DOI 10.1007/s10717-020-00287-4

Glass and Ceramics, Vol. 77, Nos. 7 – 8, November, 2020 (Russian Original, Nos. 7 – 8, July – August, 2020)

UDC 666.3.019:691.235

ANALYSIS OF THE CAUSES OF BRICKWORK EFFLORESCENCE

IN THE ARAL SEA REGION

V. S. Lesovik,^{[1], 3} L. Kh. Zagorodnyuk,^{1, 4} Z. K. Babaev,^{[2][3][4], [5]} and Z. B. Dzhumaniyazov^{2, [6]}

Translated from Steklo i Keramika, No. 7, pp. 39 – 41, July, 2020.

An analysis of the main causes of efflorescence in brickwork in the Aral Sea region is presented. It was found that as a result of alternate wetting and drying of building material, during which crystallization of salts occurs in the pores of the material, the formation of polywater crystalline hydrates with volume growth of volumes surpassing the volume of the pores in the material results in the appearance of pressure that destroys the building material. The external signs of salt corrosion are peeling and chipping of ceramic bricks. The particulars of the formation of efflorescence in ceramic bricks under the conditions extent in the Aral Sea region are indicated.

Key words: ceramic brick, clay raw material, salt corrosion, efflorescence, water-soluble salts, thermal dissociation, mixing water, crystalline hydrates.

The lasting quality of structures and buildings and reduction of their renovation costs are everpresent. This is due to the scales of industrial, residential, and individual construction. The strength, lasting quality, colorfastness, high hygienic and aesthetic qualities of brick and accessibility of clay raw material have made brick one of the most widely used manufactured products [1, 2]. Facing, porous, and clinker ceramic products are most expedient to produce under present construction conditions. However, a shortage of high-quality brick is observed in many regions of the world. The question of improving product quality remains one of the most important matters for currently operating and in-construction brick plants.

The objects of study are ancient architectural creations in the city of Khiva as well as individual construction projects in the Aral Sea region. These objects show continuous and spotted efflorescence, peeling in the form of small lamella, crumbling along the faces of the brick, permeation of water as a result of lifting of groundwater, and so forth. All these negative phenomena result in rapid destruction of ancient objects.

In this connection it becomes necessary to study the reasons why the indicated defects form and to develop a restoration technology and methods for obtaining stable ceramic material under the conditions of extreme, environmentally disastrous regions.

The defects of stone structures have attracted the attention of human beings since ancient times [3]. One of the existing defects of brickwork are so-called salt deposits or efflorescence [4]. It was determined that under the action of salt corrosion brick structures begin to break down in 15 to 20 years, whereas their service life is expected to be significantly longer. There are many works on this problem, and the issue of efflorescence in brickwork and corrosion of ceramic materials are reflected in the works of prominent scientists: N. S. Filosofov, G. K. Dement’ev, I. A. Kovel’man, A. I. Avgustinik, P. N. Grigor’ev, Ya. A. Sokolov, E. N. Rodin, G. Rais, L. Palmer, I. Melor, G. Zalmang, V. Brownel, and others [3].

The factors affecting the quality characteristics of brick work can be divided into technological and operational. The technological factors are determined by the initial raw mate-

rials and the production technology, whereas the operating factors manifest when brick interacts with the environment.

It is well known that water-soluble salts accumulate in native clays as a result of natural phenomena and human activity and lead to salinization of the clay [3]. Especially strong salinization occurs in the Aral Sea region. The salts in the clays of most deposits in the Aral Sea region accumulate

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in the form of solutions in the pores of the clay. In terms of the chemical composition salinized clays are classed as sulfates. As indicated in [5], sulfate solutions, especially sodium sulfate, are revealed by their high aggressivity, which is accompanied by up to 311% increase of their volume in the process of the transition of thenardite Na₂SO₄ into mirabilite

Na₂SO₄  10H₂O [6]. In terms of their solubility in water these salts belong to the high- and medium-solubility groups.

The sulfates Na₂SO₄ and MgSO₄ do not possess the property of hygroscopicity, but on crystallization they are capable of connecting a definite number of water molecules, forming crystalline hydrates. On crystallization 10 water molecules become attached to Na₂SO₄, which is accompanied by an increase in the volume of the salt. At 32.4°C

Na₂SO₄  10H₂O transforms into a water-free form with some volume reduction. Magnesium sulfate attaches seven molecules of water, forming the crystalline hydrate

MgSO₄  7H₂O.

The salt content in clay brick increases as it dries and is fired. This occurs when sulfur-containing fuels, specifically, coal, which contains 0.5 to 2% (by weight) sulfur, are used as fuel. In the existing clays of Uzbekistan the sulfur content can reach 10%. When this clay is burned a large part of the sulfur (up to 80%) in the form SO₂ is transferred into the atmosphere of the firing furnace. An increase in the amount of salt in a ceramic product occurs during firing when the calcium and magnesium oxides present in the clay interact with the sulfur dioxide present in the atmosphere. This process results in the formation of sulfites of the corresponding metals, which ultimately are converted into sulfates, whose thermal dissociation is very small even at the maximum firing temperature (1000°C).

Another source giving rise to efflorescence of a ceramic material is mixing water. So, under the conditions of the Aral Sea region most plants producing ceramic brick use the native water, the content of the chlorine ion can reach 1 – 2 gL, and the bicarbonate ion and, most dangerous for the ceramic material, the sulfate ion can reach 1.0 gL.

The reason why efflorescence appears, as discussed in detail in [1, 2], is atmospheric rain and subterranean moisture, interacting with the surface of the ceramic brick. As a result of the migration of dissolved salts in the capillary system of the brickwork, which occurs as a result of a change of temperature, crystalline salt formations, as a rule, appear on the facades of buildings and the interior part of the wall brick. The latter are indicators of possible processes which are destructive for the facing of buildings and the in-wall material. The destructive character of the dissolved salts is associated with the development of salt corrosion of the corresponding building material.

The products of decomposition and efflorescence formation in masonry of architectural monuments in the city of Khiva (Uzbekistan) were reported in [7].

Chemical analysis of the surface of ceramic brick with efflorescence reveal the following crystalline hydrates:

–          calcium hydrous calcium aluminum             sulfate

3CaO  Al₂O₃  3CaSO₄  (31–32)H₂O;

–          mirabilite Na₂^SO₄  10^H₂O; – epsomite MgSO₄  7H₂O;

–          chlorite NaCl  2H₂O.

In addition, other minerals were found in the interior part of the destroyed bricks:

–          aluminate Al₂(OH)₄SO₄  7H₂O;

–          thaumasite CaSiO3  CaCO3CaSO4  15H2O;

–          alunogen Al₂(SO₄)₃  18H₂O.

As a result of accumulation and alternate freezing and thawing these polywater crystalline hydrates are responsible for the development of salt corrosion as well as reduction of the frost resistance and mechanical strength, since they possess an enormous volumetric expansion on loss of water.

The crystallization of aluminate in the pores of the brickwork can occur as a result of the interaction of

3CaO  Al₂O₃  6H₂O with sulfur dioxide, oxidized to SO₃, which can be absorbed from air by the ceramic body of the brick according to the following scheme:

3CaO  Al₂O₃  6H₂O + 4SO₃ + 9H₂O =

Al₂(OH)₄SO₄  7H₂O + 3[CaSO₄  2H₂O].

The thaumasite and bricks can be formed as a result of a reaction between calcium hydrosilicate 2CaO  SiO₂  nH₂O, calcium sulfate, and carbon dioxide according to the chemical equation

2CaO  SiO₂  nH₂O + CaSO₄ + CO₂ + 15H₂O = CaSiO₃  CaCO₃  CaSO₄  (n + 15)H₂O.

Alunogen can also be formed in the pores of the brickwork as a result of a reaction between 3CaO  Al₂O₃  6H₂O and the sulfur trioxide gas, absorbed by the masonry from the air:

3CaO  Al₂O₃  6H₂O + 6SO₃ + 18H₂O = Al₂(SO₄)₃  18H₂O + 3[CaSO₄  2H₂O].

As a rule, the salt corrosion processes are a result of alternate wetting and drying of ceramic brick, where crystallization of the salts in the pores of the brick occurs [4]. The formation of polywater crystalline hydrates with an increase of volumes which exceed the volume of the pores in the material, results in the appearance of a pressure that destroys the ceramic body. The external indicators of soil corrosion are peeling and chipping of the brick.

This analysis of the reasons for the formation of efflorescence in the form of crystalline hydrate sulfate salts in the brickwork in the Aral Sea regions requires the development of serious technological measures to reduce and, if possible, eliminate them.

Analysis of the Causes of Brickwork Efflorescence in the Aral Sea Region                                                                                  279

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