Obtaining Foamed Glass-Ceramics from Diamond Concentration Tailings

: The possibility of obtaining building foamed glass-ceramic using the diamond concentration tailings of the Lomonosov deposit in Arkhangelsk Region, Russia, is demonstrated here. The effect of the tailings’ particle size distribution, feed temperature, the addition of a foaming agent, and the content of oxidizer on the feed charge foaming is established. The process conditions for obtaining foamed glass-ceramic materials are described. The specifications of the materials with the optimal composition (tailings 50 wt.%, glass waste 50 wt.%, SiC 0.5 wt.%, Fe 2 O 3 1 wt.%) foamed at 1020–1050 ◦ C were as follows: apparent density 0.23–0.51 g/cm 3 , compression strength 0.58–2.40 MPa, water absorption (by volume) 8.7–19.0%. Based on the combination of the measured properties when used in dry conditions, the obtained materials can be considered heat-insulating foam materials. The thermal conductivity was 0.060–0.066 W/m · K.


Introduction
The government strategy for the development of the building materials industry for the period through 2030, enacted by the Government of the Russian Federation (Order 868-r dated on 10 May 2016 [1]), emphasizes that the main trends in the global building materials industry in recent years have been: flux. Foamed glass-ceramic materials derived from blast-furnace slag and glass waste were obtained in [31]. For the formation of nuclei of crystalline phases, TiO 2 , ZrO 2 , and CaF 2 were used.
Obtaining various types of foamed glass is one of the promising areas for glasses' waste recycling. The main difficulty is the heterogeneity of glass in terms of chemical composition, which affects the stability of the recycling process and the quality of the finished product. Multiple publications are devoted to the use of glass waste in the production of foamed materials, including glazing glass, medical glass, and glass containers [32][33][34][35][36][37][38][39]. Progress has been made in the disposal of the so-called waste monitor glass found in the cathode-ray tubes of TV receivers, computer monitors, etc. [40]. This has been successfully processed into both block and granular foamed glass products [41][42][43][44][45].
A number of papers have been devoted to the mechanism of charge foaming based on cullet (soda-lime-silicate glass) using various foaming agents (SiC, Si 3 N 4 , AlN). In some cases, additional oxidizers have been used to intensify foam formation, for example MnO 2 , Fe 2 O 3 , and CeO 2 , leading to a more uniform microstructure compared to carbide or nitride foam agents used separately [46][47][48].
The demand for effective heat-insulating materials and the availability of significant quantities of overburden, and concentration tailings occupying large storage areas, dictate the need to develop process flows for producing new foam materials with improved properties. The purpose of this study was to evaluate the possibility of obtaining building glass-ceramic foam materials using the diamond concentration tailings of the Lomonosov deposit in the Arkhangelsk Region of Russia. The effects were established on the particle size distribution of the tailings, the sintering temperature, the presence of a foaming agent, and the content of an oxidizing agent on the foaming of the feed charge.

Materials and Methods
The feasibility of obtaining foamed glass-ceramics from the sand fraction of the concentration tailings of processing plant no. 1 of Severalmaz JSC (hereinafter referred to as tailings) was examined. For these purposes, we used an average process sample of 12 kg of dry residue, from which the <0.63 mm fraction was isolated and reground to <0.05 mm. The tailings were a red-brown powder with a specific surface area of 11.93 m 2 /g and a bulk density of 655 kg/m 3 . Part of the experiments was carried out with a fraction of <0.63 >0.05 mm.
Glass waste (crushing window and bottle glass) was used to improve the sintering process of the ceramic matrix. Chemical composition of waste, wt.%: SiO 2 74.1, Al 2 O 3 1.6, Fe 2 O 3 0.1, CaO 10.1, MgO 1.6, Na 2 O + K 2 O 12.3, SO 3 0.2. The presence of flux led to an increase in the amount of liquid phase at a lower temperature, which contributed to the compaction of the structure of the surface shell of the ceramic matrix and locked the formed gas inside the material structure. Waste glass ground to a size of −0.05 mm with a bulk density of 660 kg/m 3 was used.
In the production of foamed glass-ceramics, silicon carbide (Grade 63 C, GOST R 52381-2005) was used as a foaming agent, contributing to the production of foamed materials with a porous matrix structure.
Simultaneously with the gasifier for the complete oxidation of SiC, Grade OSCh 2-4 Fe 2 O 3 was added. With the participation of iron oxide, reactions with gaseous carbon monoxide are possible, proceeding with insignificant heat absorption: The composition, structure, and properties of the raw source and produced materials were studied using a combination of methods. The specific surface area (S sp ) of the source materials was measured by the BET method using nitrogen sorption/desorption isotherms and a TriStar II 3020 V.1.03 instrument. The particle size distribution was established using a SALD-201V particle size analyzer. X-ray diffraction analysis (XRD) was performed on a Shimadzu XRD 6000 diffractometer (Kyoto, Japan) and CuK α radiation. X-ray images were taken with an increment of 0.02 • (2θ), and the signal accumulation time at each point was 1 s. Thermal gravimetricdifferential scanning calorimeter (TG-DSC) analysis was performed using a STA409 PC Luxx NETZSCH instrument (Selb, Germany) in an argon atmosphere at a heating rate of 10 • C/min. Heating was carried out in the temperature range of 20-1300 • C. The study of feed swelling was carried out using a high-temperature optical microscope ZEISS MHO-2 (Oberkochen, Germany).
The thermophysical properties of foamed glass-ceramics were determined and evaluated based on the requirements of GOST 17177-94 (heat-insulating building materials and products; test methods) and GOST 16381-77 (heat-insulating building materials and products; classification and general specifications).
The apparent density ρ (g/cm 3 ) of a sample was measured as the ratio of its mass to volume: where m is the mass of the dried sample, g; V is the volume of the sample, cm 3 .
The apparent density was taken as the arithmetic mean of the density measurements of all samples, calculated to 0.02 g/cm 3 .
Water absorption (W,%) by volume was calculated using the following formula: where m 1 is the mass of the sample after saturation with water, g; m is the mass of the sample, previously dried to a constant mass, g; V is the volume of the sample, cm 3 ; and ρ w is the density of water, g/cm 3 . Water absorption was measured as the arithmetic mean of a series of tests of individual samples (5-10 samples). The test was carried out until the results of two successive measurements differed by no more than 0.1%. The porosity (P,%) of the samples was estimated based on the pre-determined values of true and average density according to the following formula: where ρ av is the average density, g/cm 3 , and ρ t is the true density of the sample, g/cm 3 . Tests of foamed glass-ceramics for compressive strength were carried out using a PGM-100MG hydraulic press (Russia) on cylinder samples, provided that the height and diameter of the sample were equal (20 ± 2 mm). The ultimate compressive strength (R comp , MPa) of the sample was calculated by the formula: where F is the breaking load, N, and S av is the area to which the load was applied, cm 2 .
The essence of the method was to create a stationary heat flux passing through a flat sample of a certain thickness and directed perpendicular to the front faces of the sample, measuring the sample thickness, heat flux density, and temperature of the opposite front faces. The thickness of the measured sample was 18-20 mm. To determine the thermal conductivity coefficient, an electronic thermal conductivity meter ITP-MG 4 (Russia) was used. The thermal conductivity λ (effective thermal conductivity) was estimated using the formula: where λ is the effective thermal conductivity, W/m·K; H is the thickness of the measured sample, mm; q is the density of the stationary heat flux passing through the measured sample, W/m 2 ; T H is the temperature of the hot face of the measured sample, K; and T X is the temperature of the cold face of the measured sample, K.

Results and Discussion
The XRD data of the powder used in foamed glass-ceramics production are shown in Figure 1. The diffraction pattern shows an amorphous halo and reflections of crystalline phases, mainly related to quartz, dolomite, and calcite. Small quantities of an iron mineral, herzenite, were present. the temperature of the cold face of the measured sample, K.

Results and Discussion
The XRD data of the powder used in foamed glass-ceramics production are shown in Figure 1. The diffraction pattern shows an amorphous halo and reflections of crystalline phases, mainly related to quartz, dolomite, and calcite. Small quantities of an iron mineral, herzenite, were present.
The chemical composition of the tailings is presented in Table 1. In accordance with the feed specifications for the production of building materials, including ceramics and ceramic foams, when using waste the charge composition needs to be modified, for example, with waste glass. The chemical composition of the waste glass was as follows, wt.%: SiO2 74.1, Al2O3 1.6, Fe2O3 0.1, CaO 10.1, MgO 1.6, Na2O + K2O 12.3, SO3 0.2.
Due to the fact that the fineness of the feed materials has a significant impact on the quality and properties of foamed glass-ceramics, the particle size distribution range of ground tailings was established ( Figure 2).  According to the data obtained, the reground tailings had a relatively narrow bimodal particle size distribution from 0.5 to 35 µm.
The results of the TG-DSC analysis of the tailings sample are presented in Figure 3. The chemical composition of the tailings is presented in Table 1. In accordance with the feed specifications for the production of building materials, including ceramics and ceramic foams, when using waste the charge composition needs to be modified, for example, with waste glass. The chemical composition of the waste glass was as follows, wt.%: SiO  Due to the fact that the fineness of the feed materials has a significant impact on the quality and properties of foamed glass-ceramics, the particle size distribution range of ground tailings was established ( Figure 2).  According to the data obtained, the reground tailings had a relatively narrow bimodal particle size distribution from 0.5 to 35 µm.
The results of the TG-DSC analysis of the tailings sample are presented in Figure 3. According to the data obtained, the reground tailings had a relatively narrow bimodal particle size distribution from 0.5 to 35 µm.
The results of the TG-DSC analysis of the tailings sample are presented in Figure 3.
Ceramics 2023, 6, FOR PEER REVIEW 6  The DSC data showed two main endothermic effects. The first was a low-intensity endothermic effect with a peak at 86.1 °C, accompanied by a change in mass by 1.01%, associated with the release of adsorbed water. The second, more intense endothermic effect, in the temperature range 700-900 °C with a minimum at 801.7 °C, was related to the decarbonization of dolomite and the dissociation of calcite, as evidenced by a mass change of 14.53%. A minor endothermic effect with a minimum at 578 °C was associated with the reversible polymorphic transformation of α-quartz into β-quartz. It can be noted that the TG-DSC data were in agreement with the XRD.
Preliminarily, as part of an exploratory experiment to determine the possibility of obtaining foamed glass-ceramics, the foaming process of samples of different compositions was studied using a microscope analysis. Samples for testing were prepared as follows. Feed materials of a certain composition were thoroughly mixed. Water was added as a binder in an amount of 5-8 wt.%. Cubic samples with an edge length of 3 mm were prepared in a mold, placed on a corundum substrate, and moved to a furnace. Sintering was carried out up to the temperature of the size increase in the sample, and then with an increment of 50-100 °C, with an analysis conducted at each maximum temperature. The DSC data showed two main endothermic effects. The first was a low-intensity endothermic effect with a peak at 86.1 • C, accompanied by a change in mass by 1.01%, associated with the release of adsorbed water. The second, more intense endothermic effect, in the temperature range 700-900 • C with a minimum at 801.7 • C, was related to the decarbonization of dolomite and the dissociation of calcite, as evidenced by a mass change of 14.53%. A minor endothermic effect with a minimum at 578 • C was associated with the reversible polymorphic transformation of α-quartz into β-quartz. It can be noted that the TG-DSC data were in agreement with the XRD.
Preliminarily, as part of an exploratory experiment to determine the possibility of obtaining foamed glass-ceramics, the foaming process of samples of different compositions was studied using a microscope analysis. Samples for testing were prepared as follows. Feed materials of a certain composition were thoroughly mixed. Water was added as a binder in an amount of 5-8 wt.%. Cubic samples with an edge length of 3 mm were prepared in a mold, placed on a corundum substrate, and moved to a furnace. Sintering was carried out up to the temperature of the size increase in the sample, and then with an increment of 50-100 • C, with an analysis conducted at each maximum temperature.
The effects of various factors on the foaming process, such as the particle size distribution of the powder, the heating temperature of the charge, the presence of a foaming agent, and the content of the oxidizing agent, were established.
At the first stage, experiments were carried out on the tailings without additives and with the addition of 30 wt.% of waste glass, which demonstrated insufficient foaming of the feed. Samples were heated to 1100 • C. Considering this, 50% wt.% of waste glass, a foaming agent, and an oxidizing agent (Fe 2 O 3 ) were added to the charge.
The compositions of the charges, foaming conditions, and properties of the samples after heat treatment are presented in Table 2. For each composition, three samples were prepared. From the data presented in Table 2, it can be seen that the foaming of the samples was the strongest when using a fine fraction of the feed. The introduction of an additional foaming agent into the charge contributed to a noticeable increase in the size of the sample and an improvement in its structure (pore distribution and size). Iron oxide, introduced in a dispersed state into the charge, slightly reduced its foaming capacity, which is consistent with the findings in [49], but contributed to a more ordered structure of the material. More efficient gas formation with a higher quality of the outer surface was demonstrated by samples with iron-containing additives at 1 wt.% (samples13 to 16) and 3 wt.% (samples 17 to 22).
It was established that all samples of the studied compositions had a relatively low foaming ratio not higher than 1.65, which is explained by the high content of the residual crystalline phase in the material. The highest foaming ratio was exhibited by finely ground feed with the addition of 3 wt.% Fe 2 O 3 (sample 22), which was probably due to the formation of a sufficient amount of glass phase, the temperature match of gas formation, and more complete oxidation of the SiC.
Subsequently, to study the foaming of a charge based on the tailings and to obtain foam materials, tailings sized < 0.05 mm, SiC as a foaming agent at 0.5 wt.%, and Fe 2 O 3 as an oxidant at 1, 2, 3 and 5 wt.% were used.
For the manufacture of foamed glass-ceramics, the feed charge was thoroughly homogenized, loaded into split ceramic cylinder molds, and lubricated with a suspension based on a mixture of chalk and corundum. The prepared molds with the charge were loaded into a "Nabertherm" electric furnace (Germany) for sintering (foaming) in the temperature range 900-1100 • C at an exposure time of 10-50 min at maximum temperature. The charge was loaded into a cold furnace and heated to the foaming temperature. Since the best performance in terms of foaming and lower density was demonstrated by samples placed in a furnace preheated to a given temperature, further experiments were carried out in this way. After foaming, annealing was carried out, i.e., an arbitrary decrease in temperature to ambient temperature (cooling in a furnace), for 15-20 h.
The properties of the resulting material were evaluated according to the following criteria: apparent density, compressive strength, water absorption, thermal conductivity, and macrostructure of the samples.
Key properties of the foamed glass-ceramics composed of 50 wt.% tailings, 50 wt.% waste glass, 0.5 wt.% SiC, and 1 wt.% Fe 2 O 3 are presented in Table 3. A preliminary study of the physical and mechanical properties of the samples of the above composition foamed at 1020-1050 • C showed that the apparent density of the material was 0.23-0.51 g/cm 3 , the compressive strength 0.58-2.40 MPa, and the water absorption (by volume) 8.7-19.0%. The samples had a rather low thermal conductivity of 0.060-0.066 W/m·K, matching that of foam glass.
To improve the performance and primarily reduce water absorption by the foamed glass-ceramics, the composition of the initial charge and process conditions needed to be adjusted, which required further research.
In the XRD pattern of samples sintered at temperatures of 1020-1050 • C, the presence of a glass phase, as well as diopside, quartz, and cristobalite, was noted. An increase in the halo intensity in the angle range of 15-35 • , along with the sintering temperature, indicated an increase in the content of the glass phase. With an increase in temperature, an increase in the diopside reflections intensity was observed as well as a decrease in the quartz and cristobalite peaks intensity ( Figure 4).
As is known, in order to obtain foamed glass-ceramics with improved performance, a uniform finely porous structure is required. The macrostructure of the samples foamed at different temperatures is shown in Figure 5. An analysis of the micrographs showed that the structure of the foamed glass-ceramics depended on temperature. Samples obtained at a temperature of 1020 • C were characterized by a finely porous structure (Figure 6a,b) with randomly located pores with a size of 0.16-2.67 mm. The structure of the foamed glassceramics obtained at a temperature of 1030 • C approached the optimum. The presence of evenly distributed polydisperse (in size) pores, which were separated by dense and similar cross-section interpore partitions, was recorded. As can be seen from Figure 6c,d, the pore structure was represented by cells 1.29-3.87 mm in size, predominantly close to a round shape, remotely resembling a honeycomb structure in appearance. Such a space-volume configuration is the most optimal for foamed glass-ceramics capable of being resistant to sufficiently large loads at a minimum density value. The thickness of the interpore partition in the sample was 0.14-1.16 mm. At a temperature of 1040 • C, an increase in the number of larger pores of 4.37-5.60 mm in size was observed (Figure 6e,f). An increase in temperature to 1050 • C led to a change in porosity towards the predominance of large pores with an average size of 6.70-8.00 mm (Figure 6g,h). The presence of single pores up to 10 mm in size is possible. Increasing the sintering temperature leads to enlarged pores and the breakthrough of the gaseous phase as a result of a decrease in the viscosity and surface tension of the melt. Such a structure is not optimal, which is confirmed by an increase in water absorption, as well as a decrease in the material strength. At the same time, with an increase in the sintering temperature of 30 • C, water absorption increases by a factor of 2.1.        The results obtained showed that the optimal sintering temperature was 1030 • C. Samples of the described composition, obtained at the indicated temperature, were the closest to the specifications for glass-ceramic foams. The exception was the increased water absorption, the reduction of which requires further research.

Conclusions
1. The feasibility was demonstrated of diamond processing tailings of the Lomonosov deposit in the Arkhangelsk Region, Russia, into building foamed glass-ceramics.
2. The effects on the foaming process of the feed charge of the particle size distribution of the tailings, the heating temperature, the presence of a foaming agent, and the content of an oxidizer were established. The results indicate that it is advisable to use a fine fraction of the feed materials (−0.05 mm) and introduce SiC as a foaming agent and Fe 2 O 3 as an oxidizer into the charge to improve sintering up to waste glass. The optimal process conditions for obtaining foamed glass-ceramics were identified.
3. Materials were obtained that possessed a relatively low thermal conductivity of 0.060-0.066 W/m·K, which allowed these to be classified as heat-insulating. The best properties in terms of apparent density, compressive strength, porosity, pore shape and distribution were demonstrated by the following charge, wt.%: tailings 50, waste glass 50, SiC 0.5, Fe 2 O 3 1 at a sintering temperature of 1030 • C and an exposure time of 30 min. The samples were characterized by apparent density 0.28 ± 0.02 g/cm 3 , compression strength 1.20 ± 0.02 MPa, and water absorption (by volume) 9.5 ± 0.19%. 4. Our findings are expected to form the basis of the development of processes for the production of innovative building materials that will improve the energy efficiency, reliability, and durability of buildings and structures. The recycling of industrial waste will help to reduce the environmental footprint.

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author after obtaining the permission of an authorized person.