Effect of an Addition of Calcium Chloride on the Quality of Building Brick

P. G. Usov and A. V. Petrov

By introducing calcium chloride into clay in the production of building brick by the plastic forming method, just as when sodium chloride is introduced, the aim is to lower the freezing temperature of water and thereby protect the green brick from freezing. In comparison with common salt, \(CaCl_2\), as well as magnesium chloride, are more active depressants of the freezing temperature of water, since, unlike common salt, they crystallize in the form of hexahydrates of the type \(CaCl_2 \cdot 6H_2O\) and \(MgCl_2 \cdot 6H_2O\). It is well known that in concentrated solutions salts crystallizing without water give the smallest lowering of the freezing point. Salts with the same number of ions that crystallize with two molecules of water give a greater lowering, and salts with six molecules—an even greater one.

Thus, “dissolved substances that exert the most significant influence on the freezing point of water must crystallize from solution with the greatest amount of water of crystallization” [1].

For common salt, the molecular depression (the lowering of the freezing temperature of water per one mole of dissolved substance per liter, \(\frac{\Delta t}{m}\)) changes very little with changes in the concentration of the solution. For example, when the concentration of a NaCl solution changes from 0.0 to 3.0 moles per liter, the molecular depression changes from 3.3 to 4.0. For calcium chloride, when the concentration changes within the same interval, the molecular depression changes within the range from 5.3 to 13.3.

The change in the freezing temperature of the solution as a function of the calcium chloride content in it is given in Table 1 [2].

Thus, to prevent freezing of freshly formed green brick produced by the plastic forming method, an addition of calcium chloride acts more effectively than common salt.

In principle, calcium chloride can be introduced into the clay in such quantities that the green brick can be fully protected from freezing even under Siberian conditions. We know that freezing of the green brick is dangerous only in the freshly formed state, i.e., while it contains all of the forming water. Therefore, when calcium chloride is introduced into the clay for the purpose of lowering the freezing temperature of the water, its quantity must be calculated for the full content of forming water in the clay. Thus, in order to lower the freezing temperature of the water to \(-15^\circ C\), as recommended by the instructions for introducing calcium chloride to protect the green brick from freezing, it would be necessary to introduce about 4.72% of it by weight of dry clay at a forming moisture content of the clay of 25%. Such a consumption of calcium chloride significantly increases the cost of the brick and, most importantly, the product of the reaction of calcium chloride with clay, in the presence of water vapor, destroys the structure of the brick during firing, with a large reduction in its mechanical strength.

Table 1

Calcium chloride in pure form, when slowly heated to temperatures above 260°C, passes from the crystalline hydrate into the anhydrous form, which then melts at a temperature of 772°C. But with rapid heating of the crystalline hydrate, it partially decomposes, splitting off hydrochloric acid [3]. And when calcium chloride is heated in a mixture with clay and in the presence of water vapor, it decomposes completely according to the scheme:

\[ \mathrm{CaCl_2 + SiO_2 + H_2O = CaSiO_3 + 2HCl.} \]

In this case the yield of hydrogen chloride is captured quantitatively and completely, in accordance with the reaction scheme given. This reaction with the clay materials and in the presence of water vapor proceeds already at a temperature of about 100°C and is almost completely finished at a temperature of 500°C.

Table 2 gives the results of a study of the interaction of calcium chloride with clay at various firing temperatures.

Table 2

Upon reaching the final temperatures, a holding period of 30 min was given. The amount of unreacted CaCl₂ was determined from the aqueous extract of the fired specimen by chlorine.

Thus, during the firing of bodies made from red clays containing calcium chloride, in the presence of water vapor hydrogen chloride is liberated; both while being formed and in the combustion products it acts on the clay and changes its technical properties.

Red brick clays contain complex aluminosilicates in a finely dispersed state, the composition of which has not yet been established. The presence of these aluminosilicates imparts valuable technical properties to the clays, such as plasticity, the ability to form a mechanically strong structure of products in the dry state, and also a strong structure during firing up to temperatures preceding the onset of sintering, etc.

These aluminosilicates are basic in character. They dissolve readily in dilute hydrochloric acid, even in the cold, at the melting point of ice, and do not change their properties at all under the action of a normal solution of soda when boiled.

With the decomposition of these aluminosilicates, without removal of the decomposition products, the properties of the clays change very sharply. In particular, the ability of clays to form a mechanically strong structure during firing changes.

Table 3 gives data on the change in the properties of clays from brick plant No. 10 after treatment with HCl solutions of different concentrations.

Table 3

Specimens made from clays not treated with hydrochloric acid solutions, after firing at a temperature of 900°, do not change their properties at all under the action of acids. When treated with sulfuric acid, their mechanical strength even increases considerably. Table 4 gives the results of treating fired specimens with water and acids during boiling.

Table 4

Thus, the presence of hydrogen chloride in the furnace zone has a destructive effect on the technical properties of clays, as a result of which the mechanical strength of the brick decreases very sharply.

When brick is manufactured with an addition of calcium chloride in an amount of 4.72%, calculated to lower the freezing temperature of water to −15°C, during firing 0.033 gram of hydrogen chloride is released per one gram of dry clay. This amount is quite sufficient to decompose a considerable part of the soluble aluminosilicates and to cause radical changes in the technical properties of the clays.

In the kiln zone, cases are possible in which water vapor from the combustion products condenses on the surface of dry but cold brick at the front of the standing chambers. In this case hydrogen chloride, dissolving in the condensate, will diffuse into the structure in the form of hydrochloric acid. With reverse diffusion, as the brick is heated, the formation of an azeotropic mixture is inevitable; the activity of its action is higher than that of strongly diluted acid or gaseous hydrogen chloride. In this case, along with a reduction in the mechanical strength of the brick structure, its fracture usually assumes a zonal coloration and its appearance changes greatly, as a result of severe attack of the surface by the azeotropic mixture of hydrogen chloride, evaporating from the brick surface at a temperature of 110°C.

A practical investigation to determine the effect of the calcium chloride additive on the quality of building brick was carried out in the laboratory with clays from brick plant No. 10; tests under plant conditions were also carried out at the same plant.

The clays have the following characteristics.

Table 5

Chemical composition

The amount of CaO bound in carbonate in the clay is 1.32%.

Table 6

Grain-size composition of the clays

The normal content of forming water in the working clay mass fluctuates within the limits of 23–25%. Shrinkage during drying is about 7%.

Table 7

Behavior of clay products during firing

Behavior of Products with Additions of Calcium Chloride during Forming and Drying

A study of the effect of calcium chloride additions on the forming capacity of clays was carried out by the comparison method; for this purpose, products without additions and with additions of calcium chloride were formed from one and the same clay from Brick Plant No. 10, and their properties were investigated.

Table 8

Drying of bricks with additions of calcium chloride proceeds more slowly than that of bricks without additions. The rate of evaporation of a solution containing calcium chloride decreases as its concentration in the solution increases, which was to be expected, since the vapor pressure of aqueous solutions decreases by an amount equal to approximately 2% per mole of dissolved substance [4].

The prolongation of the drying of green brick with an addition of calcium chloride is especially clearly revealed during drying in dryers at the plant. The trial batches tested at the plant were made with an addition of 3% calcium chloride and were formed at a moisture content of 18–19%.

The green brick was dried under the same regime as without CaCl₂ additions. The temperature in the common flue was in all cases maintained within the limits of 180–200°C. Despite the longer drying period, the brick was unloaded from the dryers with a higher moisture content, reaching 15%, whereas for brick without calcium chloride addition the water content after drying did not exceed 12%.

Table 9 gives data on the duration of drying of green brick, in hours by chambers, immediately before the use of CaCl₂, of green brick with CaCl₂, and immediately after the use of CaCl₂.

In appearance, the brick with additions of calcium chloride dried in the dryers did not differ from brick without additions. No cracks or other defects associated with drying were observed on it.

The dry brick was fired in a chamber kiln of the “Zig-Zag” type at a temperature of about 900°C.

At first, 16 thousand bricks with CaCl₂ were formed, and after drying they were loaded for firing mixed with brick without calcium chloride addition, approximately half and half. Its firing proceeded abnormally, since there was no front of chambers charged with green brick ahead of the fire. The charged chambers were put on fire immediately after loading, and within 14 hours their firing was completed.

From the fired batch of brick with calcium chloride, the best specimens were selected for laboratory tests.

The test results are given in Table 10.

Table 9

Table 10

After this, another 64 thousand bricks were molded with additions of calcium chloride in an amount of 3%. This time, 9 chambers of the kiln were loaded with it alone, and firing proceeded completely normally, with 2–3 chambers kept ahead of the fire. The maximum heating during firing was brought to a temperature of 900°C. The normally fired brick had no defects in external appearance. The color, in the absolute majority, was uniform, the same as that of brick without additions of calcium chloride. The best bricks from the fired batch were selected for laboratory investigations.

The characteristics of brick with an additive of calcium chloride after firing it without bricks not containing calcium chloride additives are given in Table 11.

Table 11

As is evident from Table 11, the quality of brick with CaCl₂ under normal firing is very sharply worsened. The brick cannot be certified even as grade “50.”

A comparative characterization of the physicomechanical indices of brick without calcium chloride additives, produced by the plant in the autumn of 1951, immediately before the manufacture of brick with the CaCl₂ additive, and in January 1952, immediately after the manufacture of the experimental brick, is given in Table 12.

Table 12

The firing of the bricks with CaCl₂ was carried out in such a way that externally they proved to be unblemished, unlike the brick with common salt manufactured at “Keramika,” and only in rare cases was a zonal structure observed in the fracture, whereas with common salt the entire brick had zonal coloration. This difference is explained not by a difference in the composition of the clays, but by a different firing regime.

When firing brick with the calcium chloride additive at Plant No. 10, the brick was charged into the kiln with a higher moisture content, equal to 13–15%. The drying of such brick was carried out in the kiln by combustion products having a temperature of about 100°. Under these conditions, water from the combustion products could not condense on the greenware in the chambers being dried. Therefore there was no dissolution in condensate of calcium chlorid—

hydrogen from the combustion products and the diffusion of the solution into the structure. This was also hindered by the process of evaporation of moisture from the brick itself. Therefore, zonal coloration of the bricks in the fracture was weak and occurred only in rare cases. On the whole, the brick had an even coloration—red in color and of higher intensity in comparison with brick without $\mathrm{CaCl_2}$. The introduction of calcium chloride into the batch in an amount of $3\%$ proved quite sufficient to produce such damage to the clay that, after firing, the bricks had a strength almost three times lower than those made from the same clays but without calcium chloride additives.

Conclusions

1. Calcium chloride, like sodium chloride, reacts with clay in the presence of water vapor with the formation of hydrogen chloride, which then destroys the colloidally dispersed constituent part of the clays and changes their technical properties.

2. Clays with an addition of calcium chloride, during firing, form a brick structure with lower mechanical strength. In this case, the decrease in strength is directly dependent on the amount of calcium chloride per unit weight of clay (i.e., on the concentration of hydrogen chloride). Hydrogen chloride vapors destroy clays especially strongly in the process of heating them to boiling. Thus, the greatest danger with respect to the loosening of the structure is the possibility of forming an azeotropic mixture containing $20.4\%$ hydrogen chloride and boiling at $110^\circ\mathrm{C}$.

3. The introduction of chloride salts, $\mathrm{NaCl}$ and $\mathrm{CaCl_2}$, to lower the freezing temperature of water in the clays of the city of Tomsk and certain districts of the region, in amounts from $1$ to $3\%$, leads to a reduction in brick strength by almost a factor of three. Thus, in order to prevent freezing of the green brick, the recommendation to introduce soluble salts into the clays of the city of Tomsk is not acceptable.

Literature

1. Getman F. and Daniels F. Fundamentals of Physical Chemistry, 1941 ed., p. 185, Goskhimizdat.

2. Instructional guidelines for introducing calcium chloride into the batch to protect green brick from freezing at a temperature of $-15^\circ\mathrm{C}$, Promizdat, 1951, p. 3.

3. Nekrasov B. V. Course of General Chemistry, 1948 ed., p. 678.

4. Technical Encyclopedia, vol. 3, p. 885, 1937 ed., Goskhimizdat.