Effect of cations in phosphoric acid solutions

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 

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