Slags in factory practice are usually classified according to two criteria: 1. The content of silica, which can vary from 10 (and below) up to 60%, respectively, slags are considered to be basic or acidic, and a pH of simpler and clearer just to define the size of the percentage of SiO₂; 2. Mostly on content of the basic oxide, the slags are divided into ferrous, calcium and magnesium. The composition of the slag determines its physical and chemical properties; melting point, viscosity and character of its change with temperature, specific gravity, conductivity, surface tension etc. and chemical aggressiveness of the slag melt with respect to various refractory materials of the furnace. Under the influence of the slag melt is the destruction of the refractory material of the furnace. In Figure 1(a) and Figure 1(b) shows the appearance of the linings of cooking stoves and security of the crucible after melting slag I.

In order to obtain glass based on blast-furnace slag II and III, the latter were crushed and then a dewatered silicon oxide of an analytically pure grade was added so that the glass of lots a, b, and c for each slag contains 37, 46, and 55 mol % of SiO₂, respectively. The obtained blends were stirred in a spheric grinder for several hours. The glass was melted in a platinum crucible at a temperature

of 1450˚C for 3 h. The glass mass was worked by melt pouring onto a massive mold with quenching between two steel plates. The calculated compositions of glass are presented in Table 3 and are denoted by II and III, corresponding to the slag numbers.

The obtained castings were transparent, X-ray-amorphous, the color of glass changed from dark brown with a reddish shade with the minimum content of SiO₂ to light yellow with the maximum content of SiO₂. Figure 2 shows the appearance of the original slag and glass, the synthesized on its basis.

For glass II and III, Table 4 presents densities and vitrification temperatures T_g according to the DTA method data in comparison with temperatures T₁₃ corresponding to a viscosity of 10¹³ P, as well as activation energies of frictional flow in the temperature range 700˚C - 800˚C for the melts of the a series glass with a low content of SiO₂, which were obtained from the slope of the dependence ln(η) from 1/T.

As we can see, glass densities decrease with approximately similar growth in silica content in glass II and III. The vitrification temperatures T_g and T₁₃, and, hence, the glass viscosities increase by transferring to more silica-containing glass, but to a greater degree in glass II glass than in glass III. All-in-all, glass II is less dense and more viscous than glass III, and the activation energies of frictional flow determined for the glass with the lowest silica content coincide. For the glass obtained on the base of the blast-furnace slag, high values of the viscos-

ity criterion (temperature, K) are characteristic. This primarily

causes the surface crystal nucleation. The temperature range for activation energy is 700˚C - 800˚C. The parameters of the Vogel-Fulcher-Tamman equation for glass II a: A = −8.33 ± 0.96, B = 9352.71 ± 892.40, and T₀ = 552.25 ± 22.01. The temperature is in K and viscosity in dPa・s.

Glass II readily crystallized from the surface, therefore, to make the results comparable, we used powders of the same dispersity in studying the crystallization:

the fraction with a particle size of 0.050 < d < 0.065 mm. In particular, by studying crystallization using the XPA method, glass powders of such dispersity were subjected to thermal treatment and were grated to a finely disperse state required for correct measurements using the powder technique just before the X-ray-graphical survey.

Figure 3 shows the heating curves of powders based on the glass slag II and glass III a.

The temperatures of vitrification (T_g), of melting start (T_s), and its end (T_e) were determined by the crossing of the corresponding tangents. Two exothermic effects were observed on the DTA curves, which were caused by the crystallization of two different phases; they had the form of maxima and shoulders on their slopes whose temperatures corresponding to the maximal rates of crystallization are denoted as T_cr1 and T_cr2. The temperatures of all effects increase with an increase in the silica content, while the temperature range of melting broadens.

The XPA of powders of glass II (Figure 4) preliminarily subjected to isothermal treatment at a temperature of 850˚C, corresponding to the beginning of the first crystallization peak on the DTA curves, has shown that the crystallization starts from the simultaneous formation on the surface of the melilite glass (solid solution of gehlenite 2CaO・Al₂O₃・SiO₂ in akermanite 2CaO・MgO・2SiO₂ (card-file PCPDFWIN, card 79-2423)) and diopside CaO・MgO・2SiO₂ (PCPDFWIN, card 83-1820). As can be seen from Figure 4, at temperatures above 850˚C and

comparable with the temperature of T_cr1, melilite grows much quicker than diopside. The rate of diopside formation notably increases only when the temperature approaches T_cr2.

Figure 5 illustrates the difference in the rates of formation of melilite and diopside, and also the difference in the rates of crystallization of glass with different silica content.

Thus, we consider that the first crystallization peak T_cr1 on the DTA curves corresponds to a predominant formation of melilite, while the second one T_cr2 corresponds to the formation of diopside.

A comparison of the data on viscosities of slag II (E_η = 42 kcal/mol) and the data closest to it on glass composition II of slag a shows that the used additions of SiO₂ more than doubled the activation energy of the melt’s frictional flow. The viscosity of the slag was determined at the Nizhnetagil Combinate in the temperature interval 980.5 - 1062 K. The parameters of the Vogel-Fulcher-Tamman equation for the slag II are: A = −2.64 ± 0.77, B = 1773.62 ± 179.92, and T₀ = 1425.91 ± 4.79.

The temperatures of crystallization of glass T_cr1 and T_cr2 are also marked in Figure 6. We note that in the crystallization of the powders of the glass of III series a, b and c only one clear peak was registered on the DTA curves, which was

caused by the predominant crystallization of melilite and the crystallization of diopside probably, occurred slowly, which resulted in the absence of extrema on the DTA curves.

Figure 6 shows a comparison of the effects of the temperatures of vitrification (T_g), crystallization (T_cr1, and T_cr2), and melting (T_s and T_e) on the DTA curves of the powders of glass based on blast-furnace slag II depending on the content of silica in them.

As can be seen from Figure 6, the values of the temperatures mentioned increase with an increase in silica content.

For the nucleation of catalyzing phase crystals (perovskite), samples of the initial glass were exposed in the temperature range 740˚C - 850˚C (the region between T_g and the start of crystallization on the DTA curve). The optimal temperature value for growing (visualization) nucleated crystals up to sizes observable by an optical microscope is 930˚C (the temperature corresponding to the middle of the ascending branch of the exothermal peak on the DTA curve). When the glass is heated in the interval of 740˚C - 850˚C for 24 h, two phases appear successively. Perovskite CaO・TiO₂ is precipitated first (Figure 9(a)) and the second phase is melilite, which crystallizes in the form of rosettes on the perovskite crystals (Figure 9(b)).

Electron microscopy was carried out on an EM-125 transmission electron microscope using the method of carbon replicas at an accelerating voltage of 75 kV.

The nature of crystalline phases was determined by XPA. After lasting low- temperature exposures (740˚C, for 24 h and more) diffraction maxima appeared on the X-ray pictures, which can be attributed to CaO・TiO₂ (card 83-231 in PCPDFWIN). Exposures at the development temperatures (930˚C, 30 min) resulted in melilite crystallization (PCPDF-WIN, card 79-2423). The number of

rectangular crystals is very small. They cannot be detected by the X-ray graphical method as an individual phase. It is possible that these are those crystals of perovskite at which crystals of melilite are not grown.

Then, the samples were subjected to repeated thermal treatment at a development temperature of 930˚C for 5 min.

After the thermal treatments the surface layer was ground off from the samples and the obtained surface was polished. To increase the sharpness of the interfaces between the crystals and the glass, the surface of the sample was etched in a 0.01 M solution of fluoric acid for 10 s. The number of sections of crystals N_S and their size on the section surface unit of area S were determined with the help of a Neophot optical microscope in reflected light. The number of crystals in a unit of volume and the number of crystal sections in a unit of area are connected by the relation n = N_S/(SD_av), where D_av = (D₁ + D₂)/2. (D₁ and D₂ are the maximal and minimal sizes of crystal traces on the section). We note that in order to count the crystals, we used samples in which the crystals do not fuse with each other. Figure 9(b) corresponds to the complete over crystallization of a sample.

Figure 9(c) shows dependence n(t) for the nucleation of perovskite crystals at a temperature of 760˚C. Similar dependences were also obtained for other temperatures. It turned out that the systematic dependences of the change in the saturation level with temperature are not observed.

The dependences n(t) first went through a region where an increase in the number of crystals was nonlinear and then, when a certain moment of time was achieved, they became linear. The steady-state rate of crystal nucleation I_st = dn/dt was determined for this region (Table 5).

Figure 9(c) reflects a feature of the nucleation of perovskite crystals. It consists in the fact that after certain duration of thermal treatment the number of developed perovskite crystals does not change with time. Because the nucleation proceeds in the glass sharply differs in composition from that of perovskite, the rate of nucleation of crystals strongly depends on time. It is reduced in the process of perovskite crystallization due to the reduction of the amount of titanium oxide in glass. The dependence of crystallization parameters on time is a peculiarity of the case under consideration in comparison with the crystallization of glass of stoichiometric composition.

By using the curves of the type presented in Figure 9(c) for other temperatures, we determined values I(T) and built the temperature dependence of the nucleation rate of perovskite crystals: maximal value of the rate 2 × 10³ mm⁻³・min⁻¹ at T_m = 740˚C.

The crystal growth rate was determined for every development temperature from the relation U = dR/dt’, where t’ is the time of glass exposure at the development temperature. By determining the rates of nucleation and growth of

crystals, we took the average radius of the maximal crystal trace on the section as R. The dependences of maximal spherulites radii R_max on the development time had a linear character, and this circumstance allowed us to determine the steady-state rate of growth at a given temperature using its slope.

As can be seen from Figure 10, thermal treatments to 640˚C did not substantially affect the kinetics of the nucleation of the catalyzing phase―perovskite. Starting with 640˚C, the influence of thermal treatments became noticeable; it consisted in the appearance of a certain number of perovskite crystals, which were registered both by the X-ray-graphical method and by the optical microscope.

Melilite starts nucleating from 870˚C.

To obtain the data on the kinetics of melilite nucleation on perovskite crystals, the initial glass was first exposed at the maximum temperature of the perovskite crystal nucleation rate (740˚C), in order to release the whole perovskite in the form of crystals on which melilite nucleates. In this case, melilite still did not appear in appreciable amounts. Then, the samples of glass prepared in such a way were subjected to thermal treatments at higher temperatures (Figure 10). The dependences n(t) for melilite crystals were built in a similar way. The number of developed melilite crystals does not change with time after a certain time of treatment as in the case with curves, this can be associated both with the change in the glass composition in the process of melilite crystallization and with the gradual depletion of melilite nucleation centers, crystals of perovskite. The number of perovskite crystals not actuated as catalysts can be written in the form

where α is the probability of nucleation of a spherulites of melilite on perovskite crystals in a unit of time. The value α can be referred to as the catalytic activity of perovskite crystals with regard to melilite. It depends on the relationship between the parameters of the catalyst crystalline cell and the nucleating crystal of the basic phase of melilite there. Then, nucleation rate of spherulites of melilite is equal to. This will lead to a reduction of I(t) with time as in (1). The maximum I for melilite (catalyzed phase) is located at temperature T_m = 915˚C, which is 170˚C higher than for perovskite. The rate of melilite catalyzed nucleation is approximately proportional to the general number of perovskite particles and the average area of a single particle surface and amounts at the maximum to the value 0.33 mm^?3・min^?1.

Glass based on the blast-furnace slag of Chelyabinsk Metallurgical Combinate is obtained. It is established that for glass obtained based on the blast-furnace slag;

the high values of the viscosity criterion are typical. This mainly

leads to the surface nucleation of the crystals. Only one of the nine selected compositions with the ratio of the main components TiO₂:CaO:MgO:Al₂O₃:SiO₂ = 1:7.2:1.8:2.6:7.4 was prone to volume nucleation. At 600˚C, in the bulk of the glass, the perovskite spinel phase appeared, which then served as a catalyst for the bulk nucleation of the crystals of melilite in the temperature range of 840˚C - 1100˚C. Melilite crystals were nucleated both on the glass surface and then grew into the sample and in the volume.

The crystals of diopside (CaO・MgO・2SiO₂) were nucleated only on the surface. It is established that for a glass exhibiting the volume nucleation of crystals the mechanism of crystallization is associated with three main processes. The first is the process of chemical differentiation of the glass, leading to the formation of an inhomogeneous glass structure. The second process is the homogeneous formation of centers of crystallization. It consists in the formation of the spinel phase of perovskite, which is a catalyst for the formation of the basic crystalline phase, melilite. The third process consists in the formation of the basic catalyzed crystalline phase of melilite.

A separate investigation into temperature dependences of the rates of nucleation and growth of the catalyzing phase (perovskite) and the basic phase (melilite) in pyroxenic glass is carried out. The volume parameters of the nucleation of the catalyzing and catalyzed-phase crystals are determined: the temperature dependences of the steady-state rate of nucleation and growth of crystals. The maximal values of the crystal nucleation rates for the catalyzing and catalyzed phases are found.

Using the example of the crystallization of perovskite and melilite in glass III a, for the first time, we built the complete Tamman curves for the glass of complicated industrial compositions and the ways are shown that allow us to shorten the time for searching for the chemical composition and temperature-time conditions of crystallization for the development of an expedient technology for producing glass-crystalline materials.

In order to obtain glass based on furnace slags with Cr₂O₃ content, they were pulverized, then the anhydrous silica a drum. The glass was cooked in a platinum crucible at 1450˚C for 3 h. The glass mass was produced in an array metal mold with further pressing by a metal anvil. These castings were transparent and X-ray amorphous. Figure 11 presents the appearance of glass based on furnace slags (slag in the center).

The experiment showed that the glass color has changed from dark brown with a reddish tinge (for the minimum content of SiO₂) to light yellow (with the

maximum content of the SiO₂). The glass with excess Cr₂O₃ content has a characteristic malachite color. Surface crystallization was observed in all types of glass. It started in the temperature range close to the glass transition temperature T_g for the studied glass (730˚C - 770˚C).

With increasing temperature under the action of surface tension forces, the glass melted slightly. In order to stimulate volume nucleation of the crystals, chromium oxide was used as the catalyst in amounts varying from 1 to 10 wt % (over 100). The experiments have been carried out on glass Ia. Figure 12 shows the appearance of chromium-containing slag glass. The best result was obtained for glass with 2 wt % of Cr₂O₃ (over 100%) (Figure 12(a)). The glass was green and transparent. The increase in the content of Cr₂O₃ led to the appearance of the characteristic pattern of the malachite (Figure 12(b)).

This glass is uniform in composition and, along with the surface nucleation of the crystals, appears volume. A smaller amount of Cr₂O₃ had no significant influence on the character of the nucleation and increased content led to the excess Cr₂O₃ not being digested by the glass. The excess Cr₂O₃ led to the nucleation becoming uniform in composition with a characteristic malachite pattern (Figure 12(b)). According to XRPA, the magnesiochromite phase first precipitated in the glass (MgO・Cr₂O₃, PCPDFWIN no. 10-0351), and then the diopside (CaO・MgO・2SiO₂, PCPDFWIN no. 75-1577). As in the case of the study of the influence of perovskite on the nucleation of melilite crystals in the slag glass, the kinetic and temperature dependences were obtained first of the nucleation of the catalyzing phase―of magnesiochromite and then of the catalyzed phase―of diopside.

The diopside crystals in this glass were nucleated both on the surface followed by germination into the sample and in the volume. In the latter case, the nucleation of diopside occurred on the magnesiochromite crystals formed in the glass bulk. At 720˚C, the spinel phase of magnesiochromite MgO・Cr₂O₃ appeared in the glass bulk, which then served as a catalyst for the volume nucleation of the diopside crystals in the temperature range of 800˚C - 870˚C.

For the nucleation of the crystals of the catalyzing phase―magnesiochromite ―samples of the original glass were kept in the temperature range of 720˚C - 850˚C (the area between T_g and the beginning of the crystallization on the DTA curve). The development temperature of the nucleating crystals to a size visible in an optical microscope is 880˚C, according to the DTA curve.

Upon heating the glass in this range of 720˚C - 850˚C for 24 h, two phases appear sequentially (Figure 13).

Magnesiochromite is precipitated first (740˚C, 4 h) and then at (750˚C, 4 h + 880˚C 20 min) diopside, which crystallizes in the form of sockets on the magnesiochromite crystals, is precipitated.

Glass based on the furnace slag of the Chelyabinsk metallurgical complex is obtained. It is established that the introduction of Cr₂O₃ allows achieving volumetric crystallization of slag glass and the optimum quantity of the Cr₂O₃ catalyst is 2 wt % above 100. It is established that the volume nucleation of the crystals is due to the occurrence of two basic processes. The first is the process of the formation of the crystallization centers. It consists in the formation of the metastable spinel phase of magnesiochromite, which is the catalyst for the formation of the main catalyzing crystalline phase of diopside.

A separate study of the temperature dependences of the rates of nucleation and growth of the catalyzing phase of magnesiochromite and of the main phase of diopside is performed. The parameters of the volumetric nucleation of the crystals of the catalyzing and catalyzed phases are determined: the temperature dependences of the nucleation rate of the crystals and the values of the growth rate of the crystals for the temperatures of the maximum of the growth rate. It is found that for the studied glass, the times of nonstationary nucleation are very short―about 6 min. The maximum values of the nucleation rates of the crystals of the catalyzing and catalyzed phase are determined.

On the example of the crystallization of magnesiochromite and diopside, the full Tamman curves for the glass of a complex industrial composition are constructed, and the ways that allow one to reduce the search time of the chemical composition and time temperature conditions of the crystallization upon the development of rational technology for obtaining glass-ceramic materials are shown.

It is established that an increase in the number of magnesiochromite by more than 2.5 wt % above 100% to 10 wt % above 100%, allows one to get glass with a pattern of malachite with varying degrees of coloration of the glass monolith and glass crystalline material, with diverse malachite patterns by crystallizing the glass in the temperature range of the maximum of the nucleation rate of the magnesiochromite crystals.

Thus, it is shown that the slags of the Urals Nizhny Tagil Metallurgical Plant can be used as the raw material for obtaining glass and glass-crystalline materials of a wide range of green shades with a malachite pattern. This can help reduce the slag heaps and weakening of the technogenic exposure factors on the environment.

For all of the glass studied, the dependences of the growth rate of the crystalline crust on the glass surface on temperature were determined (Figure 18).

For comparison with the growth rates of the crystalline crust, Figure 18 (3) shows the dependence of the crystal growth rate of perovskite CaO・TiO₂ in glass III a (the only one of the nine compositions in which volumetric nucleation of melilite crystals on perovskite was found). In this glass, two kinds of nucleation were observed: surface nucleation of melilite and diopside (curves a-c) and volumetric nucleation of the perovskite crystals (curve d). The perovskite crystal growth rate is obtained with the processing of the electron microscopic image shown in Figure 9. Figure 18 shows that in the slag glass, the value of the growth rate from the surface into the sample is in orders of magnitude higher than the growth rate in the volume.

Using the data of Figure 18, the dependences of the growth rate logarithm of the crystalline crust on the inverse temperature 1/T (K) for glass I a, I b, I c; II a, II b, II c; III a, III b, and III c were calculated. The activation energy values of the growth rate E_U of the crystals formed on the surface of the samples and growing into the samples are presented in Table 7.

As can be seen from Table 7, with an increase of the amount of SiO₂ in the glass the values increase. This demonstrates the deterioration of the crystalline ability of glass with the increasing content of glass forming oxides. The closeness in the values of the activation energy of the crystal growth reflects their closeness in chemical composition. The difference in the content of the oxides of calcium and titanium affects the values.

The values of density, strength characteristics, and elastic properties of the

synthesized glass are shown in Table 8 and Table 9 in comparison to the window glass.

The values in Table 8 were determined in accordance with GOST 9900-2013. The error is not more than 1%.

To determine Е, G, and σ, the following formulas were used:

Here V_l is the sound velocity of the longitudinal wave, V_t is the sound velocity of the shear wave, ρ is density, E is Young’s modulus, g is the shear modulus, and σ is Poisson’s ratio.

The data in Table 8 indicate the advantages of glass slags over the window glass and the possibility of their wide use in industry, such as glass containers of different colors ranging from light yellow to dark brown with a reddish tinge. The main advantage of slag glass over window glass is that using slag wastes as the feedstock contributes to the reduction of slag heaps and eases the technogenic impact factors on the environment.

Glass, having only surface crystallization, may be converted into a monolith with quasi-volume crystallization by grinding the original glass into powder and heating it in the range of softening temperatures, where the maximum nucleation rate of crystals lies for many types of glass. Upon exposure of glass close to

the glass transition temperature (T_g) on the surface of glassy particles, the nucleation of crystals starts to occur. Simultaneously, the particles fuse into each other due to the action of the surface tension forces and the reduction of the viscosity of glass. As a result of these processes, glass crystalline volume-crystallized material is obtained in the form of a monolith. If the crystals nucleating on the glass surface quickly grow into the depth of the glass, then the degree of crystallinity of the glass depends on the degree of dispersion of the particles of the initial crushed glass. Monoliths of the source glass were crushed, fractionated, and then the fractions were subjected to two-stage heat treatment. By changing the treatment temperature in the first stage glass crystalline material of two forms was obtained: porous material with a pore size of 0.05 mm and sintered material in the form of a nonporous monolith. In the first case, the material properties are determined by the diopside crystals bonded with a glass-forming layer; in the second case, by melilite, diopside, and a small amount of glass [5] . The study of the nucleation of crystals in glassy powders derived from slags from Nizhny Tagil allowed establishing time-temperature regimes for obtaining materials by sintering powders of porous and monolithic body-crystallized glass.

Figure 19 shows the relative content of melilite and diopside in different modes of heat treatment.

Figure 20 shows the appearance of the samples prepared by sintering the powders of glass I a and I b, as well as a picture of the porous material based on the crystallized glass I b.

According to the Ostwald rule, the phase with the highest nucleation rate is the first to fall. On the way to the formation of the critical nucleus of this phase, the lowest energy barriers are overcome. The nuclei of this phase are most easily obtained from the glass by the diffusion displacement of the atoms and further structural streamlining at relatively low temperatures. Slag glass is extremely complex in composition and solid solutions are easily formed in it, particularly, of the melilitic type. According to the data in Figure 19, one can judge the relative content of the crystalline phases. Such data can be very useful when selecting the modes of the heat treatment of glass in the preparation of glass ceramics using powder technology.

The kinetic curves in Figure 19(a) characterize the sequence of the formation of diopside and melilite at three temperatures: T₁ = 785, T₂ = 820 and T₃ = 845˚C. At T₁ diopside appears first, while at T₂ diopside with melilite fall simultaneously. With an increase of the isothermal exposure of the glass, melilite crystals are formed more than diopside crystals. At T₃ melilite is already considerably larger than diopside. Its amount sharply increases in the first quarter of an hour, while diopside becomes noticeable only after an hour’s exposure.

As was shown in [26] , near T = 960˚C, in the studied glass based on slags, intensive crystallization of diopside takes place with high rates of nucleation and crystal growth; thus, the exposure of the glass powders of the noted dispersity at temperatures close to 960˚C leads to rapid crystallization. This result in gray porous material with a pore size of up to 0.5 mm composed of diopside and a layer from the remaining uncrystallized part of the glass.

At a temperature of 860˚C for 20 min, the powder is sintered into a dark gray monolith (the main crystalline phase―melilite) with glass layers. Increasing the time of the heat treatment at 860˚C or raising the temperature to 1000˚C leads to the deposition of diopside from the glassy layer?the sintered material acquires a light gray color. The sintered materials are a dense gray monolith without pores. Upon crystallization in powders, the effect of a declining degree of crystallinity α is observed with a decrease of the particle size of the powder. It may be due to the fact that at temperatures well above the glass transition temperature T_g of the original glass, simultaneously with the crystallization, the viscous flow and fusion of the particles due to their surface tension take place. The smaller the particle size the greater the number of contacts between them in the sample. It promotes the fusion of the particles and reduces their free surface over time, which catalyzes the nucleation of the crystals. As a result of the fusion process of the fine particles upon softening of the glass, a monolithic polycrystal is formed.