1. Introduction
In the process of any mining, a huge number of overburden and host rocks is produced. Being in dumps negatively affects the whole environmental media [
1,
2,
3]. At the present stage of civilization development, it is a topical problem to involve these rocks into the industrial turnover to produce new types of marketable products. Effective mining waste management will minimize its amount and solve the problems of environmental safety at mining enterprises [
4,
5].
The main industry that uses waste from mining enterprises is construction, for which they are high-quality raw materials. Production of concrete, consisting mainly of coarse and fine aggregates, is the first in this industry.
In the literature, there are works on using dump rocks as coarse aggregates [
6,
7]. At the same time, the type of crushed stone and its properties have a great influence on the quality of the obtained material [
8,
9]. Depending on the type of coarse aggregate, the concrete compressive strength may differ, practically, by 50% from the strength of the control sample [
10].
River sand is conventionally used as fine aggregate. However, its shortage and increasing restrictions on the extraction of natural sand in connection with environmental protection necessitate the search for alternative sources of raw materials. Rock crushing sand, mine refuses of crude ore can act as this alternative [
11,
12]. Adding them to natural sand improves the physical properties of binary mixes [
13]. At the same time, the quality of the obtained concretes depends on the shape of the particles, their surface, composition and content [
14,
15,
16].
Particles of crushed rock sand have an angular shape and smooth surface. Concretes containing the optimal amount of this aggregate have higher water demand, a smaller air volume, higher density and strength [
17].
According to physical and mechanical characteristics, silicate rocks, formed as a result of phosphate raw material extraction, are suitable for producing concrete B25 (25 MPa). The tests showed that materials made of them have high strength, and their quality is not inferior to the control sample [
7].
Studies on using granite extraction and processing waste (granite sludge) showed that it could be used to produce self-compacting mortar. If the granite sludge is up to 40% of the aggregate mass, the physical and mechanical characteristics of the mortar correspond with the control sample [
18].
During coal mining, there is a huge amount of waste produced, consisting mainly of aleurolites and sandstones. After preparatory works, they can be used as a substitute for natural sand in concrete production. To obtain materials of good quality, the amount of waste should not exceed 50% of the fine aggregate mass [
19].
Keramzite from coal processing waste is the raw material for light concrete production. The frangibility of keramzite concrete coarse aggregate from carbon-veil rocks is stabilized by 28 days of hardening. The compressive strength of the obtained material is 39–42 MPa and corresponds with the strength of the control sample [
20].
Fluorite sand waste is an alternative to natural sand. Replacing this with up to 70% of the conventional aggregate, it is possible to obtain concretes with high elastic modulus, tensile and compressive strength [
21].
When using mine refuse as fine aggregate, it is necessary to consider that adding it into the concrete composition reduces its placeability. It is shown in [
22] that the maximum content of mine refuses for the beneficiation of lead-zinc and copper ores in concrete should not exceed 30% of the fine aggregate mass since a higher percentage leads to a decrease in the strength characteristics of the obtained material. However, when using iron ore mine refuses, the optimal replacement percentage of natural sand varies within different limits. In the research [
23], it is 20%, and in the research [
24] is 35%. A further increase in the waste content deteriorates the physical and mechanical characteristics of concretes. For road construction, concrete mixes can contain up to 40% of iron ore mine refuses (complete sand replacement). At the same time, slag cement activated by alkalis is used as bonding material [
25].
Copper mine refuses to have an increased content of fine fraction. Therefore, their content should not exceed 15% of the concrete composition. The obtained materials are suitable for paving stone and block production [
26].
Among the mining waste, there is a great number of magnesium–silicate rocks. They are part of ultramafic–mafic massifs distributed throughout the world.
They include mineral deposits of various types: Ni-Cu and PGE [
27], Cr [
28], Fe-Ti-V [
29], asbestos [
30], jadeite and nephrite [
31], magnesite, talc, vermiculite [
32] et al. Kimberlite and lamproite pipes contain diamonds [
33,
34]. In their natural state, the complexes do not have a significant impact on the environment. However, they become a source of negative impact on nature in the process of mining. All these deposits contain a small proportion of valuable components. This is especially true for diamond and metal ore deposits. More than 90% of the extracted rock mass goes to dumps, taking into account dilution during mining.
While there are huge reserves, magnesium–silicate rocks are practically not used, although they are of good quality. Therefore, their application in the production of commercial products is an actual task. Let us examine the possibility of their application in the production of heavy concretes.
The solution to using magnesium–silicate rocks is shown by the example of the Yoko–Dovyren dunite–troctolite–gabbro stratified massif, Northern Baikal region, Russia [
35,
36].
The Yoko–Dovyren intrusive is located in the southern folded margin of the Siberian craton, 60 km to the northeast of Lake Baikal. It lies subconcordantly in carbonate-terrigenous rocks (mainly black shale) of the Synnyr rift [
37,
38,
39]. The Yoko–Dovyren massif is 26.0 × 3.5 km
2 in size. It is part of the Synnyr–Dovyren volcanic-plutonic complex. The complex also includes underlying sills of plagioperidotites and dikes of gabbronorites [
38,
40] with similar dikes in the roof. The complex includes effusives of the Synnyr ridge, overlapping these bodies, highly titanian basalts of the Inyapuk suite, low-titanian andesites and basalts of the Synnyr suite [
41]. The geochemical and isotope data indicate a genetic relationship between intrusive and low-titanian volcanic rocks [
42].
U-Pb ages of zircons are 728.4 ± 3.4 Ma for the Yoko–Dovyren massif and 722 ± 7 Ma for the associating volcanites [
43,
44]. Using baddeleyite from pegmatoid gabbronorites in the roof,
U-Pb dating gave 724.7 ± 2.5 Ma [
45]. As a result of tectonic movements, the Yoko–Dovyren massif has an almost vertical position. The intrusive is composed of contact rocks (chilled gabbronorites and picrodolerites, further plagioclase lherzolites), there are five zones above them (bottom-up): dunite (Ol + Chr) → troctolite + plagiodunite (Ol + Pl. + Chr) → troctolite + olivine gabbro (Ol + Pl + Chr ± Cpx) → olivine gabbro (Pl + Ol + Cpx ± Chr) → olivine gabbronorite (Pl + Ol + Cpx ± Opx). Quartz gabbronorites and pigeonite-containing gabbro (Pl + Cpx ± Opx ± Pig) in the roof belong to the additional intrusion synchronously with the dikes of gabbronorites.
3. Results and Discussion
When making concretes from various aggregates, we determined the placeability of concrete mixes characterized by mobility values (cone flow and cone slump). The results are shown in
Table 4.
The cone slump and the cone flow of all the presented compositions of the concrete mixes reach satisfactory values. However, it should be noted that adding magnesium–silicate aggregates reduces their fluidity. It is influenced by the size and shape of aggregates (
Figure 7).
Quartz sand has a rounded shape without sharp corners. The shape of the dunite sand particles has a rough surface with numerous protrusions and depressions. This contributes to a tighter adhesion of the aggregates to the binder.
The average density of concrete mixes also depends on the type of aggregate and has the lowest value for the standard sample (2371 kg/m3).
The main characteristic of the concrete quality is compressive strength since, by its change, you can observe changes in the microstructure of the hardened stone. This characteristic depends on aggregates added in the concrete composition [
10]. The compressive strength of the samples (P) at the age of 7, 14, 28 and 90 days (the average value of three cubes) is shown in
Table 5. The variation of the change in strength is estimated by characteristic α, which represents the ratio of the strength of the studied sample to the strength of the control sample at the respective hardening age.
As shown in the table, the mechanical characteristics of all samples increase over the hardening time. The main strength gain occurs by 14 days of concrete hardening and is more than 70% of the strength at 28 days of age. The mechanical characteristics depend on the type and the quality of the used aggregates. Magnesium–silicate aggregates facilitate an increase in strength of the studied samples. The greatest increase in strength is observed in concretes, where dunite sand is used as fine aggregate. It differs from quartz sand in composition, size and shape of the particles. As you know, these characteristics significantly affect the mechanical properties of the obtained materials [
16,
18,
51,
52].
We found the average density of hardened concrete; it is shown in
Figure 8.
The type of aggregate has an impact on the concrete density. The highest values are observed in samples where magnesium–silicate rocks are used as coarse aggregates, and dunite sand is used as fine aggregate. Simultaneously, the concrete density decreases depending on the type of coarse aggregate in the row dunite → wehrlite → troctolite → granite and has the following values 2800 kg/m3 → 2763 kg/m3 → 2666 kg/m3 → 2358 kg/m3. It can be explained by the high specific gravity of the raw materials and the packing density of the obtained materials.
In
Figure 9, on the contact border aggregate-cement stone, cavities of small thickness are formed around the aggregate surface. The cavities have rounded edges, which indicates their formation during the hydration of the cement dough. In addition, the cement particles themselves have cavities and cracks of sufficiently small sizes that are inaccessible to water penetration. No deep cracks were found in the concrete structure; the cracks are caused by loose convergence of cement particles due to insufficient hydration.
As a result of the research, it was found that new formations that crystallize in the cement with the addition of magnesium–silicate rocks lead to the strengthening of the structure of new types of concretes and an increase in the strength of the obtained materials.
When studying the physical and mechanical characteristics of the obtained concretes, we identified their water absorption. The study was conducted following the requirements of the Russian standard (GOST 12730.3-78, “Concretes. Method of determination of water absorption”) [
53]. The results are shown in
Figure 10.
It is known that the water absorption of the concrete samples depends on the aggregate type [
54]. Aggregates, made of magnesium–silicate rocks, have a positive effect on this characteristic. Adding them into concretes decreases the water absorption. The highest value of this characteristic is observed in the concretes made of granite crushed stone and quartz sand.
The water resistance of the concretes was calculated, which is characterized by softening coefficient Csoft, equal to the ratio of the strength of the water-saturated samples to the strength of the dry samples. In the work process, some of the samples were kept in water for two days, after which their strength values were measured. The conducted studies showed that magnesium–silicate rock concretes have softening coefficient equal to 0.85–0.87. For the control samples, the water resistance was 0.82.
The obtained data are consistent with the concrete frost resistance. The values are shown in
Table 6.
After 75 freeze–thaw cycles, no damage was observed on the surface of the concrete samples. The samples withstood 50 freeze–thaw cycles without significant changes in weight. The loss of mass of the concrete samples with the addition of aggregates from dunite by 1.18%, from wehrlite—by 1.34%, from troctolite—by 1.67% was recorded. For the control sample, it was 1.83%. After 75 cycles, this characteristic was at the limit of acceptable values (2%). The frost resistance coefficient also reaches its limit values after 75 freeze–thaw cycles. Based on the obtained values, as well as the compressive strength values after the testing completion, the concrete was graded F50 for frost resistance.
The increased water absorption and frost resistance values in the concrete samples with the addition of magnesium–silicate rocks are explained by a denser structure than in the control sample. The decrease in open porosity reduces the amount of absorbed liquid, which helps to soften the structure of the obtained material.
Abrasion testing has shown that the weight loss of the samples, made of magnesium-containing aggregate, does not exceed the weight loss of the control sample (
Figure 11).
Dunite concrete has the best characteristics (0.63 g/cm2). This can be explained by the quality of the raw material. Magnesium–silicate rocks do not contain grains of weak rocks. Concretes on them made of this material have increased density and strength, which also affects this characteristic. According to the abrasion capacity, the concretes have grade G1
Thus, concretes, made of magnesium–silicate rock aggregates, have high physical and mechanical properties and can produce load-bearing and special structures.
4. Conclusions
Among mining waste, there are many magnesium–silicate rocks. They are part of ultramafite–mafite complexes, which are found everywhere and include various mineral deposits. Having high physical and mechanical characteristics, they can be used as coarse and fine aggregates in heavy concrete production.
As a result of the studies, it was found that adding magnesium–silicate rocks to the composition of concrete mixes reduces their fluidity. The use of dunite sand helps to reduce the water demand of concrete mixes and makes a dense structure of the hardened material.
The compressive strength of the concretes made of magnesium–silicate rock coarse aggregate at the age of 28 days of hardening is within 28 MPa, while for the control sample, this characteristic is 27.3 MPa. Replacement of quartz sand with dunite sand also leads to an increase in the concrete strength (~4%). Complete replacement of coarse and fine aggregates with magnesium–silicate rocks increases the concrete strength by 15–20% than the control sample.
The concrete density depends on the type of aggregate as well. The samples containing dunite have the highest density values (2800 kg/m3), the reference samples have the lowest ones (2358 kg/m3). This can be explained by the high specific gravity of the used raw material and by the packing density of the obtained material.
The concrete water absorption is within 6%. The softening coefficient is 0.85–0.87. They are marked F50 for freeze resistance and G1 for abrasion capacity.
The obtained results showed that concretes made of magnesium–silicate rocks are a promising replacement for conventional concrete in building constructions. This will contribute to developing the concept of a closed-loop economy and will facilitate the environmental safety of the mining industry.
However, to determine the possibility of using the obtained materials in special structures, it is necessary to investigate the aspects of chemical decomposition of magnesium–silicate rock concretes in an aggressive environment.