**1. Introduction**

The currently dominant model of goods production in the economy is linear. This assumes the acquisition of raw materials, the production of specific goods associated with the simultaneous production of waste, and then the goods produced after their consumption also become waste. This linear, unidirectional model begins to reach its limits due to the limited amount of natural resources. Another disadvantage is the production of large amounts of waste, which are deposited in landfills. Such landfills not only occupy a place, but can also be a source of emissions of harmful substances or radiation.

In order to be able to develop further in a harmonious manner we must follow the example of nature, which continually performs recycling processes [1,2]. Thanks to decay processes, which are an important part of its internal cycle, nature is an ideal example of a zero-waste economy. Trying to get at least a little closer to this model, it is worth making attempts to reuse post-production waste, treating it not as waste, but as raw materials of a new era. This is the basic premise of a circular economy, which is currently gaining more and more interest.

The cement and building materials industries offer great opportunities for using different mineral by-products. Materials, such as fly ash, silica fume and blast furnace slag, are commonly used as supplementary cementitious materials (SCMs) [3], the introduction of which into cement composites gives the possibility to reduce the amount of cement used and, consequently, a reduction of the adverse impacts of cement production on the environment. On the other hand, reduction of the amount of landfilled waste is possible. However, the introduction of SCMs into the concrete changes its chemical composition and rheological properties. In effect, the properties of the final composite are modified depending on the kind of SCM used, its quantity, and physicochemical properties. Therefore, obtaining hardened material with the required properties requires investigation and analysis of the physicochemical processes occurring over time in the system. In some cases, the starting material may require an additional treatment and modification procedure (e.g., chemical or physical activation) [4–7], and the composition of the mix should be optimized. It is also important that the final material does not adversely affect its user, so it is necessary to study, e.g., its natural radioactivity.

One such raw material, currently not often used in cement composite contrary to the SCMs mentioned above, is copper slag, which is a by-product from the process of copper extraction by smelting. The residues from the copper smelting process in the form of hot liquid are taken to landfills where they are cooled and then ground. The copper slag thus obtained contains a significant amount of SiO2 and if it is cooled down quickly enough, this compound takes an amorphous form and exhibits a pozzolanic activity (the ability to react with Ca(OH)2 in the presence of water to produce hydrated silicate and aluminate phases similar to those that are formed during Portland cement hydration). Additionally, its physical properties are similar to natural sand [8]. Copper slag obtained directly from smelters is a valued abrasive material used in surface blast-cleaning processes. Due to the morphology of the grains, it is more effective than sand.

Although the ground slag is, in large part (in Poland practically entirely), used as an abradant, after such use some of the material is treated and reused, but most of it is considered to be a waste, which is in major part disposed in landfills or stockpiles. It contains a small amount of corrosion products and corrosion protection coatings [9] and after the blast cleaning process its granulation is smoother. The fraction content of 0–0.125 mm and 0.125–0.25 mm is much higher than in the initial material. To distinguish between copper slag and the waste material from the blast cleaning procedure, the latter is referred to in the article as waste copper slag.

However, it can be utilised again, and its potential applications are described, amongst others, in [10,11]. Due to its composition and physical form, copper slag can be used in the production of concrete as a partial or total substitute for sand [12–15] even in lightweight concrete [16]. In contrast to e.g., fine fractions of recycled concrete aggregate, the material is also suitable for the production of high-quality concrete, without compromising its quality, and some properties even improve in comparison with concrete manufactured with sand [17,18]. Copper slag used instead of sand significantly improves the consistency of the mixture without changing the amount of mixing water which results in an increase in the compressive strength [13,17,19]. It is also possible to reduce the water content by about 20% while maintaining the same consistency, thus increasing the compression strength by up to 20%. The material used in the cleaning process does not have these particular advantages, as it deteriorates the consistency of the concrete due to its finer grain size, but it is still very useful in concrete technology. In [20] the use of blast-cleaning waste as a substitute for sand in concrete with a cement dosage of 300 kg/m<sup>3</sup> and w/c = 0.6 was tested and described. Shrinkage testing of concrete with copper slag as a substitute for sand has shown that such replacement does not have the negative consequences of increased shrinkage [12].

An important aspect of using waste materials in the production of concrete is their potential harmful impact on the natural environment. In [21] the authors suggested, that the copper slag is non-toxic and poses no environmental hazard. The slag can be safely considered for use in Portland cement and concrete manufacturing. It should be noted, however, that this material is one of the most intense sources of ionizing radiation among the materials used in construction due to its high content of natural radionuclides [22–25]. Of these, particular attention is paid to the content of radium isotopes 226Ra. As a result of its decomposition radon 222Rn is produced, which is a radioactive gas and can be absorbed into the human organism by breathing. There, it undergoes further radioactive decay, resulting in radioactive isotopes of lead and bismuth, which, as solids, accumulate in the body and act as mutagens on its cells [26]. The use of such a material as a concrete aggregate requires carrying out tests of the natural radioactivity of the concrete produced from it.

Studies on the radioactivity of building materials and waste used in their production are becoming more and more common [27–30]. So far, there is not a great deal of data about radon exhalation rate in building materials containing NORM residues [30]. For example, in [31] there are only 1100 pieces of data from 14 European countries on radon emanation/exhalation rate. The COST Action TU1301 project is being run: "NORM for Building materials (NORM4BUILDING)" with a view to promoting research into the reuse of waste containing increased concentrations of natural radionuclides (NORM) in customised building materials in the construction sector, while taking into account the impact on both external exposure of building users to gamma radiation and indoor air quality. Models are being developed to better simulate the behaviour of NORM residues in different types of building materials.

In this paper the use of waste copper slag obtained from blast-cleaning as a substitute for part of the sand in concrete with 360 kg/m3 of 42.5 class cements, and w/c = 0.45 was tested and described. Some researchers pay attention to the large impact of the packing density on many concrete properties [32–35], therefore, the concrete mixtures were prepared in two variants which differed from each other in consistency and workability. For each cement type two mixtures with waste copper slag were made. In one, the same dosage of superplasticizer as in the reference series was used. In the second, the amount of superplasticizer was experimentally determined in order to obtain consistency similar to the reference series. It was 420 ± 30 mm in table flow test (near the limit between F2 and F3 class).

According to the requirements of the Polish law [36] the tests of natural radioactivity of waste copper slag and the concrete were performed. From the results the coefficients f1 and f2 were calculated and compared to the limit values which can be found in the relevant regulations. Leachability of hazardous elements (mainly heavy metals) was also assessed.

Optimization of the manufacturing process, the purpose of which is to obtain a material with required properties, needs consideration of many variables, including knowledge of the physicochemical processes occurring during the production process, as well as the impact of raw and final materials on the natural environment and on the user. In this work, the main emphasis was placed on evaluation of the composition of the concrete, taking into account its potential natural radioactivity. To evaluate the concrete studied, the method of multi-criteria EIPI assessment presented in [37] was applied, in which as the criteria were used: compressive strength, air permeability and sorptivity as parameters determining the durability of concrete, as well as radioactive activity indices f1 and f2 used for the evaluation of building materials. Concrete made of traditional fine aggregate (quartz sand) and concrete, in which waste copper slag characterized by higher values of indices f1 and f2, used as fine aggregate, were evaluated. Due to the co-existence of both positive (improvement of durability and mechanical properties of concrete) and negative (increase in the intensity of ionizing radiation of the material) effects of the use of waste copper slag, the valuation of the applied material solution encounters objective difficulties. The EIPI method allows this judgement to be reduced to a comparison of the value of one indicator, which significantly simplifies the evaluation.

### **2. Materials and Methods**

### *2.1. Materials*

Portland cement CEM I 42.5R, blast-furnace cement CEM III/A 42.5N from the Góra ˙zd ˙ze Cement Plant located in Poland and Portland-composite cement CEM II/B-V 42.5N from the Lafarge Cement Plant located in Poland, as per PN-EN 197, were used. Basic physical and chemical properties presented by the cement manufacturer are shown in Table 1.


**Table 1.** Basic physical and chemical properties of the cement.

All concrete mixes contained 360 kg/m3 of cement by a 0.45 w/c ratio. Fractions of river sand 0–2 mm and granite from the Strzegom stone mine fractions of 2–8 mm and 8–16 mm were used. Aggregates were at laboratory air-dry condition. Waste copper slag from blast cleaning was used as a partial replacement of sand. Average chemical composition of the slag is as follows: SiO2 30–45%, CaO 10–30%, Fe2O3 <25%, Al2O3 7–15%, MgO 2–8% and the granulation was much finer than in the case of typical river sand. Waste copper slag is characterized by median diameter dm = 0.347 and the used sand by dm = 0.536. Grading of the mixes of the aggregates differed mainly in the amount of finest fractions 0–0.125 mm. The ratio of substitution was 66% of sand amount by volume. If only sand and granite were used, the portion of the finest fraction was about 0.3% while after replacing 66% of the sand with waste copper slag it increased to about 3.9%. The replacement rate allowed for the aggregate grading curves both in the reference concrete mixture and in the concrete mixture containing waste, fit between the boundary curves. Superplasticizer Chryso Optima 100 according to PN-EN 934-2 was used. Regular tap water was used as the mixing water.

Nine concrete mixtures were prepared. Mix IDs and proportions are presented in Table 2. The consistency of fresh concrete was measured by a slump test, in accordance with PN-EN 12350-2.


**Table 2.** Proportions of concrete mixtures(kg/m3).

Specimens were prepared and cured as per PN-EN 12390-2. They were cast in plastic moulds and compacted by double vibration (half and full) on a vibrating table. After one day they were stripped and then water-cured in the laboratory for 28 days.

### *2.2. Performed Tests*

The compressive strength test was conducted on 100 mm cube specimens on the 28 day of hardening. The test were carried out in accordance with PN-EN 12390-3. The strength tests were performed by using a ToniTechnik instrument of 3000 kN compression force capacity. The rate of loading was maintained at 0.5 MPa/s.

A sorptivity test was conducted on the halves of cubic specimens of 100 mm edge length by means of the mass method described in [38]. Prior to the sorptivity test, the specimens were oven-dried to a stable mass at a temperature of 105 ◦C. The measurements were conducted at the temperature of approximately 20 ◦C. The specimens were weighed and arranged in a water containing vessel. Then they were immersed up to the height of 3 mm.

Air permeability testing of concrete was performed by means of the Torrent method with use of Proceq equipment. The test was conducted on two 150 mm cube specimens, which were cured in water for 28 days and then were stored in air-dry laboratory conditions (temperature t = 20 ± 2 ◦C and RH of air equal 55 ± 10%) until they reached age of 90 days. Moisture content was measured, before conducting the air permeability test, using Tramex CMEX II, which is recommended by Swiss Standard SIA 262/1 Annex E and by [39]. The testing procedure is described in [40].

Tests for the content of hazardous substances released from waste copper slag (i.e., leaching tests) were carried out in accordance with the applicable standards and regulations by the Laboratory of Solid Waste Analysis at the Central Environmental Monitoring Department of the Mining Institute in Katowice in accordance with Annex 3 to the Ordinance of the Minister of Economy of 16 July 2015 on the approval of waste for storage at landfills (Journal of Laws of 2015, item 1277).

The PI-MAZAR01 meter was used to perform tests of natural radioactivity. It is designed to determine the concentration of natural radioactive elements, such as radium, potassium or thorium. The measuring part is located in a lead shielded cabin, which includes a type SSU-70-2scintillation probe with a NaI (Tl) (thallium-doped sodium iodide) crystal, a preamplifier and a high voltage power supply, as well as a calibration isotope source Cs 137 used to stabilize the measuring path. In the reading part there is a microprocessor controller. The analyser is adapted to work with a PC, so that it is possible to visualize the spectrometric spectrum and save the measurement results on a hard disk.

The natural radioactivity measurement procedure begins with the calibration of the analyser according to the instrument manual and the recommendations of the instructions of Building Research Institute (ITB, Poland) [41] which recommends periodical calibration at least once a year and control measurements with the use of standards once a month or as a result of a change in conditions after 24 h (e.g., change in temperature at the place of measurement). Samples (so-called qualification samples) were prepared for testing, ground to a maximum grain size of 2 mm, then dried to a constant mass at 105 ◦C and left to cool under laboratory conditions to reach an air-dry state. The prepared material was placed in the Marinelli type containers with a volume of 1700 cm3. The container and sample were then weighed, secured with adhesive tape and marked accordingly. The weight of the material of each sample was calculated on the basis of the performed weights. Afterwards, the samples were seasoned in containers for seven days at a significant distance from the measuring house (over 2 m). Before starting the measurements, the background of the samples was calculated on an aluminium mass standard and then the containers with samples were placed in the measuring chamber of the shielding house. During the study, the meter collected the measurement spectrum and then analysed the number of impulses recorded in potassium, radium and thorium windows, which were the basis for calculating concentrations of radioactive elements and qualification coefficients f1 and f2.

### **3. Results**

### *3.1. Mechanical and Durability Properties*

The results of compressive strength, sorptivity and air permeability tests are presented and discussed in detail in [40]. Table 3 presents the average values of those of all the obtained results, which were used for calculations in the EIPI analysis.

The results presented above show that compressive strength of CEM I and CEM II cement concretes containing waste copper slag increase both after the 28th and 90th days of hydration compared to the reference (CI0 or CII0 respectively). Only in the case of CEM III cement concrete, introduction of waste copper slag reduces the compressive strength. On the other hand, the presence of the sand replacement results in an improvement of the tightness of all investigated concrete compositions. The possible cause of sealing of the concrete structure is the pozzolanic reaction. The greatest share in the composition of waste copper slag is constituted by SiO2 in amorphous form, which shows pozzolanic activity. As it is commonly known, the use of pozzolanic materials in the production of concrete improves, among other things, its tightness. An additional factor is the granulation of waste copper slag—a larger share of fine fractions. In summary, the results obtained indicate a predominance of benefits from the use of waste copper slag in concrete.


**Table 3.** Test results employed in EIPI calculations [40].

Flow: R- collapse of the specimen after lifting the cone.
