The Effect of Hydrogen Chloride and Its Solutions on Clays of Different Mineralogical Composition

(On the question of the use of chloride salts in the production of building brick)

P. G. Usov

Brick plants that operate seasonally, in order to mold brick at subzero temperatures, introduce water-soluble chloride salts into the clay. The purpose of adding salts is to lower the freezing temperature of water and thereby protect the green body from freezing. Chloride salts NaCl and CaCl₂, in the presence of water vapor, when heated, react with clay with the formation of hydrogen chloride, which acts on the clay chiefly on the finely dispersed fraction, sharply altering its properties [1,2]. The evolution of hydrogen chloride upon heating clays with additions of soluble chloride salts is easily established both in the laboratory and under plant conditions.

The chemistry of the interaction of chloride salts with clay minerals has so far been insufficiently studied. In the literature one more often encounters information relating to the interaction of carbonate salts with kaolin [3] or with the oxides composing it. For example, recently M. K. Moshkina, under the direction of Professor I. S. Lileev, at the West Siberian Branch of the Academy of Sciences of the USSR, studied the chemistry of the interaction of sodium carbonate with silica and alumina. According to their investigations, soda reacts with alumina with greater difficulty than with silica. Interaction with alumina begins at a higher temperature than with silica, and this reaction proceeds at an appreciable rate at temperatures above 750°C. The temperature at which the reaction begins and its rate with silica are influenced by the physical structure of the silica. Soda begins to react with SiO₂ gel at a temperature of 300°C, and the reaction proceeds at a considerable rate at 450°C. With quartz, the reaction of soda begins at a temperature of 500°C and acquires a considerable rate above 650°C.

The interaction of chloride salts with clay, introduced into the mixture in the form of aqueous solutions, begins at lower temperatures. We investigated mixtures of clay with chloride salts upon heating to various temperatures, with a holding time of 30 minutes; the results of such an investigation are given in Table 1.

Table 1

Amount of soluble salt that reacted with the clay upon heating to a temperature in °C, %: 100, 200, 300, 400, 500, 600.

For the investigation, samples of clays with a certain amount of dissolved salt were prepared by plastic molding in the form of disks 40 mm in diameter and 0.4–0.5 mm thick. After drying in air at room temperature, the disks were heated, then treated with water, and the amount of unreacted salt was determined from the chlorine by titration of the extract. In all cases, a sample of the same composition, but not subjected to heating, was taken as the standard. The investigation was carried out with red brick clays, the composition of which is given in Table 2.

Table 2

Chemical composition

Our investigations on the use of chloride salts under factory conditions showed that when calcium chloride or sodium chloride is introduced into clay in an amount of 3%, the strength of the fired bricks decreases almost threefold. The decrease in the mechanical strength of fired bricks is directly dependent on the amount of chloride salt introduced.

It is known from periodical press materials that a number of plants in the Union are still practicing the introduction of chloride salts or even hydrochloric acid [4] into the clay in brick production, and until now there have been no reports concerning their negative effect, apart from our work. We assumed that the addition of a chloride salt to clay does not in all cases have a negative effect and, in connection with this, carried out a more detailed investigation with various clays in order to determine their relationship to salt additions.

Since the mechanism of corrosion of bricks with additions of chloride salts is reduced to the interaction of hydrogen chloride with the clay, the different relationship of a chloride salt to clay can be established by treating the clay directly with hydrogen chloride. Both gaseous hydrogen chloride and its aqueous solutions act on clay. But the action of gaseous hydrogen chloride is manifested less sharply than the action of hydrochloric acid solutions. For this reason, in laboratory investigations we adopted treatment of the clay with hydrochloric acid solutions; in this case the different relationship of the clay to the action of hydrogen chloride is revealed more sharply, and carrying out the experiment is considerably simpler and more precise.

By treating clays with aqueous solutions of hydrogen chloride of different concentrations, we succeeded in reproducing the structure and color of destroyed brick manufactured at the plant and fired in a ring kiln. This does not mean that during the firing of brick in ring kilns its destruction occurs only as a result of the action of aqueous solutions of hydrogen chloride; undoubtedly, together with them gaseous hydrogen chloride also acts on the clay. This depends chiefly on the firing regime. But under the action of gaseous hydrogen chloride and its aqueous solutions, or only gaseous hydrogen chloride, in both cases a considerable decrease in mechanical strength is observed.

structure of the brick, and the difference consists only in the fact that, when solutions of hydrogen chloride are formed on the surface of the brick, its surface, edges, and corners are corroded much more strongly, and the color changes (it becomes zonal), which is not observed under the action of hydrogen chloride gas alone.

The investigations we have carried out show that hydrogen chloride, acting on clay, changes its technical properties to a considerable degree: plasticity is worsened, shrinkage of products during drying is reduced, porosity is increased, and the mechanical strength of finished products fired at temperatures below the sintering temperature of the clays is decreased. Such a profound change in the properties of clays, with complete loss of plasticity under the action of hydrogen chloride, occurs as a result of decomposition of their finely dispersed part, composed of the most active hydrophilic minerals, capable, on their surface, in interaction with water, of forming colloidal solutions (gels and sols). The investigations of N. Ya. Denisov and P. A. Rebinder [5] established that “a substantial result of the interaction of water with hydrophilic mineral particles is the formation on their surface not of films of pure water in the ordinary state, but of films of a colloidal solution.” V. S. Sharov [6; 7; 8] also points this out in his works.

Colloidal films on the surface of particles of hydrophilic minerals provide the adhesive (binding) action of clay particles and the presence of plasticity. With the destruction of these minerals, the principal technical property of clays—plasticity—also disappears. This reasoning led us to the conclusion that the different action of hydrogen chloride on clays may be explained by the different mineralogical composition of their finely dispersed fractions. This conclusion was adopted as the main hypothesis in the practical investigations.

V. N. Razumova [9] indicates that ordinary clays contain a comparatively limited number of clay minerals. X-ray and other methods of investigation have established that the finely dispersed part of red clays is composed predominantly of minerals of three groups: montmorillonite, hydromicas, and kaolinite. The different mineralogical nature of the fine fractions of clays is confined not only to different deposits: we know of a case of occurrence of clays with different mineralogical composition of the fine fractions in individual beds of one and the same deposit. Considerably more often, the method of staining and thermal analysis establish the combined presence of minerals of all three groups in one and the same clay; then their chemical properties have a more complex character, and the action of hydrogen chloride on such clays is manifested less sharply than in cases of a monomineral composition of the fine fractions. In general, the properties of red brick clays are determined to a considerable degree by the mineralogical nature of their finely dispersed part.

The negative action of chloride salts on the formation of a strong brick structure was observed by us at the Tomsk brick plants, working with clays of the following composition.

The mineralogical composition of the coarse-dispersed fractions of both clays has no noticeable difference. The principal mass of the heavy fraction consists of feldspars—plagioclase and microcline—in the form of rounded, ovoid, and prismatic grains, from slightly brownish to brown in color. Distinct polysynthetic twins of plagioclase and microcline occur. The amount of feldspars in the heavy fraction is about 50%. The second most abundant mineral is quartz, in the form of rather rare, more often irregularly shaped grains, in an amount of 10–12% of the heavy fraction. Hornblende is frequently encountered—green and brown. Green hornblende occurs in the form of large and small prisms, and also as irregular grains. Observations—

pleochroism is observed and, sometimes, in prisms of hornblende—a scattering of magnetite. Brown hornblende occurs as single grains.

Epidote is fairly widespread in the heavy fraction; it is observed in the form of grayish or yellowish irregular grains. Apatite, pyroxene, chlorite, biotite, and others occur as individual grains. Fractions finer than one micron, separated from the clays by the sedimentation method, when the mineralogical composition is determined by the staining method according to N. E. Vedeneeva, show the following mineralogical composition.

Table 3

The heating curves and weight losses of the clays are given in Fig. 1. Characteristic for the fine fractions of both clays is the presence in them of a high content of zeolitic water, which is split off, although not at the same rate, but without sharp jumps, throughout the entire course of heating.

Fig. 1. Heating curves and weight losses of clay from the “Keramik” plant and Plant No. 10.

The chemical composition of the fine fractions finer than 0.001 mm is given in Table 4.

Table 4

Taking the iron oxides existing in the clay to be present in the form of hydrates, the composition of the fine fractions is expressed in the form of the following chemical formulas:

clay from the “Keramik” plant: \(0.11R_2O \cdot 0.3(Ca,Mg)O \cdot Al_2O_3 \cdot 3.5SiO_2 \cdot nH_2O\)

clay from Plant No. 10: \(0.1R_2O \cdot 0.3(Ca,Mg)O \cdot Al_2O_3 \cdot 3.5SiO_2 \cdot nH_2O\).

In the laboratory investigations we selected clays whose principal substance (the clayey part) consisted mainly either of one clay mineral or of one with a small admixture of others.

a) Kaolin clays.

Table 5

Chemical compositions of clays and their fine fractions (less than 0.001 mm).

When determining the mineralogical composition by the staining method, one kaolin is found in the Ivanovo and Vladimir clays; in the Belogorsk clay, kaolin with an admixture of hydromica.

The heating curves and the weight-loss curves during heating of these clays are given in Fig. 2.

Fig. 2. Heating and weight-loss curves of kaolin clays.

All three clays have very low swelling in water and an exchange cation capacity characteristic of kaolins, which does not exceed 20 milligram-equivalents per 100 g of dry clay.

b) For the study of clay with a montmorillonite base, bentonite of the following chemical composition was taken: loss on ignition—13.12%; SiO₂—59.04%; Al₂O₃—14.87%; Fe₂O₃—3.03%; CaO—6.70%; MgO—2.57%; total 99.33%. Of the calcium oxide, 5.2% in the clay is present in the form of carbonate, which is easily detected by the action of a hydrochloric acid solution on the clay and is extracted by a normal ammonium chloride solution upon boiling. By the staining method, only montmorillonite is established in the bentonite. In water it swells strongly and has a high exchange cation capacity (130 milligram-equivalents per 100 g of dry clay), characteristic of montmorillonite.

The differential heating curve and the weight-loss curve upon heating are shown in Fig. 3.

Fig. 3. Heating and weight-loss curves of montmorillonite and hydromica clays No. 118 and Kamenskaya.

c) As hydromica clays, red brick clays were taken, the fine fractions of which, when determined by the staining method, have a hydromica composition and other clay minerals; predominantly, montmorillonite is detected as an admixture, while kaolin is not detected at all.

The chemical compositions of these clays differ little from the compositions of ordinary brick clays of Tomsk Oblast, whose compositions vary within very broad limits. But their fine fractions (smaller than 0.001 mm), in terms of the content of the principal oxides included in clay minerals, have more constant values.

The chemical compositions of hydromica clays and their fine fractions are given in Table 6.

Table 6

If it is assumed that chemically bound water is released from clays at temperatures above 400°C, while up to 400° the water bound as zeolitic water is released, then the chemical compositions of the clays and fine fractions are expressed by the following equations:

\[ \text{Ivanovo clay} — 0{,}04(\mathrm{Ca},\mathrm{Mg})\mathrm{O}\cdot \mathrm{Al}_2\mathrm{O}_3\cdot 2{,}2\mathrm{SiO}_2\cdot 1{,}45\mathrm{H}_2\mathrm{O} + 0{,}64\mathrm{H}_2\mathrm{O}. \]

\[ \text{Vladimir clay} — 0{,}03(\mathrm{Ca},\mathrm{Mg})\mathrm{O}\cdot \mathrm{Al}_2\mathrm{O}_3\cdot 2{,}4\mathrm{SiO}_2\cdot 1{,}56\mathrm{H}_2\mathrm{O} + 0{,}39\mathrm{H}_2\mathrm{O}. \]

\[ \text{Fraction smaller than 0.001 mm of Belogorsk clay} — \]

\[ 0{,}09(\mathrm{Mg},\mathrm{Ca})\mathrm{O}\cdot \mathrm{Al}_2\mathrm{O}_3\cdot 2{,}1\mathrm{SiO}_2\cdot 1{,}42\mathrm{H}_2\mathrm{O} + 0{,}95\mathrm{H}_2\mathrm{O}. \]

\[ \text{Bentonite clay} — \]

\[ 0{,}61(\mathrm{Mg},\mathrm{Ca})\mathrm{O}\cdot \mathrm{Al}_2\mathrm{O}_3\cdot 6{,}6\mathrm{SiO}_2\cdot 2{,}08\mathrm{H}_2\mathrm{O} + 2{,}82\mathrm{H}_2\mathrm{O}. \]

\[ \text{Fine fraction of Kamenskaya clay} — \]

\[ 0{,}1\mathrm{R}_2\mathrm{O}\cdot 0{,}4(\mathrm{Mg},\mathrm{Ca})\mathrm{O}\cdot \mathrm{Al}_2\mathrm{O}_3\cdot 3{,}4\mathrm{SiO}_2\cdot 0{,}98\mathrm{H}_2\mathrm{O} + 3{,}1\mathrm{H}_2\mathrm{O}. \]

\[ \text{Fine fraction of Anzherskaya clay} — \]

\[ 0{,}11\mathrm{R}_2\mathrm{O}\cdot 0{,}49(\mathrm{Mg},\mathrm{Ca})\mathrm{O}\cdot \mathrm{Al}_2\mathrm{O}_3\cdot 3{,}4\mathrm{SiO}_2\cdot 1{,}7\mathrm{H}_2\mathrm{O} + 3{,}4\mathrm{H}_2\mathrm{O}. \]

In the laboratory study of the action of hydrogen chloride on clays, we used the following method: 100 g of dry clay is mixed with an aqueous solution of hydrogen chloride of known concentration. Such an amount of liquid is added that the moisture content of the body is approximately equal to the molding moisture content. The mass is thoroughly kneaded and dried to dryness in a drying oven at a temperature not higher than 110°C. From the dried clay sample, test specimens are formed by the plastic method using water; after drying at room temperature they are fired at 900°C and tested. The test specimens were prepared in the form of cubes with sides of 25 mm.

After such treatment, the following changes in the physicochemical properties were observed in clays of different mineralogical composition (Table 7).

The kaolin clays, Ivanovo and Vladimir, do not change their properties when treated with solutions of hydrogen chloride. A noticeable change in physicochemical properties is observed in the kaolin clay of the Belogorsk deposit, owing to the presence of hydromicas in its composition.

Very large changes, with complete loss of the properties characteristic of clays, occur in montmorillonite and hydromica clays when they are treated with solutions of hydrogen chloride. Plasticity is completely lost; as a result, water-tightness and drying shrinkage decrease. The porosity of the structure increases, and its strength decreases very sharply. In all cases, the degree of change in clay properties upon treat-

Table 7

1. Ivanovo kaolin clay

Table 8

Vladimir kaolin clay

Table 9

Belogorsk kaolin clay with an admixture of hydromica

Table 10

Montmorillonite clay—bentonite

Table 11

Red brick clays whose fine fractions are composed of hydromicas—Kamenskaya

Table 12

Anzherskaya

Note. The evaluation of plasticity in the tables has the following meaning: v.g.—very good, good—good, l.p.—low-plastic, and n.p.—nonplastic.

treatment is in direct dependence on the specific consumption of hydrogen chloride per unit mass of clay.

The different relation of hydrogen chloride to clays of different mineralogical composition of their finely dispersed fractions is explained by the different properties of clay minerals, which are a consequence of the unequal conditions of their formation.

It is known that, in the artificial production of clay minerals under hydrothermal conditions, different minerals are obtained depending on the chemical medium [10].

Under acidic conditions, with a ratio of alkalis to alumina and silica—$M''O : \mathrm{Al_2O_3} : \mathrm{SiO_2}$—less than $0.02 : 1 : 4$, only kaolinite is always formed. In the presence of small amounts of the alkalis $\mathrm{Ca(OH)_2}$, $\mathrm{Mg(OH)_2}$, KOH, and NaOH, kaolinite and montmorillonite are obtained simultaneously.

Under alkaline conditions, with the ratio $M''O : \mathrm{Al_2O_3} : \mathrm{SiO_2}$ greater than $0.2 : 1 : 4$, only montmorillonite is formed. Likewise in nature, kaolinite is formed as a result of acidic weathering. Montmorillonite and hydromicas are formed in an alkaline medium, at the first stages of chemical weathering; in the construction of crystal lattices, magnesium oxide is bound by montmorillonite, and potassium oxide by the micas. In the presence of water, micas hydrolyze and pass into hydromicas [11].

With regard to the chemical stability of montmorillonite, V. A. Priklonskii reports that “when alkaline conditions are replaced by acidic ones, montmorillonite is again destroyed.” S. D. Sedletskii also writes about this when discussing the phenomena of endocryptia observed in nature. Thus, montmorillonite and hydromicas, which form in nature in a basic medium, are compounds not resistant to the action of acids; and, in the practical treatment of clays whose fine fractions are composed of hydromicas and montmorillonite with dilute hydrochloric acid in the cold (at the temperature of melting ice), these aluminosilicates decompose and pass into solution, which is not observed in kaolin clays. Kaolin is more resistant to the action of acids. E. S. Dana reports that kaolin is insoluble in acids [12]. According to A. G. Betekhtin, “hydrochloric and nitric acids have almost no effect on kaolin” [13]. Kaolinite is the most studied clay mineral, and its high acid resistance is well known. For this reason, too, a different relation to the action of hydrogen chloride is found among clays with different mineralogical composition of their fine fractions.

Conclusions

1. Clays whose fine fractions are composed of hydromicas and montmorillonite possess higher pyrochemical and chemical activity; having a basic character, these clay minerals are readily decomposed by acids. The introduction into them of soluble chloride salts in brick production is accompanied by a sharp change in their physicochemical properties, with a large decrease in the mechanical strength of the brick.

2. Kaolin clays do not change their properties under the action of hydrogen chloride. The addition of chloride salts to them in brick production does not exert a harmful chemical effect.

References

1. Usov P. G. The effect of the addition of common salt on the quality of building brick. Izv. TPI, vol. 71, 1952.

2. Usov P. G. and Petrov A. V. The effect of the addition of calcium chloride on the quality of building brick. Izv. TPI, vol. 71, 1952.

3. Lileev I. S. Chemistry and Technology of Aluminum Oxide, part 1, Lensovet Chemical-Technological Institute, Leningrad, 1940.

4. Georgadze N. A., Izverova A. E., et al. Production of frost-resistant raw brick with the introduction of hydrochloric-acid solution into the batch. Rationalization and Inventive Proposals in Construction. Brick Production. State Publishing House of Literature on Construction and Architecture, Moscow, 1952.

5. Denisov N. Ya. and Rebinder P. A. DAN SSSR, vol. 54, no. 6, 1946.

6. Sharov V. S. Hydrogeology and Engineering Geology, vol. 5, 1940.

7. Sharov V. S. On the hydration of clay in water vapor and in liquid water. Kolloidnyi Zhurnal, vol. VI, issue 1, 1940.

8. Sharov V. S. Contradictions of modern colloid chemistry of soils and their source. Soil Science, no. 6, 1939.

9. Razumova V. N. Materials on the Petrography of Clays, part 1, GONTI, NKTP USSR, 1939.

10. Sedletskii S. D. Colloid-Disperse Mineralogy, Publishing House of the Academy of Sciences of the USSR, 1945.

11. Priklonskii V. A. General Soil Science, part 1, Gosgeolizdat, 1943.

12. Dana E. S. Descriptive Mineralogy (reference book), ONTI, NKTP USSR, 1937.

13. Betekhtin A. G. Mineralogy, Gosgeolizdat, 1950.