3 - Water and acid–base cements

Summary

Introduction

The setting reaction for the great majority of acid–base cements takes place in water. (The exceptions based on o-phenols are described in Chapter 9.) This reaction does not usually proceed with formation of a precipitate but rather yields a substance which entrains all of the water used to prepare the original cement paste. Water thus acts as both solvent and component in the formation of these cements. It is also one of the reaction products, being formed in the acid–base reaction as the cements set.

Water as a solvent

It is widely recognized that the solvent in which any chemical reaction takes place is not merely a passive medium in which relevant molecules perform: the solvent itself makes an essential contribution to the reaction. The character of the solvent will determine which chemical species are soluble enough to enter solution and hence to react, and which species are insoluble, and thus precipitate out of solution, thereby being prevented from undergoing further chemical change. In the case of water, as will be seen, polar and ionic species are the ones that most readily dissolve. But even so, mere polarity or ionic character is not sufficient to ensure solubility. Solubility depends on a number of subtle energetic factors, and the possible interactions between water and silver chloride, for example, do not fulfil the requirements despite the ionic nature of the silver salt. Hence silver chloride is almost completely insoluble in water.