EFFECT OF SODIUM CHLORIDE ADDITION ON THE QUALITY OF BUILDING BRICK

P. G. USOV

The addition of sodium chloride to clay in the production of building brick for seasonally operating plants in Siberia is of great importance. By lowering the freezing temperature of water, it appears possible to extend the period for forming green brick by beginning the season earlier in spring and ending it later in autumn.^1)

According to a report by the Deputy Minister of the Building Materials Industry of the RSFSR, in the winter of 1951–52 it is planned to test the manufacture of brick with additions of soluble salts on a broader scale [2].

In the city of Tomsk, the “Keramik” brick plant began forming salted brick in the autumn of 1950. Sodium chloride was introduced into the clay in the solid state at a rate of 1.5% of the dry mass. Salting of the clay is carried out in the quarry while the rail car is being loaded by the excavator. From the quarry the clay is fed to the feeder, then to the clay pug mill, the fine-grinding rolls, and the auger press. Part of the salted brick formed in the autumn of 1950 overwintered as green brick, and during the winter the green brick dried in drying sheds without any noticeable defects. The manufacture of salted brick also continued in early spring 1951; it was dried under natural conditions likewise without defects.

Fig. 1. Temperature state in the kiln

^1) Some plants manufacture salted brick by introducing sodium chloride into the clay up to 2.5% [1].

Dry salted brick is fired in a water-free annular kiln with a kiln channel located in the ground. The kiln is fired with hard coal, the fuel being charged from above through openings. Firing is carried out on two fires, so that ahead of the fire, in drying and preheating, there are usually two or three chambers.

The temperature state of the chambers of both fires, according to one of our measurements, is shown in Fig. 1.

According to the plant’s measurements, the temperature of the combustion products at the outlet to the smoke flue did not rise above 70°C. More often it is within the range of 50–60°C. After firing, all the salted brick came out with major defects. This was what made it necessary to undertake the present investigation.

Characteristics of the destruction of the bricks

The side surfaces of the bricks, to a depth of up to 5 mm, and the edges and corners, to 15 mm, are affected and constitute a material with a loose structure, which flakes off in layers when touched. No signs of sintering are observed in the corroded layer. The coloration of the brick structure in fracture has a zonal character: the inner part—pale red—corresponds to the coloration of normal, unaffected brick. It is surrounded by a layer of red-colored structure 16–17 mm thick. Closer to the surface, beyond the red layer, there is a ring 3–4 mm thick of cherry-red color and, finally, at the very surface, a layer 3–4 mm thick of pale pink color, changing to gray at the surface itself. The transitions in coloration from one color to another are sharply expressed.

Unsalted brick from the same plant, after firing without salted brick under the same schedule, does not have such defects.

When only one salted brick is fired, it is affected so severely that it is entirely rejected. After firing salted brick in a mixture with unsalted brick, the salted brick is affected less; at the same time, a noticeable surface attack is also observed on the unsalted brick. The zonal structure in fracture of the salted brick fired together with unsalted brick is clearly visible, but it is less pronounced than when only one salted brick is fired. In this case, a zonal structure is also observed in fracture in the unsalted bricks.

It is characteristic that both salted and unsalted brick, whether fired separately or in a mixture with one another, show absolutely no signs of corrosion at the places where they were in contact with one another, i.e., at the places that were not washed by the combustion products.

Specimens of salted bricks affected during firing are shown in Figs. 2 and 2′.

The outer zone of pale pink color, changing at the surface to gray, has such low strength that it easily crumbles when touched by hand, and it was not possible to test it.

Unsalted brick from the same plant, when tested for mechanical strength on small specimens, had an average compressive value of 120 kg/cm², and when tested according to GOST—a grade of “100,” with a completely regular shape. Thus, according to the test data, the inner zone of the affected brick, not only in color but also in physicomechanical properties, corresponds to normal brick made without salt additions. The strength of the red, affected zone is 25–30% lower than that of the sound part of the brick.

The plant works with ordinary red brick clays of the chemical composition (Table 3).

Fig. 2 and 2′. Salted bricks after firing.

Fig. 3. Structure of salt bricks at the fracture

Table 1

Physicomechanical characteristics of fired salt bricks

Table 2

Physicomechanical characteristics of individual zones of a salt brick
(The test was carried out on small cube specimens with a side of 20 mm)

Table 3

The clay contains a small amount of CaO—about 1.0% in the form of carbonate; when treated with hydrochloric acid it effervesces weakly.

Table 4

Particle-size composition

The swelling value according to Rutkovsky is 0.36 cm³/cm³. The exchange capacity is 21.5 milliequivalents per 100 g of dry clay. The amount of tempering water, by weight of the dry mass, ranges from 22.0 to 26.0%. Drying shrinkage is 6.5–8.0%. The tensile strength in the dry state is 10–18 kg/cm².

Table 5

Sintering temperature

The refractoriness is 1240°C.

In external appearance, dry salted brick containing about 1% common salt does not differ from unsalted brick. The introduced salt does not noticeably change the behavior of the clay in the raw state, nor the physicomechanical properties of the dry brick.

In a parallel study of salted and unsalted bodies from one and the same clay, the following results were obtained (Table 6).

Table 6

The drying of salted bricks proceeds more slowly than that of unsalted ones, especially in the final stage, when the concentration of the salt solution becomes

significant (see Fig. 4), but this is unlikely to affect the technology of producing building brick. Brick plants have fully mastered the firing of bricks with elevated moisture content (up to 10% and higher), and the drying

Fig. 4. Drying rate of salted and unsalted bricks

to this moisture content of salted and unsalted bricks has practically no difference.

In the study of raw salted bricks of factory manufacture it was established that the amount of sodium chloride in them is about 1%, i.e., less than that calculated during batching. The salt is distributed in the clay mass sufficiently uniformly, and no noticeable stratification of its concentration during drying, as is evident from the analysis, is observed.

Table 7 gives the results of the analysis of five dry salted bricks taken from different places, and the results of the analysis of different places of one separately taken brick.

Table 7

Sample No. 1. Taken from the central part of the brick in the transverse fracture, at the point of intersection of the diagonals.
Sample No. 2—from the side surface.
Sample No. 3—from the edge.
Sample No. 4—from the corner.

The content of Ca and Mg cations and of SO₄ and NO₃ anions in the aqueous extract from the salted bricks was not detected. Thus, sodium chloride

the salt used in production had no water-soluble mineral impurities.

Firing salted brick at the plant and in laboratory furnaces is accompanied by the release of hydrogen chloride vapors. Bricks fired in the laboratory furnace have the same zonal structure in fracture as bricks fired at the plant. The exception is that, in this case, there is no pale-pink layer passing into gray, located immediately at the surface of the brick, with extremely low mechanical strength, which is present in bricks of plant manufacture. We assumed that this layer is obtained as a result of the interaction of the clay on the surface of the brick not with hydrogen chloride vapors, but with its aqueous solution. And this was fully confirmed experimentally when firing specimens that had first been impregnated to a certain depth with a hydrochloric acid solution.

After preliminary treatment of clay from the same “Keramik” plant with hydrochloric acid solutions of different concentrations, its properties change very sharply. Depending on the specific consumption of hydrogen chloride per unit weight of clay, the following changes are recorded in it (Table 8).

Table 8

At the same time, at different specific consumptions of hydrogen chloride for treating the clay, the fired specimens, along with differing strength, also have different coloration.

1. At an HCl consumption of 0.166 g/g of dry clay, the coloration is light gray, considerably lighter than the outer layer of the affected plant brick. This color corresponds approximately to the coloration of the very surface of the affected brick.

2. At an HCl consumption of 0.083 g/g of clay—the color is grayish-pink. This coloration corresponds to the outer layer of the affected plant brick.

3. At a specific HCl consumption of 0.041 g/g of dry clay—the color is bright red (cherry); it corresponds approximately to the second layer from the surface of the affected brick of plant manufacture.

4. At an HCl consumption of 0.0083 g/g of dry clay—the color is red; it corresponds to the third layer of the brick.

5. At an HCl consumption of 0.0041 g/g of dry clay—the color is pale red.

1. With an HCl consumption of 0.00208 g/g of dry clay and lower, the color of the structure corresponds to the color of normal, unaffected brick.

Consequently, the color of the structure during the firing of salted brick is caused by the action of different concentrations of hydrogen chloride, both in the form of a solution and in the vapor state. Gaseous hydrogen chloride weakens the mechanical strength of the structure to a lesser degree, reducing it by 20–25%. Its aqueous solutions act especially strongly in destroying the structure.

When clay is treated with solutions of hydrogen chloride, unequal reaction activity of its component parts is observed. Calcium carbonate reacts with HCl more readily than the others. The composition of the dense residue from the hydrochloric-acid extract at concentrations of 0.25%, 0.5%, and 1.0% contains about 97% calcium chloride and only about 3% other chlorides and silica. Aluminosilicates and ferrites decompose when more concentrated HCl solutions are used, or at a higher specific consumption of it at low concentration per unit weight of clay. This relationship of clay to the action of hydrogen chloride makes it possible to conclude that products made from clays containing carbonates in large quantities will be destroyed more weakly by hydrogen chloride and its solutions formed during the firing of salted bricks, since its aggressive action will to a considerable extent be neutralized by calcium carbonate.

Thus, during the firing of salted bricks, as a result of the reaction of sodium chloride with the clay material, hydrogen chloride is released [3], and the loosening of the brick structure is the result of the action on the clay of gaseous hydrogen chloride and its aqueous solution.

Hydrogen chloride is a very stable gas. It decomposes noticeably into the elements only at temperatures above 1000°C [4].

The vapors of hydrogen chloride, together with the combustion products, move into the chambers with the green brick being preheated and act upon it. In the preheating chambers, according to our measurements and those of the plant, the temperature is below 70°C. Under these conditions, water vapors from the combustion products condense on the surface of the brick, and hydrogen chloride vapors dissolve in the condensate, forming hydrochloric acid. Hydrochloric acid diffuses into the structure as long as the brick surface remains wet, that is, as long as water vapors continue to condense on it. But as the fire approaches, the bricks heat up, their surface dries, and the movement of moisture—or, more precisely, of the HCl solution in the brick—changes its direction. Now it will move from the inner layers toward the surface. The outer surface of the brick is affected more strongly, on the one hand, because gaseous hydrogen chloride and hydrochloric-acid solutions act on it together for a longer time. And on the other hand, it is acted upon by more concentrated solutions. Since the most stable aqueous solution of hydrogen chloride is a solution with a density of 1.10, containing 20.4% hydrogen chloride, it has a constant boiling point equal to 110°C [5]. “Acid whose density is greater than 1.10 loses hydrogen chloride during evaporation faster than water, while at a density less than 1.10 water evaporates faster than hydrogen chloride. In both cases the mixture, on boiling, tends to approach the acid with a constant boiling point,” that is, a density of 1.10. For this reason, during reverse diffusion, more concentrated solutions of hydrochloric acid act on the surface layer of the brick. This is manifested especially sharply on the edges and corners.

The red layer in salted brick is the layer into which hydrogen chloride or its aqueous solution has diffused. The thickness of this layer and the intensity of its coloration depend on the rate of the fire and on the concentration of hydro-

hydrogen chloride in the combustion products. The less time the chamber spent in preheating at low temperatures and the lower the concentration of hydrogen chloride in the combustion products, the thinner the red layer and the weaker its coloration. Therefore, the most sharply expressed zonal coloration in the fracture is observed when firing a single salted brick, and a less sharp coloration when firing salted brick in a mixture with unsalted brick.

The presence of a gray layer on the surface of the affected brick, with the most weakened mechanical strength of the structure, is the result of the action of a hydrochloric-acid solution of relatively high concentration. Hydrochloric acid with a density of 1.10 evaporated from the surface of this layer at a temperature of 110°C.

In the places where bricks were stacked one on another, their surfaces were not washed by the combustion products, and no diffusion process occurred in them; therefore they also did not undergo change.

The action of hydrogen chloride and its solutions on red brick clay is manifested in the fact that it destroys certain aluminosilicates present in the clay in a highly dispersed state, with whose presence the manifestation of some of its properties is associated, such as plasticity, shrinkage during drying, the ability to form a mechanically strong structure without the participation of a melt, etc.; after treatment, even with strongly diluted solutions of hydrochloric acid, these properties disappear in the clay. The quantitative yield of aluminosilicate soluble in hydrochloric acid is small, but the properties of the clays change significantly.

We partially investigated the behavior of common salt during heating and its interaction with clay. In pure form and in the absence of water vapor, NaCl is a stable compound. It shows no signs of decomposition when heated to a temperature of 850°C. But in a mixture with clay, and especially with raw clay, common salt in considerable quantities reacts already at a temperature of about 200°C. Table 9 gives the results for the change in the NaCl content in clay with the calcination temperature (analysis by chlorine).

Table 9

As is evident from Table 9, the reaction of common salt with clay is practically completed at a temperature of 500°C.

When common salt is introduced into clay in an amount of one percent, every 1,000 bricks will release about 25 kg of hydrogen chloride. If this amount were to react completely with the clay, it would be quite sufficient for the complete damage of the entire mass of brick made from these clays, with a weakening of the structural strength by up to 50%.

Conclusions

1. As a result of the reaction of common salt with clay during firing, hydrogen chloride is released; in the gaseous state, and especially as its aqueous solution, it destroys the finely dispersed constituent part of clays, as a result of which the mechanical strength of the fired brick is sharply reduced.

1. From clays of the city of Tomsk and certain districts of the region with a carbonate content of about 1.5%, with an addition of sodium chloride of not more than 1% and firing of the green ware in ring kilns, the brick obtained is of exceptionally low quality (rejects). The damage is reduced when salted brick is fired in a mixture with unsalted brick, taken in equal proportions; nevertheless, its quality remains lower than that of brick made without additions of sodium chloride.

2. An aqueous solution of hydrogen chloride most strongly attacks the structure of the brick, with a large reduction in mechanical strength. Therefore, it is advisable to fire salted bricks in intermittent kilns (or in floor kilns); when firing in ring kilns, the combustion products must be removed from the chambers at a temperature not lower than 110°C, without allowing condensation on the green ware of the azeotropic mixture of hydrogen chloride.

3. Clays containing large amounts of carbonates will be less subject to the destructive action of hydrogen chloride. From such clays it is possible to prepare brick with additions of sodium chloride.

Therefore, it is not possible to recommend to seasonal plants the introduction of sodium chloride into clay without a preliminary investigation of the composition and properties of the raw material in each individual case.

LITERATURE

1. Gonchar P. How to preserve green ware during frosts. Newspaper “Prom. stroimaterialov,” 3/IX—1951.

2. Dobrovolsky I. A task of great importance. Newspaper “Prom. stroimaterialov,” 7/IX—1951.

3. Budnikov P. P. Ceramic Technology, part I, p. 450, State Scientific and Technical Publishing House of Ukraine, 1937.

4. Nekrasov B. V. Course of General Chemistry, 8th ed., p. 243, Goskhimizdat, 1948.

5. Treadwell F. P., Hall V. G. Qualitative Analysis, p. 335, Goskhimizdat, 1946.