of a combination of orthophosphoric acid (H3PO4) or monoaluminum phosphate [A1(H2P04)3] and aluminum oxide (fused alumina, AI2O3). Upon heating to elevated temperatures the two constituents interact, and aluminum orthophosphate (AIPO4) is formed as the final product of the reaction at temperatures above about 500 °C. This is a cementing process that results in the development of a hardened material. Aluminum phosphate binder is widely employed in refractory concrete applications, and is discussed in more detail in Chapter 23. 

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