Cement forming

Rigid slabs, pipe sections, beveled lags and molded products. Also as a powder mix for use in wet cement form. 

Cement-forming liquids are strongly hydrogen-bonded and viscous. According to Wilson (1968), they must (1) have sufficient addity to decompose the basic powder and liberate cement-forming cations, (2) contain an acid anion which forms stable complexes with these cations and (3) act as a medium for the reaction and (4) solvate the reaction products. 

Generally, cement-forming liquids are aqueous solutions of inorganic or organic adds. These adds include phosphoric add, multifunctional carboxylic adds, phenolic bodies and certain metal halides and sulphates (Table 2.1). There are also non-aqueous cement-forming liqtiids which are multidentate acids with the ability to form complexes. 

Potential cement-forming bases are oxides and hydroxides of di- and... 

Acid-base cements. Cement formation involves both acid-base and hydration reactions (Wilson, Paddon Crisp, 1979). These cements form the subject of this book. 

The nature of the association between cement-forming cation and anion is important. As we shall see from theoretical considerations of the nature of acids and bases in section 2.3, these bonds are not completely ionic in character. Also while cement-forming cations are predominantly a-... 

The cement-forming reaction is a special case of an acid-base reaction so that concepts of acid, base and salt are central to the topic. In AB cement theory, we are concerned with the nature of the acid-base reaction and how the acidity and basicity of the reactants are affected by their constitution. Thus, it is appropriate at this stage to discuss the various definitions and theories available. 

Group (1) Cations and anions which are incapable of donor-acceptor interactions. These are the large univalent ions. Bonding is purely by Coulomb and Madelung electrostatic interactions. From the Lewis point of view these are not acids or bases. They have no cement-forming potential. 

Poly(acrylic acid) and its salts have been known to have useful binding properties for some thirty years they have been used for soil consolidation (Lambe Michaels, 1954 Hopkins, 1955 Wilson Crisp, 1977) and as a flocculant (Woodberry, 1961). The most interesting of these applications is the in situ polymerization of calcium acrylate added to soil (de Mello, Hauser Lambe, 1953). But here we are concerned with cements formed from these polyacids. 

The polyelectrolyte cements are modern materials that have adhesive properties and are formed by the cement-forming reaction between a poly(alkenoic acid), typically poly(acrylic acid), PAA, in concentrated aqueous solution, and a cation-releasing base. The base may be a metal oxide, in particular zinc oxide, a silicate mineral or an aluminosilicate glass. The presence of a polyacid in these cements gives them the valuable property of adhesion. The structures of some poly(alkenoic acid)s are shown in Figure 5.1. 

The precise nature of the adhesion of the polyelectrolyte cements to untreated dental enamel and dentine has yet to be established. The earliest theory was due to Smith (1968) who speculated that the polyacrylate chains of the cement formed a chelate with calcium ions contained in the hydroxyapatite-like mineral in enamel and dentine. Beech (1973) considered this unhkely since it involved the formation of an eight-membered ring. Beech studied the interaction between PAA and hydroxyapatite, identified the formation of polyacrylate and so considered that adsorption was due to ionic attraction. 

While some formed hard, rigid cements that were stable in water, others yielded rubbery or plastic masses that were hydrolytically unstable. Minerals with cement-forming capability were found in the following classes ... 

Figure 5.6 Triangular composition diagram for SiO -Alj Oj-CaO glasses, showing that glasses with cement-forming ability fall within the gehlenite and anorthite composition region, and that only glasses with less than 61 to 62% by mass of silica have the potential to form a cement (Hill Wilson, 1988a).

Table 5.7. Properties of cements formed from glasses corresponding to the generic formula xSiO Alf). CaO...

Cement-forming liquid 45 % poly(acrylic acid)... 

Crisp, Merson Wilson (1980) found that the addition of metal fluorides to formulations had the effect of accelerating cement formation and increasing the strength of set cements the effect was enhanced by the presence of (-I-)-tartaric acid (Table 5.13). Strength of cements formed from an SiOj-AljOg-Cag (P04)2 glass, G-247, can be almost doubled by this technique. 

Cement formation with fluoride glasses - - -)-tartaric acid The presence of (+)-tartaric acid in a cement formulation exerts a profound effect on the cement-forming reaction. The nature of the underlying chemical reaction is changed and this is reflected in time-dependent changes in viscosity. 

The cement-forming reaction will be similar to glass polyalkenoate cement. The cement matrix will consist of metal polyacrylates, but boric acid will be produced instead of silica gel. Since boric acid has a water solubility oil-1 % compared with the near insolubility of silica gel, it would... 

The most important of these are the refractory cements formed by the heat treatment of aluminium acid phosphate solutions. This subject has been well reviewed by Kingery (1950a), Morris et al. (1977), Cassidy (1977) and O Hara, Duga Sheets (1972). The chemistry of these binders is extremely complex as the action of heat on acid phosphates gives rise to polymeric phosphates, with P-O-P linkages, and these are very complex systems (Ray, 1979). 

Here we are concerned with the cement-forming reaction between orthophosphoric acid solutions and basic oxides and silicates where the reaction is much simpler. Polymeric phosphates are not involved, there are no P-O-P bonds, and the structural unit is the simple [POJ tetrahedron. 

The actions of zinc and aluminium differ. In general, metal ions such as zinc merely serve to neutralize the acid and are present in solution as simple ions (Holroyd Salmon, 1956 O Neill et al., 1982). But aluminium has a special effect in contrast to zinc, it prevents the formation of crystallites during the cement-forming reaction in zinc phosphate cements. 

The early history of the material is obscure. According to Palmer (1891) it goes back to 1832, but this statement has never been corroborated. Rostaing (1878) patented a series of pyrophosphate cements which could include Zn, Mg, Cd, Ba and Ca. Rollins (1879) described a cement formed from zinc oxide and syrupy phosphoric acid. In the same paper he mentions zinc phosphate cements recently introduced by Fletcher and Weston. Similar information is given in a discussion of the Pennsylvania... 

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