Cement-forming cations

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. 

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-... 

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. 

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. 

As reaction proceeds, the polymer chain (which is in random coil form) unwinds as the charge on it grows as a result of neutralization and ionization. This contributes to thickening of the cement paste. Cations released become bound to the polymer chain. Countercations can either be bound to a polyanionic chain by general electrostatic forces or be site-bound at specific centres. More than one type of site binding is possible. Complex formation and, if the ligand is bidentate, chelate formation enhance the effect. 

Irretrievable loss of matrix-forming cations and anions can result in permanent damage to the cement surface. This is visible as milky or chalky patches or even raised blisters. For this reason it is customary to protect, temporarily, the freshly placed cement by varnish. Once hardened, attack by neutral solutions causes failure only when a cement has been poorly formulated and contains excessive amounts of soluble reaction products. In this case osmotic effects can cause blistering or even disintegration under the action of internal forces, as Figure 6.22 illustrates (Wilson Batchelor, 1967a). 

These are mainly polymeric cements formed by bonding of polyions (or macroions)which are anions with small cations called counterions. Good examples are polycarboxylate cements [9], glass-ionomer cement [10], and polyphosphonic cements [11,12]. Zinc polycarboxylate, glass polyalkenoate, and resin glass polyalkenoate are some examples... 

Whether in minerals or man-made materials, the chemical bonding in CBCs is at room or warm temperatures, and this aspect distinguishes them from conventional sintered ceramics. Most of the CBCs are formed in the presence of water, though Wilson and Nicholson [8] have discussed several nonaqueous cements. In many of the aqueous CBCs, water is bonded chemically within their stmcture, but in some cases water may be expelled during the reaction. In aU cases, their formation is based on dissolution of individual components into an aqueous phase to form cations and appropriate anions. These ions react with each other to form neutral precipitates. If the rate of this reaction is controlled, then the reaction products will form network of connected particles and produce either well-ordered crystals or disordered structures. These CBCs comprise a composite of the crystallized and partly disordered structures. 

Applicability Most hazardous waste slurried in water can be mixed directly with cement, and the suspended solids will be incorporated into the rigid matrices of the hardened concrete. This process is especially effective for waste with high levels of toxic metals since at the pH of the cement mixture, most multivalent cations are converted into insoluble hydroxides or carbonates. Metal ions also may be incorporated into the crystalline structure of the cement minerals that form. Materials in the waste (such as sulfides, asbestos, latex and solid plastic wastes) may actually increase the strength and stability of the waste concrete. It is also effective for high-volume, low-toxic, radioactive wastes. 

The first three form amphoteric oxides and are distinctly superior, as cement-formers, to the latter two which form weakly basic oxides. Data from Table 2.3b indicate that optimum cement formation occurs with cations that have / values lying between 18 and 29. 

In the context of AB cements, Al +, Mg , Ca and Zn are in class (a) while Cu is in the border region. Zn contains a completed 3d shell and forms stronger complexes with O than with S ligands, as do other class (a) cations. 

These d indices for cations and anions relevant to AB cements are shown in Table 2.5. Bases which add on through F or O and do not form i-bonds have similar hardness values they are hard bases. Soft bases form dative 7i-bonds with many cations. They have high-energy-level occupied orbitals with unshared electron pairs. 

According to Yatsimirskii, group (2) and (3) species are equivalent to Pearson s hard acids and bases, and group (4), (5) and (6) species correspond to Pearson s soft acids and bases. This classification is of more value than HSAB theory to our subject. It can be seen that all cementforming anions come from group (3) and cations from groups (3), (4) and (5). Thus, the bonding in cement matrices formed from cation-anion combinations is not purely a but contains some n character. 

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). 

Cations can be seen as acting as ionic crosslinks between polyanion chains. Although this may appear a naive concept, crosslinking can be seen as equivalent to attractions between polyions resulting from the fluctuation of the counterion distribution (Section 4.2.13). Moreover, it relates to the classical theory of gelation associated with Flory (1953). Divalent cations (Zn and Ca +) have the potential to link two polyanion chains. Of course, unlike covalent crosslinks, ionic links are easily broken and re-formed under stress there could therefore be chain slipping and this may explain the plastic nature of zinc polycarboxylate cement. 

After gelation or initial set, the cement continues to harden as cations are increasingly bound to the polyanion chain and hydration reactions continue. Recent evidence suggests that a siliceous hydrogel may be formed in the matrix. 

When fully hardened, the cement is resistant to erosion provided the solution has a pH above 4. However, the glass polyalkenoate cement is susceptible to erosion immediately after set because some of the matrixforming cations and anions are still in soluble form. In fact, the hardening process is one where these cations and anions continue to precipitate. For this reason these cements have to be protected, temporarily, by a varnish. 

All commercial examples of phosphoric add solutions used in these cements contain metal ions, whose role has been discussed in Section 6.1.2. In the case of the dental silicate cement, aluminium and zinc are the metals added to liquids of normal commerdal cements and have a significant effect on cement properties (Table 6.8) (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b). Aluminium accelerates setting for it forms phosphate complexes and is the prindpal cation of the phosphatic matrix. Zinc retards setting for it serves to neutralize the addic liquid - it... 

Setting times and hydrolytic stability of these cements are given in Table 8.3. In some cases the speed of reaction was very high, and practical cements could not be formed from ZnO or CaO even when these oxides were deactivated by heating. All the faster-setting cements exhibited good hydrolytic stability. The stability of the complexes between divalent cations and PVPA was found by a titrametric procedure to follow the order Mg Ca < Cu Zn (Ellis Wilson, 1991). This result was... 

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