Water plays a vital role in ceramics. Most ceramic raw materials are sedimentary in nature and thus formed in an aqueous environment.
Many key minerals are of secondary origin, created by water-induced weather-ing of primary rock.
Furthermore, water is added at most stages of the ceramic production process to aid completion of shaped semi-finished items.
Although water has a significant effect on any type of raw material, it plays a particularly important role in clays owing to the already-described structural char-acteristics: these include inter-particle spaces and electrostatic charges that allow intense interaction with the water molecules, which, because oxygen and hydrogen have different electronic affinities, may be seen as small dipoles with a partial nega-tive charge on the
O and a partial positive charge on the H
When water is mixed in with the powders in quantities equal to approximately one fifth the weight of the powder itself, plastic bodies are obtained. Where that proportion rises to between a third and a half, suspensions of extremely differenti-ated rheological characteristics result: given the dimensions of the clayey particles, these may be considered and treated as colloidal suspensions.
Given the complex nature of water these basic interlayer electrostatic interac-tions are accompanied by others dependent on structural or chemical irregularities, generally influencing the surface of the particles:
a) distorted ionic groupings
b) broken surface bonds, unsaturated bonds, sliding or crumbling planes
c) cations of unsaturated charge, or cations which create charge defects in the crys-tal lattice etc. If a basal plane of the cryscrys-tal lattice is theoretically in electrical equilibrium and the surface, vice versa, has a excess/insufficient charge (gener-ally caused by the high number of oxygen2- atoms in the ceramic material), a fracture will greatly disturb that equilibrium, causing different attraction/repul-sion scenarios involving the water molecules (Fig. 99).
In general, though, the particle surfaces are deemed to be sufficiently charged to attract at least one layer of water (see Fig. 100), which remains attached by a strong electrostatic bond and differs from free water in that it is denser (like ice): this first layer of molecules, oriented according to the attraction of the particle on the
di-δ-O < Hδ+
Hδ+.
Fig. 99. Kaolinite crystals with positive and negative edge charges (A and B respectively), defloccula-tion (C), edge-to-face flocculadefloccula-tion (D), face-to-face flocculadefloccula-tion (E), flocculated Ca clay (F), deflocculat-ed clay -Na (G).
Fig. 100. Distribution of charges and water, free and bound, around clay particles in suspension. The cross-hatched area in the middle is the clayey core.
free water
free water
bound water
variations caused by disturbance of these electrostatic interactions at molecular level.
Before examining these variations in more detail it is worthwhile highlighting a parameter often neglected in academic works but of great importance in industrial practice: time. That is, the contact time between water and the ceramic material.
It is a factor on which the completion of many reactions depends: such reactions can have an enormous influence on the final characteristics of the suspension to be spray-dried or cast, the body to be extruded or the semi-dry powder to be pressed.
The time factor involves both physical and chemical changes. These range from the degree of hydration significantly influencing the plastic properties to the solu-tion of substances present. The latter can result in the freeing of ions, alterasolu-tion of pH or aiding the oxidation of cations in iron and sulphides.
Another important yet often neglected alteration of significant macroscopic implications is the emission of hydration heat, which, in clayey raw materials may vary from 0.1 to more than 600 calories per gram yet is generally negligible in other ceramic raw materials (0.0015 c/g for quartz, 0.04 for Feldspars).
Zeta potential:
e = charge
D = dielectric constant
d = distance between rigid and diffuse layer
rigid layer
diffuse layer potential
resultant
R = repulsion
A = attraction
distance
As regards aqueous suspensions of ceramic bodies, the main events which need to be controlled and guided are flocculation and its opposite, deflocculation. In the former, rapid sedimentation of flaky particle aggregates of greater density easily separated from the system are created from within a homogeneous in-water particle suspension. In the latter attempts are made, vice versa, to aid dispersion of aggre-gates using appropriate additives, thus obtaining particles that remain in suspen-sion for long periods even in the absence of agitation. This last characteristic is, of course, particularly sought after in the wet grinding of clayey bodies.
Both effects are the result of disturbing the solid-water electrical particle layers:
flocculation is usually caused by the introduction of small and therefore high charge-density bi or tri-valent cations; these can seriously alter the electrostatic equilibrium of the suspension. Deflocculation, instead, is generally achieved by adding organic or inorganic polymeric molecules that isolate the particles from each other by separating them on account of their structural characteristics. Here too, the presence of appropriate cations (Na+, NH+) that alter the equilibrium of the double layer aids the effectiveness of the additive.
Because ceramic particles are negatively charged, they easily attract positively charged cations. The tendency of cations to be adsorbed has been shown, for the most common, to be: H+ > Al3+ > Ba2+ > Sr2+ > Ca2+ > Mg2+ > NH4+ > K+ >Na+ >
Li+, so the useful cations, such as sodium and potassium, have little chance to act.
The effect (which will depend on the ratio of their charge and volume) may be used to vary the electrical characteristics of a suspension or by adding excess to act on the exchange equilibrium
(CLAYn-. Xm+) + Ym+ ⇔ (CLAYn-. Ym+) + Xm+
where X and Y are the different cations.
Chapter VI