Environmentally clean construction materials from hazardous bauxite waste red mud and spent foundry sand

Construction and Building Materials 229 (2019) 116860

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Environmentally clean construction materials from hazardous bauxite waste red mud and spent foundry sand Kirill Alekseev a, Vsevolod Mymrin a,⇑, Monica A. Avanci a, Walderson Klitzke b, Washington L.E. Magalhães c, Patrícia R. Silva c, Rodrigo E. Catai a, Dimas A. Silva b, Fernando A. Ferraz b a b c

Federal Technological University of Paraná, (UTFPR), Brazil Federal University of Paraná (UFPR), Curitiba, Paraná, Brazil Brazilian Agricultural Research Corporation (EMBRAPA), Curitiba, Paraná, Brazil

h i g h l i g h t s
Were developed ceramics from industrial wastes.
Flexural strength of ceramics was till 10.54 MPa.
Values of water absorption coefficient was 2.77 and 14.41%.
New ceramics have mainly glassy structures with some crystal inclusions.
Utilization of industrial wastes will have high environment impact.

a r t i c l e

i n f o

Article history: Received 13 May 2019 Received in revised form 5 August 2019 Accepted 1 September 2019

Keywords: Bayer process’s red mud utilization Spent foundry sand Sintering till 1150 °C Mainly vitreous structure formation Environmentally clean construction materials

a b s t r a c t This paper demonstrates the possibility of producing new red ceramics composites from red mud of hazardous bauxite waste (50–100 wt%), and foundry sand (10–50%) replacing the traditional clay-sand mix and preventing the environment pollution by such industrial wastes. The newly developed environment-friendly ceramics exhibit high physical properties (flexural strength, linear shrinkage, water absorption, and density). The ceramics’ analysis by X-rays fluorescence, X-rays diffractometry, atom absorption spectroscopy, scanning electron microscopy, energy-dispersion spectroscopy, and laser micro-mass analysis revealed the synthesis of mainly glass-like structures with a small inclusion of crystalline structures. The values of flexural strength reached 10.54 MPa; after sintering at 1150 °C, linear shrinkage varied between 6.62 and 7.92%, water absorption – 2.77 and 14.41% and bulk density – 1.65 and 2.07 g/cm3. The most valuable property of the developed materials is the ecological purity due to the heavy metal’s complete neutralization from both industrial wastes. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction According to the calculations of the cosmologist Hawking [1], the pollution of the atmosphere takes such a dimension that it already jeopardizes the possibility of humankind’s survival in the next 300 years. The main polluters of all five of our planet’s environment spheres are the industrial and household wastes. Two hazardous industrial wastes - red mud of bauxite enrichment by Bayer process before aluminum smelting and spent foundry sand from machine construction – were used in this research to produce environment-friendly ceramics without the use of any traditional ⇑ Corresponding author at: 4900, Deputado Heitor de Alencar Furtado Str., Curitiba CEP: 81280-340, Brazil. E-mail address: [email protected] (V. Mymrin). https://doi.org/10.1016/j.conbuildmat.2019.116860 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

natural materials. World Aluminum’s database [2] made public that, in April 2016, 9.2 million tons of alumina and 15–20 million tons of red mud was produced. Both of these hazardous wastes were disposed of into the factory’s industrial dumps. One of these dumps brought about the ecological catastrophe in Hungary [3] in October 2010. A wave of red mud with pH = 13 and high content of heavy metals flooded the nearest city, caused the deaths of nine people, polluted 40 km2 of land and poured into the Danube. The worldwide technical literature contains information on the composition and properties, as well as methods of these wastes’ disposal, but for their industrial use, it is necessary to develop new economically and environmentally more efficient and technologically simpler methods. One of the most difficult problems regarding waste utilization, especially hazardous waste, is the consumers’ unwillingness to buy goods from hazardous wastes [4].

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To stimulate the red mud’s recycling, Liu et al. [5] suggested classifying it as a general industrial waste rather than hazardous waste. Deelwal et al. [6] showed the diversity of chemical compositions of the red mud [7] also discovered a significant difference in red mud composition from different regions of China. Some Western Australia red muds showed high levels of radioactivity [8]. Methods of magnetic separation of iron from red mud were developed by Samouhos et al. [9]. Smiciklas et al. [10] proposed a method for nickel removal from solutions; Collins et al. [11] used red mud to withdraw manganese, chromium, cobalt, nickel, copper, and zinc. Kumar et al. [12] conceived the preparation of cement-free paving blocks with a flexural strength of 3.2–4.5 MPa and water absorption of 6–7% from 10 to 20% of red mud and 80–90% of fly ash. Red mud was used by Tsakiridis et al. [13] for the production of Portland cement clinker without negatively affecting the cement quality. [14] Nithya et al. replaced 20% ordinary Portland cement by calcined red mud and hydrated lime without any loss of pozzolanic activity. The best red mud and clay ratios were determined by DodooArhin et al. [15] to attain ceramics sintering temperature of 900– 950 °C with uniaxial compression strength values of 52 MPa. Hua et al. [16] studied the role of Fe species in geopolymer synthesized from Bayer red mud, Badanoiu et al. [17] treated red mud and cullet soda-glass at 600–800 °C in order to synthesized foamed geopolymer with a compressive strength of 2.1–8.6 MPa. Singh et al [18] studied the influence of mechanical activation of red mud and curing methods on the strength of red mud - fly ash geopolymer paste. Spent foundry sand (FS) is the liquid metal casting and molding residue. As a result of thermal shocks, the sand modifies its granulometric composition and, therefore, is discarded as industrial waste with a high content of heavy metals. Coppio et al. [19] and Bhardwaj and Kumar [20] replaced fine natural sand by spent FS as an aggregate material in concrete production. Test results of Gurumoorthy and Arunachalam [21] indicated better performance of concrete with FS than control specimen and established that concrete with 30% FS is more impermeable than control concrete. Dyer et al. [22] applied FS for hot asphalt production. Mymrin et al. [23] suggested foundry sand along with the addition of other industrial wastes for the sintering of environmentally-safe ceramics at 950–1050 °C with samples’ flexural strength values of up to 14 MPa. The main objectives of the research were: (1) to develop new environment-friendly ceramic composites from hazardous red mud of bauxite processing and spent foundry sand without traditional natural material with mechanical properties corresponding to Brazilian standards; (2) to study the physicochemical processes of the developed materials structure formation.

2. Methods and raw materials characterization

(LAMMA), using a LAMMA-1000, model X-ACT. Lixiviation and solubility of metals from liquid acid solutions were studied by atomic absorption in a FAAS or ICP-OES spectrometer; granulometric composition - by a laser micro-mass analyzer, model LA-950 – HORIBA; flexural strength - by EMIC DL-10. Characteristics such as water absorption as a result of an increase in the weight of the samples after 24 h of their total immersion in water, linear shrinkage, and density of the ceramics of the samples after sintering at all temperatures were also studied. The test samples (TS) of the ceramic were prepared by homogenization of two industrial wastes used here as raw materials in different percentages. The TSs were mixed with water content 12–14%, compacted at 5 MPa in a rectangular mold of 60  20  10 mm in size, dried at 100 °C, and burned for three hours at temperatures of 800°, 900°, 1000°, 1050°, 1100°, 1150°, 1200° and 1225 °C with spontaneous furnace cooling. All physical properties were replicated ten times. Therefore, the total amount of the test samples was about 600 pieces. 2.2. Calculations Water absorption coefficient (CWA) tests were performed after ceramics’ sintering at all temperatures by the following equation:

CWA ¼ ½ðMSAT  MD Þ=MD   100

ð1Þ

where MSAT - the mass of the test specimen saturated after total immersion in water for 24 h MD - the mass of the dry test specimen The values of linear shrinkage LS (%) were performed according to the equation:

LS ¼ Li ¼ ½ðLi  LsÞ=Li  100

ð2Þ

where Li - initial length of specimen (mm); Ls - length of the specimen after the sintering (mm) The following equation was used to calculate the values of density D (g/cm3):

D ¼ Md=ðMs  MiÞ

ð3Þ

2.3. Raw materials description 2.3.1. Particles size distribution The particles of both wastes used were tiny (Table 1) due to their generation processes. The small-grained size’s complex composition of red mud was also described by Liu et al. [24]. This composition was a favorable factor for the formation of structures and mechanical properties of the developed materials. RM was the coarsest material, containing the most massive particles (0.60– 1.19 mm) in the amount of 39.33 wt%. FS exhibited a more uniform

2.1. Methods The raw materials (red mud and spent foundry sand) and the newly developed ceramics were characterized by the following methods: the particle size distribution was determined by the sieve method; the chemical composition – by XRF, using a Philips/Panalytical, model PW 2400; the mineralogical composition – by XRD with a Philips, model PW 1830 with the radiation kCuKa; the obtained diffractogram patterns were interpreted using the High Score program with the PDF-2 database. The morphological structure was analyzed by SEM, using a Jeol JSM-6360 LV; the micro-chemical composition – by EDS with a Jeol JSM-5410 LV; and the isotopic composition – by Laser micro-mass analyses

Table 1 Particle size distribution (wt%), bulk density (g/cm3) and humidity (wt%) of the raw materials.

Grain size distribution

Size (mm)

Red mud

Foundry sand

0–0.074 0.075–0.149 0.15–0.29 0.3–0.59 0.6–1.19 1.2 Bulk density (g/cm3) Humidity (wt%)

0.32 1.08 13.14 46.13 39.33 0.00 0.86 32.2

0.08 0.62 13.86 85.17 0.26 0.00 1.59 1.30

K. Alekseev et al. / Construction and Building Materials 229 (2019) 116860

size composition, with an 85.17% particle content ranging from 0.3 to 0.59 mm. The bulk density of foundry sand (1.59 g/cm3) was almost twice higher than of the red mud (0.86 g/cm3) because of the mainly siliceous composition. The foundry sand presented a minimum humidity level (1.30%) (Table 1) because it is a waste from high-temperature processes, unlike red mud, which had high (32.2%) humidity due to chemical reaction with NaOH solution. Yalcin et al. [25] determined the moisture content of the red mud between 40 and 50%. 2.3.2. Chemical composition of the raw materials Both raw materials in the study were obtained from local plants in Brazil. The representative sample of red mud (RM) was received from an aluminum factory in Sao Paulo; the foundry sand (FS) was collected in a machine-building plant in the city of Curitiba, Brazil. The main components of the bauxite red mud were Fe2O3 – 29.9% (thus this waste had a dark red color from which the material was named after), Al2O3 – 21.2%, SiO2 – 15.5%, and Na2O – 10.3%. The high (14.4%) loss on ignition (L.O.I.) value might be explained by the content of water, and hydroxide OH-group due to the Bayer thermochemical decomposition of bauxite particles in NaOH solution with pH = 13.5. High Na2O content (10.3%) was very beneficial for reducing the melting point of the ceramics’ composites. Foundry sand consisted mainly of SiO2 (91.2%). The presence of 3.7% I.L. was most likely the result of clogging with organic materials when preparing the molding form and when stored in industrial dumps. The study of leaching and solubility of metals from red mud by AAS method showed (Table 3) a high content of all metals, including heavy metals, far exceeding Brazilian standards [26]. This fact, together with the high alkalinity of the red mud (pH = 13.5), required classifying it as a hazardous material. Currently, the only form of storage of the material used in this study is open-cast industrial dumps, which inevitably causes extreme and dangerous pollution of the atmosphere, soils, and surface and ground waters. A possible path to avoid such pollution is their complete use at the industrial level as raw materials, using environmentally friendly and scientifically based methods. 2.3.3. Mineral composition of the raw materials The red mud used here presented a typical mineral composition (Fig. 1-A) for bauxite ore – namely, unbroken of Bayer process remnants of bauxite (Al2O3nH2O), hematite (Fe2O3), magnetite (Fe3O4), and quartz (SiO2). The low intensity of the X-rays peaks of the crystalline lattice with almost equal height to the background evidenced the predominance of the amorphous phase in the test samples.

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Foundry sand was depicted (Fig. 1-B) only by crystalline quartz, which accorded with the 91.2% SiO2 content detected in the chemical analysis by the XRF method (Table 2). Natural sand particles were not entirely destroyed by the thermal shocks of the casting process. Nevertheless, a significant X-ray background acknowledged a high content of amorphous material — a product of the quartz crystal structure destruction both during the long geological history of these particles and the casting’s thermal shocks, during the foundry mold shaping (Fig. 2 – B and C). 2.3.4. Micromorphology of the raw materials (by SEM method) The raw materials examination by the SEM method (Fig. 2) revealed particle sizes and surfaces somewhat different between them. The sizes of the different configurations of the red mud varied from nanoparticles to 0.2 mm, and the foundry sand ranged between 0.5 and 0.6 mm, confirming the results of the particle size analysis (Table 1). 2.3.5. Thermochemical characteristics of the raw materials The principal red mud components were (Table 2) the hydrates of Fe2O3 (29.9%) and Al2O3 (21.2%) mainly in amorphous and partially crystalline forms (Fig. 1-A). Therefore, the thermal transformations of these hydroxides, during red mud’s heating in DTA and TGA analyses (Fig. 2-A), cooccurred. The first endothermic effect during the red mud’s firing process (Fig. 3-A) indicated the loss of free and weakly bound water of the pores between 28 and 202 °C, with a total material’s weight loss of 3.53%. The second endothermic effect, between 202 °C and 296 °C, corresponded to the loss of water from the crystalline structure of gibbsite Al(OH)3 and its transformation in boehmite ɣ-AlO(OH)4, with a weight loss of 5.25%. The transformation of the amorphous gel to a, b, ɣ, and d-forms of FeOOH occurred concomitantly [27]. The third endo-effect, at 296–575 °C, coincided with the boehmite ɣ-AlO(OH)4 transition to anhydrous c-Al2O3, with a weight loss of 4.11%. At the same temperatures, the transformations FeOOH ? ɣ-Fe2O3 ? Fe3O4 began, with a total weight loss of 4.11%. A strong exothermic effect, between 575 and 1226 °C, was consistent with the chemical transformations of c-Al2O3 ? a-Al2O3 (at 850 °C) and Fe3O4 ? Fe (at 700–750 °C), with a total weight loss of 6.08% [27]. Foundry sand presented more straightforward chemical (SiO2 91.2%, Table 2) and mineralogical (quartz SiO2, Fig. 3-B) compositions. The first endo-effect, at 26–511 °C with a total weight loss of 2.43%, reflected the evaporation of all types of water with different bonding energy to the solid particles of the test samples. The combination of two small endo-effects between 511° and 595 °C might be related to two types of transformation of the sol-gel silica

Fig. 1. Diffractogram pattern of the red mud and foundry sand by XRD method.

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Table 2 The main chemical components of the raw materials. Oxides

RM

Foundry sand

Fe2O3 SiO2 SO3 Al2O3 CaO Na2O K2O MnO TiO2 P2O5 I.L. Total

29.9 15.5 0.6 21.2 4.2 10.3 0.4 0.2 2.4 0.6 14.4 99.7

1.2 91.2 0.7 2.3 0.1 0.3 0.1 0.0 0.1 0.0 3.7 99.7

L.O.I. – Loss on Ignition.

from the destructed surface part of the quartz grains (Fig. 2-B and C). A strong exothermic peak between 595° and 1226 °C with 0.09% weight loss might be the result of organic additives firing which are usually used as binders and plasticizers additives during foundry mold shaping. These additives are employed in casting factories before the foundry sand is stored as a residue [28,29]. The comparison of the DTA and TGA curves (Fig. 3-A and B), by the number of thermal effects, the weight loss in each of the effects and the total weight loss (18.92 versus 2.97%), inferred that the thermochemical activity of the red mud is much more significant. This fact is predetermined by the incomparably more complex chemical and mineral compositions (Fig. 1 and Table 2) of the red mud. Therefore, in the mixtures of ceramic composites, the red mud’s chemical influence should be more meaningful than the sand’s, which more thermochemical passive is. Table 3 3. Results and discussion 3.1. Physical properties of the developed composites Regarding physical properties, flexural strength (Table 4), linear shrinkage (Table 5), water absorption (Table 6) and bulk density (Table 7) of the developed ceramic, sintered at different temperatures, were studied. 3.1.1. Flexural strength of the developed ceramics According to the Brazilian standard [30], the flexural strength of solid bricks has the following classification: Class A < 2.5 MPa; Class B 2.5–4.0 MPa; and Class C > 4.0 MPa. Attempts to use RM as the only raw material for the production of ceramics (Table 4, comp. 1) showed that it meets the requirements of this standard only after firing at a temperature equal or superior to 1100 °C. Analysis of the results in Table 4 indicates that the maximum melting point (1050 °C) of quartz sand in a chemically neutral environment may reduce its stability in strongly alkaline red

mud environment; quartz grains begin to soften already at 800 °C (compared ceramics 1 with ceramics 2–6) and interact with red mud. This interaction explains the significant increase in the strength of ceramics 2–6 as compared to ceramics 1 at all temperatures, except 1200° and 1225 °C, due to excessive melting of quartz. The rapid decrease in the strength’s growth of ceramics 1 between 1200 and 1225 °C indicates a similar excessive melting of RM right after 1225 °C. An increase in FS content to 20% (ceramics 3) reduces the excessive melting up to 1200 °C. The following increases in FS content to 30, 40, and 50% (ceramics 4, 5, and 6) lead to a downtrend in samples strength at almost all temperatures. Nevertheless, these strength values made it possible to manufacture bricks of class A and B already after firing at 900 °C and Class C after firing at 1100 °C of all ceramics. 3.1.2. Linear shrinkage of the developed ceramics The shrinkage coefficient values for the materials after firing until 1225 °C augmented from 1.71 to 12.45% (Table 5). The highest shrinkage values were observed in the ceramics of composition 1 with 100% red mud content. The inclusion of 10 to 50% of foundry sand invariably reduced the sample’s shrinkage values, which soared with the intensification of the firing temperature. The ceramics of compositions 2 to 4 at 1225 °C and compositions 5 and 6 at 1200 °C pointed to a reduction in shrinkage, apparently due to the samples melting onset. There was a decrease in the strength of the samples (Table 4) at the same temperatures. The values of standard deviation of the shrinkage coefficient of all ceramics did not surpass 1.3%. 3.1.3. Water absorption of the developed ceramics The water absorption values (Table 6) presented a single general trend - reduction with increasing firing temperature and flexural strength. They were not associated with the linear shrinkage (Table 5) of the samples at high temperatures caused by the beginning of the samples melting and the reductions in strength (Table 4). Monteiro et al. [31] observed a similar ratio of the property’s changes. They used blast furnace sludge with 25% amount of coke for ceramics production. Herek et al. [32], differently, evaluated the growth of the ceramics strength in increasing of water absorption by the waste catalyst and foamed glass material. The authors consider that the relationship of these properties depends on the efficiency of pore formation in the compared materials. In this study, the firing of the red mud – foundry sand composites reduced their porosity and water absorption (Table 6) and increased the shrinkage values (Table 5) and strength. In accordance with national norms [28], the tolerance limit for WA in bricks is 20%. The WA values for most compositions varied from 6 to 10%, thus meeting national standards (Fig. 6). The standard deviation values of the developed ceramics’ water absorption coefficient were not superior to 1.37%.

Fig. 2. SEM micro images of red mud (A) and foundry sand (B and C).

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K. Alekseev et al. / Construction and Building Materials 229 (2019) 116860

Fig. 3. DTA and TGA curves of the: A - red mud and B – foundry sand.

Table 3 Leaching and solubility of metals from red mud and composition 6 after sintering at 1150 °C. Elements

Leaching, mg/L

As Ba Cd Pb Cr total Hg Se Al Cu Fe Mn Zn

Solubility, mg/L

Red mud

Comp. 6

[26]

Red mud

Comp. 6

[26]

7.63 94.28 7.34 4.43 18.46 1.47 2.75 28.76 16.29 98.31 55.11 68.48

0.32 1.58 0.04 0.22 1.49 0.29 0.17 0.14 0.08 1.07 0.41 0.01

1.0 70.0 0.5 1.0 5.0 0.1 1.0 * * * * *

9.56 95.47 15.41 7.66 22.67 3.81 3.35 36.44 30.08 108.75 69.43 84.13

<0.001 0.032 0 <0.01 0.02 <0.001 <0.001 0.05 0.87 0.06 0.05 0.69

0.01 0.7 0.005 0.01 0.05 0.001 0.01 0.2 2.0 0.3 0.1 5.0

Table 4 Flexural strength (MPa) of ceramics after sintering at different temperatures (°C). N°

1 2 3 4 5 6

Compositions, wt%

Flexural strength (MPa) of ceramics after sintering at T °C

RM

FS

800

900

1000

1050

1100

1150

1200

1225

100 90 80 70 60 50

0 10 20 30 40 50

0.00 1.83 1.48 1.80 1.25 1.03

0.00 2.17 1.79 2.53 1.83 1.55

0.45 3.03 2.12 3.05 2.48 2.13

0.93 4.37 4.61 3.67 3.47 2.79

4.34 5.27 6.33 5.76 4.24 4.10

6.06 8.43 8.26 8.02 7.93 6.85

12.17 10.54 7.51 7.90 7.70 6.13

12.32 9.07 6.91 6.64 6.28 5.69

The standard deviation values of the flexural strength of all ceramics tended to increase with the sintering temperature rise but did not outpace 0.74 MPa.

Table 5 Linear shrinkage (%) of ceramics after sintering at different temperatures (°C). №

1 2 3 4 5 6

Linear shrinkage (%) of ceramics sintered at T °C

Compos., wt% RM

FS

800

900

1000

1050

1100

1150

1200

1225

100 90 80 70 60 50

0 10 20 30 40 50

1.89 1.84 1.83 1.81 1.74 1.71

2.27 2.23 2.17 2.09 2.03 2.00

3.42 3.24 3.12 3.03 2.99 2.89

4.33 4.14 4.09 4.84 4.80 4.65

5.29 5.15 5.02 4.93 4.90 4.70

7.24 7.19 7.11 6.97 6.75 6.62

11.67 11.45 11.27 11.05 6.62 6.43

12.45 11.23 11.04 10.89 6.02 6.26

This decline means that the sand might not only stabilize the red mud samples sizes but also act as a flux due to a large number of heavy metals (Table 3) in highly alkaline medium.

3.1.4. Bulk density of the developed ceramics The bulk density values (Table 7) of ceramics 1, with 100% of RM, stepped up slowly until 1225 °C. The add-on of 10, 20 and

30% of FS demonstrated the beginning of excessive melting of the test samples at 1225 °C; increasing FS content up to 40 and 50% diminished the melting point to 1200 °C. All these changes are in

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Table 6 Water absorption of ceramics after sintering at different temperatures (°C). №

1 2 3 4 5 6

Compositions, wt%

Water absorption (%) of ceramics at temperatures (°C)

RM

FS

800

900

1000

1050

1100

1150

1200

1225

100 90 80 70 60 50

0 10 20 30 40 50

33.78 28.37 25.87 23.79 22.39 18.12

31.36 25.17 22.19 20.11 16.94 14.75

28.68 23.45 19.60 16.23 13.80 11.24

25.79 19.67 16.19 12.39 11.40 8.60

24.62 16.54 12.01 9.22 6.32 5.54

19.98 14.41 10.99 8.93 5.96 2.77

11.79 9.45 8.89 6.70 4.60 2.69

9.31 6.48 7.20 4.47 2.42 2.35

Table 7 Bulk density of ceramic compositions at different firing temperatures. №

1 2 3 4 5 6

Composites, wt%

Density (g/cm3) of ceramics sintered at T °C

RM

FS

800

900

1000

1050

1100

1150

1200

1225

100 90 80 70 60 50

0 10 20 30 40 50

1.28 1.40 1.42 1.52 1.60 1.71

1.31 1.43 1.48 1.57 1.64 1.74

1.35 1.47 1.52 1.64 1.68 1.80

1.42 1.50 1.57 1.68 1.74 1.86

1.44 1.55 1.65 1.74 1.87 1.97

1.55 1.65 1.78 1.95 2.09 2.07

1.89 1.81 2.11 2.09 1.97 1.92

1.96 1.75 2.09 2.04 1.95 1.81

effective agreement with the previously described changes of flexural resistance, water absorption, linear shrinkage, and flexural resistance strength. The obtained bulk density values are in reasonable conformity with the results of Antonovicˇ et al. [33]. The standard deviation values of the bulk density of the ceramics did not exceed 0.12%. 3.2. Physical-chemical processes of the ceramics 6’s structure formation Composite 6 was selected to look into the physicochemical processes of the ceramics structure formation due to the higher foundry sand (50%) and red mud (50%) contents as well as the excellent physical properties, which went beyond the requirements of national standards. The ceramics sintered at 1000° and 1150 °C were compared by complementary methods. 3.2.1. Mineral transformation during ceramics 6’s structure formation The comparison of the ceramics 6’s diffractogram patterns, after firing at 1000° and 1150 °C, showed (Fig. 4) the presence of the sin-

Fig. 4. Diffractogram pattern of the composition 6 after firing at: A – 1000° and B – 1150 °C.

gle mineral of the foundry sand (quartz SiO2) at the angle 2H = 26.6° (Fig. 1-B), as well as low intensities peaks of quartz at 2H° = 45.8°, 50.1°, and 54.9° with high-temperature modification of quartz - SiO2. Hematite Fe2O3 peaks barely visible also appeared in ceramic 6, besides a minimal quantity of two newly synthesized minerals fayalite Fe2SiO4 and albite NaAlSi3O8 (Fig. 4-A). However, after sintering at 1150 °C (Fig. 4-B), fayalite minerals peaks disappeared along with a substantial intensification of the xRays background, evidenced by a drop in the total intensity scale from 4000 to 3000 counts per second with a decrease of quartz SiO2 peak at the angle 2H = 26.6°. It is reasonably possible that the decrease in the intensity of the quartz peaks and the destruction of the crystalline structures of fayalite have contributed to such a substantial growth of the amorphous phase of ceramics 6 at 1150 °C. The lowering in the flexural strength of ceramics 6’s samples (Table 4) endorsed the motive for the increase in the crystal peaks intensity drop and the samples’ amorphous phase growth.

3.2.2. Thermochemical transformation of composite 6 during sintering The first endothermic effect of composition 6’s thermochemical analyses (Fig. 5) by DTA and DTG methods was feeble and revealed the loss of free water present in the pores until 105 °C with a weight loss of 2.33%. The second endo-effect in the range of 105°–291 °C with a weight loss of 6.33% indicated the removal of the water. The third endo-effect, between 291 and 574 °C, corresponded to the transition of boehmite ɣ-AlO(OH)4 to anhy-

Fig. 5. Thermochemical reactions of the composition 6 during sintering.

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drous c-Al2O3 and FeOOH ? ɣ-Fe2O3 ? Fe3O4 with a total weight loss of 5.36%. The strong exothermic effect between 574 and 1226 °C reflected the transformation, at 850 °C, of c-Al2O3 ? aAl2O3 and Fe3O4 ? Fe (700–750 °C) [27], with a total weight loss of 2.38%. Composition 6 contained two raw materials of ceramics in the same weight ratio (50: 50%). Nevertheless, as mentioned earlier in Section 2.2.5, red mud is much more thermochemical active and therefore it is more influential than the foundry sand, which is chemically more passive. As a result of the thermochemical reactions, composition 6 developed the highest flexural strength (6.85 MPa) after firing at 1150 °C, reducing the strength to 5.69 MPa at 1225 °C. The linear shrinkage at 1150 °C reached 6.62%, density – 2.07 g/cm3 at 1150 °C, and the water absorption was 2.77%. 3.2.3. Morphological structure development of composition 6 SEM photomicrograph (Fig. 6-A) of ceramics 6 after firing at 1000C showed a large number of particles of different configurations and sizes within 2–100 mm. Some of them were joined by molten material, but a much more considerable amount was unrelated. There were vast quantities of well visible pores between these particles also of various sizes and configuration. That was the main reason for the very low flexural strength values (2.13 MPa) and the rather high-water absorption value (16.23%) of ceramics 6 after sintering at 1000 °C. An increase in the firing temperature to 1150 °C (Fig. 6-B) led to the appearance of extensive fields of almost entirely molten material, pores were very rare, and the material was almost entirely monolithic. In many spots, the molten film seemed to be very thin, but still, it might serve as a thin film of adhesive material. Therefore, the flexural strength value increased almost thrice (up to 6.12 MPa) and of water absorption decreased almost twice (8.93%). 3.2.4. Micro-chemical analysis of the ceramic 6 by EDS and LAMMA analyses The micro chemical composition of the ceramic 6 provided information, by EDS method (Fig. 6-B), about the processes occurring in the temperature of 1150 °C. Points 1 and 2 (Table A1) of the big particle with very similar (81.37 and 82.65%) Si content is the grain of quartz of foundry sand. The content of each element varied significantly; for example, Al varied between 1.32 and 19.42%, Si between 17.60 and 82.65%, and the Fe content was also not uniform (between 8.41 and 62.49%). All of this confirmed the high level of chemical heterogeneity of the ceramic 6, which is typical for amorphous materials.

A

keV

B

keV

C

keV

Fig. 7. Isotopes composition of the ceramic 6 after sintering at 1150 °C by laser micro-mass analysis.

The results of laser micro-mass analyses (LAMMA) of isotopes composition ratified the conclusion on the results of XRD, DTA, TGA, SEM and EDS methods on the heterogeneity of the composited 6 after sintering at 1150 °C. All three points of LAMMA demonstrated (Fig. 7) a very different set of isotopes with very different values of their intensities (peak height).

3.3. Environmental properties of the developed ceramics The high leaching and solubility values of heavy metals from red mud (Table 3) obliged to control these values in the developed ceramics. The leaching assay was performed according to the procedure of NBR 10,005 [34]. The samples were dried and milled in order to obtain particles smaller than or equal to 9.5 mm. A pretest was performed using 5 g of the sample, which was transferred into a beaker where 96.5 mL of deionized water was added. The mix was vigorously stirred for 5 min. Then the pH was measured. Since the pH was inferior to 5, the extraction solution was prepared with 5.7 mL of glacial acetic acid and 64.3 mL of NaOH 1.0 mol L1, to which distilled, deionized, organic-free water was added until 1 L volume. The 100% solids residue leaching procedure was followed. For this purpose, 2 g of the sample was transferred into leaching flasks to which 40 mL of the extraction solution previously described was added, so that the solution quantity was 20 times the used mass. The flask was closed and kept under stirring for 20 h to complete (18 ± 2) h, at 20 °C of temperature with a rotation of (30 ± 2) rpm on the rotatory stirrer.

Fig. 6. Morphological structure of the composition 6 after sintering at: A – 1000 °C and B – 1150 °C.

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K. Alekseev et al. / Construction and Building Materials 229 (2019) 116860

Table 8 Leaching and solubility of composition 6 after sintering at 1100 °C. Elements

As Ba Cd Pb Cr total Hg Se Al Cu Fe Mn Zn

Leaching, mg/L

Solubility, mg/L

Red mud

Comp. 6

[26]

Red mud

Comp. 6

[26]

7.63 94.28 7.34 4.43 18.46 1.47 2.75 28.76 16.29 98.31 55.11 68.48

0.32 1.58 0.04 0.22 1.49 0.29 0.17 0.14 0.08 1.07 0.41 0.01

1.0 70.0 0.5 1.0 5.0 0.1 1.0 * * * * *

9.56 95.47 15.41 7.66 22.67 3.81 3.35 36.44 30.08 108.75 69.43 84.13

<0.001 0.032 0 <0.01 0.02 <0.001 <0.001 0.05 0.87 0.06 0.05 0.69

0.01 0.7 0.005 0.01 0.05 0.001 0.01 0.2 2.0 0.3 0.1 5.0

Solubilization assay was performed according to the procedure of NBR 10,006 [35]. In order to obtain the solubilized extract, the samples were dried at a temperature of 40 °C. A dried sample of 250 g was placed in a 1500 mL flask, and 1000 mL of distilled and deionized water was added. The mixture was stirred at low speed for 5 min and then left resting for 7 days at a temperature of up to 25 °C, protected by a plastic film (PVC). The suspension was filtered, obtaining the solubilized extract. The contents of the elements present in the leaching and solubilization extracts were determined by atomic absorption (FAAS or ICP-OES). Analyses of the filtrate showed (Table 8) a reduction of tens and hundreds of times in the metal content compared with the initial red mud. These results suggested the complete sustainability of the developed ceramics both for the construction materials for various purposes and their rubble at the end of the materials service.

4. Conclusions 1. The current study has shown the real possibility of using two hazardous industrial wastes as valuable raw materials for the production of environmentally friendly ceramic materials. The widespread reuse of industrial wastes at the industrial level will make it possible to stop and prevent, in the future, the extremely dangerous pollution of the environment and the death of the Earth’s population, scientifically calculated and proven by S.W. Hawking [1]. In the developed materials there are no traditional natural components such as clay and sand, which enables to halt the destruction by the quarries of unrecoverable natural bonds; 2. The physical properties of the developed ceramics far exceeded the demands of Brazilian technical norms. The values of flexural strength reached 10.54 MPa, after sintering at 1150 °C, linear shrinkage varied between 6.62 and 7.92%, water absorption between 2.77 and 14.41%, and bulk density between 1.65 and 2.07 g/cm3; 3. The study of physical-chemical processes of the structure formation showed the synthesis of mainly amorphous materials, closure of pores and bonding particles of the original components. Thus, high mechanical properties of ceramics and chemical bonding or strong encapsulation in the vitrified mass of metals, including hazardous heavy metals, are achieved. 4. The calculation of the economic efficiency of the developed ceramics production on an industrial scale was out of the authors’ scope. However, from a common point of view, free raw materials (or even surcharges for the use of wastes from their producers) should be very profitable compared to expensive natural raw materials, whose price is continuously growing.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.116860.

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