Bentonite

Bentonites are highly plastic secondary clays that are used in small amounts as absorbents or as binders/plasticizers in batches of other materials [3]. Bentonites are formed from volcanic ash or tuff rather than igneous rocks [6]. The most significant commercial deposits of bentonite in the U. S. are in Wyoming, but bentonite deposits are widespread. The main crystalline constituent of bentonites is montmorillonite, with Mg and Fe substitution onto the octahedral sites (Fig. 6). Bentonites swell significantly when mixed with water. Also, bentonites form highly thixotropic gels, even in low concentration [14]. Because of swelling and extremely high drying and

Table 7 Typical compositions (weight percent) of other commonly used clays

Type

SiO2

AlA

Fe2O3

TiO2

CaO

MgO

K2O + Na2O

H2O

Bentonite

49.6

15.1

3.4

0.4

1.1

7.8

23.0

Talc

56.3

3.2

5.4

0.4

27.9

0.9

5.7

Shale

54.6

14.6

5.7

5.2

2.9

5.9

4.7

firing shrinkages, bentonites are rarely used as a major constituent of traditional ceramics; applications are confined to additives in a variety of processes. The typical composition of a bentonite is given in Table 7 [6].