Renowned cosmologist [1] believed that pollution and human stupidity linger as the most severe threats to humanity because, over the past five years, air pollution has increased by 8% in all five spheres of our planet. To prevent the environmental collapse of our civilization, it is essential to neutralize all forms of waste and eliminate their harmful effects on the atmosphere. 

Bauxite red mud (RM) is a byproduct of bauxite processing that contains high levels of iron and aluminum. Usually, one ton of aluminum is produced from 6 tons of bauxite, generating from 2 to 5 tons of RM in India (2000) with high alkalinity (pH = 13.5) and hazardous elements contents such as As (110 mg/kg), Hg – 1.2 mg/kg and Cr – 660 mg/kg. There is a significant number of works on the disposal of RM in the world’s scientific and technical literature.

The notorious environmental tragedies in Hungary and Brazil [2, 3] convincingly confirm the correctness of Hawking’s [1] conclusions. Therefore, the authors of this article have been developing economically and environmentally efficient composites using raw materials only from industrial and municipal waste or with a minimum number of natural components [4] for six decades.

RM has been studied as an essential component by many researchers for a variety of purposes, in particular [2] developed ceramics consisting of 50% clay and 50% RM to produce concrete, cement, geopolymers from RM and glass waste; as a sorbent of cations from aqueous solutions, particularly as a low-cost adsorbent for wastewaters with Pb (II) or Cr (VI) ions [5, 6] used RM as a sorbent for fluoride ions; its performance as an arsenic sorbent was demonstrated in comparison to a commercial material [7]. RM was applied as a catalyst to produce glycerol carbonate efficiently [8].

A mixture of RM with fly ash might prepare foam ceramic. Zn-alginate beads doped with hydrogen peroxide-treated RM were investigated as adsorbents for their sorption nature towards the fluoride ions from water [9]. RM was also used for cement production [10].

Due to its fine granulometric composition, RM is frequently utilized as an adsorbent of heavy metals and toxic substances. Notably, it can be used as an effective and low-cost adsorbent for treating wastewater contaminated with Pb (II) ions [5]. Moreover, RM was successfully applied as an active photocatalyst to remove Cr (VI) and malachite green from wastewater [11].

Ferrous slag (FS) is the principal waste of the ferrous iron-smelting industry, with a worldwide production of about 400 million tons [12] which is constantly increasing. Similar to RM, FS can be used with high environmental and economic efficiency for various purposes. One such goal is as a binder for partially replacing Portland cement [3]. Additionally, FS can be employed for removing heavy metals (Zn, Fe, Ni, Pb, Cr (VI), vanadium, and Co) from wastewater [13].

The lime production waste (LPW) was used for cement manufacturing, acidic soil neutralization, and drinking water production as a highly alkaline component. LPW can be mixed with other types of industrial and municipal waste [14] in different combinations for various purposes, including the one previously mentioned.

The Principal Aims of This Study Were

• Use bauxite red mud as a key component (52–78%) in new construction materials, along with ground-cooled converter slag (20–45%) and lime production waste (2%) as an alkaline activator for RM and a chemical activator for slag in a humid environment;
• To study the developed materials’ structure formation’s physical-chemical processes during their hydration and interaction;
• To ensure a proper chemical bonding of heavy metals from the hazardous initial components to prevent environmental pollution by the developed materials.

2.1 Methods

All raw materials and developed materials were characterized by a set of complementary methods: chemical composition - by X-ray fluorescence (XRF) and energy-dispersive spectroscopy (EDS) analyses;  granulometry composition  - by laser diffraction analysis of the particle size distribution (LAMMA); mineral composition - by X-ray diffraction (XRD); the isotopic composition of new formations that strengthen ceramics - by laser micro-mass analysis (LAMMA); changes in the morphological structure of the test samples during hydration and cure - by scanning electron microscopy (SEM); solubility and leaching of different elements - by atomic absorption spectroscopy (AAS) method. The changes in axial resistance strength, water and frost resistance of the test samples, water absorption, and linear shrinkage of the developed cementless concrete were determined at different hydration and cure ages of the test samples.

2.2 Calculations

• The water absorption (W_A) values of cementless concrete were determined according to the standard (CN-25-74), which uses the following equation:

W_A = [(R_SAT – R_D)/R_D] x 100                                               Eq. 1

Where RSAT is the water-saturated specimen mass after 24-hour water immersion, and RD is the dry specimen mass after 24-hour drying at 100°C.

The water resistance coefficient (CWR) was calculated (GOST 9179-77) for 90-day-old samples using the equation:  

   C_WR = R_W /R_D,                                                                      Eq. 2

Where R_W is the axial compressive strength after 24-hour water saturation, and R_D is the dry samples’ axial compressive strength.

Frost resistance coefficient (C_FR) was calculated for 90-day-old test samples using the equation:

     C_FR=R_F/ R_SAT,                                                                   Eq. 3   

Where R_F is the axial compressive strength after 25 freezing-thawing cycles, each freeze occurs at negative 25° C for 8 hours; thawing also takes 8 hours in water at + 20°C; R_SAT is the compressive strength after 24-hour water saturation at 20°-25°C.

2.3 Characteristics of the Raw Materials

The representative samples of the raw materials under study were composed of red mud (RM) from a Kamensk-Uralsky Aluminum Smelter, ground-cooled converter slag (CS) from the Chelyabinsk Metallurgical Plant, and lime production waste (LPW) from a local lime production plant, all in the Ural Mountains, Russia.

The granular composition of the RM consisted of 75% particles smaller than 75 microns, depending on the degree of bauxite grinding prior to the Bauer thermochemical process. LPW presented only 41.67wt. % of particles smaller than 75 microns; slag was the coarsest raw material, with only 6.35% of particles in the same range.

2.3.1 Chemical Composition of the Raw Materials (By the XRF Method)

The CS chemical composition expresses its high basicity modulus Mbas, which is well characteristic of its bonding properties: CS has the most significant amount of CaO+MgO content (63.8%), followed by LPW (55.4%), and RM has the minimum (29.4%). However, RM presented the maximum sum of SiO2+Al2O3+FeO+Fe2O3 (56.4%) and LPW the minimum (10.3%), while CS exhibited an intermediary value (28.6%). As a result, the minimum basicity value Mbas = (CaO+MgO) / (SiO2+Al2O3+FeO+Fe2O3) was calculated as 0.52 for RM, then CS (2.20) and LPW (5.4). Therefore, LPW was used as an alkaline activator in this study to promote the chemical interaction of RM with CS in a humid environment [15]

Table 1: Chemical components of the raw materials (by the XR Method).

Note: C.L. – calcination loss at 1000°C.

Such a high calcination loss (C.L.) value at 1000°C (C.L. = 23.9%) is likely due to CO₂ content in LPW, which strongly indicates the high carbonate rock content as poorly burned raw materials for lime production.

Heavy metals in RM (Table 2) significantly surpass the permitted amounts under Russian sanitary norms (CN-25-74). Specifically, the XRF method revealed the following values: Zn - 0.72%, Ni - 1.26%, Ba - 0.79%, Cu – 1.32%, Sn - 1.18%, and Cr - 0.54%. These findings and the extremely high alkalinity level (pH = 13.5) classify RM as an environmentally hazardous material.

Table 2: Leaching and solubility of metals from RMand composition 9 after 730 daysOf sample hydration (by AAS method).

Note: n.d. – not detected.

2.3.2 Mineral Composition of Raw Materials

Mineral composition of the raw materials was analyzed through the XRD method (Fig. 1).

Fig 1: X-ray diffractograms patterns of the raw materials:A - red mud, B - slag, and C – lime production waste.

RM contained minerals such as Al₂O₃·H₂O, Al₂O₃·3H₂O, Fe₂O₃·SiO₂, anatase TiO₂, minerals of iron oxides hematite Fe₂O₃ and magnetite Fe₃O₄, and quartz SiO₂. The slag had ackermannite Ca₂MgSi₂O₇ and quartz SiO₂. LPW consisted of poorly burnt raw limestone calcite CaCO₃ and magnesite MgCO₃, with small peaks of lime CaO and relatively strong peaks of hydrated lime portlandite Ca(OH)₂ and quartz SiO₂.

All raw materials also contained varying amounts of amorphous substances. The XRD patterns from the slag (Fig. 1-B) reveal a considerable amount of amorphous materials (until 250-300 cps) with very weak peaks of crystalline structures, except for the coincident peaks of ackermannite and quartz. The large halo between 21° and 34° in 2θ° λCu-Kα indicates the existence of a glass structure in the slag.

2.3.3 Micromorphology of the Raw Materials (By the SEM Method)

SEM analysis revealed distinct particle configurations and shapes for the three raw materials investigated (Fig. 2). Conglomerates of bauxite ore waste particles (Fig. 2-A) suggest its hydrometallurgical decomposition into tiny particles by the Bauer hydrochemical method. Slag particles displayed a monolithic structure (Fig. 2-B), confirming their high melting point. The rounded surface of all LPW particles (Fig. 2-C) indicates their hydration and carbonization during lime storage. Most of the particles in all three components fall within the 1 - 5 µm range.

Fig 2: Microstructure of raw materials (SEM method): A - red mud, B - slag and C - Lime production waste.

2.3.4 Research Methodology

All raw materials were dried, ground using a ball mill, and sieved through a 1.14 mm sieve. All raw materials combinations listed in Table 3 were homogenized, hydrated at optimum humidity (10-12%), and compacted into cylindrical molds under 10 MPa of axial pressure to obtain samples sizing

20 x 20 mm.

Compositions 1 and 2 were prepared with a 0 and 2% addition of LPW to control the binding properties of slag and its alkaline activation efficiency. Compositions 3 – 8 (Table 3) demonstrate that RM sludge (52-78%) and slag (20-45%), activated by 2% and 3% LPW additives, can result in effective binding. The initial humid mixes were compacted with 10 MPa and kept in an air-humid atmosphere with 94-96% moisture. The mechanical properties tests and the study of physicochemical processes of structure formation were carried out in samples with 3, 7, 28, 60, 90, 180, 365, and 730 curing days. Data analysis was performed on 600 test samples, with average and standard deviation values calculated from 10 measurements.

3.1 Mechanical Properties of the Developed Materials

The changes in the mechanical and physical properties of the test samples during their hydration and compaction processes were investigated: strength under axial compression, water resistance, linear expansion, and water absorption coefficients.

3.1.2 Uniaxial Resistance of the Test Samples

The axial resistance of all compositions’ test samples (Table 3) increased over hydration time. On the 7^th day, the resistance values were between 0.5 - 1.4 MPa; by the 730^th day, they had reached 6.3 - 14.6 MPa. Under the Russian standard GOST 379-2015, the bricks’ strength should be between 2 and 4 MPa.

Table 3: Changes in axial resistance strength during hydration.

Based on the results, the strength of composition 2 was 1.8 times higher than the requirement for the maximum quality bricks already on the 14th day. Comparing the TSs’ resistance values of compositions 1 and 2 showed that using 2% LPW for slag alkaline activation was highly effective at all ages. However, introducing RM in compositions 3 - 7 with amounts ranging from 58 - 78% and reducing slag to 20 - 40% resulted in a sharp decrease in strength at all ages of the test samples. 

During the timeframe of 28 to 60 days, a temporary decrease in strength was observed, with its subsequent growth at 90 days and further at 730 days. At 90 days, the strength of all compositions, except composition 3, had reached or surpassed 4 MPa. Furthermore, within 180 days, the requirement outlined in [16] was significantly exceeded.

3.1.3 Water and Frost Resistance of the Materials

Water and frost resistance of the test samples were determined (Table 4) according to the national standard [17] and were calculated using equations 2 and 3, respectively. Water and frost resistance values increased with LPW and slag contents. Composition 1, with 100% slag content, achieved a very high water absorption coefficient (1.05), which means an increase in axial resistance during the 24-hour water immersion. Following [17] demands, the water-saturated samples of the 1st class must demonstrate the strength values between 4-6 MPa and those of the 2nd class – 2-4 MPa. Based on these guidelines, compositions 1, 2, 5-8 meet the first-class requirements, and composites 3 and 4 comply with the second-class ones.

All composites, except control composite 1, meet the 1st class requirements (Table 3) of the frost resistance coefficient, which must be at least 0.75. The increase in materials strength is due to the water-saturated samples enduring 50 thermal shocks, which consist of sudden temperature changes from +25 to –25 °C and vice versa. The manifestation of the inevitable desquamation of solid particles causes an increase in the specific surface area of alkali corrosion and the synthesis of additional sol-gel structures, further strengthening the samples [14].

Table 4: Water and frost resistance of 90-day-old samples.

3.1.4 Water Absorption (WA) During Test Sample Hydration

The water absorption coefficient (WA) directly depends on all initial components’ contents (Table 4). Specifically, WA decreases in compositions 3–9 as the number of binders (slag and LPW) increases in the initial compositions and as the test samples hydrate in an alkaline medium (28 and 90 days)  due to the increase in hydraulic binders, their chemical interaction with sol-gel new formations and densification in the pore space of the test samples. The new formations’ compaction is the main reason for the increase in strength of all developed materials (Table 3).

Table 5:  Changes in water absorption (WA) during test samples hydration.

Therefore, among the six compositions (2 -7) with equal 2% LPW, composition 2 with 98% slag content has the highest WA values (12.48 and 12.01%), and the samples of composition 7 have the lowest WA values (10.54% and 10.29%).  

Composition 9 (with WA = 5.69 and 7.32%) and the nearest composition 8 (WA = 9.26 and 8.11%) exhibit the minimum WA values after reducing the LPW amount from 4% to 3%. All these changes can be attributed to the synthesis and densification of an increased number of new sol-gel formations in the pore space due to the chemical destruction of slag and RM particles in an alkaline LPW medium and their chemical interaction during sample hydration.

3.1.5 Coefficient of Linear Expansion (CLE) Of Samples

Composites 1 and 2 had the maximum CLE values (Table 5) due to the highest alkaline corrosion of the slag and LPW (whose CaO+MgO contents were 63.85% and 55.4%, respectively). These two highly alkaline solutions can interact with each other, maximally filling the pore space compared to other compositions, resulting in a decrease in water absorption (Table 4) and an increase in resistance (Table 3) and linear expansion (CLE, Table 5).

When comparing test samples of composition 3, with maximum RM (78%) and minimum slag (20%) contents, to composition 2, there was a sharp decrease in the expansion rate. However, when increasing slags and decreasing RM’s contents in compositions 3 to 7, there was a gradual increase in linear expansion coefficient over 730 days.

Adding 3 and 4% LPW to compositions 8 and 9 noticeably increases test samples’ CLE, indicating alkaline activation of the slags and RM’s particles with increasing LPW amounts, which also raises the samples’ strength. Table 3 provides more details on the relationship between LPW amount and sample strength.

Table 6: The coefficient of linear expansion (CLE) during the process of test sample cure.

The replacement of 78% of slag with red mud (composition 3) triggered a sharp drop in CLE and strength resistance to the lowest values ??among all 3-component materials at all ages, suggesting that the red mud had possibly absorbed the alkaline corrosion products to the maximum extent, thus, delaying all processes of synthesis of new formations and hardening of the test samples. However, gradually replacing the red mud with an equal slag amount in compositions 4-8 led to increased CLE and strength of the test samples.

Composite 8, composed of the minimum quantity of red mud (52%) and the maximum of slag (45%) and lime (3%), had the highest CLE among the 3-component materials in the stage of 3-90 days; its resistance strength was inferior only to composite 2 at all samples ages. Composite 6 (63% RM and 35% slag) exhibited the maximum CLE value among the 3-component systems at 189 – 730 days of hardening ages.

The magnitude of the average deviation of CLE values of all composites increased with hydration time, staying between 0.06 and 0.14%

3.1.6 Water Absorption during the Test Sample Hydration

Water absorption coefficient (WA) values are directly related to the slag and LPW contents and the samples’ hydration time (Table 6). Therefore, samples of composition 2 recorded the highest WA value (4.68 – 7.19%) at 28 and 90 hydration days, followed by the values of composite 9, which contained the maximum LPW (4%) and slag (45%) contents among compositions 3 – 9 due to the increased water absorption (5.69 – 7.32%) by these two hydraulic binders.

Table 7: Changes in the materials’ water absorption (WA) coefficient (%).

Water absorption coefficient WA values (Table 6) increase with slag and LPW contents, ranging from 4.68 to 7.32%. The WA of mixes 3-9 increased with the TSs hardening, varying from 3.29% on the 28^th hydration day to 7.32% on the 90^th curing day. The increase in water absorption was proportional to the increase in strength (Fig. 3).

The values of mechanical properties of all materials under study have standard deviations that follow clear tendencies. Specifically, standard deviations are directly correlated with an increase in both strength and shrinkage values, but do not exceed 1.2 MPa and 0.2%, respectively. Additionally, water absorption has a standard deviation that does not exceed 0.08%.

The physicochemical processes of structure formation were investigated on test samples of composition 9. This composition exhibits superior mechanical and physical properties compared to the other ones. Based on this evidence, all these processes are expected to be more prominent and intense than in the other compositions.

4.1 Changes in the Mineral Composition of Composite 9 during Hydration by the XRD Method

The main minerals of the initial mix 9 (Fig. 3-A) are calcite Ca?O₃, ackermannite Ca₂MgSi₂O₇,

anatase TiO₂, magnetite Fe₃O₄, hematite Fe₂O₃, bauxite (Al₂O₃·H₂O, Al₂O₃·3H₂O, Fe₂O₃·SiO₂). And quartz SiO₂. The X-ray background of the test samples of the initial mixture (Fig. 6-A) reaches approximately 50 cps (counts per second), which indicates a relatively low content of amorphous components. However, after 90 hydration and hardening days (Fig. 6-B), the background increases to 400 cps and slightly more in the two-year-old samples (Fig. 6-C).

A very high X-ray background is visible on the diffractogram patterns, compared to the intensity of most of the crystalline structures’ peaks of the minerals, denoting a high content of amorphous materials and crystalline structures with low peak intensity.

Fig 3: XRD patterns of composition 9: A - dry mix, B - on the 90^th day and C – on the 730^th day of cure.

In the presence of potent oxidizing agents in an alkaline environment, Fe₂O₃ exhibits reducing properties and oxidizes to iron derivatives: Fe₂O₃ + 3KNO₃ + 4KOH = 2K₂FeO₄ + 3KNO₂ + 2H₂O. After 90 hydration days in a highly alkaline environment and chemical interaction of the components, the diffractogram pattern (Fig. 6-B) points out the disappearance of three mineral peaks - magnetite, hematite, and bauxite without replacing them with other minerals, but with a substantial increase in X-ray background.

The absence of new minerals with high iron oxides content in the diffractograms of Fig. 6-B and C suggests that magnetite, hematite, and bauxite turned into the amorphous state as a result of their crystalline structures destruction in a moist alkaline environment from the mixture of three highly alkaline wastes. The background intensity practically reaches 500 counts per second, which exceeds the weak intensity of the crystal peaks of the original blend (Fig. 6-A). This fact may have occurred due to the transition mentioned above. Specifically, the calcite peak intensity at 2Θ° = 28.5° increased from 120 to 1100 cps on the 90^th day and more than 1200 cps on the 730^th day. Additionally, the calcite peak at 2Θ° = 47° increased from 100 (Fig. 6-A) to 600 cps on the 90^th day and 850 cps on the 730^th day (Fig. 6-C).

The quartz peaks’ intensity increases at 2Θ° = 20.5°, 56.5°, and 69° due to the dissolution of the mechanically destroyed surface layers of this mineral’s particles in an alkaline medium (pH about 13.5), shielding their undamaged nuclei with a more perfect structure. The destroyed quartz layer may engage in the sol-gel process generating new amorphous formations, such as calcium-silicate-hydrate (CSH). CSH’s role in concrete structure formation is well established in cement hydration chemistry.

4.2 Morphological Changes in Composite 9 during the Hydration Process

Morphological structure formation processes changes during composite 9 hydration and cure were studied by the SEM method (Fig 7). The initial dry mix (Fig. 7-A) mainly consists of particles with different sizes and configurations. After 60, 180, and 730 hydration days (Fig. 7-B, C, and D), it was evident that most  particles merged to create vast fields of sol-gel-like new formations with a significant number of individual particles on the surface of these fields.

Fig 4: Changes in the microstructure of composition 9: A - initial dry mix,B - after 60, C – 180, and D - 730 hydration days (by the SEM method);chemical composition of the points and area (by the EDS method).

These sol-gel-like surfaces are the products of the initial components’ surface chemical destruction by a highly alkaline pore solution and their chemical interaction and densification. This process is the only reason for the change in all properties (Tables 3, 4, 5, 6, and Figures 6 and 8) of the developed materials, including hardening up to 6.5 MPa in 60 days and 14.9 MPa in 730 hydration days. The amorphous materials formed in the pore space between the solid particles’ grains may remain stable. Their natural amorphous analogs (such as flint, hisingerite, limonite, and opoka) retain an amorphous structure for long geological eras.

4.3 Chemical Composition of Composition 9’s New Formations after 60 and 730 Hydration Days

Test samples from the 60 and 180-day hydration periods have a very high degree of inconsistency in their chemical compositions (Fig. 7-C and Table 7) for all chemical elements. Points 1, 2, and 3 of the new formations have a carbon content variation between 41.84% and 11.25%, with an average content of 27.40%. The total value of the carbon C content in area 1 (14.56%) is much lower, which indicates the carbonization process’ non-uniformity at the plotted points.

Table 8: The chemical composition of composite 9’s new formations after 60 and 730 hydration days (by the EDS method, Fig. 7-C).

After half a year of hardening, the heterogeneity levels for all chemical elements remain almost the same, as shown in Table 5, except for carbon C (23.43, 24.29, and 25.25%) at points 4, 5, and 6. The average carbon content in these three points (24.32%) almost coincides with the total carbon content in area 2 (24.63%). This suggests relative alignment of the synthesis of new crystalline and amorphous carbonate formations over the entire bulk of the samples and endorses the XRD analysis results (Fig. 6-C).

4.4 Isotopic Composition of Composite 9’s New Formations

Composite 9’s new formations were studied using laser micro-mass analysis (LAMMA), as shown in Fig. 8. Samples on the 730^th day of hardening showed results that confirmed the heterogeneity of elemental composition obtained by XRD, SEM, and EDS methods (Fig. 6, 7 and Table 7).

Fig 5: Isotopes composition of composition 9’s new formations by laser micro-mass analysis (LAMMA) on the 730^th curing day.

The analyzed points’ chemical composition in the new formations showed very different qualitative (isotope numbers) and quantitative (peak intensity) compositions in all isotope spectra. High contents of Cr, Zn, and Cu were revealed, consistent with the solubility and leaching outcomes of metals from red mud (Table 2).

The environmental properties of the developed materials were determined due to the presence of mobile heavy metals (Ba, Cr, Ni, Pb, and Hg) and other metals (Fe and Al) and as in the red mud (Table 2) in quantities that significantly exceed the requirements of Russian sanitary norms. Standard metals solubility and leaching analyses in an acidic medium were conducted to ascertain the binding degree of metals and As in the formed structures of the developed formulations.

When comparing leaching and solubility values ??from composition 9 with those of RM (Table 2), measured experimentally applying the standard GOST 379-2015, it is possible to observe significantly lower values ??of all metals, especially heavy metals. This finding shows that the materials’ compositions and manufacturing techniques reliably bind heavy metals. As a result, the developed materials’ dissolution and leaching values are significantly lower than the original red mud. Hence, the developed materials have various potential uses, such as constructing road and airfield foundations, industrial and municipal dumps, as the core of hydraulic dams, building foundations, and producing tiles and bricks. Kamensk-Uralsky alumina smelter red mud might be used in mixtures with other industrial waste, such as dump slag from Chelyabinsk Metallurgical Plant or in combination with other similar wastes with the addition of various alkaline activators that chemically interact with red mud and slag. Small amounts of building lime for mixes can accelerate the materials’ hardening process. The production technology for these materials is straightforward and does not create additional waste. Moreover, using these industrial wastes as raw materials will diminish the extraction of natural materials from open pits.

• In the present work, the possibility and feasibility of sustainable building materials producing from mixtures of red mud from a bauxite enrichment plant and slag from the manufacture of ferrous metals, with minor additives of lime production (2, 3 and 4 wt. %) are experimentally proved. The compressive strength of the test samples increases from 5.3 MPa at 7 days to 7.2 MPa at the hydration age of 14 days and continues to grow steadily up to 14.9 MPa over a year.

           The materials have very high water and frost resistance values. The samples’ maximum linear expansion and water absorption in one year do not surpass 3.54%. These properties of the developed materials allow for recommending them in applications              such as bases of roads, airfields, and foundations of industrial and municipal landfills. The technology for producing these materials is elementary, and its implementation generates no new waste.

• When studying the physicochemical processes of structure formation, it was established that all the developed materials’ properties are explained by the occurrence of sol-gel processes in the pore space of the samples with the creation of new amorphous formations, their syneresis and their transitions to a stone-like state.
• Producing materials industrially using red bauxite sludge in composites with ferrous slag and a small addition of lime or commercial lime production waste can be highly cost-effective due to the low price of the raw materials used — industrial waste — and the simplicity of the process.
• To retain environmental pollution in densely populated industrial areas, large-scale use of the developed materials is an environmentally essential and economically highly profitable initiative. The solubility and leaching tests of the metals from these materials in acidic solutions showed values far below the requirements of Russian sanitary standards as a consequence of the chemical bonding of heavy metals. The most meaningful advantage of the present method is the application of industrial dumps of bauxite red mud, converter slag, and waste from lime production, which are notorious pollutants. This approach of using waste from these and other
• Company’s offers an alternative to extracting natural materials from Siberian open pits.

The authors thank the Laboratory of Minerals and Rocks (LAMIR) and the Laboratory of Ceramics at UFPR, Curitiba, Brazil, for their dedicated technical assistance.

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