Applied Clay Mineralogy Phần 9 ppt

goethite, the firing temperature and degree of vitrification, the propor￾tion of alumina, lime, and magnesia in the clay material, and the com￾position of the fire-gases during burning (Butterworth, 1953). The best

white firing clays contain less than 1% Fe2O3. B tan-burning clays con￾tain between 1% and 5% Fe2O3, and red-firing clays contain 5% or

more Fe2O3.

Common clays occur in a variety of environments and in many differ￾ent rocks across all time periods of the geologic record. The source clay

material can be glacial clay, soils, alluvium, loess, shale, weathered and

fresh schist, slate, argillate, and underclays or seat earths.

2. LIGHTWEIGHT AGGREGATE

Certain common clays are used to produce lightweight aggregate. The

American Society for Testing Materials has published a standard spec￾ification governing lightweight aggregates for concrete (Code No. C330-

53 T, 1955). The unit weight of fine lightweight aggregate cannot exceed

70 lb/ft3 and the unit weight of coarse lightweight aggregate cannot

exceed 55 lb/ft3

.

Clay and/or shale, which is a common clay, is used as the raw ma￾terial to make lightweight aggregate. The raw material is crushed and

fed into a rotary kiln or sintering machine. The raw material is heated

rapidly up to the range between incipient and complete fusion. The

bloating and vesiculation require the presence of substances that release

gas after fusion has developed a molten jacket around the particles to

prevent the escape of the gas. The molten jacket must be viscous enough

to prevent the escape of the expanding gas. Conley et al. (1948) and

Riley (1951) have investigated the causes of bloating. Several factors are

important, most of which are based on chemical and mineralogical

composition.

Shales and clays containing illites, chlorite, some montmorillonite, and

mixed-layer clays are the most promising sources to make lightweight

aggregate. A close relationship exists between the chemical composition

and the bloating characteristics of clay and shale. Riley (1951) concluded

that the viscosity of the melt produced by firing is determined essentially

by the bulk chemical composition based on SiO2, Al2O3, and the total of

CaO, MgO, FeO, Fe2O3, K2O, and Na2O in which optimum viscosity of

the molten jacket might be expected. Fig. 19 shows the limits of bloating

established by Riley (1951). This was verified by Murray and Smith

(1958) in their study of some Indiana shales. Clays and shales of various

144 Applied Clay Mineralogy