Development of Cement-Free Binder Systems Based on Metallurgical Waste: Hardening by Forced Carbonation

Abstract

This article explores the possibility of using metallurgical waste slags formed during the smelting of cast iron and steel as cementless binders that harden due to forced carbonization and the subsequent hydration processes of some minerals that form the basis of these slags. This study presents the results of multi-objective optimization using statistical methods of mathematical experimental design, with the purpose of obtaining a carbonized material with good mechanical and physical properties. As a result of the research, carbonized stone with compressive strength up to 116.5 MPa was obtained. Water absorption by weight is within the range of 6.0–17.0%, and quantitative CO₂ binding was 6–11.9%, depending on the type of slag. A pilot batch of wall product samples (hollow bricks and paving elements of various territories) was manufactured under production conditions. During the tests, we found that the compressive strengths of products based on BOF and EAF slags were 96.3 and 81.1 MPa, respectively, and that of bricks based on BS slag was 37.1 MPa. A comprehensive analysis of the performance properties of products from the pilot batch showed that these samples meet the requirements of national standards.

1. Introduction

In the context of rapidly increasing instability in the global economy and growing signs of the depletion of primary natural resources, there is a growing interest among international agencies in a new economic model. In the process of evolution and diversification, the industrial economy has almost never gone beyond one basic characteristic it acquired at the dawn of industrialization: a linear pattern of natural resource consumption that follows the “take–make–waste” principle. However, this linear model of production and consumption eventually led to environmental and climate problems on regional and global scales [1,2]. The rejection of linear principles of economic management, along with the decarbonization of the industrial and energy sectors of the economy, is one of the main factors determining the transformation of the current linear economy into a cyclical economy, which should become environmentally sustainable and fit into the material framework existing on our planet.

The essence of a cyclical economy is the attempt to create a system in which everything that is produced or used is fully recycled within the system, so that no environmental problems arise. Thus, those products that are considered waste in the traditional (linear) economy are assets in the cyclic economy; the main goal in this context is to lessen the consumption of primary resources from the environment and reduce to zero the amount returned to it in the form of waste, including gaseous waste. This model for the further development of the global community is, in fact, an innovative political challenge, which is focused on the development and implementation of new knowledge, taking into account the peculiarities of systems functioning and the restructuring of the existing linear system, without causing economic damage to the system.

An important aspect here is obviously the support of scientific research aimed at developing new production methods and technologies that can meet modern environmental and climatic challenges. Thus, one of the ways to reduce the carbon load on the atmosphere is the conversion of a number of industrial enterprises to “green” technologies. The most important strategy for reducing the carbon footprint is the development of effective methods of utilizing anthropogenic carbon dioxide as a raw component for the production of certain products. To maximize CO₂ binding and substantially reduce the carbon footprint, products should be in demand and involved in mass consumption, such as building materials and products. In this case, the carbon dioxide produced during the technological processes of production should be absorbed and bound into thermodynamically stable compounds.

It is necessary to emphasize the special role of the industry surrounding construction materials and products, which is the most material-intensive of industries in terms of the volume and variety of resources consumed, both on its own and as part of a range of other industries, including those that determine the current state of the economy and the potential for its development.

Various technogenic wastes of ferrous and non-ferrous metallurgy (i.e., slags and sludge) are perspective raw materials with the ability to bind gaseous CO₂. Conducting scientific research in this area is especially relevant for the countries that are world leaders in the production of steel and non-ferrous metallurgy products. The metallurgical industry generates 7–9% of the world’s CO₂ emissions [3]; at the same time, it is the source of a significant amount of solid slag. Meanwhile, the generated CO₂ is emitted into the atmosphere, and the utilization rate of slags, especially steelmaking slags, remains rather low. There are two main reasons for the low utilization rate of steelmaking slags: first, low or absent hydraulic activity, and, second, the content of a large amount of free calcium and magnesium oxides (CaO, MgO) and thermodynamically unstable phases (β-C₂S), which can cause harmful expansion and volume instability [4,5]. According to recent research [6,7], the forced carbonization of various types of steelmaking slag increases the volumetric stability of the final product; thus, it is an effective means of overcoming these problems. Thus, a synergistic approach to utilizing slag from the steel industry, in order to absorb CO₂ from the waste gases of the steel industry for the production of a wide range of building products, is an effective strategy within the transition to a cyclical economy.

A number of studies have shown that the forced carbonization of steelmaking slag products is a potential method for producing environmentally friendly building materials and CO₂ binding [8]. The main chemical reactions that occur during forced carbonization of steelmaking slags are described by the following equations [9,10,11,12]:

Ca(OH)₂ + CO₂ + H₂O → CaCO₃ + 2H₂O

(1)

3CaO·SiO₂ + (3 − x)CO₂ + yH₂O →xCaO SiO₂·yH₂O + (3 − x)CaCO₃

(2)

2CaO·SiO₂ + (2 − x)CO₂ + yH₂O →xCaO SiO₂·nH₂O + (2 − x)CaCO₃

(3)

Furthermore, calcium silicate hydrate continues to undergo carbonation involving the decalcification of C–S–H, which eventually produces silica gel and CaCO₃. A significant portion of global research into the carbonization of metallurgical slags is devoted to exploring phase and structural transformations. Most of the slags formed during the smelting of steels [13,14] and ferroalloys [15,16] have been studied. All researchers indicate that carbonization leads to the reduction of β-C₂S and γ-C₂S, C₃S, with the simultaneous formation of various polymorphs of calcium carbonate. The existing research shows that, on the one hand, carbonization curing is an ideal method for safely isolating and storing CO₂ [17,18] as thermodynamically stable products such as CaCO₃; on the other hand, the reaction products fill the pores and improve the mechanical performance and durability of carbonized materials [19,20]. According to [21], after two hours of carbonization at a constant temperature of 80 °C and a CO₂ pressure of 20 bar, experimental samples with compressive strength up to 50 MPa were produced.

There is also research devoted to the study of the carbonization features of minerals constituting various steelmaking slags. W.J. Huijgen et al. [22] studied CO₂ sequestration using minerals constituting the basis of BOF slags during the direct carbonization process. The results showed that the temperature and CO₂ pressure required for the carbonization of steelmaking slag was lower than, for example, natural minerals such as serpentine and wollastonite. Slag with a particle size smaller than 38 μm was subjected to carbonization reactions for 30 min at 100 °C and a CO₂ pressure of 1.9 MPa, and the calcium conversion efficiency reached 74%. It was shown in [23,24,25,26] that the efficiency of CO₂ binding can be increased by reducing the particle size of slag and selecting its optimum moisture content. According to [24], increasing the Blaine number from 125 to 529 m²/kg resulted in a significant increase in compressive strength from 28.8 to 72.1 MPa under identical conditions of curing by forced carbonization.

Other studies [27,28,29,30,31,32,33] found that the CO₂ pressure in the carbonization reactor, temperature, liquid-to-solid ratio, and forced carbonization time have a significant effect on CO₂ conversion, as well as the physical and mechanical characteristics of the resulting carbonized material. For instance, using electric arc furnace slag, the amount of bound CO₂ reached 52 kg/t slag at an ambient temperature and a CO₂ pressure of 1.1 MPa with a reaction time of only 10 min. When the temperature was increased to 160 °C and the CO₂ pressure to 4.8 MPa during a 12 h reaction, the amount of bound CO₂ reached 283 kg/t of slag.

On the basis of the scientific studies presented above, and taking into account the authors’ own studies [12,34,35,36],

Table 1. Factors affecting the carbonization of steelmaking slags.

No.	Factors	Control in the Technological Cycle
1	Slag phase composition	Raw material requirements
2	Particle size	Raw material requirements
3	Chemical additives (activators, catalysts)	Raw material requirements
4	Parameters of forming materials, products	Forming condition
5	The water content of the molding mixture (W/T)	Forming condition
6	Carbonization temperature	Forced carbonization conditions
7	Carbonization time	Forced carbonization conditions
8	CO2 pressure	Forced carbonization conditions
9	CO2 concentration	Forced carbonization conditions
10	Relative humidity	Forced carbonization conditions
11	pH of the environment	Forced carbonization conditions

presents a list of factors affecting the forced carbonization of various steelmaking slags, as well as the qualitative and quantitative results at the end of the process.

Thus, a comprehensive analysis of existing research in the field of forced carbonization of steelmaking slags indicates that this area of research is promising in terms of the management of both the solid waste from metallurgy (slag) and its gaseous emissions (carbon dioxide). It should be noted that some of the factors presented in Table 1 (CO₂ pressure, carbonization temperature) make the technological cycle significantly more complicated in terms of the equipment used in the proposed technology. In this regard, the purpose of this study was to develop cement-free binder systems based on curing metallurgical waste using forced carbonization. The main priority of the research was the selection of factors from the list (see Table 1), which eventually contributed to the development of the forced carbonization technology that incurs the lowest material and energy costs during operation, allowing us to obtain quality building products.

2. Materials and Methods

Various steelmaking slags and slimes from metallurgical enterprises located in the Russian Federation were used as raw materials for this scientific research:

-

BOF slag from Cherepovets Steel Mill of the public joint stock company “Severstal” (CherSM PJSC “Severstal”) with an initial particle size 5–30 mm (hereafter BOF slag);

-

Slag from the electric arc furnace of the A.A. Ugarov Oskol Electrometallurgical Plant JSC. LLC MC Metalloinvest with an initial particle size of 0.16–10 mm (hereafter referred to as EAF slag);

-

Nepheline sludge, which is a large-tonnage byproduct of producing alumina using the alkaline method from nepheline concentrates of the Kolsky Peninsula of LLC Pikalevsky Alumina Plant, with an initial particle size of 0.16–2 mm (hereafter referred to as BS slag). A general view of the initial slags under consideration is presented in Figure 1.

All slag samples were taken from the production shops of the above-mentioned enterprises; they were then placed in hermetically sealed packaging and delivered to the laboratory for further studies. After delivery to the laboratory, the slags were pre-dried to a constant mass at the temperature of 95 °C. The input control of the initial characteristics of the studied slags was carried out after drying and preliminary preparation (grinding). A chemical analysis of the slags was carried out via X-ray fluorescence analysis (XRF) on an ED spectrometer Epsilon 3XLE (PANalytical, Eindhoven, The Netherlands). The results of the analysis are presented in

Table 2. Chemical composition of the slag.

Name of Slag	Content if Recalculated as Oxides (%)
	CaO	MgO	SiO2	Al2O3	Fe2O3	Na2O	K2O	MnO	TiO2	Cr2O3	SO3	Total (%)
BOF	45.88	5.38	20.22	5.44	17.65	-	0.30	2.22	1.07	0.34	0.76	99.26
EAF	41.48	8.76	25.63	5.29	16.82	-	0.09	0.77	0.31	0.37	0.23	99.75
BS	62.07	1.27	28.79	3.77	2.45	0.38	0.86	0.08	0.32	-	-	99.99

.

To conduct the analysis, all slag samples were pulverized in a drum ball mill with a capacity of 100 L (LBM 100, Saint Petersburg, Russia). The specific surface of the obtained slag powders ranged from 280 to 310 m²/kg. The specific surface was controlled using the air permeability method on an automatic Blaine apparatus manufactured by model 1.0297E (Testing, Berlin, Germany). The granulometric composition of the slag particles after pulverization was determined using a laser diffraction analyzer Partica LA-960, (Horiba, Kyoto, Japan). The granulometric composition of the slag samples is shown in Figure 2, Figure 3 and Figure 4 in the form of graphs of the integral and differential particle size distributions.

The geometric dimensions of the pulverized slags were 19–27 μm, and the numerical values of the fractional distribution are presented in

Table 3. Particle size distribution in pulverized slag.

Name of Slag	Particle Size, µm									Average Geometric Particle Size, µm
	≤1.0	≤2.0	≤4.0	≤10.0	≤20.0	≤50.0	≤100.0	≤200.0	≤300.0
BOF	1.5	5.1	11.5	31.2	50.0	68.5	86.0	97.7	100.0	21.0
EAF	0.5	7.0	14.0	32.5	50.5	68.5	84.0	95.6	100.0	19.8
BS	1.1	2.7	6.1	24.0	48.0	63.0	77.0	94.0	100.0	27.3

.

Studies were conducted on cylindrical samples that were made by pressing a molding mixture consisting of pulverized slag, which was thoroughly mixed in a high-speed mixer, and the required amount of water by model 1.0206.07, 5L (Testing, Berlin, Germany). The refined compositions of the molding mixture were determined by the experimental conditions and are presented below. Experimental sample cylinders with a diameter of 30 mm and height of 30 ± 2 mm were molded by squeezing the prepared raw material mixture in metal molds on a hydraulic press with simulated double-sided counter pressing by model 21.2401.01-SV-i20 15/30 kH (Testing, Berlin, Germany).

The analysis of the complex influence of the technological elements of sample fabrication, as well as the mode of their forced carbonization, was carried out using statistical methods for the mathematical planning of the experiment [37,38]. Rotatable central compositional planning (RCCP) was used. In the rotatable central compositional planning of the experiment, the plan points are located on three spheres: cube points (Nφ), “star points” (Nα), and center points (No) (sphere of zero radius). Rotatable plans produce model coefficients that predict object output values with equal accuracy in all directions at the same distance from the center of the plan. The processing of the obtained experimental data was carried out using the Stat Soft STATISTICA (Version 12) software package. As a result of experimental data processing, the experimental–statistical (ES) models of each studied parameter are formed, which are second-order equations with the following form:

$$\mathrm{Y}={\mathrm{b}}_0+\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{b}}_{\mathrm{i}}{\mathrm{z}}_{\mathrm{i}}+\sum _{\mathrm{i};\mathrm{l}=1}^{\mathrm{n}}{\mathrm{b}}_{\mathrm{i}\mathrm{l}}{\mathrm{z}}_{\mathrm{i}}{\mathrm{z}}_{\mathrm{l}}+\sum {\mathrm{b}}_{\mathrm{i}\mathrm{i}}{\mathrm{z}}_{\mathrm{i}}^2.$$

(4)

where b₀, b_i, b_il, b_ii are correlation coefficients determined based on the mathematical and statistical processing of the experimental data; z_i and z_l are the values of varying formulation and technological factors, respectively.

The significance of the correlation coefficients of the ES models was determined using the Student’s criterion. The adequacy of the object description by the second-order equation was assessed using Fisher’s F-criterion.

The factors in the experiment were chosen based on the conclusions obtained when analyzing the formation of the structure and the properties of artificial stone, according to previous studies [12,36]. Taking into consideration the influence of multiple factors (see Table 1) affecting the process of the forced carbonization of various slags and the formation of material properties, the following factors were varied in the present experiment: the molding-specific pressure for pressing the experimental cylinders (Z₁), the initial water content of the raw material mixture (Z₂), and the time of forced carbonization (Z₃). The experimental planning conditions are presented in

Table 4. Evaluation of the quantitative content of the main minerals in the studied slags before carbonization.

Factor	Unit of Measurement	Code	Variation Levels					Variation Interval
			−1.682	−1	0	1	1.682
Pressing pressure	MPa	Z1	3.18	10.0	20.0	30.0	36.82	10.0
Water content of the mixture	%	Z2	4.95 (1.95)	7.0 (4.0)	10.0 (7.0)	13.0 (10.0)	15.05 (12.05)	3.0
Carbonization time	min	Z3	49.08	90.0	150.0	210.0	250.92	60.0

Note: (7.00)—values in brackets are for BOF slag only.

.

The experiment planning matrix in the coded and natural expression of variables is presented in

Table 5. Experiment planning matrix in coded and natural terms of variables.

No.	Plan of Experiment
	Point Groups	Planning Matrix			Natural Values of Variables
		Z1	Z2	Z3	Pressing Pressure of the Mixture (MPa)	Water Content of the Mixture (%)	Forced Carbonization Time (min)
1	Nφ (cube points)	−1	−1	−1	10.00	7.00 (4.00)	90.00
2		1	−1	−1	30.00	7.00 (4.00)	90.00
3		−1	1	−1	10.00	13.00 (10.00)	90.00
4		1	1	−1	30.00	13.00 (10.00)	90.00
5		−1	−1	1	10.00	7.00 (4.00)	210.00
6		1	−1	1	30.00	7.00 (4.00)	210.00
7		−1	1	1	10.00	13.00 (10.00)	210.00
8		1	1	1	30.00	13.00 (10.00)	210.00
9	Nα (star points)	−1.682	0	0	3.18	10.00 (7.00)	150.00
10		1.682	0	0	36.82	10.00 (7.00)	150.00
11		0	−1.682	0	20.00	4.95 (1.95)	150.00
12		0	1.682	0	20.00	15.05 (12.05)	150.00
13		0	0	−1.682	20.00	10.00 (7.00)	49.08
14		0	0	1.682	20.00	10.00 (7.00)	250.92
15	N0 (center points)	0	0	0	20.00	10.00 (7.00)	150.00
16		0	0	0	20.00	10.00 (7.00)	150.00
17		0	0	0	20.00	10.00 (7.00)	150.00
18		0	0	0	20.00	10.00 (7.00)	150.00
19		0	0	0	20.00	10.00 (7.00)	150.00
20		0	0	0	20.00	10.00 (7.00)	150.00

Note: (1.95)—values in parentheses are for BOF slag only.

. The value of the analyzed optimization parameter was determined as the arithmetic mean of six experimental samples.

The forced carbonization of the experimental samples was carried out in the carbonization unit designed by the authors, with the automatic control and maintenance of the required CO₂ concentration. The carbonization of the samples was carried out under normal conditions (~293 K, atmospheric pressure) at ~60% vol. of CO₂ concentration. The scheme of the developed chamber and its general view are presented in Figure 5. The carbonization chamber is a metal container made of stainless steel with a hydraulic jacket and a hermetically sealed lid; it can take overpressure up to 1.0 MPa. There is a fan inside the chamber for the mixing and uniform distribution of carbon dioxide in the internal space. A vacuum pump is connected to the chamber to pump out air from the internal volume and create the required concentration of CO₂ in the chamber. The system of pressure regulation and carbon dioxide supply to the carbonization chamber includes a cylinder with carbon dioxide and a gas pressure reducer. High-pressure liquid carbon dioxide in tanks was used as a source of carbon dioxide in the laboratory studies.

According to the experimental plan (see Table 5), the following parameters were analyzed as the key parameters of the experimental carbonized samples: compressive strength (R_c), average density (ρ_o), water absorption by mass and volume (W_m and W_v), water resistance (K_S), and total porosity (P). Compressive strength, average density, water absorption by mass, and volume were determined according to standard methods for building materials. The water resistance of the test samples, i.e., the ability of the material to retain its operational properties under prolonged exposure to water, was evaluated according to the so-called softening coefficient, K_S, which is the ratio of the compressive strength of the sample in the water-saturated state, R_w, to the compressive strength of the material in the dry state, R_d, according to the following formula:

K_S = R_w/R_d

(5)

Materials with a K_S value greater than 0.8 are considered water resistant. The amount of CO₂ absorbed by samples in % by weight per unit mass of slag was determined for each individual sample according to the following formulas:

$${\mathrm{m}}_{{\mathrm{C}\mathrm{O}}_2}=\left({\displaystyle \frac{{\mathrm{m}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}.}-{\mathrm{m}}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{b}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}}}{{\mathrm{m}}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}^{\mathrm{d}\mathrm{r}\mathrm{y}}}}-1\right)\times 100\mathrm{\%}$$

(6)

where m_carb. is the mass of the dry carbonized sample, g;${\mathrm{m}}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}^{\mathrm{d}\mathrm{r}\mathrm{y}}$ is the mass of dry slag powder in the sample, g; and ${\mathrm{m}}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{b}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}}$ is the mass of the sample including chemically bound water as a result of hydration of the sample material, g, which is determined by the formula:

$${\mathrm{m}}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{b}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}}={\mathrm{m}}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}^{\mathrm{d}\mathrm{r}\mathrm{y}}\times {\displaystyle \frac{{\mathrm{W}}_{\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{y}\mathrm{b}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}}}{100}}$$

(7)

where W_{chamically bound} is the amount of chemically bound water in the sample, % by weight, as determined by the following formula:

$${\mathrm{W}}_{\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{y}\mathrm{b}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}}=\left({\displaystyle \frac{{\mathrm{m}}_{\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}^{\mathrm{d}\mathrm{r}\mathrm{y}}-{\mathrm{m}}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}^{\mathrm{d}\mathrm{r}\mathrm{y}}}{{\mathrm{m}}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}^{\mathrm{d}\mathrm{r}\mathrm{y}}}}\right)\times 100\%$$

(8)

where ${\mathrm{m}}_{\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}^{\mathrm{d}\mathrm{r}\mathrm{y}}$ is the mass of dry carbonized sample, g.

All the samples were dried in a desiccator to constant weight at 90 °C before testing.

3. Results

Thermal analysis of the original slags, as well as the carbonized samples, was carried out using synchronous TG-DTA/DSC analysis on a high-temperature analyzer STA 8000 (Perkin Elmer, Waltham, MA, USA) in the temperature range of 30–1000 °C at a heating rate of 20 °C/min in a dynamic nitrogen environment. This analysis makes it possible to simultaneously record the change in heat flux and sample mass as a function of programmable temperature under a controlled atmosphere, which allows the temperature ranges of physicochemical transformations and phase transitions to be determined with high accuracy. The calculation of mass change on the TG curve was carried out in the Pyris (Version 11) program complex (Perkin Elmer). Thermograms of original slags are presented in Figure 6, Figure 7 and Figure 8.

The thermogram of BOF slag (see Figure 6) has two minor endothermic effects at temperatures of 468 and 740 °C. The endothermic effect in the temperature range of 450 to 560 °C, with a maximum at 467.8 °C, probably corresponds to the removal of chemically bound water, and the endothermic effect with a maximum at 740 °C with a mass loss of about 1.1% corresponds to the decomposition of calcium carbonate. It is notable that the mass of the sample increases above 450 °C and 750 °C. Apparently, under the influence of temperature, phase transformations occur in the sample, accompanied by a change in the crystal lattice and a possible change in mass. The total mass loss of the sample in the range of 30–780 °C was 1.7%.

The thermogram of EAF slag (see Figure 7) identifies endothermic mass loss effects at the following temperatures: 417; 480; and 734 °C. The endothermic effects at 417 and 480 °C are caused by the removal of chemically bound water. The endothermic effect with a maximum at 734 °C corresponds to the decomposition of calcium carbonate. The slight exothermic effect at 840 °C is probably related to the phase transition of dehydrated calcium silicates into wollastonite. The total mass loss of the sample in the range of 30–780 °C was 6.1%.

The thermogram of BS slag (see Figure 8) contains two minor endothermic effects with peaks at 143 and 275 °C, corresponding to the removal of bound water from hydrate compounds, as well as an endothermic effect with an extremum at 728 °C, corresponding to the decomposition of calcium carbonate. The slight exothermic effect at 846 °C can probably be attributed to the phase transition of dehydrated calcium silicates into wollastonite. The total mass loss of the sample in the range of 30–780 °C was 3.2%.

X-ray phase analysis of slags before and after carbonization was conducted on a high-resolution diffractometer “Ultima IV” (Rigaku, Tokyo, Japan) with a cobalt anode for the precise determination of the qualitative and quantitative phase composition of polycrystalline materials, including those with high iron content. Qualitative and semi-quantitative analyses were carried out using the PDXL program package (version 2) with the ICDD PDF2 database. The corundum number method (RIR) was used to determine the semi-quantitative ratio of each phase in the studied sample.

Table 6. Estimation of the quantitative content of basic minerals in the studied slags before carbonization.

		Weight (%)
Phase	Formula	BOF	EAF	BS
Okermanite	Ca2(Mg0.50Fe0.20Al0.30)(Fe0.25Al0.21)Si1.54O7	61.5	-	-
γ-Belite (Shannonite)	γ-Ca2SiO4	23.6	11.0	-
β-Belite (Larnite)	β-Ca2SiO4	-	-	90.0
Merwinite	Ca3Mg(SiO4)2	-	30.0	-
Hedenbergite	CaFeSi2O6	-	9.0	-
Fayalite	γ-Fe2SiO4	-	11.0	-

presents the results of the X-ray phase analysis in the form of the minerals that make up the basis of the studied slags.

According to the XRD analysis data, the examined slags are represented by different minerals, which are formed in the melt depending on the technology of steel production, as well as alumina. Thus, BOF slag is mainly represented by ockermanite and belite of γ modification (shannonite). EAF slag is mainly represented by mervinite, belite γ modification (shannonite), and gedenbergite and fayalite. BS slag is essentially a monomineral and consists mainly of belite of β modification (larnite).

The experimental data regarding the properties of the test samples made of BOF slag, according to the experimental plan (see Table 5), are presented in

Table 7. Experimental data of the properties of the test samples made of BOF slag.

No. g	Variable Factors in the Natural Expression			Optimized Sample Parameters
	Z1 (MPa)	Z2 (%)	Z3 (min)	Non-Carbonized			Carbonized
				Rc (MPa)	ρo (kg/m3)	P (%)	Rc (MPa)	ρo (kg/m3)	mCO2 (%)	P (%)	KS	Wm (%)	Wv (%)
1	10.00	4.00	90.00	Destroyed in the drying	2338	33.5	36.8	2445	4.8	28.8	1.00	10.9	26.5
2	30.00	4.00	90.00		2487	29.4	78.1	2615	5.0	23.7	0.84	8.3	21.6
3	10.00	10.00	90.00		2409	32.1	65.7	2526	6.9	24.6	0.59	8.6	21.6
4	30.00	10.00	90.00		2558	28.5	74.3	2664	6.0	20.6	0.59	6.9	18.4
5	10.00	4.00	210.00		2346	33.6	47.4	2448	5.1	27.9	0.68	10.6	25.8
6	30.00	4.00	210.00		2515	29.5	81.7	2616	5.2	22.8	0.72	8.3	21.7
7	10.00	10.00	210.00		2402	31.6	66.6	2542	7.4	24.4	0.60	8.3	21.0
8	30.00	10.00	210.00		2542	27.8	93.8	2689	7.4	19.4	0.70	6.3	16.8
9	3.18	7.00	150.00		2183	38.0	27.3	2286	7.0	32.1	0.59	12.4	28.2
10	36.82	7.00	150.00		2529	27.4	99.8	2681	7.0	19.5	0.73	6.5	17.4
11	20.00	1.95	150.00		2442	30.4	29.2	2504	3.3	27.5	0.73	10.6	26.5
12	20.00	12.05	150.00		2585	27.2	- **	2583	1.7	26.1	- **	- **	- **
13	20.00	7.00	49.08		2446	30.8	74.0	2563	6.0	24.0	0.60	8.4	21.6
14	20.00	7.00	250.92		2574	30.6	79.2	2574	6.6	22.5	0.66	8.0	20.6
15 *	20.00	7.00	150.00		2453	30.1	80.2	2587	6.6	22.8	0.66	7.9	20.3

Note: 15 *—average values of indicators of the whole group of “zero” points from 15 to 20 are indicated. - **—samples were destroyed in the process of drying, after their carbonization.

.

The calculated correlation coefficients of the ES models, obtained through the statistical processing of the experimental data, are presented in

Table 8. Correlation coefficients of ES models of changes in the basic properties of experimental samples made of BOF slag.

Coefficient	Coefficients of ES Models of the Studied Parameters
	Rc, MPa	ρo (kg/m3)	mCO2 (%)	P (%)	KS	Wm (%)	Wv (%)
b0	76.6	2576.7	6.57	22.9	0.69	8.0	20.6
b1	34.1	188.5	−0.09	−5.91	0.03	−2.7	−5.1
b2	2.1	62.9	0.74	−2.42	−0.13	−1.19	−2.4
b3	6.3	9.3	0.50	−0.83	−0.03	−0.28	−0.66
b11	−2.4	−50.3	0.59	1.46	0.00	0.60	0.72
b22	−35.5	−7.9	−2.58	2.17	0.03	1.42	3.63
b33	6.8	9.8	0.14	−0.33	−0.02	−0.25	−0.48
b12	−9.9	−13.3	−0.33	0.34	0.06	0.30	0.41
b13	2.9	7.8	0.23	0.22	0.08	0.00	−0.05
b23	1.6	9.3	0.36	0.09	0.14	−0.17	−0.35

.

The analysis of the test results of the BOF slag samples that underwent forced carbonization (see Table 7) shows that R_c ranges from 36.8 to 100 MPa depending on the technological factors of production. According to the experimental data, the maximum values of the amount of bound CO₂ reaches 7.4% wt. According to the ES model (see Table 8), the parameter of the amount of CO₂, binding mostly depends on the initial water content of slag (Z₂) and the duration of forced carbonization (Z₃).

The strength characteristics of the cylinder samples increase with increasing specific pressing pressure and the duration of carbonization. The most significant technological factor affecting the strength properties of the samples that underwent forced carbonization is the pressing pressure (Z₁). According to the data, the optimum water content of the molding mixtures to achieve maximum strength properties is in the range of 7 to 10 % by weight. The K_S parameter mostly depends on the factor (Z₂): as it increases, the water resistance decreases. The lowest value of water absorption by mass is 6.3 % by weight (see Table 7). According to the ES model (see Table 8), water absorption by mass and volume decreases with increasing values of all technological factors (Z₁, Z₂, Z₃).

Experimental data regarding the properties of the test samples made of EAF slag, according to the experimental plan (see Table 5), are presented in

Table 9. Experimental data of the properties of the test samples made of EAF slag.

No. g	Variable Factors in the Natural Expression			Optimized Sample Parameters
	Z1 (MPa)	Z2 (%)	Z3 (min)	Non-Carbonized			Carbonized
				Rc (MPa)	ρo (kg/m3)	P (%)	Rc (MPa)	ρo (kg/m3)	mCO2 (%)	P (%)	KS	Wm (%)	Wv (%)
1	10.00	7.00	90.00	2.7	1958	37.4	57.0	2112	7.2	29.1	0.8	13.2	27.7
2	30.00	7.00	90.00	4.8	2076	33.1	86.5	2263	7.3	24.5	0.9	10.3	23.2
3	10.00	13.00	90.00	3.1	1997	35.8	71.0	2177	15.2	26.6	0.8	11.0	24.1
4	30.00	13.00	90.00	5.1	2154	31.0	7.3	2180	8.0	29.7	0.7	12.4	27.0
5	10.00	7.00	210.00	2.9	1941	37.2	54.6	2110	7.3	30.0	0.9	12.7	27.0
6	30.00	7.00	210.00	6.7	2030	32.2	116.5	2275	7.1	23.7	0.7	10.4	23.7
7	10.00	13.00	210.00	2.5	1992	36.0	72.9	2185	16.1	26.5	0.8	11.0	24.1
8	30.00	13.00	210.00	6.2	2145	31.0	8.4	2202	9.2	28.1	0.2	11.9	26.2
9	3.18	10.00	150.00	0.4	1826	41.7	37.0	1967	11.6	37.2	0.8	16.0	31.3
10	36.82	10.00	150.00	7.8	2178	30.6	28.8	2234	6.6	27.8	0.6	10.5	23.5
11	20.00	4.95	150.00	4.5	2057	33.3	77.2	2194	4.3	27.1	0.8	12.1	26.5
12	20.00	15.05	150.00	3.2	2116	29.8	7.7	2183	10.8	28.9	0.6	12.4	27.1
13	20.00	10.00	49.08	5.3	2066	33.5	58.4	2194	9.4	28.2	0.8	11.4	25.0
14	20.00	10.00	250.92	5.4	2056	32.9	101.8	2242	11.4	25.8	0.7	10.1	22.8
15 *	20.00	10.00	150.00	5.3	2060	33.1	96.4	2235	11.7	24.7	0.8	10.5	23.5

Note: 15 *—average values of indicators of the whole group of “zero” points from 15 to 20 are indicated.

.

Calculated correlation coefficients of ES models, obtained through the statistical processing of the experimental data, are presented in

Table 10. Correlation coefficients of ES models of changes in the basic properties of the experimental samples made of EAF slag.

Coefficient	Coefficients of ES Models of the Studied Parameters
	Rc, MPa	ρo (kg/m3)	mCO2 (%)	P (%)	KS	Wm (%)	Wv (%)
b0	95.4	2231	11.6	24.96	0.78	10.45	23.4
b1	−7.73	102.4	−3.29	−3.22	−0.17	−1.78	−2.33
b2	−40.15	7.54	4.48	0.99	−0.15	0.04	0.11
b3	15.52	5.08	0.79	−0.84	−0.11	−0.43	−0.69
b11	−40.28	−79.0	−1.45	4.47	−0.04	1.74	2.49
b22	−33.54	−16.8	−2.54	1.29	−0.05	1.03	2.07
b33	−6.92	4.03	−0.56	0.59	−0.016	−0.005	0.02
b12	−55.49	−95.5	−3.51	3.91	−0.135	1.85	3.21
b13	8.46	28.5	0.02	−1.799	−0.185	0.02	0.09
b23	−5.58	−16.5	0.54	−0.465	−0.095	−0.05	−0.13

.

The analysis of experimental data presented in Table 9 shows that forced carbonization significantly changes the properties of the experimental samples (cylinders of semi-dry pressing from EAF slag). The compressive strength of the samples after their forced carbonization increases by 1.35 (point 8)–92.5 (point 9) times. From the entire dataset, points 4, 8, and 12 stand out: these are the samples that, according to the plan, were obtained at the maximum values of water content and specific pressing pressure. The R_c of these samples is much lower than that of the other matrix samples and slightly exceeds the strength of the non-carbonized samples. The increased water content of the molding mixture and the density of the initial structure of the samples prevent the diffusion of CO₂ in the samples and the carbonization reaction. In addition, the maximum values of the amount of bound CO₂ are observed in the samples obtained at higher values of slag water content and at the minimum forming pressures of the experimental samples (points 3 and 7), which indicates the significant influence of slag water content on the effective course of the carbonization reactions of slag minerals.

As a result of the forced carbonization, the porosity of the samples is significantly reduced in comparison with the non-carbonized samples. The size and number of pores in the material of the samples are reduced due to the filling of the pores with new carbonate formations. Therefore, these samples have higher values of compressive strength, average density, and water resistance, and lower values of water absorption. It should be noted that the porosity of the samples is mainly (about 90%) open, which is evident from the water absorption by volume, i.e., the samples remain permeable to CO₂ and further carbonization.

The evaluation of the effect of individual technological factors on the formation of the properties of the experimental samples from EAF slag (see Table 10) shows that the most significant technological factors affecting the formation of compressive strength are the initial water content of the molding mixture Z₂ and the duration of forced carbonization Z₃. With the increase in the factor Z₂, compressive strength decreases, and the increase in the factor Z₃ contributes to the increase in compressive strength. The most significant factors affecting the amount of bound CO₂, according to the ES model, are Z₁ and Z₂. As Z₁ increases, the parameter of bound CO₂ decreases, and, with increasing Z₂, it increases. Of all the technological factors considered, the duration of forced carbonization (Z₃) of the experimental samples has the least influence on the increase in this parameter.

The water resistance (K_S) of samples depends on all of the studied factors almost equally. The most significant factor is Z₁, followed by Z₂ and Z₃. According to the ES model, with the increase in all of the investigated factors, the K_S of experimental samples decreases. The water absorption by mass of the experimental samples falls within the range of 10 to 13%. Samples obtained at the minimum value of Z₁ are characterized by the highest water absorption (16.0 wt. %), which is explained by the higher porosity of these samples (more than 37%). In general, according to the ES model, the water absorption of the experimental samples increases with an increase in the pressure used to form the samples and decreases as the carbonization time decreases.

Experimental data regarding the properties of the experimental samples made of BS slag, according to the experiment plan (see Table 5), are presented in

Table 11. Experimental data of the properties of the test samples made of BS slag.

No. g	Variable Factors in the Natural Expression			Optimized Sample Parameters
	Z1 (MPa)	Z2 (%)	Z3 (min)	Non-Carbonized			Carbonized
				Rc (MPa)	ρo (kg/m3)	P (%)	Rc (MPa)	ρo (kg/m3)	mCO2 (%)	P (%)	KS	Wm (%)	Wv (%)
1	10.00	7.00	90.00	1.6	1750	41.4	32.0	1899	8.9	34.7	0.92	17.3	32.9
2	30.00	7.00	90.00	4.7	1867	41.0	70.6	2044	6.8	28.4	0.81	13.9	28.4
3	10.00	13.00	90.00	1.8	1733	45.5	69.3	1972	16.2	28.6	0.77	13.7	27.1
4	30.00	13.00	90.00	5.3	1859	41.5	26.6	1991	9.5	31.1	1.13	15.0	30.0
5	10.00	7.00	210.00	1.6	1746	44.7	31.6	1896	9.0	34.3	0.92	17.6	33.4
6	30.00	7.00	210.00	6.3	1874	40.5	63.3	2042	9.4	27.9	1.01	13.9	28.5
7	10.00	13.00	210.00	1.8	1702	45.2	69.3	1985	16.3	27.1	0.91	13.6	27.0
8	30.00	13.00	210.00	5.9	1862	40.5	43.5	2030	10.0	29.3	0.84	11.5	23.3
9	3.18	10.00	150.00	0.4	1595	48.8	23.1	1774	11.2	37.0	0.75	19.9	35.3
10	36.82	10.00	150.00	7.0	1899	39.6	55.1	2068	9.6	28.0	0.85	11.3	23.4
11	20.00	4.95	150.00	3.3	1821	42.2	34.4	1951	6.7	33.9	0.89	16.7	32.7
12	20.00	15.05	150.00	4.2	1831	41.8	59.2	2068	15.4	25.8	0.84	11.5	23.8
13	20.00	10.00	49.08	3.4	1817	41.8	72.3	2034	12.2	27.2	0.77	13.3	27.0
14	20.00	10.00	250.92	3.7	1832	41.3	89.9	2067	14.1	26.0	0.63	12.1	25.0
15 *	20.00	10.00	150.00	4.3	1815	41.6	79.4	2058	13.6	25.4	0.72	12.1	24.9

Note: 15 *—average values of indicators of the whole group of “zero” points from 15 to 20 are indicated.

.

Calculated correlation coefficients of the ES models obtained through the statistical processing of the experimental data are presented in

Table 12. Correlation coefficients of ES models of changes in the basic properties of the experimental samples made of BS slag.

Coefficient	Coefficients of ES Models of the Studied Parameters
	Rc, MPa	ρo (kg/m3)	mCO2 (%)	P (%)	KS	Wm (%)	Wv (%)
b0	79.1	2058.8	13.62	25.34	0.7	10.11	24.82
b1	8.0	123.5	−2.52	−3.38	0.04	−2.5	−4.42
b2	6.7	42.1	4.77	−3.35	0.02	−1.23	−4.52
b3	6.0	15.9	0.94	−0.94	−0.03	−0.01	−1.43
b11	−30.4	−100.6	−2.54	5.19	0.11	3.38	3.51
b22	−22.6	−38.1	−2.06	3.29	0.13	2.68	2.76
b33	1.26	−9.1	−0.58	0.99	0.04	0.5	1.14
b12	−33.6	−58.3	−2.83	4.37	0.03	1.57	2.15
b13	2.3	8.3	0.76	−0.12	−0.02	−0.92	−1.73
b23	6.2	15.8	−0.52	−0.59	−0.08	−1.03	−1.87

.

According to the data (see Table 11), the R_c of the experimental samples (cylinders made of BS slag after forced carbonization) increases from 5 (point 4) to 58 (point 9) times. When evaluating the obtained dataset, it is impossible to estimate unambiguously and reveal a clear pattern of the dependence of R_c on individual varying technological factors of the experimental samples. In general, the strength characteristics of the cylinder samples increase with the increase in each of the studied technological factors separately; however, the coefficient of regression dependence b₁₂, describing the joint influence of factors (Z₁) and (Z₂) on the parameter R_c, has a negative value (−33.6). Accordingly, by simultaneously increasing the pressing pressure and water content, the final strength of the samples decreases.

It is important to note the high values of the index of the amount of CO₂ bound by the experimental samples during their forced carbonization. This value reaches 16.2% by weight. This is mainly due to the high content of belite (see Table 4) in the mineralogical composition of this slag, as well as the high initial porosity of the experimental samples, which ranges from 39.6 to 48.8%. According to the ES model (see Table 12), the parameter of the amount of bound CO₂ depends to the greatest extent on the initial water content of BS slag (Z₂), as well as on the pressing pressure of the samples (Z₁). The amount of bound CO₂ increases with increasing Z₂ and decreasing Z₁. The high porosity of test samples resulting from their forced carbonization is significantly reduced. Depending on the factors involved in obtaining the experimental samples (cylinders), porosity decreases by at least 25%, and up to 37%. Similarly to the samples based on EAF slag, the porosity of the BS slag samples is mainly open, which has a significant effect on the rather high water absorption values. Thus, the W_m of the test samples ranges from 11.3 by weight % (point 10) to 17.3, 17.6 by weight % (points 1 and 5, respectively) and 19.9 by weight % (point 9). However, the softening coefficient of the cylinder samples is close to 0.8 or higher, which makes it possible to classify the sample material as water resistant. Some samples (see Table 11, points 4 and 6) have a K_S > 1, i.e., they are stronger after soaking in water than in a dry state. This can be explained by the activation of the process of belite hydration during water storage in the samples. The water resistance of the BS slag samples increases both with increasing Z₁ and with increasing Z₂ (see Table 12).

In addition to changing the properties of the material after carbonization, the efficiency of the carbonization reaction is also related to the quantitative absorption of carbon dioxide per unit mass of this material. This indicator is a determining factor in understanding the prospects of using a particular material that is capable of binding CO₂ and thereby contributes to [1] reducing the emission of the greenhouse gas into the atmosphere. To determine the amount of CO₂ in our study, we used the methods of thermal and X-ray phase analyses. According to the experiment plan, the highest calculated value of bound CO₂ after carbonization with different combinations of technological factors for samples production was characterized by the point of the plan No. 7 (see Table 7, Table 9 and Table 11). Accordingly, thermal and X-ray phase analyses were performed for these samples. The thermal analysis data are shown in Figure 9, Figure 10 and Figure 11. The comprehensive analysis of the thermogram data shows that, regardless of the type of slag, the TG and DSC curves are similar and have two weight-loss zones accompanied by certain endothermic effects. The first zone is in the temperature range of 30–500 °C and corresponds to the removal of chemically bound water from hydrated compounds. Thus, after carbonization, a stretched endothermic effect appeared for BOF slag in the range of 30 to 420 °C, with an extremum at 138 °C. The total mass loss of the sample in this range was 2.52 % by weight, as opposed to 0.73 % by weight for this slag without carbonization (see Figure 6). The second mass reduction zone is, on average, in the temperature range of 450–850 °C and corresponds to the decomposition of CaCO₃. For BOF slag, the total mass loss in this range was 6.0 %, which characterizes the quantitative content of bound CO₂ and is fairly consistent with the results of determining this indicator using the calculation method (see Table 7, point No. 7).

A similar situation is observed for EAF and BS slags. The amount of chemically bound water in the range from 30 to 420 °C increases in comparison with these samples without carbonization (see Figure 7 and Figure 8), and the content of bound CO₂ increases in the temperature range of 450–850 °C. For EAF slag, the total amount of bound CO₂ was 9.4%, and for BS slag it was 11.9%. Notably, the shape of the DSC curve in this temperature range shows the presence of various calcium carbonate polymorphs in the slags after carbonization. The decomposition of the amorphous CaCO₃ phase probably occurs first. According to the DTG curve, this process occurs in the temperature range of 450 to 600 °C. The decomposition of waterite and aragonite can occur in the range of 600 to 720 °C. Only the last distinct endothermic effect in the range from 720 to 850 °C corresponds to the decomposition of the stable carbonate phase, calcite. These data are fairly consistent with the authors’ previous research [39,40,41].

The thermal analysis data are consistent with the X-ray-phase analysis data for the same samples, the results of which are presented in

Table 13. Estimation of the quantitative content of the main minerals in the studied slags after their carbonization.

		Weight (%)
Phase	Formula	BOF	EAF	BS
Okermanite	Ca2(Mg0.50Fe0.20Al0.30)(Fe0.25Al0.21)Si1.54O7	54.6	-	-
γ-Belite (Shannonite)	γ-Ca2SiO4	11.2	5.3	-
β-Belite (Larnite)	β-Ca2SiO4	-	-	63.4
Merwinite	Ca3Mg(SiO4)2	-	17.2	-
Hedenbergite	CaFeSi2O6	-	7.5	-
Fayalite	γ-Fe2SiO4	-	7.1	-
Calcite	CaCO3	16.9	23.1	31.3

. According to these data, the main mineral formed during the carbonization process for 210 min is the calcium carbonate phase, with a simultaneous decrease in the amount of initial slag minerals.

Thus, the results obtained make it reasonable to suggest that the artificial stone obtained via the carbonate hardening of the studied slags can be used as a matrix for various composite materials and building products. The data analysis (see Table 7, Table 9 and Table 11) shows that, with a combination of certain technological parameters, it is possible to obtain a material with a compressive strength of more than 100 MPa, low water absorption, high water resistance, and, consequently, significant durability. With such a set of positive performance properties, this material is suitable for the production of a certain range of building products: for example, paving elements for sidewalks, roads, and wall bricks. It is reasonable to use the fine-grained waste from the mining and processing of sedimentary and inverted rocks up to 5 mm in size for the production of these building products. Applying the results obtained in the scientific laboratory, an experimental industrial batch of building products in the form of paving elements for sidewalks and wall bricks was produced under the conditions of the existing industrial enterprise. Fine-grained waste from the extraction and processing of igneous rocks (diabase) up to 5 mm in size was used as a filler for paving elements. Fine-grained waste from the extraction and processing of sedimentary rocks (limestone) up to 5 mm in size was used as a filler for wall bricks. The ratio of the materials was 1:1. The water content of the mixture was in the range of 6–10% by weight. The carbonate hardening of the products was carried out in a model shell chamber made of rolled polyethylene. The film was 700 µm thick, 2 m wide, and 25 m long. The exposure time of the products in this chamber was 5 h. High-pressure liquid carbon dioxide in cylinders was used as a source of carbon dioxide. Construction products were produced on an industrial production line designed to produce a certain range of construction products via double-sided pressing. The overall process of producing these building materials is shown in Figure 12.

Since the carbonization reaction is exothermic, a Testo 882 (Testo AG, Titisee-Neustadt, Germany) portable thermal imager with a standard lens was used to monitor the reaction. The results of the step-by-step control of the carbonization reaction are shown in Figure 13.

As shown in Figure 13, the forced carbonization reaction proceeds with the release of a large amount of heat. At the same time, the reaction rate is very high, which is confirmed by a rapid increase in temperature in the first 90 min of the process; this is followed by the temperature’s stabilization and then its slow decrease. After 5 h of exposure, the temperature inside the chamber was 15 °C at an ambient temperature of +2 °C. The construction products obtained in this way were delivered to the laboratory to determine the complex of operational properties. The test results are presented in

Table 14. Test results for the construction products from the experimental industrial batch depending on the type of slag and the purpose of the products.

Indicator	BOF (Tile)	EAF (Tile)	BS (Brick)
Compressive strength, MPa	96.3	81.1	37.1
Flexural strength, MPa	8.3	7.1	3.8
Water absorption by weight, %	6.0	8.3	9.9
Water resistance, KS	0.83	0.87	0.88
Frost resistance, cycles	>200	>200	>300
Average density, kg/m3	2550	2320	1670

. The data presented in Table 14 were obtained during tests in accordance with national standards [42,43]. These were used in conjunction with general specifications.

4. Conclusions

We conducted an analytical review of the literature on methods of industrial waste disposal: in particular, solid steelmaking slags and gaseous carbon dioxide gas. Our analysis revealed that solid waste is mainly accumulated in industrial waste dumps (various types of slag) and is released into the atmosphere (carbon dioxide). The experimental results obtained suggest that the potential of using these types of raw materials in the production of building materials and products has not been fully utilized, particularly in terms of developing materials with low CO₂ emissions. We carried out an optimization of the technological parameters of carbonized material production on the basis of different types of slags with high performance characteristics by means of the method of mathematical planning of the experiment. Carbonized stone with a compressive strength of up to 116.5 MPa was obtained. Its water absorption by weight is in the range of 6.0–17.0 % and depends on the technological factors used to obtain the material. The formulation and technological factors used in the production of carbonate-hardened building products based on the studied slags were examined: we studied wall and paving areas using sedimentary and volcanic rocks as a filler, respectively. An experimental batch of samples of wall products, hollow bricks, and paving elements was produced in a manufacturing environment. The tests revealed that the compressive strength of products based on BOF and EAF slags is 96.3 and 81.1 MPa, respectively, and, for bricks based on BS slag, it is 37.1 MPa. The integrated analysis of the results suggests that the proposed method of forced carbonization can be used to produce the building products in the form of paving elements and wall bricks that meet the requirements of the relevant regulatory documents. Moreover, the use of recyclable raw materials (steelmaking slags, CO₂ emissions) and a fast production cycle will reduce the production costs of the ready-made construction products of carbonate hardening.