Cement-forming reaction

The liquid is an aqueous solution of phosphoric acid, always containing 1 to 3 % of aluminium, which is essential to the cement-forming reaction (Table 6.2). Zinc is often found in amounts that range from 0 to 10% to moderate the reaction. Whereas zinc is present as simple ions, aluminium forms a series of complexes with phosphoric acid (Section 6.1.1). This has important consequences, as we shall see, in the cement-forming reaction. 

The setting reaction is an acid-base one and the course of the reaction is shown by pH changes in the cement. Two minutes after mixing the pH is as low as 1-6, after 60 minutes it increases to about 4 and reaches between 6 and 7 after 24 hours (Plant Tyas, 1970). 

The nature of the setting reaction and the set cement remained imperfectly understood for many years. This is not surprising, for the products of the reaction depend on a number of factors, including the phosphoric acid concentration and the presence or absence of aluminium in the solution. These complexities have caused considerable confusion in the literature. 

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. 

In the earliest attempt to explain the reaction, Crowell (1929) used, in part, arguments based on the phase diagrams of the ZnO-PjOj-HjO system constructed by Eberly, Gross Crowell (1920). Later, Darvell (1984) advanced similar arguments using the phase diagrams of Salmon  

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The cement-forming reaction is a special case of an acid-base reaction so that concepts of acid, base and salt are central to the topic. In AB cement theory, we are concerned with the nature of the acid-base reaction and how the acidity and basicity of the reactants are affected by their constitution. Thus, it is appropriate at this stage to discuss the various definitions and theories available. 

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. 

Cement formation with fluoride glasses - - -)-tartaric acid The presence of (+)-tartaric acid in a cement formulation exerts a profound effect on the cement-forming reaction. The nature of the underlying chemical reaction is changed and this is reflected in time-dependent changes in viscosity. 

The cement-forming reaction will be similar to glass polyalkenoate cement. The cement matrix will consist of metal polyacrylates, but boric acid will be produced instead of silica gel. Since boric acid has a water solubility oil-1 % compared with the near insolubility of silica gel, it would... 

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. 

The actions of zinc and aluminium differ. In general, metal ions such as zinc merely serve to neutralize the acid and are present in solution as simple ions (Holroyd Salmon, 1956 O Neill et al., 1982). But aluminium has a special effect in contrast to zinc, it prevents the formation of crystallites during the cement-forming reaction in zinc phosphate cements. 

The chief problem with these cements, as with many AB cements, is to moderate the cement-forming reaction. If the reaction is over-vigorous then a crystalline mass rather than a cement is formed (Komrska Satava, 1970 Crisp et al., 1978). Therefore, the zinc oxide used in these cements... 

Crisp et al. (1978) were able to follow the course of the cement-forming reaction using infrared spectroscopy and to confirm previous observations. They found that the technique could be used to distinguish between crystalline and amorphous phases of the cement. Hopeite shows a number of bands between 1105 and 1000 cm this multiplicity has been explained by postulating a distortion of the tetrahedral orthophosphate anion. (Two-thirds of the zinc ions are tetrahedrally coordinated to four phosphate ions, and the remainder are octahedrally coordinated to two phosphate and four water ligands.)... 

The addition of aluminium to the liquid slowed down the reaction. An amorphous cement was formed and there was no crystallization in the bulk of the cement. However, after some time crystallites were formed at the surface. Thus, the presence of aluminium exerts a dedsive influence on the course of the cement-forming reaction. This effect is to be attributed to the formation of aluminophosphate complexes (see Sections 6.1.2 and 4.1.1). These complexes may delay the predpitation of zinc from solution and also introduce an element of disorder into the structure, thus inhibiting crystallization. It is significant that zinc, which does not form complexes, has little effect on the nature or speed of the reaction. 

Vashkevitch Sychev (1982) have identified the main reaction product of the cement-forming reaction between copper(II) oxide and phosphoric acid as Cu3(P04)2. SHjO. The addition of polymers - poly(vinyl acetate) and latex - was found to inhibit the reaction and to reduce the compressive strength of these cements. However, impact strength and water resistance were improved. 

Cement formation between MgO and various acid phosphates involves both acid-base and hydration reactions. The reaction products can be either crystalline or amorphous some crystalline species are shown in Table 6.5. The presence of ammonium or aluminium ions exerts a decisive influence on the course of the cement-forming reaction. 

Ando, Shinada Hiraoka (1974) examined cements formed by the reaction between magnesium oxide and concentrated aqueous solutions of aluminium dihydrogen phosphate. Later, Finch Sharp (1989) made a detailed examination of the cement-forming reaction and reported that the reaction yielded cements of moderate strength. 

A number of infrared spectroscopic studies have been made which have thrown light on the cement-forming reaction (Copeland et al., 1955 Gemer et al., 1966 Wilson Mesley, 1972). Wilson Mesley (1972) used ATR spectroscopy to follow the course of the reaction and showed that major spectral changes were almost entirely associated with loss of the... 

The CHgO-Zn coordinate bond in the zinc eugenolate chelate is very weak (Gerner et ah, 1966) and the chelate has poor stability thus, the cement-forming reaction [9.1] can be reversed. This occurs when the cement is placed in water, when the matrix is easily hydrolysed to eugenol and zinc hydroxide (Figure 9.2a) (Wilson, 1978 Wilson Batchelor,... 

There is little information available on their setting and structure. Bagby Greener (1985) used Fourier transform infrared spectroscopy (FTIR) to examine the cement-forming reaction between zinc oxide and a mixture of EBA and n-hexyl vanillate. Although they found evidence for reaction between zinc oxide and EBA, they were unable to find any for reaction between zinc oxide and n-hexyl vanillate because of peak overlaps, the minor concentration of n-hexyl vanillate and the subtle nature of the spectral changes. 

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