Introduction

The construction industry demands the massive exploitation of natural resources. An alternative to mitigate the impact of these processes and protect the environment is to use some solid waste from mining processes as partial substitutes for raw materials to generate new engineering materials (^{Ferreira et al., 2015}). This contributes to reduce the extraction of virgin materials and to mitigate the impact of the generation of waste that, due to its large scale production, has become a crucial problem to solve regarding with environmental issues (^{Novais et al., 2015}).

Mining activities generate economic benefits worldwide; however, they also produce large amounts of solid and liquid waste that negatively impact the environment if they are not properly managed (^{Benzaazoua et al., 2008}; ^{Sivakugan et al., 2006}). Annual global production of mining solid waste is estimated to be between 20 and 25 billion tons, of which 5 to 7 billion tons are mine tailings (^{Mudd & Boger, 2013}). For the disposal of mining waste there are various methods (i.e., storage in piles or deposits, filling of underground mines and open pits, underwater disposal and disposal of slurries); depending on the method, there will be migration of contaminants into the soil, water or air, generating environmental and health problems (^{Arcos, 2017}).

^{Orozco and Orozco (1992)} define a mine tailing or waste as the non-commercial residue of mining processes with characteristics generally similar to a sand, which are deposited in storage dams.

The reuse of mining tailings might help to mitigate the problems mentioned, and could have different applications depending on their physical and chemical characteristics that depend on their mineralogy and geochemistry, type of mining equipment, particle size of the material and moisture content (^{Lottermoser, 2010}). Since the middle of the last decade, the effectiveness of the reuse of mining waste in ceramic products, glass, glass ceramics, cement blocks, bricks, concrete and high performance concrete has been investigated with positive results, resulting in an increase in the tendency to use them for geotechnical and construction purposes (^{Roy et al., 2007}; ^{Choi et al., 2009}; ^{Ramesh et al., 2012}; ^{Zhu et al., 2015}; ^{Zhao et al., 2014}). However, ^{Benzaazoua et al. (2002)} make evident the potential risk of leaching trace elements from untreated tailings, which makes their application in geotechnical engineering as substitute aggregates and fillers for landfills, structural fillers and road construction materials difficult, indicating that stabilization treatment is necessary before their use for construction purposes. The stabilization consists of encapsulating the contaminants, making them insoluble in a stable pH environment and propitiating chemical stabilization, particularly by breaking the bonds of the metals, which reduces their potential pollution and toxicity (Voglar & Leštan, 2011). This stabilization can be performed by biological, chemical and/or physical methods, impeding the spread of contaminants in the soil by reducing their leaching and bioavailability.

In regards to this, several investigations (^{Harter, 1983}; Gerriste y Van Driel 1984; ^{Naidu et al., 1994}) indicate that there is a direct correlation between soil pH and the retention of heavy metals, resulting that the basic soils present a greater adsorption of heavy metals than those with acidic pH.

In particular, the adaptation of these waste for their use in road construction is easy and appropriate because they can present good engineering characteristics (shear, compressive and tensile strength, compaction, compressibility, permeability and low erodibility by rain) whether or not their properties are modified by the addition of Portland or asphalt cement (^{Sultan, 1978}; ¹⁹⁷⁹). ^{Mahmood and Mulligan (2010)} studied the possible use of mine tailings mixed with Portland cement as unpaved road bases, finding that the mixtures produced with the materials from five different mines, of the six sampled, have enough characteristics and are suitable for use as support layers based on unconfined compressive strength tests. ^{Qian et al. (2011)} used granite mill tailings stabilized with 5 % of cement in a subbase layer, which presented simple compressive and tensile strengths similar to cemented stabilized stone aggregates, and with higher static and dynamic modulus than them; they constructed a 20.38 km test section using these tailings as a subbase, proving the viability of their use as a substitute to virgin aggregates. ^{Ojuri et al. (2017)} experimented with the use of mine tailings from an iron mine to form the subgrade layer; they made test specimens with different percentages of lateritic soil, mine tailings and a combination of lime-cement with ratio 1: 2, which were tested for CBR, simple compression, leaching and mineralogical composition; the addition of mining tailings to the layer decreased the consistency limits and the percentage of material passing the no. 200 sieve, as well as increasing the CBR values and simple compression resistance; to guarantee the safety of the mixture, they found that stabilization with cement and lime at a rate of 8 % brought the quantity of leachates to levels accepted by the regulations.

The results of the investigations suggest that the use of mine tailings in the formation of support layers in the pavement structure is feasible, as long as the characteristics of each material are determined and it is placed accordingly within the structure (^{Oluwasola et al., 2015}; ^{Mahmood y Mulligan, 2010}; Quian et al., 2011; ^{Ojuri et al., 2017}). However, the index, grain-size distribution and engineering properties vary significantly among the samples studied by each author. Due to this discrepancy, it is not possible to generalize the expected results for all materials.

In México, there are hundreds of millions of tons of mine tailings dispersed without an adequate control of storage conditions and environmental affectations, but few resources are assigned to carry out the diagnosis and risk evaluation of the existing tailings dams (^{Ramos y Siebe, 2006}). The sites that are most affected by heavy metal contamination derived from mining are the states of Zacatecas, Querétaro, Hidalgo and San Luis Potosí (^{Covarrubias y Peña, 2017}). Zacatecas is the second largest mining producer in the country. In the period between 1982 and 2014, for the extraction of gold, silver, copper, lead and zinc minerals, 17025 million tons of solid waste were generated, producing 1897.9 million kilograms of cyanide that have contaminated 55.78 million cubic meters of water per year (^{Guzmán, 2016}).

Therefore, this research focuses on studying mining waste from the state of Zacatecas, in order to determine its possible use within the pavement structure or as an aggregate in hydraulic concrete mixtures and to mitigate the regional problem. In contrast to previous investigations, the material extracted from the sampled tailings dam will be used in the pavement structure or as hydraulic concrete aggregate in its natural state, this means without adding virgin aggregates, in an effort to use as much of the waste as possible and minimize the extraction of virgin aggregates.

Methodology

In order to characterize the mining waste and determine its potential use as a subgrade material in the pavement structure or as an aggregate for hydraulic concrete, the sampling and nine laboratory tests described below were performed.

Results and discussion

The mine tailings sample for this investigation was obtained from a mining deposit from a mine located in Zacatecas, where four main minerals are extracted: copper, silver, zinc and lead by the flotation extraction process. Due to the nature of the work carried out and the sensitivity of some data, the complete confidentiality of the sampled tailings dam company was arranged, as it does not influence the analysis of results.

The chemical composition obtained by X-ray diffraction shows the presence of quartz, calcite, sanidine, microcline, amesite, clinochlorine and grossular. The X-Ray diffraction pattern of the sample is detailed in Figure 1.

Figure 1: X-ray diffraction pattern resulting from the analyzed mine tailings. Source: self-made 

The sample is mostly composed by quartz and calcite, while the remaining compounds present are characteristic of the rock containing the minerals with commercial value (^{Moreno et al., 2011}).

The results obtained from X-ray diffraction show us a general overview of the components that constitute the sample; however, for this investigation it is crucial to determine the amount of heavy metals present in the sample as they are potentially contaminating elements for the environment. The 64 elements that compose the material were analyzed, emphasizing the twelve elements contemplated in the Mexican Official Standard NOM-147-SEMARNAT/SSA1-2004 that establishes the maximum permissible concentrations in contaminated soils by heavy metals, the results are shown in Table 1 (^{SEMARNAT, 2004}). It can be determined that, in its natural state, the sample of mine tailings is not adequate as soil for agricultural, residential or commercial use because it exceeds the permissible limit of arsenic.

To emulate the transport of heavy metals by exposure to rain, the material was subjected to a leaching test. Subsequently, the pH of the leachate generated was measured, obtaining a value of 7.29, considered to be neutral. The eight heavy metals considered in the Mexican regulations were analyzed to determine the innocuousness of the material through the three atomic absorption techniques. The values are below the permissible limits, so it is concluded that the material does not require stabilization. Even with this, and based on the previously mentioned investigations (^{Mahmood y Mulligan, 2010}; Quian et al., 2011; ^{Oluwasola et al., 2015}; ^{Ojuri et al., 2017}) the same tests were done for two mixtures of mining waste stabilized with percentages of 1 and 3 % ordinary Portland cement in order to analyze the effect of encapsulation on those elements that were detectable in the leachate. The results obtained with the reference values of the regulations based on NOM-001-SEMARNAT-1996 are shown in Table 2 (^{SEMARNAT, 1996}).

Leachates obtained from cement-stabilized samples show an increase in pH as a result of the percentage of cement, which increases the adsorption of heavy metals from the material, decreasing its leaching potential, as reported in the literature (^{Harter, 1983}; ^{Gerritse y Van Driel 1984}; ^{Naidu et al., 1994}). The increases in the levels of chromium compared to the leachate in its natural state are caused by the presence of this element in the portland cement (analysed by the atomic absorption spectroscopy technique), so it is recommended in future research to experiment with different alternatives of cements.

The sample of the residues were subjected to geotechnical and mechanical tests to evaluate their possible use as a subgrade layer in a pavement. The results of the tests, as well as the compliance values for the layer mentioned according to the Mexican Standard N-CMT-1-03/02, are summarized in Table 3 (^{SCT, 2002b}).

Based on Table 3, it is determined that the material has a maximum size, liquid limit, plastic index and expansion below the maximum specifications according to the regulations. It presents a CBR greater than the minimum required to satisfy the characteristics required to be used as a subgrade layer and, due to the fact that it does not run the risk of potential leaching, it is not necessary to apply a stabilization treatment. The characteristics obtained from the tests carried out were compared with the current Mexican Standard N-CTM-2-002/02 for quality of hydraulic concrete aggregates. Table 4 shows the values required for fine aggregate; the liquid limit is in the range of 26 to 35, it does not present plasticity and its fine content is less than 15 % recommended for non-plastic materials (SCT, 2002a).

Figure 2 shows the resulting particle size curve of the mining waste sample compared to the particle size limits for fine aggregate of hydraulic concrete materials. Based on this, it is possible to use the material, after adjusting its particle size as fine aggregate in the fabrication of hydraulic concrete. Due to the fact that there is an excess of material passing through the 200 sieve, two methods of particle size correction are recommended. The first consists of sieving the material through a mesh with an opening of 0.075 mm in order to remove the fine particles from the material, while the second consists of washing the material using industrial methods in mills specifically designed for this purpose. In both cases, the residues must be returned to the dam from which they were initially extracted and which complies with the regulatory requirements for this purpose. It is important to note that additional tests for reactivity with cement alkalis and accelerated weathering are required to confirm the potential use of the waste as fine aggregates in hydraulic concrete mixes. With respect to the grain-size distribution required to comply as a subgrade layer, the material has the necessary characteristics, considering that the regulations that were taken as reference only establish the maximum particle size which is 76 mm and there is no minimum limit of compliance, as shown in Table 3 (^{SCT, 2002b}).

Figure 2: Particle size of the sample compared to standards. Source: self-made with data of SCT (2002a) 

Conclusions

The sampled residues have the necessary characteristics of grain-size distribution, resistance and consistency limits to be used as subgrade material according to the Mexican Standard N-CMT-1-03/02 (^{SCT, 2002b}). The material can also be used as fine aggregate in the elaboration of hydraulic concrete according to standard N-CMT-2-02-002/02 (SCT, 2002a), being subjected to a method of particle size correction, either sieved through a 0.075 mm mesh or washed in mills, in order to reduce the content of fines in the material and performing the rest of the necessary tests such as reactivity with the alkalis in the cement and accelerated weathering.

The stabilization of mine tailings was not necessary because the material transported by leaching is below the permissible heavy metal limits according to NOM-155-SEMARNAT-2007 (^{SEMARNAT, 2007}). This may be due to the neutral pH of the sample, which indicates a high adsorption of heavy metals that would avoid their migration. On the contrary, there was an increase in the chromium content of the leachates after the addition of different percentages of Portland cement. Therefore, the chemical composition of the cement used for the stabilization and its effect on the transport of contaminants by leaching should be considered before employing it as a building material in engineering works.

The construction industry requires materials and soils for different purposes, which means the massive extraction of virgin resources. The characterization of mine tailings as possible substitutes for these raw materials represents an opportunity to mitigate the environmental impact generated by mining activity and the overexploitation of material banks. However, it is necessary to characterize each mine tailings dam to determine the possible uses of these waste.