Water and acid-base cements

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 has three possible roles in acid-base cements. First, it acts as the medium for the setting reaction of these materials, and second, it is one of the components of the set cement, actually becoming incorporated into the cement as it hardens. Third, water may act as plasticizer in these cements. All of these roles are reviewed here. 

Oxychloride and oxysulfate cements are another class of acid-base cements. These are formed by reaction of a metal oxide such as that of magnesium oxide with a chloride or sulfate of a metal in the presence of water. Magnesium and zinc based oxychloride cements have been developed fully. 

Within clinical dentistry, there are several types of cement available, including the zinc phosphate and the zinc polycarboxylate. They share with glass-ionomers the feature of being acid-base cements and setting as the result of a neutralization reaction, and consequently they are hydrophilic by nature [7]. These cements differ from each other in that they have different acid and base components, but they resemble each other in that the acid is always an aqueous solution and the base is a water-insoluble soUd metal oxide powder. The setting reaction, which begins immediately when the components are mixed, involves acid attack on the solid powdered base, and leads to the release of metal ions into the aqueous phase. In this phase, the metal ions interact with the acid (or its anion) to form metal salts, and these are rigid and insoluble. As these salts form, so the overall cement hardens and becomes insoluble in saliva and other aqueous media [7]. 

The glass-ionomer cement is an acid-base cement based on weak polymeric acids and powdered glasses that are basic in character [1], Their setting takes place within water and resnlts in a complex polysalt matrix, formed by chemical reaction of the acidic polymer solntion with the basic glass. The structure also contains a snbstantial amount of nnreacted glass that acts as reinforcing filler [2], In terms of materials classification, this makes them composites, but convention in dentistry is to consider them distinct from composites, and to classify them as cements. 

Zinc oxide-eugenol is a type of acid-base cement, and forms by reaction of eugenol with zinc oxide powder. The matrix of the cement is a chelate substance consisting of zinc eugenolate. For this reaction to occur, at least a trace of water is essential [21]. 

Petroleum. Apart from its use ia petrochemicals manufacture, there are a number of small, scattered uses of lime ia petroleum (qv) production. These are ia making red lime (drilling) muds, calcium-based lubricating grease, neutralization of organic sulfur compounds and waste acid effluents, water treatment ia water flooding (secondary oil recovery), and use of lime and pozzolans for cementing very deep oil wells. 

Water as the solvent is essential for the acid-base setting reaction to occur. Indeed, as was shown in Chapter 2, our very understanding of the terms acid and base at least as established by the Bronsted-Lowry definition, requires that water be the medium of reaction. Water is needed so that the acids may dissociate, in principle to yield protons, thereby enabling the property of acidity to be manifested. The polarity of water enables the various metal ions to enter the liquid phase and thus react. The solubility and extent of hydration of the various species change as the reaction proceeds, and these changes contribute to the setting of the cement. 

Two matrices are formed a metal polyacrylate salt and a polymer. There is a lack of water in the system because some of it has been replaced by HEM A, and lack of water in glass polyalkenoate cements is known to slow down the ionomer add-base reaction (Hornsby, 1977). Thus, the initial set of these materials results from the polymerization of HEMA and not the characteristic acid-base reaction of glass-ionomer cements. The later reaction serves only to harden and strengthen the already formed matrix. 

Magnesium (or magnesia) phosphate cements are based on the reaction between ignited magnesium oxide and acid phosphates, which are generally modified by the addition of ammonium and aluminium salts. The phosphates may be either in solution or blended in solid form with the magnesium oxide. In the latter form the cement is formed by mixing the powder blend with water. 

Cement formation is the result of an acid-base reaction between zinc oxide and eugenol, leading to the formation of a zinc eugenolate chelate. Water plays a vital role in the reaction. 

Not surprisingly, the use of acidified water increased the level of fluoride release from the glass, and this effectively models what happens in a setting cement. The acid-base reaction between the glass and the water-soluble polymeric acid liberates fluoride from the glass, causing it to move to the matrix, from where it is gradually leached as the cement releases fluoride [227,228]. 

Fig. 1.33 Reductions in water-cement ratio as a function of aggregate-cement ratio for lignosulfonate and hydroxycarboxylic-acid-based water-reducing agents.

---