Stabilization of the Properties of Building Brick

P. G. Usov, N. F. Voronova

(Presented by the scientific seminar of the Department of Silicate Technology)

Insufficient study of the physicochemical processes [1, 2] occurring during the firing of products made from low-melting clays leads to the firing regime being established without taking into account the capabilities of the material being fired. Firing is the final stage for all ceramic materials. During firing, the mechanical strength of the products is also formed.

The existing theoretical explanations of the formation of a strong structure during the firing of ceramic products [3], based mainly on the processes of melting of the material and on reactions involving the melt, do not encompass all the phenomena that take place in the formation of a strong structure from low-melting clays, and are not even the principal ones. In the development of the mechanical strength of products made from loams, the mineralogical compositions of the finely dispersed fractions play the main role. Loams containing a large quantity of finely dispersed fractions (fractions smaller than 0.001 mm) possess higher mechanical strength after firing. In addition, a clay particle contains on its surface colloidal shells, which are formed as a result of the loosening or dissociation and dissolution of the surface layer of the clay particle by water, or as a result of the peptization of hydrophilic clay material [4].

When loam is dissolved in water, the colloids pass into sol and gel formation, owing to which the clay particles are bonded.

Thus, the clay substance, along with the clay minerals proper—montmorillonite and hydromicas, which are crystalline formations with a stoichiometric ratio of elements—also contains amorphized material: colloids of varying degrees of maturity, without stoichiometric ratios of elements. The presence of colloidal shells on clay particles accounts for the plasticity of loams and their cohesiveness. Especially strong and dense bodies are obtained upon drying masses that contain a sufficient quantity of particles of colloidal size, which during treatment with water had been brought to “primary sizes,” to a colloidal solution.

Bringing clay particles during treatment to colloidal sizes is a necessary condition for red clays for the development of reactions in the solid state during firing, as a result of which the strong structure of building brick is formed.

The entire firing process of building brick can be divided into three stages.

The first stage is firing up to 800°C. In this temperature interval, mechanical strength increases gradually. Moreover, the values of mechanical strength depend on the material composition of the loams and on the quality of preparation of the body. At the temperatures of molecular breakdown of clay minerals, no weakening of the structure is observed [1, 2].

The second stage is firing at a temperature of 800–1050°C. In this temperature interval, the mechanical strength remains the same as at 800°C, but this period is characterized by a radical change in the structure of the shard, associated with the destruction of the crystal lattice of the clay substance and the formation of spinels.

The third stage is firing at a temperature of 1050–1100°C. In this temperature interval, the increase in mechanical strength proceeds very rapidly, owing to intensive melting of the coarsely dispersed component (fractions larger than 1 micron), consisting mainly of feldspar, quartz, hornblende, etc. But because of the sharp decrease in the apparent viscosity of the material, the products are easily deformed. Therefore, the low-melting component of the coarsely dispersed part of the loam does not make it possible to use melting processes to strengthen the structure of the products.

Thus, the principal value of the mechanical strength of building brick is created during firing up to a temperature of 800°C, but its resistance to water and atmospheric agents is low. Therefore, to stabilize the chemical resistance of the structure of building brick, it must be fired at a temperature of 900–1000°C.

The studies we carried out on the hydrophilicity of fired loams made it possible to establish that noticeable expansion of products made from loams occurs even after firing them at a temperature of 800°C. Above 900°C, practically no expansion of clays occurs.

Fig. 1. Expansion of fired specimens after saturation with water

To determine the magnitude of clay expansion and the mechanical strength after firing at the corresponding temperatures, specimens were made from loams in the form of bars with dimensions: length 5 cm, width and height 1 cm, and cubes with an edge of 2.5 cm. After drying, the specimens were fired in a muffle furnace at temperatures of 100–1000°C with an interval of 100°C and a holding time of 1 h at each temperature.

To determine the degree of expansion, the bars, in a suspended state, were immersed in a vessel with water. The magnitude of expansion was measured with a micrometer to an accuracy of 0.5 micron. The study established that noticeable expansion of specimens occurs in the first hour of their saturation with water (Figs. 1, 2). Specimens fired at a temperature of 100–200°C disintegrated after immersion in water in 10–20 seconds. Specimens fired at a temperature of 300–1000° expand in the first hour of saturation with water.

from 0.014 to 2.8% of their length.

The mechanical compressive strength of the cubes was determined in the dry state and in the water-saturated state. Saturation with water was carried out for 24 hours. The change in mechanical strength is shown in Fig. 3. It is evident from the figure that the mechanical strength increases uniformly up to 800°C. Wet specimens have lower mechanical strength than dry ones. In the temperature range 800–1000°C, the mechanical strength of both dry and water-saturated specimens remains approximately the same.

Fig. 2. Expansion of a specimen fired at 300°C after saturation with water

When products are fired to a temperature of 800°C, the decomposition products of the clay substance in loams possess a high adsorption capacity and a tendency to swell. Water, entering the pores of the products, pushes the particles apart and thereby expands the product and reduces the mechanical strength of the specimens.

At temperatures above 800°C, a primary melt appears [5], which partially dissolves the decomposition products of the clay substance and stabilizes the structure. As the firing temperature is increased up to 1000°C, the amount of melt increases only slightly.

The mechanical strength of the structure of products formed by firing at a temperature below 800°C is unstable with respect to the action of water.

Thus, in order to stabilize the properties, firing of building brick must be carried out at a temperature of 950–1000°C, at which a higher chemical stability of the structure of the products is achieved. Mechanical strength, however, is created by preparing the mass in the plastic state by reducing the colloids to the sizes of elementary particles, by drying the products, and by firing them at a temperature below 800°C.

Fig. 3. Change in mechanical strength of specimens.
1 — dry specimens,
2 — water-saturated specimens.

References

1. P. G. Usov. Pyrochemical activity of red brick clays. Izv. SO AN SSSR, No. 8, 1958.

2. M. G. Lundina. Investigation of the physicochemical processes occurring during the firing of products made from low-melting clays. In: Collection of Scientific Papers, issue VII. Materials of the scientific session of NIISM BSSR, Minsk, 1958.

3. A. A. Baikov. Physicochemical conditions for the production of refractory products. State Scientific and Technical Publishing House, Moscow, 1931.

4. N. Ya. Denisov, P. A. Rebinder. On the colloid-chemical nature of the cohesion of clayey rocks. Dokl. AN SSSR, vol. 54, No. 6, 1946.

5. P. G. Usov, A. V. Petrov. Deformation of products made from red clays. Izv. TPI, No. 83, 1956.