Statistical Design of Eco-Friendly Mortar Mixtures Containing Scheelite Tailings and Quartzite Sand: Evaluation of Resistance to Alkali-Aggregate Reaction

Abstract

This study focuses on addressing the challenge of society’s consumer demands through sustainable production processes, as outlined by Sustainable Development Goal 12 established by the United Nations. In this context, this study aims to assess the durability of eco-friendly mortars with mineral waste as alternative raw materials, considering the alkali-aggregate reaction (AAR). For this purpose, scheelite tailing (ST) was used to partially replace Portland cement (PC), and quartzite sand (QS) was used to fully replace conventional sand. The ST was ground and sieved (<75 μm), and part of it was used in its natural form, while the other part was calcined (1000 °C for 1 h). A mixture experimental design was created to select the compositions with the best mechanical performance. All the mortar mixtures were produced with a cementitious material to QS ratio of 1:3. Three mortar compositions (0% ST, 30% natural ST, and 30% calcined ST) were selected to study the resistance to the AAR. Linear expansion measurements, compressive strength tests, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy were conducted to evaluate the phases formed and the mechanical behavior of the mortars in relation to the AAR. The expansion results demonstrated that QS does not exhibit deleterious potential. Regarding the use of ST, the results indicated that it is possible to partially replace PC with calcined ST without significantly compromising the mechanical performance and durability of the mortars. However, the use of non-calcined ST is not recommended, as it presents deleterious effects on the mechanical properties of the mortars. This study highlights a new sustainable mortar alternative for use in construction without future degradation of its properties.

1. Introduction

In the past 30 years, the concept of sustainable development has gained global recognition, with a comprehensive definition referring to practices that allow the current population to meet their basic needs without compromising future generations [1]. However, the production of Portland cement (PC), which plays a crucial role in mortars and concretes, is not environmentally sustainable because it emits a significant amount of carbon dioxide (approximately 7% of global emissions), one of the main greenhouse gases [2,3,4]. Furthermore, cement production requires large quantities of natural raw materials such as limestone and clay. The extraction of these resources can lead to the depletion of non-renewable resources, soil degradation, and damage to local ecosystems [5].

Considering this scenario, research on cleaner production processes and using alternative materials to replace conventional ones becomes increasingly important to minimize the environmental impacts generated by the construction industry [6,7]. Furthermore, such research also contributes to the implementation of Sustainable Development Goal number 12 (SDG 12) of the 2030 Agenda established by the United Nations (UN), which aims to promote sustainable patterns of production and consumption [8]. In the context of this pursuit for sustainability, the exploration of mineral resources offers an opportunity to transform large volumes of improperly disposed of solid waste into alternative raw materials in the production of sustainable construction materials [9,10].

Currently, numerous studies are being conducted to investigate the feasibility of incorporating industrial and mineral waste into cementitious matrices, such as concretes and mortars, with the aim of promoting sustainability in the construction sector. Some of these studies report promising results regarding the partial replacement of cement in mortars and concretes with copper tailings [11], marble and granite waste [12,13,14], kaolin waste [15], perlite waste [16], and agricultural waste [17,18,19]. Overall, these studies have demonstrated that it is feasible to replace natural resources with industrial and mineral waste in the production of cementitious materials, reducing environmental impacts associated with the excessive exploitation of natural resources and the disposal of large volumes of waste.

In the northeastern region of Brazil, specifically in the states of Paraíba, Rio Grande do Norte, Ceará, and Pernambuco, there is a significant potential for non-metallic minerals, with emphasis on scheelite (CaWO₄) and quartzite (SiO₂) deposits. The waste generated from scheelite extraction is abundant in these areas but is often improperly disposed of, leading to environmental impacts [10]. In this context, the chemical composition and availability of scheelite tailings (STs) make it attractive as an alternative raw material to produce sustainable mortars and concretes [20]. Medeiros et al. [21] used STs as a total substitute for natural sand in mortar compositions for coating. The researchers found that the tailings exhibited similar chemical and physical characteristics to natural sand, indicating their potential for substitution. Additionally, the mortars containing ST demonstrated satisfactory mechanical results. In another study [20], the durability of eco-efficient concretes produced with partial substitutions of CP by porcelain polishing waste (0, 5, 10, 15, 20, 25, and 30%) and sand by ST (81%) was evaluated. The authors found that the production of eco-efficient concrete resulted in improvements in properties compared to the reference concrete, while promoting sustainability by reducing CP and sand consumption by 15% and 81%, respectively.

However, most studies have focused on using ST to produce ceramic masses [10,22] or in the partial or total replacement of conventional aggregates [20,21,23]. Partial substitution of CP with ST was little explored in previous studies. The existing studies [24,25] have used ST with chemical compositions containing approximately 50% SiO₂ and 13% Al₂O₃ and Fe₂O₃. However, ST from the Brejuí mine, considered the largest scheelite mine in South America [9], has a distinct chemical composition, being rich in CaO, with approximately 40–48% CaO, 16–22% SiO₂, and 7–10% Al₂O₃. These differences in chemical composition highlight the importance of specifically studying ST rich in CaO as a partial substitute for CP in the development of mortars and concretes. Furthermore, there is also a scarcity of studies involving the use of quartzite tailings for the development of alternative mortars, as well as understanding the behavior of these eco-friendly mortars in the face of the alkali-aggregate reaction (AAR).

The evaluation of the durability of mortars produced with waste materials is of utmost importance to ensure their acceptance in the consumer market, as it is essential for these mortars to exhibit satisfactory properties that remain throughout their service life [6]. Thus, this study aims to provide an environmentally conscious and technically viable alternative for the disposal of scheelite and quartzite extraction tailings by using them in alternative mortars for construction applications. To achieve this objective, partial replacement of PC with ST and total replacement of conventional sand with quartzite sand were performed. Additionally, a durability study was conducted to understand the phenomena related to using these waste materials as alternative raw materials, focusing on the analysis of the AAR.

2. Materials and Methods

2.1. Raw Materials and Their Characterizations

The eco-friendly mortars were produced using high-initial-strength Portland cement (PC) CPV-ARI MAX, following the requirements of ASTM C150 [26], quartzite sand (QS) (Figure 1a) as a total replacement for natural sand (NS), and scheelite tailings (STs) (Figure 1b) as a partial substitute for PC. The QS was used as a fine aggregate (<4.8 mm), as shown in Figure 1c, and it was obtained from a company in Várzea City, Paraíba state, Brazil. The ST was obtained from a tungsten mining company located in Currais Novos, Rio Grande do Norte state, Brazil. Sodium hydroxide (NaOH 97% PA, Synth) was used in the alkali-aggregate reaction (AAR) resistance test.

After collection, the ST was ground in a roller mill for 2 h and sieved (<75 μm) to obtain a particle size similar to PC (see Figure 1d). Finally, part of the ground tailing was used in its natural form (Figure 1e), and the other part was calcined at 1000 °C in a muffle furnace for 1 h, with a heating rate of 5 °C·min⁻¹ (Figure 1f), to evaluate the effect of calcination on the properties. The as-received and ground scheelite tailings were denoted ST_G, and the as-received, ground, and calcined tailings were designated ST_C. Figure 1a–f show images of QS, ST_G, and ST_C, along with their respective particle size distribution curves obtained by sieving method (sieves with openings of 4.8 mm, 2.4 mm, 1.2 mm, 0.6 mm, 0.3 mm, and 0.15 mm) for QS and by laser diffraction (Malvern, Mastersizer 2000 model, Malvern, United Kingdom) for ST_G and ST_C.

Table 1. Chemical composition of QS, PC, ST_G, and ST_C obtained by XRF.

Raw Materials	Oxides (%)
	SiO2	Al2O3	Fe2O3	CaO	K2O	MgO	SO3	MnO	Others	* LOI
QS	66.4	17.2	3.0	1.5	7.7	1.8	0.3	0.1	0.6	1.4
PC	15.0	4.0	3.1	66.9	1.5	1.2	4.8	–	0.3	3.2
STG	16.4	6.1	5.0	48.1	1.0	3.4	0.1	0.3	0.6	19.0
STC	20.3	6.3	6.1	57.3	0.3	4.2	1.5	0.4	1.0	2.6

* LOI—Loss on ignition measured after drying at 110 °C and firing at 1000 °C.

shows the chemical compositions of PC, QS, and ST (ST_G and ST_C) obtained by X-ray fluorescence (XRF) (Shimadzu, EDX 720, Kyoto, Japan). In contrast, Figure 2 shows the mineralogical composition of PC, QS, and ST (ST_G and ST_C) obtained by X-ray diffraction (XRD) (Shimadzu, XRD6000, Kyoto, Japan). The chemical compositions of ST (ST_G and ST_C) suggest that these materials do not have the potential to be used as pozzolanic material since SiO₂ + Al₂O₃ + Fe₂O₃ is less than 70%, ASTM C618 [27].

2.2. Experimental Design, Mortar Production, and Curing

A statistical design of mixture experiments, {3,2} centroid simplex-lattice design augmented with a central point, was used to define the compositions and amounts of PC, ST_G, and ST_C to be used as matrix/binder materials in the development of the mortars. The maximum substitution content of PC was fixed at 30% for both tailings. Figure 3 illustrates the graphical representation of the composition triangle estimated by the experimental design for mortars. The triangle of interest is highlighted in red within the green circle. The experimental design consisted of seven experimental compositions (

Table 2. Compositions obtained from experimental design for constrained mixtures with three factors (PC, ST_G, and ST_C).

Components (%)	Compositions
	1	2	3	4	5	6	7
PC	100	70	70	85	85	70	80
STC	0	30	0	0	15	15	10
STG	0	0	30	15	0	15	10

), represented by points 1, 2, 3, 4, 5, 6, and 7 in Figure 3. Two replicas were performed for each point, resulting in 21 experimental runs. Compressive strength (CS) and water absorption (WA) were the response variables evaluated to determine the optimal percentage of CP replacement with ST. For each experimental run, the CS and WA results correspond to the average of three specimens. The experimental matrix and statistical data analysis were conducted using Statistic 14 software (TIBCO Software Inc., Palo Alto, USA) [28].

It is important to emphasize that the evaluation of mechanical behavior is a crucial stage, as the mechanical strength of a material is closely related to its future application. This evaluation can be conducted on a small scale, using test specimens, or in a more complex way, like that of the authors Li et al. [29] and Zhang et al. [30] who evaluated the failure and damage characteristics in different rock masses using analytical models.

All formulations used QS as a total replacement for natural sand. The mortar mixtures were produced with a cementitious material to QS ratio (PC + ST_G: QS, PC + ST_C: QS) of 1:3. A reference mortar was prepared using natural sand (MR). The water-to-cement (w/c) ratio was adjusted until the consistency index of the mixtures reached 260 ± 10 mm, as recommended by ASTM C1437 standard [31]. The mortar mixtures were homogenized in a planetary mechanical mixer for 5 min. Cylindrical specimens with dimensions of 50 mm × 100 mm (diameter × height) were molded for CS tests, while prismatic specimens (25 mm × 25 mm × 285 mm) were produced for alkali-aggregate reaction (AAR) resistance tests. The samples were demolded after 48 h and cured in a moist chamber (100% humidity) for 28 days. WA and CS tests were conducted on the developed mortars.

The WA of the mortars was determined according to ASTM C642 standard [32]. To perform the test, the specimens were dried in an oven at 105 ± 5 °C for 72 h to determine the dry mass. Subsequently, the samples were immersed in water for 72 h to determine their wet and saturated mass. The WA was determined by calculating the difference between the wet mass and the initial dry mass of the specimens and expressed as a percentage relative to the initial dry mass. This test is important to assess the porosity of the mortars and their water absorption capacity, which can directly affect their strength and durability. The CS was determined on cylindrical specimens (50 mm × 100 mm), following ASTM C39/C39M [33]. The results represented the average of three specimens and were determined using a universal mechanical testing machine (SHIMADZU, AG-IS 100KN) with a loading rate of 0.25 ± 0.05 MPa/s.

2.3. Resistance to Alkali-Aggregate Reaction

To evaluate the alkali-aggregate reaction (AAR) resistance, linear expansion measurements were carried out on mortar bars, following ASTM C1260 standard [34]. After curing in a moist chamber (100% relative humidity), the samples were immersed in a 1N NaOH solution and kept at 80 ± 2 °C for 28 days. Linear expansion measurements were taken at equal intervals of 0, 1, 7, 14, and 28 days. The linear expansion results represented the average of three specimens.

To evaluate the evolution of mechanical behavior, the best mortar compositions were exposed to water and 1N NaOH solution for 28 and 56 days and subsequently tested for compressive strength. One-way ANOVA (analysis of variance) was used to determine if there were significant differences among the groups (i.e., mortars with and without ST immersed in water and 1N NaOH solution for 28 and 56 days), followed by Tukey’s multiple comparison tests at a significance level of 5%, which were conducted to determine which mortar samples differed significantly from the others. All statistical analyses were performed using Statistica 14 software [28].

The samples were also analyzed by X-ray diffraction (XRD) using a Shimadzu XRD6000 instrument, with Kα-Cu radiation (40 kV/30 mA) and a step size of 0.02°. Thermogravimetric analysis (TGA) was performed using a Shimadzu TA–60H instrument, with a heating rate of 10 °C·min⁻¹ in an air atmosphere. Scanning electron microscopy (SEM) was conducted using a VEGA3 TESCAN instrument. Cross-sectional sections (10 mm × 10 mm) were obtained for SEM analysis. A thin film (~50 nm) of gold was deposited on the cross-section of the mortar bars for 4 min at a current of 10 mA using a SANYU SC–701 device. Figure 4 presents a flowchart summarizing the steps of the methodology used in this study.

3. Results and Discussions

3.1. Experimental Design Results

Table 3. Compressive strength (CS) and water absorption (WA) values measured from the mortar compositions with different component proportions (PC, ST_C, and ST_G).

Compositions	Components (%)			CS Values * (MPa)			WA Values * (%)
	PC	STC	STG	1	2	3	1	2	3
1	100	0	0	6.56	7.41	8.25	13.1	13.9	13.5
2	70	30	0	8.89	10.39	9.46	12.6	12.6	12.6
3	70	0	30	3.56	4.48	5.07	13.3	13.1	13.2
4	85	0	15	6.95	6.17	6.56	12.9	13.0	12.9
5	85	15	0	7.29	7.16	7.15	13.4	13.0	13.2
6	70	15	15	6.79	6.00	6.40	14.7	14.7	14.7
7	80	10	10	7.21	7.10	7.16	12.7	13.3	13.0

* Each CS and WA value corresponds to the average of three samples.

summarizes the CS and WA values for each composition defined in the experimental design. In general, it can be observed that composition 2 (30% ST_C) exhibited the highest values of CS. Based on these data, statistical analysis was performed on the results.

Table 4. Statistical parameters (F_test, F_cal/Ftab, P_value, R², and R²_ajust) calculated from different regression models (linear, quadratic, and special cubic) for the CS and WA.

	Regression Models	Ftest	Fcal/Ftab	Pvalue	* R2 (%)	** R2ajust (%)
CS	Linear	10.13	2.85	0.00002	70.26	66.95
	Quadratic	21.26	6.46	0.00067	90.17	86.90
	Special Cubic	1.02	0.22	0.32915	90.84	86.92
WA	Linear	0.32	0.09	0.73367	3.38	0.01
	Quadratic	19.33	5.88	0.00002	80.14	73.53
	Special Cubic	24.88	5.41	0.00012	92.85	89.79

* R²: determination coefficient; ** R²_ajust: adjusted determination coefficient.

presents the main statistical parameters obtained from the analysis of variance (95% confidence level) of the experimental design conducted to determine the compositions of mortars containing ST_G and ST_C as partial replacements for PC. Linear, quadratic, and special cubic regression models were fitted to the experimental data for statistical analysis. The significance and predictive capability of the models can be evaluated through the F-test (F_cal). For a model to be significant and predictive, the calculated F-value (F_cal) should be at least four to six times greater than the tabulated F-value (F_tab) in the Fisher–Snedecor distribution [35].

For the analysis of CS, the quadratic model showed the highest values of F_cal/F_tab (6.46), R² (90.17%), and R²_Adj (86.90%), making it the most suitable for predicting CS results. While for WA, the special cubic model showed the best results (F_cal/F_tab = 5.41, R² = 92.85%, and R²_Adj = 89.79%). Figure 5a,b show the correlation between the observed and predicted values for CS (Figure 5a) and WA (Figure 5b). It can be observed that the models are suitable for describing the mechanical behavior and WA of the mortars containing ST_G and ST_C as partial replacements for PC. Equation (1) represents the quadratic model obtained from the regression analysis for CS, while Equation (2) represents the cubic model obtained for the WA of the mortars.

In Equation (1), it was observed that all components (PC, ST_C, and ST_G) and their interaction (CP × ST_C, CP × ST_G, and ST_C × ST_G) are statistically significant (p_value < 0.05, with 95% confidence) and exhibit coefficients greater than zero, indicating that they all contribute to increasing the CS values. This behavior suggests that the replacement of PC with ST_G or ST_C did not, by itself, reduce the CS values of the mortars. Thus, any observed decrease is due to PC being the component with the highest contribution coefficient (7.337) to CS. In Equation (2), the CP × ST_C interaction was not statistically significant (i.e., p_value (0.3498) > 0.05). Therefore, this interaction was disregarded during model adjustment. Furthermore, it was observed that the interactions between the PC and the ST (PC × ST_G and CP × ST_C × ST_G) had coefficients lower than zero, indicating that these interactions favor the WA value reduction.

The response surfaces obtained from the proposed models for CS and WA are illustrated in Figure 6a,b. In Figure 6a, it can be observed that regions with higher ST_C content exhibited higher CS results compared to regions with higher ST_G content, indicating that the calcination of ST favored the mechanical behavior of the mortars. However, within the studied range (maximum substitution of 30% of PC with ST), all mortar compositions showed CS values above the minimum limit of 2.4 MPa established by the ASTM C270 standard [36]. Figure 6b indicates that the highest WA values are in regions with higher ST_G content. It is known that WA is directly related to the porosity of the material, so it is likely that the incorporation of ST_G increased the porosity of the mortars.

$$\begin{array}{c}Compressivestrength\left(\mathrm{MPa}\right)=7.337\times \left(PC\right)+6.284\times \left(S{T}_C\right)+4.714\times \left(S{T}_G\right)+\\ \left[1.822\times \left(PC\right)\times \left(S{T}_C\right)\right]+\left[2.402\times \left(\mathrm{PC}\right)\times \left(S{T}_G\right)\right]+\left[3.855\times \left(S{T}_C\right)\times \left(S{T}_G\right)\right]+0.105\end{array}$$

(1)

$$\begin{array}{c}Waterabsorption\left(\%\right)=\{0.136\times \left(PC\right)+0.126\times \left(S{T}_C\right)+0.132\times \left(S{T}_G\right)-\\ \left[0.017\times \left(PC\right)\times \left(S{T}_G\right)\right]+\left[0.071\times \left(S{T}_C\right)\times \left(S{T}_G\right)\right]-\left[0.194\times \left(\mathrm{PC}\right)\times \left(S{T}_C\right)\times \left(S{T}_G\right)\right]\}\times 100\end{array}$$

(2)

Based on the results of the experimental design, three mortar compositions named M0 (0% ST), M30S_G (30% ST_G), and M30S_C (30% ST_C) were selected to study the resistance to the AAR. M30S_G and M30S_C were chosen because they allowed for the highest amount of PC substitution with ST, while M0 was selected for comparison purposes.

3.2. Resistance to Alkali-Aggregate Reaction

Figure 7 shows the linear expansion values measured from the M0, M30S_G, and M30S_C samples immersed in a 1N NaOH solution for 28 days. The expansion values for the M0 mortar (without ST) were below the limits specified in the ASTM C1260 [34] standard, which sets a maximum expansion value of 0.10% at 14 days and 0.20% at 28 days. This indicates that the QS does not exhibit deleterious potential. These results are in line with the chemical composition (Table 1) and mineralogy (Figure 2a) of QS, which indicates that the present silica is predominantly in the form of non-reactive silica, quartz, and mica materials. On the other hand, the potassium present in QS is associated with the mica structure, which imparts a low-reactivity alkaline character [37]. For the M30S_G and M30S_C samples, it was observed that the use of calcined ST allows for the production of materials with deleterious potential but within the acceptable range (<0.20%) established by the ASTM C1260 [34]. However, the material in its non-calcined form presents a value within the range considered deleterious by the ASTM, indicating that its use in mortar formulation should be avoided.

The MR, M0, M30SG, and M30SC samples were exposed to a 1N NaOH solution for 28 and 56 days to assess the evolution of mechanical behavior, and their compressive strength was determined. Figure 8 presents the CS values of the mortars after immersion in distilled water and the 1N NaOH solution for 28 days (Figure 8a) and 56 days (Figure 8b). Different letters indicate significant differences in CS values at a confidence level of 95%. The ascending order of the letters indicates the order of CS values, from highest to lowest. For the samples immersed in water, the CS values of M0 are statistically equal to those of MR (p = 0.5244 at 28 days and p = 0.6234 at 56 days), indicating that the substitution of commercial sand with QS did not cause significant changes in the mechanical behavior. When immersed in the 1N NaOH solution, the CS values of M0 were higher than those of MR (82.7% higher than MR at 28 days (p = 0.0003) and 46.5% at 56 days (p = 0.0103)). The partial replacement of PC with 30% ST_G resulted in a significant reduction in the CS values. Compared to the control group (M0), which does not contain tailings, the CS values decreased by 36.2% at 28 days (p = 0.0211) and 33.7% at 56 days (p = 0.0013). When immersed in the 1N NaOH solution, the reduction in CS values was even more significant (54.7% at 28 days (p = 0.0283) and 37.2% at 56 days (p = 0.0083)). On the other hand, the CS values of M30S_C were statistically equal to those of M0 at a 95% confidence level (p = 0.9111 at 56 days in water and p = 0.0924 and p = 0.0670 at 28 and 56 days in the 1N NaOH solution). This result indicates that substituting PC with ST_C by up to 30% will not significantly affect the mechanical behavior of the mortars. In other words, this result suggests that ST_C is a sustainable and economically viable partial substitute for PC.

An increase in CS between the ages of 28 and 56 days was observed for the MR mortars (50.4% in water (p = 0.0274) and 94.6% in the 1N NaOH solution (p = 0.0008)), M0 mortars (52.7% in water (p = 0.0046) and 55.3% in the 1N NaOH solution (p = 0.0127)), and M30S_G mortar immersed in the 1N NaOH solution (47.1%, (p = 0.0245)). This behavior was expected and can be attributed to the formation of calcium silicate hydrate (C–S–H) during the mortar curing process, contributing to the increase in mechanical strength. C–S–H has a three-dimensional gel structure that fills the empty spaces between the particles in the mortar, forming a cohesive and resistant structure. Over the curing time, C–S–H continues to develop, increasing its density and improving the mechanical properties of the mortar [7,38]. On the other hand, the M30S_G mortar immersed in water and the M30S_C mortar did not show a significant increase in CS values between the ages of 28 and 56 days (p = 0.4714). This suggests that all the relevant reactions for gaining mechanical strength occurred during the first 28 days.

An increase in CS values was observed for the M0 mortar (39.6%, (p = 0.0188)) and M30S_G mortar (93.8%, (p = 0.0030)) after exposure to the 1N NaOH solution for 56 days. On the other hand, the other samples did not show significant differences in CS values (MR p = 0.3702 and M30S_C p = 0.0508). This behavior indicates that the mortars containing tailings (QS, M30S_G, and M30S_C) developed in this study were not susceptible to the aggression of the alkaline solution, which is consistent with the results obtained in the expansion test (Figure 7). This behavior suggests that M0, M30S_G, and M30S_C were able to withstand the negative effects of the 1N NaOH solution, which is a positive aspect in terms of the durability and performance of these materials in chemically aggressive environments.

On the other hand, it is noted that M30S_C showed higher CS values compared to M30S_G (p = 0.0011 and p = 0.0005 at 28 and 56 days in water and p = 0.0036 and p = 0.0126 at 28 and 56 days in the 1N NaOH solution). This difference can be attributed to the higher content of CaO (57.3%) present in the ST after calcination (CaCO₃ → CaO + CO₂). Previous studies [39,40,41] have shown that CaO can accelerate hydration reactions due to increased alkalinity caused by the reaction between CaO and water to generate Ca(OH)₂. Consequently, increased alkalinity accelerates the dissolution of Ca²⁺, Al^3+, and Si⁴⁺ ions present in calcium-, aluminum-, and silicon-rich phases, resulting in the formation of hydration products such as C–S–H, which plays a crucial role in pore filling and strength gain. In fact, M30S_C exhibited lower porosity (17.7%) and higher density (2.19 g·cm⁻³) compared to M30SG (18.9% and 2.11 g·cm⁻³) (

Table 5. Density and porosity values of mortars MR, M0, M30S_G, and M30S_C. Different letters show significant differences (p < 0.05, Tukey test).

	MR	M0	M30SG	M30SC
Density (g·cm−3)	2.05 ± 0.01 c	1.95 ± 0.01 d	2.11 ± 0.01 b	2.19 ± 0.01 a
Porosity (%)	17.2 ± 0.4 b	11.5 ± 0.6 c	18.9 ± 0.5 a	17.7 ± 0.3 b

).

Figure 9a,b show the differences in the microstructure of the mortars. In Figure 9a, representing M30S_G, a higher presence of pores can be observed compared to M30S_C in Figure 9b. This behavior is associated with the better packing provided by the calcined tailings, as indicated by the mortar density results (Table 5). In fact, it can be observed that M30S_C had the highest density value among all the evaluated samples. This effect can be attributed to the smaller particle sizes exhibited by the ST after calcination (D₉₀ = 8.43 µm) compared to the natural tailings (D₉₀ = 16.38 µm). It is known that smaller particles provide a pore-filling effect, promoting better particle packing [42,43].

Still, in Figure 9a, it is possible to observe the presence of elongated crystals in the form of needles, which can be associated with the ettringite phase. Ettringite is known to appear as clusters of elongated crystals that can group together, forming bundles or compact clusters. These crystals generate internal stresses and cracks around the aggregates, creating points susceptible to crack propagation. As a result, the material weakens [35,44]. On the other hand, in Figure 9b, representing M30S_C, the presence of pores is less evident, the structure is more compact, and there is no presence of elongated crystals characteristic of ettringite. Therefore, M30S_C exhibits a structure less prone to crack formation compared to M30S_G. It is important to highlight that in the SEM images, the characteristic alkali-aggregate reaction gel was not observed in the pores or cracks, which is consistent with the results of the expansion test (Figure 7), demonstrating expansion values within the acceptable limit (<0.20%).

The XRD results of the mortars immersed in distilled water and the 1N NaOH solution for 56 days (Figure 10) confirm the presence of the ettringite phase (Ca₆A_l2(SO₄)₃(OH)₁₂·26H₂O), in addition to quartz (SiO₂) and portlandite (Ca(OH)₂), which are predominant phases. It is also possible to observe the presence of calcite (CaCO₃) and calcium silicate (Ca₂SiO₄) in smaller quantities. The peaks corresponding to the ettringite and quartz phases show an intensification with the exposure of the mortars to the 1N NaOH solution. This intensification is more pronounced in M30S_G. It is worth noting that the quartz present in the mortars comes from the aggregate (QS), while portlandite is formed during the hydration process of PC. Ettringite probably formed during the hydration of PC, as QS, ST_G, and ST_C do not contain sulfides in their composition (see Figure 2a,b). There is also a reduction in the intensity of the peaks corresponding to portlandite with exposure to the 1N NaOH solution, indicating that the hydrate has been consumed. This reduction can be attributed to the chemical interactions between the alkaline solution and portlandite. These interactions can lead to the formation of more stable compounds, such as C–S–H, which is one of the main contributors to the mechanical strength development of the mortars [38].

Figure 11 shows the TGA curves of the samples after immersion in water and after immersion in the 1N NaOH solution for a period of 56 days. It can be observed that MS30_G and M30S_C immersed in water exhibit more intense mass loss stages compared to those immersed in the solution. The most significant mass loss stages are observed in the following temperature ranges: 26 °C–385 °C, 385 °C–500 °C, and 500 °C–800 °C. These mass loss stages occur due to the dehydration or decomposition of the phases present in the mortars. The mass loss in the first region, between 26 °C and 385 °C, corresponds to the evaporation of free water as well as the decomposition of the C–S–H, ettringite (AFt), and AFm phases present [45]. In the temperature range between 385 °C and 500 °C, dehydroxylation of portlandite occurs, which decomposes into calcium oxide and releases water (Ca(OH)₂ → CaO + H₂O). The mass loss observed in this region is related to the removal of water released during the dehydroxylation of portlandite. In the range of 500 °C to 800 °C, decarbonation of calcite takes place, decomposing into calcium oxide and carbon dioxide (CaCO₃ → CaO + CO₂). The mass loss measured in this temperature range is associated with the release of CO₂ during decarbonation [35].

Based on the TGA/DTGA results, the quantification of the main phases present in the mortars up to 1000 °C was performed, and the results are presented in

Table 6. Percentage estimation of the hydrated products present in the mortars investigated in this study, immersed in distilled water and 1N NaOH solution for 56 days.

Condition	Mortar	Weight Loss (%)		Estimated Amount (%)
		Total	Free Water, C-S-H, AFt, and AFm *	Ca(OH)2	CaCO3
Water	M0	17.5	10.0	14.4	6.1
	MS30G	17.9	6.9	10.7	15.4
	M30SC	17.0	8.8	15.6	7.0
1N NaOH solution	M0	17.1	9.3	11.1	9.8
	MS30G	9.3	4.3	4.5	7.5
	M30SC	10.7	6.5	7.0	4.8

* The percentages correspond to the gross decomposition without applying any multiplication factor since the combined water loss occurred due to the decomposition of distinct phases.

. The percentages of the portlandite and calcite phases were calculated according to Equations (3) and (4). Regarding the C–S–H, ettringite, and AFm phases, it was not possible to calculate their individual percentages due to the overlapping decomposition peaks, as well as the simultaneous removal of free water during the decomposition process. Therefore, the percentages presented in Table 5 for these compounds correspond to the gross decomposition, i.e., the overall weight loss in the first region (between 26 °C and 385 °C).

$$\%\mathrm{Ca}{\left(\mathrm{OH}\right)}_2=\frac{\mathrm{MMCa}{\left(\mathrm{OH}\right)}_2}{{\mathrm{MMH}}_2\mathrm{O}}\times {\%\mathrm{H}}_2\mathrm{O}=\frac{74.09}{18.02}\times {\%\mathrm{H}}_2\mathrm{O}=4.11\times {\%\mathrm{H}}_2\mathrm{O}$$

(3)

$${\%\mathrm{CaCO}}_3=\frac{{\mathrm{MMCaCO}}_3}{{\mathrm{MMCO}}_2}\times {\%\mathrm{CO}}_2=\frac{100.09}{44.01}\times {\%\mathrm{CO}}_2=2.27\times {\%\mathrm{CO}}_2$$

(4)

In these equations, $\mathrm{MMCa}{\left(\mathrm{OH}\right)}_2$, ${\mathrm{MMCaCO}}_3$, ${\mathrm{MMH}}_2\mathrm{O}$, and ${\mathrm{MMCO}}_2$ represent, respectively, the molar masses of portlandite, calcite, water, and carbon dioxide; ${\%\mathrm{H}}_2\mathrm{O}$ and ${\%\mathrm{CO}}_2$ are the calculated mass losses [46].

Overall, a reduction in mass loss can be observed in the temperature range corresponding to the formation of C–S–H (26 °C–385 °C) with the partial substitution of PC by ST_G and ST_C. This behavior indicates that using 30% ST_G and ST_C as partial replacements for PC reduced the formation of C–S–H, which plays an important role in the mechanical strength gain of mortars. The reduction in C–S–H formation was more pronounced in M30S_G (reduction of 44.9% and 116.3% compared to M0 when immersed in water and the 1N NaOH solution, respectively) than in M30S_C (reduction of 13.6% and 43.1%). This difference justifies the lower CS values presented by M30S_G. It is important to note that the mass loss values related to the C–S–H formation region are combined with ettringite and AFm compounds. Therefore, although M30S_C showed a reduction in the mass loss in this temperature range (26 °C–385 °C), there was no reduction in the CS values compared to M0, indicating that many of the compounds formed in this region are C–S–H. In the CaCO₃ decomposition range, a 152% increase in its content was observed in M30S_G compared to M0 when immersed in water. However, when exposed to the 1N NaOH solution, there was a 30.7% reduction in this content. This increase in CaCO₃ content suggests that M30S_G is more susceptible to the carbonation phenomenon.

In general, it can be observed that the substitution of PC by ST_C did not cause significant changes in the mechanical behavior compared to the M0 sample (without tailings). This indicates that the ST_C can partially replace the PC without significantly compromising the mechanical performance of the mortars. On the other hand, the non-calcined ST should be used cautiously, as it reduced the CS values.

4. Conclusions

The results demonstrated that it is possible to replace natural sand with quartzite sand without altering the mechanical behavior and the evolution of the mechanical properties of the developed mortars. The results of the experimental design demonstrated that the quadratic (R² = 90.17%) and special cubic (R² = 92.85%) models, proposed to predict the mechanical behavior and water absorption of the mortars, were effective in explaining the observed variations and showed good predictive capability. Overall, substituting 30% of Portland cement with calcined scheelite tailings did not significantly compromise the long-term mechanical performance of the mortars. However, using non-calcined scheelite tailings is not recommended as it showed detrimental effects on the mechanical properties of the produced mortars. Compared to the control group, which did not contain tailings, the compressive strength values decreased by 36.2% at 28 days and by 33.7% at 56 days. Furthermore, it was found that the mortars with scheelite tailings exhibited good resistance to the detrimental effects of the 1N NaOH solution, indicating satisfactory durability in chemically aggressive environments. These results highlight the feasibility of using mineral tailings as alternative raw materials in mortars without compromising their mechanical and durability properties, thus promoting sustainability in the construction industry.