Clay-Water Interactions

The processing methods for clay-based ceramics can be categorized by the water content and the resulting rheological behavior. The methods that will be discussed in this section, in order of increasing water content, are: (1) dry pressing (2) stiff plastic forming, (3) soft plastic forming, and (4) casting. Most clay compositions can be fab­ricated using any of the forming processes by simply changing the water content of the batch. As such, the choice of forming methods is often dictated by the desired shape of the product and will be discussed in that context. Water contents and shape limitations for the four forming methods are summarized in Table 8. The overlap in the water contents for the different techniques is due to the varying water require­ments for different clays, which is caused by differences in composition, structure, and physical characteristics of the clays.

Forming techniques used for clay-based ceramics require control of water con­tent in the batch. Water content, in turn, affects the response of the clay during forming [27]. As the water content of the batch increases, the yield point of the clay-water mixture, and thus the force required to form the desired shape, gener­ally decreases [26]. However, the relationship is complex and depends on the composition of the clay, its structure, additives to the batch, and other factors [14]. One method for quantifying the behavior of clay-water pastes is to measure the plastic yield point as a function of water content [14]. The water contents and maximum yield points in torsion are compared for several clays in Table 9. Kaolins and plastic fire clays require the least amount of water to develop their maximum plasticity, ball clays require an intermediate amount, and bentonite requires the most.

The interactions between water and ceramic particles are complex and important for processes ranging from the rheology of slurries to the drying of particulate solids. An in-depth discussion of water-particle interactions is beyond the scope of this chapter. For the discussions that follow, it is sufficient to understand the forms that water takes within a particulate ceramic [27]. At the lowest contents, water is present as partial, complete, or multiple layers adsorbed (physical) on the surface of the particles. After the surfaces are covered with a continuous adsorbed film, liquid water can condense in the pores between particles. Finally, at the highest water

Table 8 Water contents and pressure range used during the four common forming methods used for clay-based ceramics [22,26]

Method

Water (wt%)

Pressure range (MPa)

Dry pressing

0-15

100-400

Stiff plastic

12-20

3-50

Soft plastic

20-30

0.1-0.75

Slip casting

25-35

None

Table 9 Water content and maximum yield point for different types of clays [14]

Clay

Water content (wt%)a

Yield torque (g cm 1)

Kaolin

19.2

472

Plastic fire clay

19.0

442

Ball clay

34.4

358

Bentonite

41.9

254

aDetermined as weight of water added to clay dried at 105°C for 24 h

contents, free water that does not interact with particle surfaces begins to separate individual particles, eventually leading to a stable dispersion of fine, separated particles.