9 - Non-aqueous cements

Summary

General

The non-aqueous or chelate cements are an exceptionally diverse group of materials (Wilson, 1975a,b, 1978; Smith, 1982b). The term chelate cement is not strictly speaking correct, as a minority of them do not form chelates, and some aqueous AB cements do. However, the term is a convenient one. They are of interest in that the reaction media for the acid–base reaction are non-aqueous, although sometimes water may play a role in cement formation. In these cements water is replaced by an organic acid that is liquid at room temperature and generally has chelating ability.

The low permittivity of these liquids compared with water inhibits dissociation of the acids so that cement formation demands much more reactive basic oxides. Oxides and hydroxides that are capable of cement formation are ZnO, CuO, MgO, CaO, Ca(OH)₂, BaO, CdO, HgO, PbO and Bi₂O₃ (Brauer, White & Moshonas, 1958; Nielsen, 1963). In practice these are confined to two: calcium hydroxide and special reactive forms of zinc oxide.

Examples of liquid organic acids suitable for cement formation are:

(1) Alkoxyphenols, for example the 2-methoxyphenols, which include eugenol, guaiacol and vanillates. Also, 2,5-dimethoxyphenol.

(2) β-diketones, e.g. acetylacetone.

(3) β-keto esters, e.g. β-keto-ethylate.

(4) Keto acids, e.g. acetyl acetic acid.

(5) Other difunctional aliphatic carboxylic acids, e.g. lactic acid, pyruvic acid, ethoxyacetic acid.

(6) Aldehydic aromatic acids, e.g. salicylaldehyde.

(7) Alkoxy aromatic acids, e.g. 2-ethoxybenzoic acid.