High-strength wall ceramics based on phosphorus slag and bentonite clay

Original article

O ne of the promising building materials with a variety of colors, textures, and shapes that can develop the quality of building cladding and improve the architectural appearance of buildings is ceramic brick [1–3].

In Kazakhstan, the demand for high-quality ceramic products, such as clinker, the facade facing bricks is increasing. The limited raw material base of high-quality clay raw materials hampers the production of the abovementioned efficient materials. The main raw materials for the ceramic bricks manufacture are loams, which are low-plastic raw materials with a short sintering interval, which does not allow obtaining high-quality bricks. The maximum strength of products made of loam, fired at a temperature of 1100oС, does not exceed 15 MPa. In this regard, the transition to a semi-dry pressing method using industrial waste is relevant. To obtain high-strength ceramic materials, it is of interest to use “coarse component – finely ground binder” formula. With such a structure, the clay fraction consumption in the formula content will be in the range of 25–30%, and 75–100% in traditional ceramics of plastic molding [4–6].

Nowadays, the attention of ceramic building materials researchers is increasingly attracted by nanosized clay binder usage [7, 8].

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Fig. 1. Micrograph (a, ×100) and size distribution of bentonite clay particles (b): ■ – distribution by fractions; ^____ – cumulative distribution

METHODS AND MATERIALS

Bentonite clay of the Darbazinsky deposit of the Turkestan region was used as a plastic material for the studied ceramic masses, and dense crystallized phosphorus slag was used as a coarse material.

Bentonite is a nanostructured material with a high specific surface area, which has the property of self-dispersion, which helps to bind a large amount of non-plastic material. It is well known that the clay mineral montmorillonite consists of three store packages. Crushed particles of the phosphorus slag, located in the interpacket layer of montmorillonite form strong structures [4, 6, 8, 9].

Determination of the granulometric composition of bentonite (Fig. 1) showed that the highly dispersed powder is equally distributed over fractions from 1 to 32 micrometers, with a predominant distribution in the range from 1 to 12 micrometers. Particles form aggregates of irregular shape [10].

Table 1 shows the chemical composition of clay and phosphorus slag.

The slag is characterized by a high content of calcium oxide (47.5%), as well as the phosphorus pentoxide presence and fluorine compounds up to 4.1%, which have a fluxing effect and help to reduce the calcination temperature of ceramic materials. In addition to basic oxides of silicon, aluminium, and iron, clay contains low-melting oxides such as CaO, MgO, Na2O and K2O [11–13].

According to the number of plasticities (38–40), clay belongs to highly plastic, according to the content of clay particles (62.5%) to highly dispersed, according to the index of refractoriness to fusible clays (1250oC).

The phase composition of bentonite clay is characterized by the quartz ( d = 0.335; 0.425 nm), illite ( d = 0.319; 0.256 nm), biotite ( d = 1.000; 0.353 nm), montmorillonite ( d = 0.950; 0.225 nm), kaolinite ( d = 0.709 nm) presence. Carbonate inclusions are represented by calcite ( d = 0,303; 0.228 nm). Potassium feldspar and ferritic phases are also present. The content of montmorillonite is 80–85% [14–16].

Phosphorus slag from the Taraz phosphorus plant is a dense, crystallized mass that melts at a temperature of 1320–1350oC. The phase composition of phosphorus slag is represented mainly by wollastonite and cuspidin [17, 18].

Samples of wall ceramic bricks were made as follows. The slag was subjected to crushing and screening through a 1.25 mm sieve. To obtain a binder, slag and clay were subjected to grinding in a ball mill until they completely passed through a 0.063 mm sieve. Slag granules less than 1.25 mm in size were subjected to moisture until a moisture content of 8% was reached, then a binder was applied

Table 1

Chemical composition of raw materials

Raw material	Composition, mass. %
	SiO2	Al 2 O 3	Fe 2 O 3	CaO	MgO	Na2O	K2O	SO3	F + P 2 O 5	calcination losses
Bentonite Clay	60.51	16.06	6.43	1.27	2.23	2.41	2.8	1.3	–	8.7
Phosphorus slag	41.3	2.8	0.4	47.5	1.2	0.3	0.1	0.6	4.1	0.2

APPLICATION OF NANOMATERIALS AND NANOTECHNOLOGIES IN CONSTRUCTION to their surface. The prepared press powder was poured into a mold and subjected to pressing at a pressure of 15–25 MPa. The strength of the molded raw material is 0.35–0.4 MPa, which satisfies the requirements of the automatic stacker. The sintering process of the composition was studied on cubes samples of 50×50×50 mm in size. Samples were dried at a maximum temperature of 105oС for 3 hours; calcination was carried out in an electric muffle furnace at a temperature of 1050–1100oС with an isothermal exposure of 30 min [19–21].

In order to determine the optimal ratio of the core and the shell, the structures were modeled at a ratio of 5, 10, 20. The amount of the shell substance varied from 20 to 50%. The size of the core aggregates was changed from 0.5 to 1.25 mm, the thickness of the shell was changed from 0.05 to 0.3 mm. The strongest structures were obtained at a ratio of their sizes equal to 10–20.

In the technology of semi-dry pressing, it is important to establish the maximum grain sizes of the coarsely dispersed component. To do this, the temperature stresses at the grain boundaries were calculated according to the formula of W.D. Kingery [7]:

where: σ – stress in the contact layer, MPa; E1 and E2 – moduli of elasticity, MPa; µ1, µ2 – Poisson’s ratios; ∆α is the difference in the temperature coefficient of the linear expansion of the phases; ∆T is the temperature interval in which stresses arise; d – grain size; V – are the volume fractions of the contacting phases.

From the analysis of changes in the thermal stresses developing at the grain boundaries, the maximum sizes of slag grains (1.25 mm) were selected.

Studies of the microstructure of ceramic samples were carried out on a JEOL JSM7500 scanning electron microscope with an X-ray spectral analysis attachment [20].

RESULTS

The influence study of recipe factors on the basic physical and mechanical properties of ceramic wall materials was carried out by the simplex-lattice planning of experiments using the properties of an incomplete cubic form model as a mathematical model [21]:

Y = A ₁ + A ₂ + A ₃ + A ₄ xy + A ₅ xz + A ₆ yz +

+ A ₇ xyz + A ₈ xy ( x–y ) + A ₉ xz ( x–z ) + A ₁₀ yz ( y–z ). (2)

The ultimate compressive (R _com ) and bending strength (R _bend ), shrinkage (U, %), and water absorption (W, %) were studied as Y response functions.

The followings were used as raw materials for the ceramic bricks production:

• Х ₁ – the content of phosphorus slag fraction less than 1.25 mm, 55–70%;

Х ₂ – the content of finely ground bentonite clay, 25–40%;

Х ₃ – the content of finely ground phosphorus slag, 5–20%.

The experiment planning matrix is given in the Table 2.

Table 2

Experiment Design Matrix

Composition number	Content of components,%			Properties
	Phosphorus slag fraction less than 1.25 mm, %	Bentonite clay, %	Finely ground phosphorus slag, %	U, %	Rcom, МPа	Rbend, МPа	W, %
	Х 1	Х 2	Х 3
1	70	25	5	0.9	26.1	2.8	13.8
2	55	40	5	1.9	54.3	4.7	5.2
3	55	25	20	1.6	36.7	3.6	8.2
4	65	30	5	1.2	32.3	3.4	11.8
5	60	35	5	1.6	51.5	3.7	10.4
6	65	25	10	1.4	30.8	3.2	11.8
7	60	25	15	1.5	34.7	3.7	11.2
8	55	35	10	1.1	45.3	3.6	7.4
9	55	30	15	1.2	38.6	3.7	8.6
10	60	30	10	1.4	43.4	3.9	10.9

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Fig. 2. Equal values lines of high-strength ceramics properties changes:

a – compressive strength; b – bending strength; c – water absorption; d – shrinkage

The figurative points of the binder compositions based on slag and clay are in the CaO–Al ₂ O ₃ –SiO ₂ state diagram and are located in the anorthite-wollastonite-quartz triangle with eutectic at a temperature of 1165^oC.

The regression equation of the compressive strength dependence of ceramic bricks on the composition of the charge has the form:

R_c = 27,1 x ₁ + 54,3 x ₂ + 49,7 x ₃ – 17,1 x ₁ x ₂ +

+ 18,225 x ₁ x ₃ – 61,875 x ₂ x ₃ + 315 x ₁ x ₂ ( x ₁ – x ₂ ) +

+ 400,275 x ₁ x ₃ ( x ₁ – x ₃ ) + 282,375 x ₂ x ₃ ( x ₂ – x ₃ ) +

+ 228,15 x ₁ x ₂ x ₃ .                                     (3)

The clay content in the amount of 25% guarantees the packing density, the sintering effect becomes sufficient, and the samples strength is 27.1 MPa (Fig. 1, b). If the clay amount increases from 25% to 40%, the sintering effect continues to grow and the samples strength reaches 54.3 MPa.

According to the obtained “composition-property” diagrams (Fig. 2, a, b, c, d), ceramic bricks have low shrinkage (0.9–1.6%), water absorption (7.4–12.1%), high bending strength index (2.8–4.7 MPa).

DISCUSSION

The obtained results show that the most active sintering and dense structures formation in coarse-grained formulas with high-calcium slag occur at a binder content of 35–45% and core-to-shell ratios of 10–20 in accordance with the mixed ceramic structures modeling [17–19].

According to thermodynamic calculations, the most likely anortite (CaO Al ₂ O ₃ 2SiO ₂ ), quartz, and wollastonite formation is in the calcination process at temperatures of 400, 800, and 1000^oC; such phases with the largest negative values of the Gibss free energy (∆Z), with densities that do not change during the synthesis process and contribute to the increase in the brick strength.

The frame structure creation was ensured through the use of a polyfractional composition (phosphorus slag) and finely ground bentonite and phosphorus slag binder.

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Fig. 3. Frame structure model of ceramic materials according to the “core – shell” type:

1 – slag granule; 2 – binder; 3 – pores; 4 – binder and wollastonite and anorthite crystals

Figure 3 shows frame structure models of the material constituent elements (crystal frame and binder).

The samples structure from the granular slag with binders formula is the most pronounced variant of “core – shell” type structures (Fig. 4, a, b). And the nature of slag with binders structures most clearly reflects the process of their formation. It can be seen from the structure of the samples that the binder forms shells not only around large grains but also around medium and small slag grains located between them. It may be viewed that the shells interact with the surface of the grains to form the crystalline phases of wollastonite and anorthite. The shell and grain interaction proceeds very actively. As a result of this interaction, a fairly noticeable transition zone is formed from the slag grain surface to the shell. The transition zone formation determines the slag grain and the shell consistency. Therefore, the samples strength is high and amounts to 49–54.3 MPa in compression and 4.3–4.7 MPa in bending [15].

From the structure of the densely sintered samples, achieved by using the maximum (40%) amount of clay in the binder and after calcination at a high temperature

Fig. 4. Ceramics microstructure from the phosphorus slag and bentonite clay composition: a – calcination temperature 1050^oC; b – calcination temperature 1100^oC

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(1050–1100oC), it is seen that the smallest slag grains are involved in the liquid phase formation. The amount and reactivity of the glass phase increases due to the influence of fluorine-containing components of the slag. As a result, the thickness of the zone between the large slag grains surface and the shells grows and the crystallization of the new phases in the form of wollastonite and anorthite became active in the shell. Furthermore, glass phase viscosity decrease due to calcium ions, a growth in its penetrating ability and the glass phase affinity for slag grains contribute to its spreading over the samples’ surface and the vitreous coating formation, which crystallizes upon cooling [14].

Interaction products are observed along the slag grains edges and between them, as well as on their surface.

CONCLUSION

To get high-strength ceramic bricks, the content of coarsely dispersed component in the form of phosphorus slag with a fraction of less than 1.25 mm should amount to 55–65%, finely ground phosphorus slag should be 5–10%, and bentonite clay is to be 30–40%. The most favorable technological parameters are: calcination temperature 1050–1100oС, pressing pressure 20–25 MPa, press powder moisture content 7–8%.