Effect of Temperature and Aging Time of Bodies Made from Red Clays on the Properties of the Paste and of Products Made from It

P. G. Usov

In studying methods for preparing plastic bodies from red brick clays, we sought to determine the effect of temperature and aging time on the formation of the structure of brick and on its mechanical strength in the fired state.

In the practice of producing building brick by the plastic method of shaping, the parameters—temperature and aging time—vary over wide limits. In winter, when working with cold water and quarry clay, the temperature of the molding body as it leaves the auger is close to \(0^\circ\mathrm{C}\), whereas with the use of steam moistening it rises to \(40^\circ\mathrm{C}\). At mechanized brick plants, the process of extracting clay from the quarry is usually combined in time with its processing into brick, and preliminary preparation of the bodies with aging in the wet state is not practiced; this is usually not permitted at small collective-farm or cooperative plants that process the body into brick manually. We studied the interaction of clay with water from changes in the properties of clay paste prepared under different conditions.

As an evaluation parameter we adopted the limiting structural strength, previously applied to clay bodies by P. P. Budnikov and G. S. Blok. The limiting structural strength of the paste is determined from the expression

\[ \tau = k \frac{P}{h^2}\ \text{kg}/\text{cm}^2, \]

where

\[ \begin{aligned} \tau &\text{— limiting value of shear stresses, in kg/cm}^2,\\ P &\text{— load, in kg,}\\ h &\text{— depth of immersion of the cone, in cm,}\\ k &\text{— coefficient depending on the angle of inclination of the rubbing surfaces}\\ &\quad \text{of the cone (for our cone with an angle of inclination of }45^\circ,\ k=0.685). \end{aligned} \]

The plastometer was made so that any depth of immersion of the cone into the paste and the corresponding load could be recorded on it. The rate of introduction of the cone was always maintained the same by means of precise regulation of the water supply to a small hydraulic press that transmitted pressure to the cone. This makes it possible to calculate the limiting value of the shear stresses for any depth of cone immersion. For the study, in all experiments the clay was taken in an air-dry state, ground and passed through a sieve with openings of \(1\ \text{mm}\). The preparation of specimens for the study of each series was carried out simultaneously from one batch and, as far as possible, under identical conditions. In all cases the clay bodies were prepared with molding moisture and the quantity

water was added by weight dosing. The paste was processed manually to a forming state.

For the study of structural-mechanical properties, paste with identical compaction was placed in metal rings 60 mm in diameter and 30 mm high. The lower open surface of the ring was set on a glass plate—the bottom—and a cone was pressed into the center of the upper surface. To age it, the open surfaces of the clay in the rings were covered with a layer of paraffin and, after weighing, were kept in desiccators at a specified temperature in thermostats. Before testing, each specimen was weighed again to check constancy of weight, after which, from the surface into which the cone had been pressed, the layer of paraffin was cut off and the structural-mechanical strength of the paste was determined.

In all cases, the specimens subjected to testing were those whose weight did not change during aging.

The study was carried out with two red brick clays, the grain-size and chemical compositions of which are given in Tables 1 and 2.

Table 1

Grain-size composition

Table 2

Chemical composition

The mineralogical composition of the fractions finer than one micron in both clays is composed of montmorillonite with an admixture of hydromicas. The exchange capacity and pH of the clays have the following values:

for the first clay—21.5 milliequivalents per 100 g of dry substance; for the second—33.3 milliequivalents per 100 g of dry substance; pH is 7.1 for the first and 7.2 for the second.

The change in the physical state of clay paste under different conditions of its formation was studied in detail by A. I. Avgustinik [2]. He was the first to establish the spontaneous reversible strengthening of clay paste during aging, which, upon repeated kneading, disappears to its initial values. He called this phenomenon thixotropic strengthening.

In studying the influence of temperature on the thixotropic strengthening of clay paste during aging at normal and above-normal temperatures, we obtained data analogous to those of A. I. Avgustinik, according to which, at a temperature of 20°C, the maximum thixotropic strengthening of the structure of clay paste is reached after three to four days, while with an increase in the aging temperature to 40°C the rate of thixotropic strengthening of the paste increases. The change in the structural strength of the paste at different aging temperatures is presented in the tables.

At a temperature of 20°C

Table 3

At a temperature of 30°C

Table 4

At a temperature of 40°C

Table 5

For practical purposes it is also important to know the interaction of clay with water and the behavior of clay paste during aging at low temperatures.

The investigations we have carried out show that at temperatures close to zero, water is assimilated by clay less readily. A paste made from one and the same clay, with the same weight quantity of molding water, after identical working has different ultimate structural strength if it was prepared at different temperatures.

When water with a temperature close to 100°C is used, the structural strength of the paste is 22% higher than that of a paste of the same composition prepared from water and clay cooled to 2°C.

At low temperatures the clay particles disperse in water less colloidally and form solvate shells of smaller thickness. For this reason, at low temperatures, despite the increase in the viscosity of water, the structural-mechanical strength of the paste has a lower value.

Table 6 gives the results of determining the ultimate structural strength of a clay paste prepared from one and the same clay at different temperatures. The first was made from water and clay cooled to 2°C, the second from clay at normal temperature and boiling water. In both cases the masses were worked by hand to the molding state, and during testing the first mass had a temperature of 8°C and the second, 30°C.

Table 6

Experiments also show that at low temperatures, along with the binding of a smaller quantity of water by the clay, no thixotropic strengthening of the clay paste occurs during aging.

Table 7 gives data on the change in the structural strength of a clay paste prepared from clay and water cooled to 2°C, with aging at temperatures from 2 to 5°C.

Table 7

The amount of water bound by the clay decreases very sharply when the paste is frozen. Before freezing, the clay paste is a homogeneous system in which the water is distributed rather uniformly throughout the whole mass of the clay. During freezing, the colloidal shells of the clay particles become compacted, and, under the action of crystallization, the excess water is squeezed out and crystallizes in the cracks in pure form (Fig. 1).

Fig. 1

For our clays, the amount of water squeezed out during freezing is about 6%, and when products are made from these clays with a water content 6% below the molding water content, no structural changes are observed during freezing. After the paste thaws at temperatures close to zero, the water squeezed out is again not absorbed by the colloidal shells of the clay, but remains in the form of a mechanical impurity; as a result, the structural-mechanical strength of the paste decreases sharply, which is easily confirmed by experiment.

Table 8

Modern views on the interaction of clay with water affirm the formation on the surface of clay particles of films of colloidal solution [3], solvation shells [4 and 5]. The presence of the latter ensures

plasticity and the binding action of the clay paste. With unequal colloidal solubility in water of the clay particles at different temperatures, solvate shells of different thickness are formed, which cannot but affect the formation of the structure of the products.

To verify the proposition stated, we prepared products from one and the same clay with the same amount of molding water, taken by weight batching. All batches were processed on a small laboratory belt press, with a round-section mouthpiece 24 mm in diameter. From the extruded rods, specimens were cut for bending and compression tests.

The first batch was prepared by mixing cold clay with cold water. The temperature of the batch on leaving the mouthpiece was 5°C.

The second batch—clay and water were mixed at room temperature. The batch temperature was 20°C.

The third batch—clay at room temperature was mixed with boiling water. The batch on leaving the mouthpiece had a temperature of 30°C.

The fourth—clay and water at room temperature were mixed, kneaded by hand to a molding paste, and aged in a desiccator for three days to the greatest possible thixotropic strengthening of the batch, after which it was passed through the belt press. All specimens were dried and fired under identical conditions. The cold batch was worked on the press worse than the others. The batch after aging molded best of all.

The mechanical strength of specimens from all four batches after firing to a temperature of 950° is given in Table 9.

Table 9

We had occasion to observe a decrease in the mechanical strength of brick and in its grade when the temperature of the processed batch was lowered in the winter of 1953–1954 at one of the brick plants in the city of Tomsk.

In the autumn the plant worked on clay of summer preparation from the reserve pit. The clay prepared in the reserve pit was not sufficient for the whole winter period of operation. And in December the plant was forced to extract clay from an open pit, first thawing it in the face by electric current. Cold water was used to make the batch. When quarry clay was used, the temperature of the batch being processed on the presses fell sharply, and the quality of the brick produced declined.

The quality of the brick for the autumn–winter months is given in Table 10.

Table 10

The positive effect of hot treatment of bodies on the mechanical indices of the fired bricks has also been noted by other investigators [6].

Thus, when clay interacts with water at different temperatures, solvate shells of different thickness are formed around the clay particles; these shells are directly related to the mechanical properties of the finished products.

LITERATURE

1. Budnikov P. P. and Block G. S. DAN SSSR, vol. 89, No. 5, pp. 897—900, 1953.

2. Avgustinik A. I. Self-steaming management of clay bodies. Proceedings of the session of VNITO on the achievements of Soviet science in the field of silicates. Promstroiizdat, pp. 255—283, 1949.

3. Denisov N. A. and Rebinder P. N. DAN SSSR, vol. 54, No. 6, 1946.

4. Sharov V. S. The nature of clay and its relation to water and aqueous solutions. Geology and Engineering Geology. Collection of articles No. 5, edited by P. V. Bogomolov, pp. 24—59, 1940.

5. Sharov V. S. Contradictions of contemporary colloid chemistry of soils and their sources, journal Pochvovedenie, No. 6, pp. 121—175, 1939.

6. Ellis-Anwyl R. H. Ryck and Clay Record, No. 1, pp. 54—60, 1946.