DEFORMATION OF PRODUCTS MADE FROM RED CLAYS DURING FIRING

P. G. Usov and A. V. Petrov

Products made from red clays, when fired in the high-temperature range, undergo plastic deformation. Deformation at high temperature of products made from refractory clays has been studied in detail, and this characteristic is one of the main ones in evaluating the quality of products. Red clays have not been studied at all from this point of view, despite the fact that data on fluidity could be very useful in processing them into products.

Plastic deformation of products made from red clays during firing, as with other products of ceramic compositions, occurs as a result of the gradual melting of the material, taking place within a certain temperature interval. Unlike refractory clays, which in most cases have a long melting interval, red clays, owing to their more complex chemical and mineralogical composition, melt more intensely, and products made from them during firing deform to the point of complete destruction within a narrow temperature interval.

In our work we set ourselves the task of practically determining the fluidity of products made from red brick clays and tracing the influence of additions of calcium carbonate and soda on fluidity during firing.

For the investigation, red clays were taken with a chemical composition typical of the regions of Western Siberia.

Table 1

Chemical composition of clays

Table 2

The coarse-dispersed portion, with grain size larger than 0.01 mm, in all three clays consists predominantly of feldspars.

In the Anzhero clay, the grains of feldspars are predominantly brown; to a considerable degree they have been transformed into clayey material. Fresher grains with distinct microcline twinning are observed more rarely. Quartz, chlorite, green hornblende, etc., are present as admixtures in these fractions.

In the Kamenka clay, plagioclase predominates in fresh, transparent, and little-altered brownish or grayish grains. A complex twin structure and inclusions of small prisms and grains of colorless minerals and magnetite are often observed. Orthoclase is most often brown, but little-altered grains are also observed. Microcline is always fresh, transparent, or slightly pinkish.

The Malinin clay contains a high percentage of strongly kaolinized orthoclase, and to a lesser extent its chloritization is observed. Plagioclase occurs in the clay in small amounts. Unlike the others, this clay contains a high percentage of limonite, reddish-brown to black in color. Quartz occurs in the form of rare angular grains.

By their refractive index, birefringence, and extinction angle, the plagioclases of all three clays belong to the oligoclase-andesine series.

The content of the principal minerals of the coarse-grained fractions of the clays is given in Table 3.

Table 3

The selected clays differ in ceramic properties. The Anzhero clay contains a higher percentage of particles of colloidal size. An aqueous suspension of this clay remains turbid after 60 hours of standing, whereas the Malinin and Kamenka clays, under identical conditions, become completely clear. The plasticity of the clays decreases from Anzhero to Kamenka. The clays differ especially in pyrochemical activity (the ability to build up a strong structure during firing): in the Anzhero clay it is expressed to a greater degree, while in the Kamenka clay it appears weakly. The Malinin clay occupies an intermediate position.

The chemical compositions of the fine fractions (finer than 0.001 mm) of the clays studied are given in Table 4.

Table 4

The clays contain small amounts of water-soluble salts: Anzhero—0.08%, Malinin—0.06%, and Kamenka—0.1%, and have exchangeable

capacity respectively: Anzherka—31.45; Malinino—8.5 and Kamenka—15.0 milliequivalents per 100 g of dry clay.

The study of the deformation of products during firing was carried out on specimens having the form of a ring, with dimensions: height 16 mm, outside diameter 16 mm, inside diameter 12 mm.

The specimens are formed by the plastic method from clays of molding moisture content, under the same load. Excess mass is removed from the mold through a side opening, which makes it possible always to prepare specimens of the same height and of comparatively identical density. A specimen in the form of a ring with a wall thickness of 2 mm was adopted with allowance for the smaller temperature gradient within the material itself, which makes it possible to study more accurately the temperature dependence of deformation. Firing is carried out in a tube furnace. The specimen (1) is placed on a sillimanite support, at the center of which is the outlet of the hot junction of the thermocouple (3). Pressure on the specimen is trans-

Fig. 1. Apparatus for studying the deformation of products made from red clays during firing.

mitted through a second sillimanite rod (2). The arrangement of the apparatus is shown in Fig. 1. Heating is controlled with the aid of a potentiometric regulator. The heating period, at a comparatively uniform rate, is 65–70 minutes. The deformation of the specimen during heating is measured with a dial indicator with an accuracy of up to 0.01 mm. The study of deformation was carried out with a load on the specimen of 1.0 kg/cm². The deformation curves of specimens made from clays of natural composition as a function of temperature are given in Fig. 2.

Specimens from all three clays are deformed differently in the region of their failure. All three clays begin to flow actively at a temperature of 1100°C. Deformation occurs because at this temperature the skeletal part of the structure, composed of grains of coarse fractions, melts. The coarse-dispersed part of all three clays, as indicated above, is composed of feldspars; in the Anzherka and Kamenka clays, chiefly of plagioclases of the acid group

with a small content of orthoclase, while the Malin clay, on the contrary, has a high content of orthoclase and a small admixture of acid plagioclases. Plagioclases of acid composition begin to melt at a temperature of about 1100°C. For this reason, specimens made from all three clays begin to flow actively at 1100°C. The high content of orthoclase in the Malin clay considerably increases the internal friction of the structure of the articles, owing to the higher refractoriness and lower chemical activity of this feldspar.

With respect to the temperature at which deformation begins, the clays also behave differently.

Fig. 2. Deformation curves of specimens during firing from clays:
1 — Anzher clay, 2 — Kamenka clay, 3 — Malin clay.

At lower temperatures, beginning at 860°C the Anzher clay begins to flow; from a temperature of 930°C the specimen made from Kamenka clay deforms; and, finally, from a temperature of 1040° the specimen made from Malin clay deforms.

From these temperatures, in all the clays a uniform deformation is observed, proceeding at a constant rate over a certain temperature interval. The magnitude of the temperature interval of such deformation is:

for Anzher clay . . . . . from 860 to 1100°C

for Kamenka clay . . . . . from 930 to 1080°C

for Malin clay . . . . . from 1040 to 1100°C.

Above these temperatures, the rate of deformation of the specimens increases. The first deformation, proceeding at a constant rate, occurs under the action of the primary melt, the appearance of which causes weakening of the structure. The primary melt, at the temperature of its appearance, evidently has a sufficiently high viscosity, and the dissolving power of such a melt is small.

But with increasing temperature, the decrease in the structural strength of the article is associated with a lowering of the viscosity of the melt or of the apparent viscosity of the composition, as a result of which, under the action of external pressure, the article deforms; and as the particles come closer together during deformation of the article, the magnitude of internal friction in the structure increases. Therefore, in this interval, at each definite temperature, a definite degree of deformation of the specimen corresponds to a constant external load. Above these temperatures, as a result of melting of the composition, the amount of melt increases, its viscosity decreases, and the structural strength of the article is reduced. In this interval the specimen deforms even when the load and temperature are kept constant. The data of these experiments show that sintering of the structure of articles, accompanied by shrinkage, without external loads, occurs at higher temperatures than the temperature at which the primary melt appears, and with the participation of larger amounts of melt. And the appearance of the primary melt is sometimes recorded by an insignificant increase in the porosity of the structure of the article.

Fig. 3. Deformation of specimens from Malinino clay during firing:
1 — with an addition of 2% $\mathrm{Na_2CO_3}$; 2 — with an addition of 10% $\mathrm{CaCO_3}$; 3 — in the natural state; 4 — treated with an $\mathrm{NH_4Cl}$ solution.

From an analysis of the curves in Fig. 2 it also follows that the temperature interval of deformation and the absolute magnitude of deformation of the specimens are directly related to the amount of fine fractions in the clays when their chemical and mineralogical compositions are the same. Specimens from Anzherka clay, with a high content of fine and colloidal particles and a high exchange capacity, deform at lower temperatures and to a greater degree, owing to the formation of a larger amount of primary melt, than specimens from Kamenskaya clay and, still more, from Malinino clay, in which the amount of fine fractions and the exchange capacity are the lowest. Primary melt in red-clay compositions is formed as a result of the interaction of exchangeable alkaline cations with the sorbing base, since—

exchange of natural sorbed cations for \(\mathrm{NH}_4^+\) sharply changes their fluidity during firing, chiefly in the temperature range in which the primary melt appears.

Fig. 4. Deformation of specimens of Anzher clay during firing:
1 — with an addition of \(2\%\ \mathrm{Na}_2\mathrm{CO}_3\), 2 — with an addition of \(10\%\ \mathrm{CaCO}_3\), 3 — in the natural state, 4 — treated with an \(\mathrm{NH}_4\mathrm{Cl}\) solution.

The fluidity of articles made from clays increases considerably when an alkali cation—sodium—is introduced into their composition, owing to the formation of large amounts of primary melt. But in clays with a low content of fine

Fig. 5. Deformation of specimens of Kamensk clay during firing:
1 — with an addition of \(2\%\ \mathrm{Na}_2\mathrm{CO}_3\), 2 — with an addition of \(10\%\ \mathrm{CaCO}_3\), 3 — in the natural state, 4 — treated with an \(\mathrm{NH}_4\mathrm{Cl}\) solution.

fractions and low exchange capacity, the replacement of natural exchangeable cations or the introduction of an excess of sodium and calcium affects the change in deformation of products during firing less sharply (Fig. 3, Malinovskaya clay), as compared with clays containing a higher percentage of fine fractions (Figs. 4 and 5).

The low-temperature deformation of specimens, caused by the appearance of a primary melt, disappears in the presence of calcium carbonate in the clays. In this case the specimens deform at higher temperatures and considerably more abruptly as a result of the destruction of shape. In the presence of calcium carbonate, the sintering temperature rises and the sintering interval of red clays is sharply reduced with a decrease in their melting temperature, which was first noted by D. N. Poluboyarinov.

Conclusions

Products made from red clays containing calcium carbonate have a higher structural strength during firing and do not deform up to the sintering temperature. The deformation of such products proceeds abruptly and over a short temperature interval.

With the participation of alkalis, clay products deform smoothly, over a long temperature interval. Such compositions are more favorable for technological processing.