Study of feasibility of using copper mining tailings in mortar production

1.1 Theoretical background

Mine tailings have been used in mixed concrete but mostly as partial or complete substitutes for fine aggregate, given that tailing particles have diameters of less than 1 mm [13,14,15]. The physical characteristics of tailings notably influence their workability, density, and strength [16]. In general, tailings have a rough and irregular surface [17] due to the crushability of various mineral phases of which they are composed. Then, adding tailings to other materials increases the specific surface area of the new mixture and changes its rheological properties such as workability.

Workability, which is defined as the ease of transportation, placement, compaction, and finishing of the mixture, is an important property of fresh mortar or concrete mixtures [18]. The flow of mortar has been found to decrease with increasing tailing substitution levels because tailings have a finer particle-size distribution that raises the total surface area of fine aggregates [19,20,21]. Given the characteristics of tailings, steps before incorporating them into concrete as supplementary cementitious materials must be evaluated. These steps include drying, grinding, and/or calcining. Regrettably, only a limited number of research papers have delved into the drying process of tailings. Onuaguluchi et al. [22,23] tested tailing samples that underwent air drying and subsequent sieving under a 600 μm size sieve. Other researchers [24] have proposed that treating the tailings could involve passing them through a 45 μm size sieve before utilization in concrete. Additionally, few investigations [25,26] have employed grinding to treat tailings. Table 1 summarizes the chemical composition of tailings as reported in the literature used in mortar studies of tailings replacing cement.

In general, mine tailings exhibit a high silica content, as detailed in Table 1, which increases the molar ratio of SiO₂/Al₂O₃ in geopolymer composites derived from mine tailings, negatively affecting the geopolymerization process. For this reason, metakaolin is more frequently used in the chemical industry as an additional source of aluminum [37,38,39] due to the purity and uniformity of its composition, as well as its high interaction capacity [40]. Furthermore, the addition of supplementary alumina-silicate source materials enhances the compressive strength of copper mine tailings-based geopolymer composites [41,42]. Similarly, increasing the concentrations of copper slag, raising the sodium silicate-to-sodium hydroxide ratio up to 1.0, and increasing the molarity of sodium hydroxide solutions to 10 molars all contribute to an improvement in the compressive strength of copper mine tailings and copper slag blended geopolymers [43,44].

Some studies have observed that the use of tailings as a cement substitute has resulted in a decrease in compressive strength with an increase in the proportion of tailings, regardless of the age of the concrete or mortar [25,26,30,35,45,46]. In these studies, the decline was attributed to the low pozzolanic activity of tailings as supplementary cementitious materials, even when treated as ground powder. However, in a few cases, a specific concentration of fine tailings could enhance the compressive strength of concrete with tailings as supplementary cementitious materials because the fine tailings powder could fill minuscule pores, accelerating cement hydration [26,30,35,45]. Indeed, Franco de Carvalho et al. [47] demonstrated that if fine basic oxygen furnace slag tailings were used as supplementary cementitious material, the compressive strength was highest with a 20% fine tailings replacement. If 40% coarse tailings and 20% fine tailings were added, the strength was lower than that of the control concrete but still close to it. By using calcined tailings, in which metakaolin was created from the kaolinite, the compressive strength of concrete improved [48,49]. Also, in tailings used as an additive to concrete, without replacing cement, it was found that the compressive strength was higher than the control concrete due to the filler effect [23,50,51].

Compressive strength and flexural strength tests with copper tailings as replacements for sand were evaluated in previous studies [32,50]; these studies suggested that the higher strengths of the samples mixed with copper tailings are due to the filling effect and the moderate reactivity of the fine particles of the tailings along with the cement particles. Furthermore, the setting time of cement replacement with tailings has been analyzed, in which a comparison case between tailings from the Nalunaq and Zinkgruvan mines [52] showed that the time from the initial setting to the final setting decreased for the tailings-cement mixture.

The promising results found when mixing tailings in concrete mixtures prompted to study the impact of mixing Chilean copper tailings on the mortar samples’ mechanical strength and workability. Different proportions – 10, 20, and 30% of cement replacement – were tested based on the above literature, which specified replacement percentages.

2.1 Cement

A pozzolanic-class cement was used based on its composition and strength [53]. The results of an Energy-dispersive spectroscopy (EDS) analysis provided information about the elemental composition of the cement. Overall, EDS findings for the cement reveal the presence of abundant calcium, silicon, aluminum, and iron, which are responsible for providing strength and bonding characteristics to the cement from hydration reactions and processes during the setting.

Figure 2 shows the weighted concentration of elements in the pozzolanic cement. EDS results primarily specify the chemical composition of the materials present in the cement. Besides oxygen (O), calcium (Ca) was the most abundant element, representing 14.27% of the total weight. Silica and alumina, also abundant, had a positive impact on the cement due to their potential pozzolanic and semi-cementitious properties. The chemical and mechanical properties of the mortar will strengthen, depending on the dispersion of the elements. Here the dispersion clearly indicates good C–S–H formation in the mix. This directly influences increased pozzolanic activity during the hydration process. Figure 3 shows Scanning electron microscope (SEM) morphologies of the cement at scales of 250 and 5 µm, demonstrating a wide size-range of particles with irregular morphologies and small spherical pozzolana particles.

Figure 2

EDS of pozzolanic cement.

Figure 3

SEM of pozzolanic cement: (a and b) scale of 250 μm (mapping), and (c) scale of 5 μm.

In addition, Table 2 shows the mineral phases present in the cement used, determined from X-ray diffraction (XRD) analysis. In order of abundance, Hatrurite, Quartz, Plagioclase, and Akermanite appear. Both Hatrurite (Ca₃SiO₅) and Akermanite (Ca₂MgSi₂O₇) confer cementitious properties to the material. This mineralogy is consistent with the abundant calcium observed through the EDS analysis, as well as the rest of the determined elements. The overlap of diffraction peaks of the main components in the range of 2θ = 3°–70° can also be observed in Figure 4.

Figure 4

XRD of pozzolanic cement.

2.2 Fine and coarse aggregates

As shown in Figure 1, the maximum particle size used in this study was 2 mm for fine aggregate and under 4.5 mm for coarse aggregate. The fine and coarse aggregates have a density of 2.76 and 2.77 g·cm⁻³, respectively [54]. Figure 5 shows the weighted concentration of elements in the aggregates (through EDS). EDS results primarily specify the chemical composition of the materials. Besides oxygen, silicon (Si) is the most abundant with more than 10% and aluminum (Al) with about 6%. Also, there is a high presence of other elements such as Ca, Fe, Na, and Mg, in addition to the organic content of the sample.

Figure 5

EDS of fine aggregate (left) and coarse aggregate (right).

Additionally, Figure 6 shows SEM morphologies of the coarse and fine aggregate at scales between 500 and 5 µm. The irregular morphology of the surface of the grains and their angularity can be observed, as well as the gradation in their particle size. Fine aggregates (Figure 6a) have a higher uniformity coefficient than coarse aggregates (Figure 6d). Based on the roundness scale of Powers [55], the particles are subrounded and with low sphericity. These characteristics, together with the aforementioned mineralogical composition, point to a mafic volcanic origin of the source rock of both types of aggregates, which have been fragmented, transported, and accumulated naturally by streams of water.

Figure 6

SEM of fine and coarse aggregate. Fine – (a)–(c): scale of 500, 10, and 5 μm, respectively. Coarse – (d)–(f): scale of 500, 200, and 10 μm, respectively.

Table 3 shows five types of mineral components obtained by XRD: Pyroxene, Quartz, Plagioclase, Olivine, and Garnet. The overlap of diffraction peaks of the main components in the range of 2θ = 3° to 70° can also be observed in Figure 7.

Figure 7

XRD pattern of coarse and fine aggregate (red – plagioclase, blue – quartz, green – pyroxene, pink – olivine, and maroon – garnet).

2.3 Copper mine tailings

The EDS results primarily specify the chemical composition of the materials present in the copper mine tailings. After oxygen (O), silicon (Si) is the most abundant cation, representing 12.38% of the total weight, followed by silicon aluminum (Al) with 7.23% (Figure 8).

Figure 8

EDS of copper tailings.

Atomic absorption spectroscopy (AAS) is commonly employed to identify the concentration of metals/metalloids in environmental studies. To evaluate whether the use of these copper tailings could pose a potential environmental hazard, analyses of heavy metals present in the material were carried out, using a total sample digestion process and then AAS. The AAS results are shown in Table 4.

Figure 9 presents SEM images, showing morphologies and sizes of the copper mine tailings. The morphology of the particles is irregular due to the comminution processes, and their size is mostly less than 500 µm, with finer sizes predominating, data that agree with the particle-size-distribution are presented in Figure 1. Additionally, tailings exhibited a range of densities because of the various mineral phases present. Tailings used in this study had a bulk density of 2.67 g·cm⁻³ [54].

Figure 9

SEM with EDS of copper tailings. (a)–(c): Scale of 500, 10, and 1 μm, respectively.

The chemical composition of the mine tailings used in this study is shown in Table 5. Silica and alumina are the main components of these tailings. In lower concentrations, alkalis (K and Na) also stand out. The measurement of Total Organic Carbon (TOC) is also presented in Table 5, where a value above 4 is shown for the copper mineral tailings used in this study.

Table 6 displays the semi-quantitative mineralogical composition obtained for the copper tailings, along with the corresponding diffractogram of the copper tailings depicting their respective phases as shown in Figure 10. It is noteworthy that the mineral phases present are quartz (SiO₂), muscovite (KAl₂(AlSi₃O₁₀)(OH)₂), albite (NaAlSi₃O₈), and kaolinite (Al₂ Si₂O₅(OH)₄). This composition indicates a felsic-type parent rock, with a certain degree of alteration that gives rise to the appearance of kaolinite, unlike the aggregates used, which originate from mafic rocks. The percentage of muscovite explains the high concentration of potassium observed. On the other hand, the presence of kaolinite and muscovite is important for this work, due to their potential pozzolanic behavior. However, quartz, the main component of the tailings, is a resistant mineral even in the alkaline conditions of the medium presented by the mortars [56]. Nevertheless, quartz will not have an important contribution to the cementing properties.

Figure 10

XRD pattern of copper mine tailings (Q – quartz, M – muscovite, A – albite, and K – kaolinite).

2.4 Samples

The mortar samples were fabricated in steel molds. The steel molds used have dimensions of 50 mm × 50 mm × 50 mm for cube samples, 40 mm × 40 mm × 160 mm for prism samples, and 70 mm diameter × 150 mm length for cylinder samples. Figure 11 shows mortar samples before mechanical tests. Initially, a total of 60 mortar samples were prepared (including the control samples without tailings) with different dosages corresponding to a 10% (RC10C), 20% (RC20C), and 30% (RC30C) replacement of cement with copper mine tailings with fine aggregate, and 12 mortar samples (only cubes) with coarse aggregate.

Figure 11

RC30C samples.

Then, 24 samples were prepared by adjusting the initial dosages to increase the use of cement and copper tailings and decrease the water/cement (w/c) and water/(cement + tailing) ratios by almost half, exclusively using fine aggregate.

Each initial mortar components is summarized in Table 7. Samples containing coarse aggregate are indicated by the term “AG” (e.g., Control_AG); in the other samples, only fine aggregate was used. The samples with an asterisk correspond to the adjusted dosage.

The variation in adjusted dosage involves the quantity of cement and tailings and consequently fine material amounts. Additionally, different water-to-cement ratios were considered to achieve a suitable workability mixture (evaluated through flow test). The adjusted dosage for the mortar was developed to investigate the effect of cement replacement while simultaneously increasing the overall cement content and reducing the water–cement ratio. This approach was specifically designed to evaluate how an optimized mixture could compensate for potential reductions in mechanical performance or durability often associated with partial cement replacement in an effort to obtain favorable results in mechanical resistance using mining tailings.

In the experimental plan, not all dosages underwent the same number of tests because of a shortage of tailings material. Table 8 presents the test matrix for each sample, indicating which tests were conducted. Three samples were prepared for each dosage. This triplicate testing approach helps mitigate any potential variability and provides a more robust dataset for analysis to ensure the reliability and accuracy of the results.

2.5 Flow tests

The mixing process begins with a mixer in the starting position, water poured into a container, and the continuous addition of cement. Once the mixture is prepared, the flow test is conducted following ASTM C1437 [57]. The test begins by introducing the mixture into the cone, giving it a height of approximately 25 mm, compacting it with 20 strokes made by rotating the handlebar of the testing device to finally determine the height difference. The results obtained are shown in Table 9.

To complete the flow test, the mixture was put in steel molds as shown in Figure 12. After setting for 24 h, the samples are demolded and then cured for 7 and 28 days for cubes and prisms, and 28 days for cylinders. After the curing period, samples were dried at a temperature of 110°C for 24 h. Once the drying was complete, samples were cooled to room temperature to conduct the mechanical tests. The samples were dried because some studies [59,60] indicate that the presence of moisture in the samples can reduce their mechanical strength in both compression and flexure.

Figure 12

Equipment used for the flow test (left) and cubic-prismatic samples (right).

2.6 Mechanical test: Compression, flexural, and tensile tests

Compressive strength is a key parameter in concrete and related materials. This parameter indicates the mix's compressive strength after hardening and is indirectly related to other properties such as compactness, toughness, and durability [61]. In this study, the Chilean Standard #1037 is used, which has an adaptation in the Chilean Manual [62].

Figure 13 shows an example of the compression test carried out. To validate the test, after the sample fails, a load equivalent to 60% of the maximum load is applied to determine the type of failure. Sixty compression tests were conducted: 24 cubic samples with fine aggregate (7 and 28 curing days), 24 cubic samples with coarse aggregate (7 and 28 curing days), 12 cubic samples with fine aggregate and adjusted dosage (28 curing days).

Figure 13

Example of compression test.

Flexural strength corresponds to the theoretical maximum tensile stress reached at the extreme tensioned part of a test beam under a three-point bending test (Figure 14). The Chilean standard NCh 1038 has an adaptation corresponding to section 8.402.12 in the Highway Manual [62], and this adapted standard was used in this study. At 28 days of curing, 36 flexural tests were conducted, including 12 with fine aggregate, 12 with coarse aggregate, and 12 with adjusted fine aggregate.

Figure 14

Example of flexural test.

Tensile cracking is a common failure mode in mortars. To test for this failure, split tensile testing were done following the 1170 standard, as adapted in the Highway Manual, corresponding to section 8.402.13 [62]. This standard establishes the procedure for conducting the splitting tensile strength test on cylindrical specimens until failure. This method employs various loading patterns classified into four typical modes: flat plates, loading jaws, modified wooden pads and loading plates. Among these loading patterns, the flat loading was selected (Figure 15). Twelve splitting tests were conducted at 28 days of curing.

Figure 15

Example of splitting tensile strength test.

3.1 Flow test results

Figure 16 shows the flow test of different samples, and Table 10 shows their workability ranges. Of all the samples, just the first two mixtures (RC10C and RC20C) entered the plastic workability range, defined in Table 9, which is useful for considering shotcrete, helping to prevent possible segregation in the case of being too dry or too wet. The RC30C replacement had a flow percentage of 59.24%, indicating a rigid workability, which could complicate possible shotcreting. All the samples of coarse aggregate remained in a moist workability condition (Table 9). A greater segregation of material was observed in the mixture made with coarse particles and 30% replacement of tailings (Figure 16h). In this case, the coarse material was found to concentrate in the central part with migration of cement toward the external parts. Otherwise, the mixtures with adjusted dosages of cement, tailings, and water improved the workability of the prepared mortars compared to the control sample, as evidenced by the samples with 10, 20, and 30% replacement.

Figure 16

Consistencies of mortar from left to right: Top row – fine aggregate. Second row – coarse aggregate. Last row – adjusted doses of mortar.

3.2 Compression test

Figure 17 shows the compression test results for 7 and 28 days of curing for the Control, RC10C, RC20C, and RC30C samples and x_AG samples. Here samples of 28 days of curing with adjusted dosages are also included (*). The highest values for the three dosages (fine aggregate, coarse sand, and fine aggregate adjusted) are in the control samples. The compressive strength in all samples decreased as more tailings were added to replace cement.

Figure 17

Compression test results in all samples. Red dashed line represents the minimum suggested for shotcrete applications [61].

When comparing the results obtained from the initial dosage and the adjusted dosage, an increase in the compressive strength value was observed, maintaining the trend of decreasing compressive strength value as the dosage of cement replacement by tailings increased. Additionally, it was noted that the control sample continued to maintain the highest compressive strength value. The compressive strength values increased on average by 199% when adjusting the dosages as described in Table 7, and the difference between the initial and final values obtained for compressive strength averaged 31 MPa.

3.3 Flexural test

Figure 18 shows the results of flexural strength in which the samples corresponding to 7 and 28 days of curing with initial dosages exhibit the same behavior as in compressive strength: increasing the percentage of tailings caused a decrease in strength. In the RC20C and RC30C samples, there was a slight decrease from 7 to 28 days of curing. The control sample reached a strength of 5.79 MPa, followed by the 10% replacement of cement with tailings, which had a strength of 4.59 MPa. After 28 curing days, the replacements of 20 and 30% demonstrated strengths of 3.10 and 2.88 MPa, respectively.

Figure 18

Flexural strength result.

The results obtained for the adjusted dosages (*) are shown alongside the initial dosages. A change in the trend of the flexural test results with the adjusted dosages was observed, with this trend increasing as the amount of tailings replacing cement in the mortars increased. In this particular case, the highest flexural strength value was presented by the 30% cement replacement (10.2 MPa), and the lowest recorded value was that of the control sample (7.3 MPa), with a difference of 2.9 MPa between the control and RC30C values. Comparing the results obtained using the initial and adjusted dosages, it can be observed that the increase in these flexural strength values was on average 142%. In general, the flexural strength is about 10–20% of the compressive strength of concrete. In our result, the flexural strength results are about 12–31% of the compressive strength.

3.4 Splitting tensile strength

Figure 19 shows the results of splitting tensile strength in samples with 28 days of curing. A clear decrease in tensile strength was observed as the cement replacement increased, exhibiting the same behavior as in the previous compressive mechanical tests and decreasing from 2.25 to 1.6 MPa when comparing the control with RC30C. The splitting tensile strength results were about 11–14% of the compressive strength.

Figure 19

Splitting tensile strength results.

4.1 Chemical and mineralogical stability

To evaluate the results obtained by AAS of potentially hazardous elements present in tailing (Table 4), a review of different international regulations regarding the presence of these elements in soils was carried out. It should be noted that in Chile, the country where the study was carried out, there is no specific soil regulation in this regard. Table 11 displays various chemical elements along with the standards from different countries, indicating the maximum permissible limits for each element in soils for industrial or similar use. Comparing the values obtained here with those of the international regulations shown in Table 11, copper (Cu) and arsenic (As) values surpass the limits allowed in some cases. Despite this, given that these tailings would be integrated into the structure of the mortar, the possibility of these elements leaching and being released into the environment is much lower than the possibility of leaching from tailings dump; although to guarantee this, other types of tests would have to be carried out on the already-formed mortar.

It should be noted that the mineralogical composition of the tailings differs from that of the aggregates, the parent rock of the former being of the felsic type, while the aggregates used come from mafic rocks. However, this composition does not imply chemical incompatibility with that of the cement.

4.2 Workability

Workability, which is defined as the ease of transportation, placement, compaction, and finishing of the mixture, is an important property of fresh mortar or concrete mixtures. The physical properties of tailings have a significant impact on this parameter [16]. Since plasticity and cohesion are difficult to measure, consistency is frequently used as a measure of workability [69].

Analyzing the workability of mortars with fine aggregate dosages (Table 10), the mixtures lose fluidity as the percentage of tailings increases. Parthasarathi et al. [70] in a study in which they used gold mine tailings to make mortar determined that the workability of the mixture decreases with the higher the percentage of tailings used, which can be attributed to the presence of very fine particles in the tailings. Other researchers [19,20,21] also found that mortar flow decreased with increasing levels of tailings substitutions, justifying this because the tailings had a finer particle size distribution. This is probably not only due to the presence of smaller particles since they are in the size range of cement particles but also due to the presence of minerals from the clay group among the tailings components, which have a greater specific surface area than cement particles, given their laminar crystalline structure. These minerals adsorb water in abundance in their internal structure, preventing them from participating in the fluidization of the mixture, thus reducing its viscosity.

In this study, comparing the results of the samples prepared with fine and coarse aggregates, the fine preparations behave in a more viscous manner. Considering that the proportions of cement, water, aggregates, and tailings are equal, this can only be due to the friction generated between the aggregate particles in the mixture, which should be greater in the fine ones. Fine aggregates have a greater specific surface area than coarse aggregates; therefore, for the same proportion in the mixture, the larger particles will have fewer points of contact with each other, thus reducing friction. This lower friction between coarse aggregates translates into lower viscosity of the mortar. Other studies have also identified a lower workability of the mortar when it has fine aggregates compared to coarse ones [69].

Finally, the mixtures with adjusted dosages, despite having a lower water/solid ratio than the former (Table 7), present higher fluidity values. Taking into account the mixture components, this is justified by a lower proportion of fine aggregates in the mixture, especially concerning the cement content, which would cause the aggregate grains to have less friction, mainly between themselves due to greater dispersion, causing the viscosity of the mortar to decrease.

According to what was observed in this work, with the variables used, the main determining factor in the workability of the mortar seems to be the size of the aggregates. Various authors report the adverse effect of tailings used as a partial replacement of aggregate or cement on the workability of fresh concrete and mortar. Although the fluidity of the mortar can be modified by varying the dosage of water or using additives, these can have other effects such as a delay or acceleration in the beginning of setting [61]. For some kinds of applications, for instance, in the case of shotcrete, the incorporation of tailings material into the mixture can be useful to reduce the fluidity of the mortar due to the greater water retention of tailings material, which can then be adjusted to optimal workability conditions in a range between wet and plastic for use as shotcrete.

4.3 Mechanical behavior

Superior mechanical strengths were observed in samples with coarse aggregate compared to those containing fine aggregate at the same dosage, with a range of improvement from 20 to 23%. The larger particle size contributed to enhanced compressive strength by promoting greater interaction with the mortar matrix, facilitated by improved hydration of calcium silicate. However, the adjusted dosage using fine aggregates (*) considerably improved the mechanical strength, which would be expected, since the proportion of cement is doubled compared to the previous samples.

The influence of fine and coarse aggregates on compression tests has been evaluated in a variety of studies. The use of coarse sand improved the compression results compared to fine sand, as observed by Meddah et al. [71] suggesting that the compressive strength increases in correlation with the particle size distribution of the aggregate employed. Also, Ruiz [72] discovered that the compressive strength of concrete increased as the volume of coarse aggregate increased; however, when the aggregate volume fraction rises within the range of 45–75%, the compressive strength of concrete could slightly decrease (15%) and can be practically considered constant. Gou et al. [16] determined that for the same workability of mortar or concrete with tailings as fine aggregate, the compressive strength decreased since tailings need more water resulting in a higher water–cement ratio.

In addition to the direct effect of the significant increase in cement in the samples with dosage adjustment (*), indirectly, this would lead to the formation of high amounts of calcium hydroxide, which could have triggered an early pozzolanic reaction of certain minerals in the tailings, contributing to improving the strength of the mortar. On the other hand, the decrease in the water–cement ratio could also have had a favorable effect on the compressive strength of the mortars tested. In the work conducted by Haach et al. [69], all hardened properties evaluated (compressive strength, elastic modulus, and flexural strength) decreased with the increase in the water–cement ratio, while sand grading appeared to have no influence on compressive strength; however, sand granulometry had a significant impact on flexural strength. In their study, mortars made with coarse sand showed higher flexural resistance, as observed in this work and as Haach et al. pointed out, coarse sand likely promoted better particle entanglement.

A progressive increase in flexural strength value was observed in the mortar samples in which the adjusted dosage was used in comparison to the control sample (Figure 18) followed by 10, 20, and 30% replacement of cement with copper tailings. Other researchers [32,73] also found better results in flexion tests at higher proportions of tailings (with the adjusted dosage), which may be because the lower proportion of water favors the formation of calcium silicate bonds, increasing the bending strength. It is also suspected that this behavior is due to a slightly higher degree of hydration induced by the release of internal curing water.

The physical and chemical attributes of tailings utilized as fine aggregate in concrete have a direct impact on the mechanical properties of the concrete [51,74,75]. The general decrease in mechanical strengths is attributed to the fact that the replacement of tailings used lacks cementitious properties that would contribute to strength improvements. Therefore, progressively reducing the cement content in the mixture results in significant decreases in strength. It is also important to note the value obtained for the TOC in the copper tailings, which is above 4. The TOC content (Table 5) could have negatively affected the mechanical strengths in compression and flexure, as organic carbon may have formed a thin layer around cement particles, hindering their proper hydration and resulting in a potential decrease in strength. This effect is heightened in cases where copper tailings were extensively used as a substitute for cement. This behavior mirrors findings by researchers [76,77], who agree that higher TOC values lead to reduced durability and hardening properties of mortar samples due to partial inhibition or delay in cement hydration. On the other hand, international standard EN 197-1 [78] specifies that to achieve optimal behavior in mortar mixes, the TOC should not exceed 0.20 wt% for CEM II/A-LL and CEM II/B-LL types while the maximum TOC for CEM II/A-L and CEM II/B-L should not exceed 0.50 wt%.

Table 12 shows the results from various studies on replacing cement with tailings, based on different types of tailings, tests, and tailings content. The potential decrease in strength when adding tailings of all types may be attributed to the dosage used since not even the results of the control samples surpass any of the results from the references in Table 12. However, our study suggests that by adjusting the dosage, the mechanical properties of the mortar can be improved. Therefore, increasing the cement content should lead to an increase in strength – at least for the control sample – since there is no pozzolanic activity in the hydration process, there is no release of internal curing water from pre-moistened tailings as shown in [50], for the initial dosage used. The results of this study are in agreement with the findings from previous studies.

For tailings to exhibit pozzolanic material properties, it is necessary to have calcium hydroxide, typically supplied by hydrated Portland cement, which reacts with siliceous or silico-aluminous minerals. The tailings used in this work have high concentrations of silica and alumina (Table 5), although the high SiO₂ content in these is present in the form of quartz, which is a chemically stable phase, so it does not generate this type of reaction. On the other hand, the presence of aluminosilicates such as kaolinite and muscovite (Table 6) could potentially give rise to this type of reaction although the resistance results obtained clearly do not reflect their pozzolanic activity. Despite this mineralogy present, due to the granulometry of the tailings, it would be expected that pozzolanic reactions would begin to take place in curing time periods greater than the 28 days analyzed here. Wu et al. [84] carried out studies with ground quartz particles, determining that the finest particles, with sizes less than 1 µm, are mainly responsible for the reactivity at early stages (up to 7 days). At later stages (28–90 days), particles of this mineral with diameters between 1 and 10 µm contribute to the pozzolanic reaction, providing compressive strength after 28 days of curing.

To develop the pozzolanic capacity of these minerals, abundant in these tailings, it would be useful to carry out interventions on them, such as calcination in the case of kaolinite, as described in numerous works [85,86,87], or mechanical activation by reducing the size of the muscovites by grinding, which increases their specific surface and reactivity [88]. Although the results of the resistance of the mortars have not been as expected by replacing cement with tailings, the abundance of aluminosilicates such as kaolinite and muscovite in these materials could potentially be used through thermal or mechanical treatments, developing their pozzolanic capacity to achieve improvements in their mechanical behavior.