Steel-smelting slags of Kuzbass in production of clinker-free fine-grained slag concrete

. Article presents the results of experimental studies of fine-grained slag concrete based on mechanically activated steel slag of West Siberian Metallurgical Plant (Novokuznetsk). The purpose of research was to develop scientific principles for obtaining clinker-free fine-grained concrete based on activated steelmaking waste resources. For slag crushing for the purpose of binding properties initiation commercial equipment have been selected; coal industry waste of the Abashevskaya mine (Novokuznetsk) was used as activators, i.e. the pit dump burnt rocks, and sludge, obtained by neutralization of used acid batteries, batteries for industrial vehicles with lime electrolytes. Disposal of steelmaking waste remains a challenging issue and possibility of using this type of waste in production of clinker-free fine-grained concrete for industrial and civil construction will create an industrial resources base for the construction industry to improve ecological situation in the region. Change in structure of steelmaking resources using mechanical chemistry has increased its structural instability and activity and has enabled obtaining waterproof clinker-free fine-grained slag concrete of matrix structure (softening factor 0,84 – 0,95) and В 7 strength class. The developed slag concrete has been tested at construction companies in the region.


Introduction
Kemerovo region with a territory of 95.7 thousand km 2 and population of 2592 thousand people has a large mineral and raw material potential, which development allowed it to become one of the leading industrial regions of Russia. The regional enterprises are part of large transnational mining and metallurgical companies: Evraz Group SA (Luxembourg), United Company RUSAL, Ural Mining and Metallurgical Company, Mechel, Siberian Mining and Metallurgical Company, Industrial Metallurgical Holding Management Co. All companies are integrated into the global metal market.
Development of resource-intensive industries is associated with significant environmental costs: the total volume of waste generated annually at the enterprises of the region is already about 1,700 million tons. The situation is aggravated by high concentration of unsustainable industries in a relatively small area.
Development of local industrial resource base for construction industry in the region involving secondary products in production process is a prospect research area that allows reducing negative environmental impact. It is driven by both economic and environmental requirements.
Electric steel-smelting (ladle) slags of one of the metallurgical the Kuzbass enterprise (Novokuznetsk Metallurgical Plant (NKMK)) is the object of the study.
Physical and mechanical properties and composition of slags are influenced by mineral resource base, represented by iron ore deposits of the Altai-Sayan folded region and the Angaro-Ilimsk iron ore region (ores with magnesian waste rock and a high content of Al2O3), Salair limestones, Gornaya Shoria dolomites and Antonovsky mine quartzites. The slag composition also depends on ash and sulfur, which content in Kuzbass coals coke is 0.4-0.6%.
Slags are highly basic (CaO content up to 60%), with a high content of aluminum oxides (10-18%) and magnesium (8-12%). Despite their chemical composition close to the Portland cement, they do not have cementing properties, are unfit for hardening and maturing, since C2S phases present in their composition are in a crystallized state and do not hydrate when interacting with water [6].
The theoretical and methodological basis of the presented research was the research results of leading scientists in the field of building materials science in the field of structure formation processes control in the dispersed mineral systems and study of mechanochemical technologies for technogenic waste materials processing in production of building materials and products for various functional purposes with desired properties [7][8][9][10][11][12][13][14][15][16][17].
Analyzing the research results, it can be concluded that application of secondary largetonnage mineral resources that are not inferior to natural raw materials in their mineralogical and chemical composition is the fundamental concept of modern building materials science [18][19][20].
However, steel-smelting slags continue to be the least demanded in production of building materials, and are considered unpromising resources due to instability of chemical composition, tendency to silicate decomposition, inability to self-harden and mature.
The studies conducted have shown that combined mechanochemical activation is an effective technology for processing structurally unstable electric steel-smelting slag resources [21].

Materials and methods
The purpose of research was to develop scientific principles for obtaining clinker-free finegrained concrete based on activated steelmaking waste resources.
To achieve this goal, the following tasks have been solved: 1. Justification of the choice, study of material composition and technological characteristics of technogenic waste resources (mineral waste from metallurgical and mining industries) for the purpose of its application in clinker-free fine-grained concrete production.
2. Determination of the optimal composition and basic technological parameters for clinker-free slag concrete production with the required physical and mechanical properties.
3. Study of the processes of phase formation and formation of the matrix structure of a clinker-free binder system based on mechanically activated steelmaking waste resources. 5. Feasibility study of design solutions with pilot testing in the factory.
The following acid technogenic resources were used as activating additives: burnt waste from coal mining and processing, i.e., burnt rocks of the Abashevskaya mine dumps; sludge (flooded highly plastic gel) obtained as a result of lime neutralization of spent acid batteries of special types of transport (electric locomotives, electric cars, forklift trucks, electric tractors and bulldozers) of the same enterprise have been chosen as a dispersion medium.
The chemical composition of technological samples of resources is presented in Table  1. Resource mixture composition was calculated by coefficient of basicity. To find ratio of components (slag and burnt rock), the following equation has been solved: Resource with Кbas > 1 / Resource with Кbas< 1 = = In the numerator -slag chemical composition, in the denominator -burnt rock chemical composition.
where n is a value of Kbas = 1.6 set for the binder mixture; x -the number of mass parts of resources with Kbas > 1 (slag) per a weight part of the resource with Kbas < 1 (burnt rock).
The proposed slag concrete is a composite of matrix structure, consisting entirely of secondary resources from mining and metallurgical production with complex composition of resources, which makes it difficult to choose grinding equipment.
It has been established experimentally that impact method is the most effective of various mechanisms for the mill charge particles loading (static, quasi-static, cyclic, and impact), when strain accelerations reach large values. Impact loading is carried out in various mills: vibrating, ball, jet, centrifugal, hammer, and under electro-hydraulic impact.
To grind slag in order to initiate its binding properties, commercially available equipment was selected ( Table 2).

*Types of mills used in experiments
The grinding time depends on the selected grinding unit. The greatest fineness of grinding when dispersing slag in an acidic environment is achieved in planetary mills with less grinding time than in other units. The advantages of such mills are: energy efficiency, no vibration, easy adaptation to production.
Preparation of the slag-concrete mixture has been carried out by dispersing electric steel-smelting slag together with burnt rock and sludge.
The type of equipment chosen for production of slag concrete mixture affects the grinding time. Thus, the grinding time to a specific surface of 340-350 m2/kg in a drum mill is 30-35 minutes, in a planetary mill -8-10 minutes.
As a result of mechanochemical activation of a mixture based on steel-smelting slags, a clinker-free binder of matrix structure has been obtained [22] with compressive and flexural strength of 6.60-10.64 and 1.94-5.62 MPa, respectively.
The research performed resulted in accumulation of significant experimental material for study of the processes occurring during hydration of a clinker-free mixture [23]. Generalization of data on generation of hardening systems based on activated steel-smelting waste resources have been implemented in development of high-slump fine-grained cinder concrete (with a cone draft of 12-16 cm).

Results
Samples of 70×70×70 mm were made out for the activated mixture of different grinding duration. To determine water resistance of the material, one batch of prototypes was kept in natural conditions (t = 18 ± 200), the other in the state saturated with water to determine the softening coefficient. The strength and water resistance of samples at the age of 28 days are shown in Table 3. It is known that in the process of fine grinding of bulk materials, only a part of the total energy supplied is spent on generation of a new surface, the rest of the energy is accumulated in the processed material in form of stressed structural defects. This accumulated energy subsequently has a significant impact on the rate of various technological processes, as well as on basic physical and mechanical properties of the resulting materials.
With ultra-fine (specific surface 370 m2/kg) and long-term grinding of mixture (treatment duration, more than 10 minutes), a moment came when large internal stresses arose in the material, which led to appearance of cracks and a decrease in strength characteristics.
In our opinion, during chemical activation of mixture, the following interaction of minerals is possible: 1. Formation of dihydrate calcium sulfate: As a result of the reaction of lead ions substitution (in lead sulfate sludge) for calcium ions (free calcium oxide of slag and burnt rock).
2. Formation of calcium orthosilicate: 2CaOfree. + SiO2 → Ca2SiO4   Table 4. The optimal mixture by strength was chosen out of the 3 studied compositions (No. 1). The optimal mixture of fine-grained slag concrete is protected by a patent for the invention "Concrete mix" [24].
During 2 years, slag concrete of optimal composition was studied for basic physical, mechanical and deformative properties (Table 5). It has been found that clinker-free fine-grained slag concrete increases its compressive strength and prismatic strength by 13 and 19%, respectively, within 2 years. The maximum growth in the first year is up to 15% and for the subsequent period less than 5%. Duration of the hardening process is explained by presence of CaSO4 × nH2O in the slag (the course of hardening process is similar to gypsum binders, B group -slowly fastening: start of fastening is not earlier than 20 minutes, the end is not standardized), GOST 125-79.
Increased shrinkage is one of the disadvantages of fine-grained concrete. However, studies have shown that over a 2-year period, shrinkage was only 0.46 mm/m. On the first day, shrinkage is insignificant, and amounts to 0.12 mm/m, then up to 28 days it is within 0.23 mm/m. At the age of 28 days to 180, it continues to increase (0.35 mm / m), and over 180 days it practically stops.
The cinder concrete mixture supplied to the place of laying must have the required average density with deviations of no more than ± 20%, P3 workability grade of 10-15 cm, GOST 7473-2010.
The modulus of elasticity of slag concrete has been determined. During the experiment, the sample was subjected to a continuously increasing load, until complete destruction with the help of a loading device. Data have been entered into the diagram showing the effect of loads on degree of deformation (Fig. 2). At the final stage, the average modulus of elasticity of all samples has been calculated. It has been found that over 2 years it had increased by 23% and does not exceed the requirements of SNiP 52-01-2003 for fine-grained cement concrete.
To determine the shrinkage and creep deformations of cinder concrete, samples were tested -prisms with a size of 10 × 10 × 40 cm. The prisms were in an air-dry environment with a relative humidity of 50-75% at a temperature of 18-220C. Deformation shrinkage was measured using a tripod device with a dial indicator with a division value of 0.001 mm.
The study of concrete time yield has been carried out on special spring installations, starting from the age of 28 days. Prisms 10×10×40 cm were loaded with a long-term load, the stress from which is 50% of the prism strength. Time yield develop most noticeably during the first day after the load application (up to 0.57 mm/m), then increase slightly (up to 0.63 mm/m) with gradual attenuation. For 2 years, compressibility of slag concrete has increased by 6%, and extensibility by 18%.
Stress of reinforcement adhesion with concrete has been determined by axial pulling out of a reinforcing bar with a diameter of 12 mm from a cinder-concrete cube sample of 250 × 250 × 250 mm with an embedment length of 5d, since when pulling out, the adhesion strength of a compressible bar is greater than when indenting, due to resistance of the surrounding concrete layer to the transverse expansion of the compressible rod.
A total of 9 samples have been made and tested (3 samples of each composition). Cube specimens have been tested on a GMS-20 tensile testing machine. Forces have been applied to the rod in stages and maintained at each stage for 3 minutes. When the rod has been pulled out of the sample, it splitted.
Shear stresses for axial pull-out tests were calculated by the formula: where P is the applied load, (L); C is the length of the rod circumference, (mm); С = π×d; L is the length of the embedment of the rod in concrete, (mm); d is the rod diameter (mm).
The test results presented in Table 6 show that the bond strength values with reinforcement varied from 1.9 to 2.62 MPa. Analysis of changes in structure during generation of slag concrete stone, carried out using an X-ray microanalyzer (JEOL JSM-6390 LV scanning electron microscope with a JED 2300 energy-dispersive attachment) showed that after completion of crystallization processes, increase in strength of slag concrete slows down (28 days), but does not stop. A further increase in its strength is due to evaporation of moisture: crystallization centers are generated, making the structure more thermodynamically stable. An x-ray phase analysis of concrete samples has been carried out: characteristic x-ray spectrum of the sample has been recorded from an area of 9 mm2 at an accelerating voltage of 20 kV and a probe current of 1 nA; resolution of the energy dispersive detector is 133 eV; calculation of the percentage of each element in the analyzed substance has been carried out on the basis of the obtained spectra using JEOL Engineering Analysis Station software of 3.62.07. version by means of non-standard ZAF method; mapping of chemical elements has been carried out at an accelerating voltage of 20 kV and a probe current of 1 nA (the letter to the right of the name of the element is the line of characteristic radiation spectrum from which the signal has been recorded). The color scale on the left reflects the intensity of the characteristic radiation signal.  It has been found that the structure of stone (Fig. 4) is a volumetric spatial frame -a matrix of continuous structure, formed from fine-grained masses of gypsum dihydrate, which perform function of a "binder". The matrix combines a "filler" -crystalline columnar  It is likely that their randomly oriented structure provides reinforcing effect leading to an increase in strength and modulus of elasticity. Real crystals of the filler of dispersed phase contain various defects in a lattice structure: distortions and irregularities on the faces and deformations; have reduced symmetry due to various internal defects and influences.
They envelop aggregate crystals, structured particles of etringite, which are an interfacial layer in the matrix structure.

Discussion
Fine-grained concretes based on clinker-free binders are widely used in various fields of building construction [25,26].
A comprehensive study of local mineral waste from metallurgical and mining industries has shown that current state of science and technology makes it possible to obtain clinkerfree fine-grained slag concrete based on activated, structurally unstable electric steelmaking wastes.
Physical and chemical studies have established specific features of transformation of the matrix structure of a clinker-free binder system: the interaction of mineral components of the system with identification of the phase composition of neoplasms and staging of their changes due to mechanochemical synthesis. The presence of fine-grained masses of twowater gypsum in the system in the early stages of hardening contributes to generation of a matrix with high-density structure and low porosity; reinforcing effect, leading to an increase in strength and modulus of elasticity, is provided by granules of quartz, magnesium oxide and calcium orthosilicate, interfacial layer in the matrix structure is formed by structured particles of etringite.
During industrial testing of scientific results, their reliability and technological efficiency of application of the developed slag concrete mixture for general construction purposes have been documented. Based on comparative calculation of the final product cost, economic feasibility of its use has been established (35 to 61% reduction in cost of construction products with its application).

Conclusion
The optimal composition of the slag concrete mixture has been determined, which satisfies the criteria for its application in construction: compressive and bending strength, respectively, 10.00-11.73 and 5.97-7.00 MPa; average density 1950-2000 kg/m3, modulus of elasticity 7960-9330 MPa. Slag-concrete mixture is waterproof, it maintains uniformity of volume change. The beginning of fastening is 20 hours, which can be convenient for transportation over long distances, when using it to make monolithic sections during installation of prefabricated structures, as well as large-scale repair work where preliminary preparation of concreting sites is needed. A test for physical, mechanical and deformation characteristics has shown that it meets all the requirements of SNiP and GOST for finegrained concrete and can be used for the manufacture of internal fencing elements in prefabricated and monolithic designs.
The main provisions and results of the work were used in development of the regulatory framework for implementation of the proposed technologies at the enterprises of building construction industry.

Acknowledgments
The author expresses her gratitude to Professor Berdov G.I. Dr. Sci. Eng., Honored Worker of Science and Technology of the Russian Federation for his help during performance of experimental work and discussion of their results, to Lukhanina T.M., General Director of Avangard Trade and Production House LLC for assistance in pilot testing of the developed slag concrete.
The author expresses her gratitude to Khodosov E.V., Director of KUZBASS Testing Center for Building Materials and Structures LLC for assistance in conducting experimental studies, to Volynkina E.P., Dr. Sci. Eng., Academician of the Russian Ecological Academy, a Director of Ecological Regional Center LLC, Chairman of the Presidium of the "Kuzbass Association of Waste Processors" SRO for support, positive feedback and recommendation for implementation of the developed technology for production of fine-grained slag concrete.
The author expresses her gratitude to Krzysztof Klimkowicz, Director of the European innovation company "Europejska Grupa Innowacyjna Sp. z o.o. (Rzeczpospolita Polska, Kędzierzyn-Koźle) for his interest to the developed technology and proposal for cooperation.