Physicochemical Studies of Opoka as a Raw Material Component of Sodium Silicate Mixture for Subsequent Synthesis of Foam Glass Material Based on It

Abstract

The present article presents the results of physical and chemical studies of opoka. In particular, the opoka was subjected to chemical analysis, X-ray phase, differential thermal analysis, scanning microscopy, and X-ray energy dispersive elemental microanalysis. The opoka was studied with the aim of using it as an available raw material for obtaining a sodium silicate mixture and, in the future, developing an energy-saving technology for obtaining a building heat-insulating and sound-insulating foam glass material based on it, using synthesis. As a result of the studies, the chemical composition of the opoka was determined, which is 69–80% represented by silica. The elemental composition of the opoka was established, which is represented by 94.25% oxides of Si, Al, and Fe. The presence of such oxides makes it an ideal raw material component of a silicate-sodium mixture for the subsequent synthesis of foam glass material from it. Experimental exploratory studies on the synthesis of foam glass based on opoka have been carried out. The experimentally obtained sample of foam glass material consists of 93.37% Si, Al, Mg, and Na oxides, has a porous structure with a pore size of 2–5 microns, an average density of 375 kg/m³, thermal conductivity of 0.063 W/(m °C) at 25 °C, and noise absorption of 51.6 Db.

1. Introduction

In the modern world, one of the priority areas of energy saving in the construction industry is the minimum level of heat loss in buildings. In the Republic of Kazakhstan, energy consumption in residential areas reaches up to 40% in places. Heat consumption in the republic averages between 170 and 185 Gcal annually, while more than 45 million different fuels are consumed during the heating season. In a number of cities in Kazakhstan, as in all cities of the CIS countries, an urban central heating system is used, which was designed and put into operation in the last century, and currently, most of it has a high degree of wear. Statistical analysis over the past 5 years indicates that, at the moment, heating networks have losses of slightly more than forty percent, which is many times more than similar heat losses in the networks of European cities [1,2,3,4,5,6,7,8,9,10,11,12].

In modern conditions, high demands are placed on the thermal and energy saving of enclosing products and structures, both in terms of raw materials and manufacturing technology, to form a high degree of thermal and noise insulation for various purposes of buildings.

Today, an important parameter of economic efficiency is the tendency to reduce the energy consumption of buildings and structures. Based on statistical data, in the modern world, a significant part of various energy resources is spent on maintaining heat in buildings, depending on the country, from 8% to 53%. When used in the combustion process, carbon dioxide (CO₂) is formed and released into the atmosphere as a result of human activity.

About 70% of various heat losses through enclosing structures are accounted for by windows, which, in residential premises, occupy 25–40% of the wall area. The introduction of energy-saving technologies in the construction industry can lead to savings of up to 31–72% of energy and fuel in the production of building materials, housing, and communal services [1,4,5,6,7,8,9,10,11,12,13,14,15].

In Kazakhstan, the main types of insulation are mineral wool products, which make up 63% of the total volume of production and consumption. Another 9% are glass wool materials, 22% are polystyrene foam and other foam plastics. The share of heat-insulating cellular concrete is no more than 2.5-3.2%, and expanded perlite is less than 0.35% [1,7,8,9,10,11,12,13,14,15,16].

Currently, there is an increasing trend in the construction industry of the Republic of Kazakhstan towards the demand for high-quality cellular concrete as a thermal insulation material. However, its production facilities are unable to dramatically increase their capacity to meet the existing demand in the construction market. In developed economies, particularly in Western Europe, the production of aerated concrete and products based on it reaches up to 282–295 per thousand people [12,13,14,15,16,17,18,19].

According to various data, approximately, in the CIS countries, the market volume of cellular concrete and products made from it in the field of civil engineering is projected to reach 4.7–5.2 million m³ by 2025. This forecast is based on the experience of countries with developed economies and achievements in this area by the leader of the CIS countries, the Republic of Belarus, the level of production and use of aerated concrete and its products varies between 168–172 m³ per 1000 people [1,2,3,4,7,8,9,10,11,12,13,14,15,16,17,18,19].

Basalt fiber products are a dynamically developing and promising thermal insulation material. However, in the modern realities of the CIS countries, and Kazakhstan in particular, all industrial production facilities were built in the middle of the last century, and the equipment that is located there does not meet modern standards and requirements, neither in quality nor in terms of electricity consumption for the better. Existing melting furnaces, in particular rolling furnaces, due to insufficient melting point capacity, are simply not capable of melting basalt. Due to the fact that they do not correspond technologically, such capacities increase the cost of production and are unable to compete, neither in terms of economic indicators nor in quality [1,2,3,4,7,8,9,10,11,12,13,14,15,16,17,18,19].

Over the past few years, entrepreneurs in Kazakhstan have been actively attracting European technological experience. Thus, technological facilities with sufficiently high-performance thermal insulation materials were built. equipped with high-tech equipment from European countries. Especially successful Kazakhstani enterprises in this area are the limited liability partnerships in the cities of Almaty region (MISOT FLEX), Ekibastuz (MVI Plant LLC), Temirtau (Ecotherm LLC), Makinsk (Makinsky Thermal Insulation Plant LLP) and Oskemen (Isotherm). According to experts, these leaders are able to increase the production capacity of thermal insulation materials in the country and compete with the world’s leading manufacturers in the market of Kazakhstan [1–4, 7–19].

The growth in the share of national companies producing thermal insulation building materials is due to the systematic transition to new European standards in the field of construction and the use of high-tech equipment of technological lines producing basalt products. If, according to statistics on the productivity of mineral wool products of national enterprises of Kazakhstan in 2007, productivity was at the level of 10.25%, the upward trend reached 32.56% already in 2008, and it was about 48% in 2009. Today, in Kazakhstan, the share of high-tech companies and technological lines of a number of production facilities that meet global standards reaches no more than 67% [1,2,3,4,7,8,9,10,11,12,13,14,15,16,17,18,19].

Today, the production of mineral wool products in our country is due to the economic activities of about 12 industrial enterprises. According to forecasts of the Ministry of Industry, by 2025, the production of various mineral wool products based on fibrous basalt will reach approximately 3800–4000 m3, which should lead to a balance between supply and demand for these products. This will be possible due to the development of the design capacities of existing plants built over the past 5 years and the increase in their total capacity to 880 thousand m³ per year, as well as the construction of new plants in the cities of Almaty, Shymkent, Pavlodar and Makinsk [1,2,3,4,7,8,9,10,11,12,13,14,15,16,17,18,19].

Increasing the level of thermal and noise insulation and energy efficiency will help reduce energy consumption and ensure the necessary sanitary, hygienic, and climatic standards for residential buildings, which will have a beneficial effect on the livelihoods of people. The use of advanced technologies and materials helps to effectively protect buildings from heat loss, saving conventional fuel. Research shows that each cubic meter of thermal insulation can save approximately 1.4–1.5 tons of conventional fuel per year [1,2,3,4,7,8,9,10,11,12,13,14,15,16,17,18,19].

The main types of insulation in Kazakhstan at the present stage are divided into the following types:

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mineral wool products (65%);

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glass wool materials (9%);

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polystyrene foam and other foam plastics (21%).

The share of thermal insulation cellular concrete is no more than 3%, and expanded perlite is less than 0.29% [1,2,3,4,7,8,9,10,11,12,13,14,15,16,17,18,19].

Despite efforts to expand the range and improve the quality of thermal insulation materials, there is still a shortage of these products on the construction market in Kazakhstan.

Contractors often match one or another new building material without taking into account the necessary natural and climatic factors of various regions of large countries, which leads to the disruption of the essential living conditions and daily activities of the population quartered in newly erected buildings.

In this regard, there is a need to develop physical-technical and design-technological foundations for the creation of heat-efficient enclosing structures that increase the efficiency of thermal protection of residential and public buildings in accordance with sanitary standards for high-quality provision of life support for the population in new housing construction. The introduction of resource-saving technologies requires the use of highly efficient, domestically produced insulation materials, along with known thermal insulation materials. These insulation materials should be represented mainly by inorganic cellular materials, such as block foam glass [15,16,17,18,19,20,21,22,23,24,25,26,27].

Foam glass is a highly porous material (80–95%) consisting of air-filled, cellular closed or communicating pores, separated by partitions of glassy substance [26,27]. Foam glass is traditionally obtained on the basis of secondary glass cullet or specially welded glass (glass granulate), with its full or partial introduction into the composition of the foaming batch [26,27].

The structure of foam glass resembles an ordinary rubber sponge or solidified foam, hence its name.

Due to its porosity, foam glass is very light, it can be used to insulate not only floors, roofs, and attics, but also as a filler for the walls of frame buildings. Which, by the way, allows buildings to be constructed even on weak soil, because the load on the foundation is significantly reduced.

Foam glass is an ideal material for widespread use in residential construction. The combination of its environmental friendliness and excellent thermal insulation qualities allows to quickly and independently insulate any private property, whether it be a residential building, cottage, summer house, or garage, as well as the installation of a heated floor in an apartment or the insulation of a loggia or attic.

At the same time, foam glass has retained all the qualities of its “parent”, glass, so it does not burn (its fire safety class allows its usage even in the nuclear industry); it is impermeable to water; it does not shrink (expanded clay, for example, must be added 2–3 times when used in frame walls); it is resistant to aggressive environments, including acids; and it does not contain any organic compounds (which means that mice and cockroaches will not infest it, and there will be no fungus or mold). Therefore, foam glass is effective for the construction of summer cottages, cottages, grain storage facilities, and warehouses [28,29,30,31,32,33,34,35,36,37].

Foam glass has excellent soundproofing properties. In addition, it is not subjected to any temperature changes, as it withstands any changes in any climate zone at any time of the day. Furthermore, it is not subjected to corrosion, and it is resistant to aggressive environments, including acids and alkalis. Today, foam glass is the only insulation material permitted by EU standards.

In essence, foam glass is an environmentally friendly glass foam, so it is also environmentally safe. This allows it to be widely used for any type of construction, even for food and pharmaceuticals, replacing hazardous toxic materials such as asbestos, environmentally harmful and fire-hazardous foam plastic, polyurethane (PUF), expanded polystyrene (EPS), and short-lived mineral slabs (mineral wool) [38,39,40,41,42,43,44,45,46].

Moreover, the production of this material itself is environmentally friendly: it can use both natural and man-made mineral raw materials, such as broken glass and any waste from glass production.

It is not difficult to work with foam glass: unlike such traditional materials as aerated concrete or polystyrene foam, it combines well with cement mortars. Furthermore, it is easily processed with cutting tools, drilled, nailed, glued with mastics, and plastered.

Traditionally, foam glass is produced on the basis of secondary cullet or specially welded glass (glass granulate), with its full or partial introduction into the foam-forming batch. Limitations in the volumes of secondary glass of constant composition determine the need to expand and involve the mineral resource base and develop alternative technologies for the production of foam glass materials due to accessible, cheap, natural, and man-made silicate raw materials [1–10, 25–27, 32–44]. Taking into account a number of technological and economic features of foam glass production, an urgent task is as follows: determining the possibility of obtaining foam glass by low-temperature heat treatment of the mixture at temperatures below 900 °C, directly from a silicate-sodium mixture based on opokas, bypassing the glass melting process.

The technological and economic aspects of foam glass production determine the purpose of this study—to determine the possibility of using local raw materials, in particular opoka. In this regard, there is a need to conduct physico-chemical studies of opoka, followed by experimental and exploratory synthesis of foam glass based on it.

2. Materials and Methods

In the current study, amorphous siliceous rocks from the Orangay deposit located in the Turkestan region of the Republic of Kazakhstan were used for the subsequent production of foam glass. More than 25 different opoka and opoka-like clay deposits associated with the Suzak and Khanavat horizons of the Lower and Middle Paleogene were discovered in this area.

Opokas are light, dense, fine-pored siliceous rocks consisting mainly of tiny (less than 0.005–0.001 mm) isometric and irregular opal-cristobalite particles. They are characterized by greater hardness and average density compared to diatomites and tripoli. Opokas often contain traces of diatoms and other siliceous organisms, as well as their remains after dissolution [14]. Opokas are greenish-gray in color, have a dense structure, and sometimes contain clay layers. The mineralogical composition of opokas varies, but the main component is opal. Opokas consist of two main components: opal silica, represented by small rounded and irregular particles less than 0.005 mm in size, and clay material. The content of opal silica may vary depending on the amount of clay component, which usually makes up 32–47% of the rock. In crossed nicols, opal silica is characterized by bright interference and a fine-flaky structure. The clay part of opokas and opoka-like clays of the Tasaran suite includes montmorillonite, hydromicas, and kaolinite.

The estimated reserves of opokas suitable for producing thermal insulation foam glass in the Turkestan region alone are about 9–11 million tons. In Kazakhstan, this figure exceeds 1.0–1.25 billion tons.

The amount of certain oxides allows us to preliminarily determine the specific properties of opoka and opoka-like rocks, as well as to judge the mineralogical composition of the raw materials using physicochemical research methods.

The second component for obtaining the sodium silicate mixture was technical sodium hydroxide, which belongs to the alkali class. It consists of a sodium cation and a hydroxyl group OH’, and has the ability to dissolve in water and alcohols. A 13% alkaline solution was used in the studies.

Sodium hydroxide is a strong base, and its reactions are intense. The small radius of the sodium ion causes deformation of the crystal lattices of the interacting compounds, increasing their reactivity.

When studying opokas, their features were taken into account: the phase state, structure, and various formations. A range of modern physico-chemical analysis methods were used in the study.

Differential thermal analyses were performed on a D-1500Q derivatograph (Budapest, Hungary) from the system developed by F. Paulik, I. Paulik, L. Erdey, which allows for the simultaneous recording of four heating curves: the DTA curve of differential thermal analysis, showing the change inenthalpy of the sample under study; TG—thermogravimetric curve recording the change in mass; DTG—differential thermogravimetric curve characterizing the rate of mass change; and T—temperature curve showing temperature change over time. The temperature inside the studied substances was measured with a platinum-platinum-rhodium thermocouple. The sample was heated from 20 °C to 1000 °C at a rate of 10 °C/min. Calcined alumina a-Al₂O₃, which has no thermal effects in this temperature range, was used as an inert substance. Alundum crucibles were used. Room air served as the atmospheric environment. Sample weights were 950-960 mg.

X-ray phase studies were carried out by us on an X-ray fluorescence wave-dispersive spectrometer ARL 9900 Intel WorkStation, (Thermo Fisher Scientific, Basel, Switzerland) with an X-ray tube and a nickel filter on pea-shaped samples. The range of detector movement angles (diffraction angles from 4° to 64°) was counted on the scale of the goniometric device and by marks on the diagram or table of peaks output to a personal computer. The counter rotation speed was 4 °C/min. X-ray diffraction patterns were taken at a tube voltage of 20 kV and a current of 20 A. The samples were crushed until they passed through a sieve of 10,000 holes/cm².

Electron microscopic studies were carried out on a scanning electron microscope JSM-6490LV (JEOL, Tokyo, Japan). These devices use field emission guns operating under conditions of ultra-high vacuum (up to 10⁻⁸ Pa), providing sufficient current in a small-diameter probe (0.15–0.35 nm), with a range of electron-optical magnifications from ×100 to ×60,000 and an accelerating voltage to 100 kV.

Experimental exploratory research on the synthesis of foam glass based on quartz sand and opoka was carried out as follows. To foam the mixture, specially made cube molds with a size of 5 × 5 × 5 cm were used, the walls of which were coated with clay solution before firing to avoid sticking of the samples to the metal walls. The cuboid shapes themselves were installed on a metal pallet with alumina filling. Prepared, averaged, and homogenized sodium silicate mixtures (with sand and opoka) were pre-uniformly loaded into prepared molds, filling them to 40–45% capacity and compacting the mixtures manually. Then, the molds were placed in a muffle furnace preheated to 590-610 °C in order to obtain a foam glass material when foaming the samples. In the furnace chamber, the temperature was adjusted to 850 °C in a time interval of 35–45 min, at a temperature rise rate of 6–10 °C/min, with the sample exposed for 25–35 min. To fix the foam, the temperature in the oven was sharply reduced by opening the oven door briefly. After that, the foamed samples were subjected to slow spontaneous cooling in an oven from 850 °C to 25–50 °C for 16–19 h, which was due to the need to prevent the occurrence of thermal stresses during abrupt cooling. After cooling, the samples were removed and analyzed.

3. Results and Discussion

During the physical and chemical studies of the opoka as the main raw material component of sodium silicate mixture for further production of foam glass, the opoka was subjected to chemical, thermal, X-ray phase, and microscopic analysis. The chemical analysis of the opoka from the Orangay deposit in the Turkestan region is given in

Table 1. Chemical composition of opoka from the Orangay deposit in the Turkestan region (mass, %).

SiO2	Al2O3	Fe2O3	TiO2	CaO	MgO	Na2O	K2O	SO3	H2O	PPP
68.97–79.57	6.11–10.38	2.37–3.44	0.25–0.45	0.3–2.19	0.98–1.82	0.55–1.07	0.69–1.12	0.76–2.75	3.34–4.39	3.97–4.53

.

As can be seen from the presented data, the content of the main oxide SiO₂ in the opokas varies between 69% and 80%, which determines its role as the main raw material for obtaining the mixture and foam glass based on it.

The total silica content consists of three components: opal silica, silica that is part of clay minerals, and silica represented by terrigenous quartz.

The presence of clay minerals in the opoka indicates the presence of alumina. By the presence of alumina in the chemical composition of the opoka, it seems possible to assume its “clay content”, which in turn can reflect certain arguments about technological properties. The opoka contains clay minerals, expressed by hydrosludes, as well as aluminum, sodium, potassium, iron, and magnesium oxides.

The alkaline group in the opokas is represented by the sum of Na₂O and K₂O, the content of which varies from 0.42 to 1.47%.

In the opokas, iron is mainly present in the bound state (from 2.38 to 3.47%) and is present in clay minerals in compounds in the form of hydroxide and in small amounts in the form of clay sulfides. It is also in the bound state of hydroxides and, in small amounts, in sulfide form. The concentration of iron hydroxides in the opokas is due to the resulting microcracks. The presence of iron oxides in the opoka can be judged by the colors of the opoka during firing in a palette of colors from orange to dark brown, while iron oxides help to reduce the firing temperature.

Calcium in the opoka is represented by carbonates; in particular, it is calcium carbonate. Sulfur anhydrite in the opokas is represented by gypsum and iron sulfide minerals and, in total, it reaches 3.72%.

Differential thermal studies of opoka from the Orangay deposit show that the thermogram shows characteristic signs of changes in the properties of silica, where four endoeffects are observed. The first of them is recorded at a temperature of 99.5 °C, which is accompanied by a loss of mass and is associated with the release of adsorbed water. Then, two endothermic effects in the range of 440–550 °C are associated with the release of constitutional water of clay minerals, and, in particular, hydromicas and mixed-layer formations, which causes the endoeffect of their decomposition (Figure 1).

As a result of the analysis, it was concluded that the silica of the studied opokas is represented mainly by a-cristobalite, and also that in the temperature range from 400 to 800 °C, not only polymorphic transformations occur, but also intensive changes in the crystal lattice of silica, leading to the formation of reaction-free bonds [1,12,16].

Analyzing the obtained results of DTA and DTG, it is possible to note the endoeffect in the studied opokas in the temperature range of 80–100 °C, which is typical for the processes of removing free water adsorbed by opal clay minerals, with an amount of up to 2.4–3.0%. The maximum amount of adsorbed water is contained in the clay varieties of opokas. The exoeffect in the region of 300–400 °C is associated with the burnout of finely dispersed organic impurities.

The X-ray phase analysis of the selected opoka sample contributed to finding the most accurate data of interplanar distances, and to identifying the diffraction maxima of silica contained in the opoka. The X-ray phase parameters of the studied opoka are presented in

Table 2. X-ray phase indices of diffraction maxima of different intensity of the studied sample of opoka from the Orangay deposit.

	Pair	Corner	Square	Intensive	Half-Width	Interpol	% Max
No.
1		6.840	469.821	367	1.2802	12.9225	11.76
2		9.840	84.770	459	0.1847	8.9884	14.70
3		20.800	757.996	1079	0.7025	4.2704	34.56
4		21.500	1283.277	939	1.3666	4.1329	30.08
5		26.620	548.748	3122	0.1758	3.3485	100.00
6		29.860	242.550	330	0.7350	2.9921	10.57
7		32.000	110.006	414	0.2657	2.7968	13.26
8		36.520	167.571	380	0.4410	2.4603	12.17
9		42.440	73.787	291	0.2536	2.1298	9.32

.

The set of diffraction maxima of different intensities of the analyzed opoka in the above table identifies their quartzite-predominant composition and character. The most intensely pronounced are quartz peaks, with diffraction values of 3.3485, 4.2704, and 4.1329 A.

From the X-ray phase analysis data (Table 2), it is evident that the opoka of the Orangay deposit consists almost entirely of amorphous silica, mainly in the form of opal SiO₂·H₂O, which is confirmed by the peaks of interplanar distances found.

Based on the results obtained using scanning electron microscopy and X-ray energy-dispersive microanalysis JSM-6490LV with magnification (×250–×2000), it was established that the bulk of the studied opoka from the Orangay deposit is represented by globules.

In particular, opoka is composed of tiny spheroidal mineral aggregates (Figure 2a–d), which are formed from rounded phases of amorphous silica, having dimensions at the micro level, undergoing further internal condensation and restructuring to a more compacted state, leading to the formation and growth of colloidal particles of large sizes, the core of which consists of SiO₂ (spectrum 1), and the surface is covered with SiOH groups.

The general analysis of the selected opoka sample using scanning electron microscopy and X-ray energy dispersive elemental microanalysis on a scanning electron microscope is shown in Figure 3.

From this, it is evident that the studied opoka is represented by the following elements: 91.98% oxygen, silicon, aluminum; 2.27% iron;—and 5.75% other various elements, as shown in Figure 3.

Elemental microscopic analysis confirms the main results of chemical and X-ray phase analysis, which characterizes the correct approach chosen for the study of modern physical and chemical studies of opoka as a raw material component in a mixture with caustic soda for further synthesis of foam glass material from it, used in various thermal and noise insulation structures [23,47,48,49,50,51,52], depending on climatic conditions in civil and industrial construction [20,52,53,54,55,56,57,58,59].

The prospects for further research will be aimed at using silica amorphous rocks in foam glass technology [16,37,38,39,40,41,42,43,44,45,46] instead of traditional raw materials with a crystalline structure, which allows intensifying the glass formation process due to the high reactivity of amorphous silica [16,60,61,62,63], while eliminating the energy-intensive and labor-intensive process of high-temperature cooking and granulation of complex multi-component glass mass.

In the conducted exploratory experiments, quartz sand and opoka were taken as silica-containing raw materials. The sodium–silicate mixture was prepared in a ratio of 6–20% to opoka and quartz sand for subsequent comparison.

In the course of the conducted exploratory studies, the optimal holding time of samples in a muffle furnace at 850 °C was experimentally established, which was 25-35 min. When comparing two silicon-containing materials (quartz sand and opoka) subjected to foaming, it was found that mixtures prepared on the basis of quartz sand, at all studied concentrations of an alkaline NaOH solution, formed only loose specks without visible pores, which are characteristic of foams. This is due to the fact that quartz grains do not have time to dissolve in sodium hydroxide and at foaming temperatures, the amorphous phase is not enough for foaming. Gases released from the mixture through a loose sinter escape into the atmosphere, and do not contribute to the formation of bubbles-cavities that formed in samples of mixtures with opoka. Based on this, quartz sand was excluded from further research on the synthesis of foams from it. From further studies with a silicate–sodium mixture based on opoka, the optimal concentration of sodium hydroxide was established, which was 12%, since with an alkali content of 10% or less, the structure of the foam glass material was insufficiently porous and the samples had a high density of >500 kg/m³. Higher concentrations (15; 17; 19%) led to the formation of a strongly friable structure of samples with large pores and cavities up to 10 mm connected by labyrinthine channels. At the same time, it is known that the optimal structure for a foam glass material is its structure, characterized by a large number of small pores up to 5 microns in size and isolated from each other by the thinnest walls. At the initial stage of foam glass formation, its structure resembles spherical foam, which is characterized by a slight saturation with gases and a large thickness of the separation walls [8,9,14,16]. Foam glass is a highly porous material (82–96%) consisting of airy cellular closed or communicating pores separated by partitions of a vitreous substance [14,16,32,33,34,35,36].

In our case, in the SiO₂-NaOH system, the formation of gases and the creation of a cellular structure of foam glass occurs through several chemical reactions. The main process involves the interaction of silica opoka (SiO₂) and sodium hydroxide (NaOH) at high temperatures. When a mixture of SiO₂ (silica opoka) and NaOH is heated, sodium silicate is formed, resulting in the release of water: SiO₂ + 2NaOH→Na₂SiO₃ + H₂O. This is the main reaction, which results in the formation of water-soluble sodium silicate (Na₂SiO₃), which can then participate in further processes. Hydrolysis of silicate or aluminosilicate compounds is also possible, which can lead to the release of hydrogen or carbon dioxide. It also helps to increase the porosity of the glass. But it should be noted here that the process of controlling gas release and bubble formation requires precise control of temperature, additive composition, and melt cooling rate, in order to obtain a uniform cellular structure of foam glass, which gives it its distinctive properties: lightness and thermal insulation characteristics [32,33,34,35,36,37,38,39,40,41,42,43,44,45].

Later, during the formation of foam glass, the crystalline phase at the foaming stage inhibits the uniform development of cells, and at the stabilization stage increases the structural and mechanical strength of the pyroplastic foam.

An experimentally obtained sample of foam glass material from a sodium silicate mixture based on opoka was subjected to physico-chemical and physico-mechanical studies. A micrograph and an elemental chemical analysis of an experimentally searchable optimal sample of a foam glass material obtained on the basis of an opoka is shown in Figure 4.

In Figure 4, a micrograph of the structure of the foam glass material clearly shows a porous structure with a pore size of 2–5 microns. The resulting sample was subjected to compression testing on a laboratory press (average—19.53 MPa, with an error of ±3–5%), and its thermal conductivity (average—0.063 W/(m °C) at 25 °C, with an error of ±2–5%), noise absorption (average—51.6 Db, with an error of ± 2–5%) and density, the average of which was 375 kg/m³ (with an error of ±1–3%), were determined. according to GOST 33949-2016, foam glass products are heat-insulating for buildings and structures [42].

The conducted studies correlate with similar, previously conducted studies by scientists and research centers in the field of building materials [14,16,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69]. However, in order to clarify and obtain accurate technological characteristics of the process for obtaining foam glass material from a silicate-sodium mixture, research will have to be conducted using multifactorial mathematical modeling.

4. Conclusions

Based on the conducted physical and chemical studies of the opoka from the Orangay deposit as a raw material component of the raw mix for obtaining foam glass, the following conclusions and proposals can be made:

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all the conducted physical and chemical studies of the studied opoka contribute to the complementarity of each other, and in most cases, they coincide and confirm the obtained results by correlating;

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using chemical analysis, the interval of the content of basic silicon oxide (SiO₂) in the studied opoka was established, which is presented in the range of 69–80%, contributing to the use of opoka as a raw material for the production of foam glass material;

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based on the X-ray phase analysis, the phase composition of the studied opoka was established with pronounced quartz peaks, the main diagnostic lines of which are the values of 3.3485, 4.2704, and 4.1329 A;

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as a result of X-ray energy-dispersive elemental microanalysis of the opoka, its elemental composition was established, which is represented by the sum of O₂, Si, and Al by 91.98%, Fe by 2.27%, and various impurities by 5.75%;

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the experimentally obtained sample of foam glass material consists of 93.37% Si, Al, Mg, and Na oxides, has a porous structure with a pore size of 2–5 microns, an average density of 375 kg/m³, thermal conductivity of 0.063 W/(m °C) at 25 °C, and noise absorption of −51.6 Db, and, according to GOST 33949-2016, foam glass products are heat-insulating for buildings and structures;

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the results obtained from the conducted complex of modern physical and chemical studies of opoka determine the role of opoka as the main raw material for obtaining a sodium silicate mixture and further synthesis of foam glass material based on this mixture with the possibility of eliminating the energy-intensive and labor-intensive process of high-temperature cooking and granulation of complex multicomponent glass mass and obtaining in the future an ideal inexpensive local building material used for heat and sound insulation of housing construction, increasing the level and quality of life of the population.

Experimental exploratory studies on the synthesis of foam glass based on opoka have been carried out. In this paper, the limitations are due to an experimental exploratory study of the production of foam glass. In the near future, a number of experiments will be conducted to obtain foam glass through mathematical modeling.