Cement formation

Table 2.1. Examples of acids used for cement formation...

Condensation cements. Here, cement formation involves a loss of water and the condensation of two hydroxyl groups to form a bridging oxygen ... 

Acid-base cements. Cement formation involves both acid-base and hydration reactions (Wilson, Paddon Crisp, 1979). These cements form the subject of this book. 

Cement formation requires a continuous structure to be formed in situ from a large number of nuclei. Moreover, this structure must be maintained despite changes in the character of the bonds. These criteria are, obviously, more easily satisfied by a flexible random structure than by one which is highly-ordered and rigid. Crystallinity implies well-satisfied and rigidly-directed chemical bonds, exact stoichiometry and a highly ordered structure. So unless crystal growth is very slow a continuous molecular structure cannot be formed. 

In random structures, stoichiometry need not be exact and adventitious ions can be incorporated without causing disruption. Bonds are not highly directed, and neighbouring regions of precipitation, formed around different nuclei, can be accommodated within the structure. Continuous networks can be formed rapidly. Thus, random structures are conducive to cement formation and, in fact, most AB cements are essentially amorphous. Indeed, it often appears that the development of crystallinity is detrimental to cement formation. 

A final point needs to be made. Theory has indicated that AB cements should be amorphous. However, a degree of crystallization does sometimes occur, its extent varying from cement to cement, and this often misled early workers in the field who used X-ray diffraction as a principal method of study. Although this technique readily identifies crystalline phases, it cannot by its nature detect amorphous material, which may form the bulk of the matrix. Thus, in early work too much emphasis was given to crystalline structures and too little to amorphous ones. As we shall see, the formation of crystalUtes, far from being evidence of cement formation, is often the reverse, complete crystallinity being associated with a non-cementitious product of an acid-base reaction. 

The first three form amphoteric oxides and are distinctly superior, as cement-formers, to the latter two which form weakly basic oxides. Data from Table 2.3b indicate that optimum cement formation occurs with cations that have / values lying between 18 and 29. 

Crisp, S., Prosser, H. J. Wilson, A. D. (1976). An infra-red spectroscopic study of cement formation between metal oxides and aqueous solutions of poly(acrylic acid). Journal of Materials Science, 11, 36-48. 

Many divalent and trivalent oxides form cements with PAA (Crisp, Prosser Wilson, 1976 Hodd Reader, 1976 Hornsby, 1977). Cement formation was observed using infrared spectroscopy and physical and chemical tests. Of these cements that of ZnO (Smith, 1968) was the first and remains by far the most important it is given detailed treatment in Section 5.7. 

He compared the infrared spectra of cements with that of zinc polyacrylate salt and found differences. Inspection of his data shows that, unlike the cements, the salt was purely ionic, so that it seems here that cement formation is associated with the formation of coordination complexes. There are no ligand field stabilization effects with the Zn ion because it has a completed d shell (Cotton Wilkinson, 1966). For this reason the... 

Using this information. Crisp et al. (1977, 1979) and Hornsby et al. (1982) selected candidate minerals for cement formation with poly(acrylic acid) and found a number of minerals that formed cements (Table 5.4). 

Crisp, Merson Wilson (1980) found that the addition of metal fluorides to formulations had the effect of accelerating cement formation and increasing the strength of set cements the effect was enhanced by the presence of (-I-)-tartaric acid (Table 5.13). Strength of cements formed from an SiOj-AljOg-Cag (P04)2 glass, G-247, can be almost doubled by this technique. 

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. 

In a classic study, Kingery (1950b) examined a large number of oxides for cement formation with orthophosphoric acid. He observed three types of reaction no reaction, violent reaction with crystallization, and controlled reaction with cement formation. 

Although an acid phosphate matrix cannot be excluded it is not essential for cement formation. In fact, it must be remembered that when these cements are prepared the oxide or silicate powder is normally in excess of that required for the reaction. Under these conditions most oxides (MgO... 

As we have already shown, the presence of cations in orthophosphoric acid solution can have a decisive effect on cement formation. As noted above, Kingery (1950b) found it necessary to modify orthophosphoric add, by the addition of calcium, to obtain cement formation with calcium oxide. Also, Finch and Sharp (1989) had to modify orthophosphoric add, with either ammonium or aluminium, to achieve cement formation with magnesium oxide. 

Even when modifiers are not necessary for cement formation, they can lead to improved cement properties. Kingery (1950b) also examined this effect. He found that optimum bonding was achieved with cations that had small ionic radii and were amphoteric or weakly basic, such as beryllium, aluminium, magnesium and iron. By contrast, cations that were highly basic and had large ionic radii, for example calcium, thorium and barium, had a detrimental effect on bonding. 

We have noted earlier that aluminium is unusual in forming alumino-phosphate complexes in phosphoric acid solution which may be of a polymeric nature. Bearing in mind the analogies between aluminium phosphate and silica structures, it may well be that during cement formation an aluminium phosphate hydrogel is formed. Its character may be analogous to that of silica gel, where a structure is built up by the... 

Surprisingly, the effect of aluminium on the reaction had been anticipated by van Dalen many years before in his thesis of 1933, but had not made its way into the scientific literature. The authors are indebted to Dr L. J. Pluim of the Rijksuniversiteit te Groningen for this information. Van Dalen (1933) was convinced that aluminium had an important role in cement formation and that Crowell had been wrong to ignore it. Van Dalen found that the reaction between zinc oxide and phosphoric acid was... 

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. 

Cement formation with ammonium dihydrogen phosphate... 

Cement formation with diammonium hydrogen phosphate... 

Cement formation with aluminium acid phosphate... 

They considered that cement formation was the result of an acid-base reaction leading to the formation of hydrates by a through-solution mechanism, by nucleation and precipitation from pore fluids. Two phases were found in the matrix, one amorphous and the other crystalline. The crystalline phase was newberyite. Finch Sharp concluded that the amorphous phase was a hydrated form of aluminium orthophosphate, AIPO4, which almost certainly contained magnesiiun. They ruled out a pure AlP04.nH20, for they considered that the reaction could not be represented by the equation... 

There have been a number of studies aimed at understanding the chemistry of the curing and setting of magnesium oxychloride cements and at identifying the phases that are present in the final material. Investigations in the first half of the twentieth century revealed that cement formation in the MgO-MgCla-HaO system involves gel formation and crystallization of... 

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. 

Silica gel is formed in the reaction but is not associated with cement formation. 

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