Coordination of water to ions

The ions that tend to be involved in AB cements include such species as Al , Mg, Ca and Zn. These are all capable of developing a coordination number of six, and hexaquo cations are known to be formed by each of these metal ions (Huckel, 1950). The typical requirements for an ion to develop such coordination characteristics are that the ion should exist in the -I- 2 or -1-3 oxidation state, and in this state should be of small ionic radius (Greenwood Earnshaw, 1984). 

Another feature of the metal ions that are typically involved in cementitious bonding in AB cements is that most of them fall into the category of hard in Pearson s Hard and Soft Acids and Bases scheme (Pearson, 1963). The underlying principle of this classification is that bases may be divided into two categories, namely those that are polarizable or soft, and those that are non-polarizable or hard. Lewis acids too may be essentially divided into hard and soft, depending on polarizability. From these classifications emerges the useful generalization that hard acids prefer to associate with hayd bases and soft acids prefer to associate with soft bases (see Section 2.3.7). 

Of the ions most often implicated in cementitious bonding in AB cements, Ca , Mg and AF are classified as clearly hard Zn by contrast falls into the category that Pearson designated borderline , as 

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