Reuse of Mine Tailings Through Geopolymerization Applied to 3D Printing: A Review of Progress, Challenges and Perspectives

Abstract

The increasing global demand for metals, driven by technological progress and the energy transition, has led to an acceleration in the expansion of the mining and metallurgical industry, resulting in an increase in the generation of mine tailings. This waste, which is of heterogeneous composition and has high contaminant potential, represents significant environmental and social challenges, affecting soils, water, and the geotechnical stability of tailings. The accumulation of these mine tailings poses a problem not only in terms of quantity, but also in terms of physicochemical composition, which exacerbates their environmental impact due to the release of heavy metals, affecting ecosystems and nearby communities. This article reviews the potential of geopolymerization and 3D printing as a technological solution for the management of tailings, offering an effective alternative for their reuse as sustainable building materials. Alkaline activation of aluminosilicates facilitates the formation of N–A–S–H and C–A–S–H cementitious structures, thereby providing enhanced mechanical strength and chemical stability. Conversely, 3D printing optimizes structural design and minimizes material consumption, thereby aligning with the principles of a circular eco-economy and facilitating carbon footprint mitigation. The present study sets out to compare different types of tailings and their influence on geopolymer reactivity, workability, and mechanical performance. In order to achieve this, the study analyses factors such as the Si/Al ratio, rheology, and setting. In addition, the impact of alkaline activators, additives, and nanoparticles on the extrusion and interlaminar cohesion of 3D printed geopolymers is evaluated. These are key aspects of their industrial application. A bibliometric analysis was conducted, which revealed the growth of research in this field, highlighting advances in optimized formulations, encapsulation of hazardous waste, CO₂ capture, and self-healing geopolymers. The analysis also identified technical and regulatory challenges to scalability, emphasizing the necessity to standardize methodologies and assess the life cycle of materials. The findings indicated that 3D printing with tailings-derived geopolymers is a viable alternative for sustainable construction, with applications in pavements, prefabricated elements, and materials resistant to extreme environments. This technology not only reduces mining waste but also promotes the circular economy and decarbonization in the construction industry.

1. Introduction

The present and increasing demand for metals to meet technological, energy, and commercial needs has exerted significant pressure on the global mining and metallurgical industry [1,2]. The management of mine tailings, by-products generated during extraction processes and mineral refinement, poses a significant environmental challenge [3,4]. This residual material poses significant environmental risks, as well as threats to the health and safety of communities residing in proximity to mining operations [5]. Furthermore, it has been identified as a contributing factor to the escalation of social conflicts in mining regions, particularly in Latin America [4,6,7]. For example, in Peru, a significant proportion of socio-environmental conflicts are associated with water management and environmental degradation resulting from mining activities, which adversely impact agricultural production and the social and economic stability of communities [6,7,8].

In this context, geopolymerization—a process that utilizes alkaline activation of aluminosilicates—has emerged as a promising solution [9,10]. This approach facilitates the transformation of mine tailings into sustainable construction materials, thereby chemically confining heavy metals and other potentially polluting elements [11,12,13]. The resulting geopolymers exhibit suitable mechanical properties for utilization in the construction industry [14,15]. Additionally, they contribute significantly to the reduction of CO₂ emissions and energy consumption, thereby positioning themselves as a viable and environmentally friendly alternative to Portland cement [11,13,16].

The utilization of geopolymers in the fabrication of materials such as bricks, along with their capacity for advanced applications including 3D printing, introduces novel prospects for the reuse of industrial waste in construction [17,18]. Moreover, the incorporation of supplementary industrial byproducts, such as fly ash and blast furnace slag, during the geopolymer synthesis process enhances their environmental and mechanical efficiency [19,20,21]. These materials have the potential to drastically reduce CO₂ emissions compared to conventional cementitious materials, while requiring less energy in their production [22,23]. Geopolymers’ capacity to immobilize heavy metals and stabilize contaminated soils renders them a compelling solution for the management of mine tailings, thereby reducing their storage and the environmental risk associated with their disposal [3,4,12,24,25]. Furthermore, the rheological and thixotropic properties of geopolymers can be modified to facilitate their use in 3D printing technologies, which expands their applications in the construction of complex structures [18,26,27].

The development of geopolymers for construction and the encapsulation of hazardous waste is an area of ongoing research [10,22,28]. This approach not only fosters more sustainable construction but also offers a comprehensive solution for industrial waste management, facilitating the transition towards a circular economy [29]. The reuse of mining tailings in the production of construction materials could play a crucial role in reducing the environmental impact of mining, while providing new economic and technological opportunities for developing countries and alternatives for decarbonizing construction [30,31,32,33]. Despite the advances in research on geopolymerization and its application in the reuse of mine tailings, there are still gaps in the literature regarding the optimization of formulations for 3D printing and the standardization of methodologies to ensure mechanically, chemically, and environmentally viable performance on an industrial scale. The present study aims to evaluate the potential of mine tailings-derived geopolymers for use in 3D printing by analyzing the influence of Si/Al ratio, types of alkaline activators and the incorporation of additives on their mechanical, rheological, and chemical stability properties. In addition, the challenges associated with their industrial scalability and their integration in the framework of the circular economy will be reviewed. The objective of this work is to contribute to the development of innovative strategies for the valorization of mining tailings, with a view to promoting sustainability in the construction industry through the design of geopolymeric materials with high structural performance and lower environmental impact.

2. Bibliometric Analysis

Bibliometric analysis and network visualization were employed to examine the extant literature on the reuse of mining tailings through geopolymerization and its application to 3D printing, along with the challenges and opportunities in this domain. A comprehensive search strategy was implemented in prominent scientific databases, including Scopus, Google Scholar, and Web of Science. The search terms were meticulously combined with Boolean operators to ensure the maximal relevance of the results. The search combinations that were utilized were as follows: (“Geopolymer” OR “alkali-activated material”) AND (“mine tailings” OR “mining waste”) AND (“reuse” OR “recycling” OR “sustainability”) AND (“3D printing” OR “additive manufacturing”) AND (“rheology” OR “extrusion” OR “shaping”). The selection of these keywords was driven by the objective of identifying studies that concentrate on the reuse of mining tailings through geopolymerization, 3D printing, and the rheological properties of these materials over a 10-year timeframe. In addition, the following inclusion and exclusion criteria were applied to ensure the relevance and quality of the studies:

The inclusion criteria are as follows: (a) Peer-reviewed studies published in the last decade; (b) research that addressed the reuse of mining tailings through geopolymerization and its application in 3D printing; (c) articles that analyzed the mechanical, rheological, and durability properties of geopolymers.

Exclusion criteria included articles that (a) did not directly address the combination of mining tailings, geopolymerization, and 3D printing technologies; (b) did not present empirical data.

The results of the search yielded a total of 104 articles; however, the application of the inclusion and exclusion criteria reduced this number to 57 studies. This selection process is represented in a PRISMA flow chart to ensure the transparency of the process. Utilizing these data, a comprehensive analysis of the research trends was conducted, and the co-occurrence of keywords in the selected studies was performed using the VOS Viewer software (version 1.6.20), thereby creating the co-occurrence networks of key terms and density maps.

The visualization of the network enables the demonstration of the interrelations between the key concepts of the research (nodes) and the frequent co-occurrences (edges) present in the scientific literature (Figure 1). The most prominent terms, such as geopolymerization, mine tailings, sustainability, and 3D printing, appear in the center of the network, reflecting their fundamental role in current research. Thematic clusters further reveal the grouping of terms related to rheological properties, mechanical properties, and additive manufacturing, thereby demonstrating the close linkage between these subfields in the development of advanced geopolymers (Figure 2).

Figure 2 presents the overlay visualization, introducing the temporal component in which the terms that have gained relevance more recently are observed. The most prominent terms, including “3D printing”, “tailings”, and “rheological properties”, indicate an expansion of the field of geopolymerization towards more sophisticated and specialized applications. This shift suggests a transition from the fundamental study of mining tailings and sustainability to the development of novel materials with technological applications, such as additive manufacturing and environmental remediation.

The visualization depicted in Figure 3 facilitates the identification of terms that are the focal point of research endeavors and the identification of the publications in which these terms are most prevalent. The high-density areas encompass pivotal concepts such as geopolymerization, mine tailings, sustainability, and 3D printing, denoting their pertinence and constituting the thematic focal point of contemporary research endeavors. Conversely, the high density observed in terms such as rheology and mechanical properties suggests that optimizing these characteristics remains a priority research objective, with the aim of ensuring that geopolymers are suitable for technological applications, particularly in the domain of additive manufacturing.

The multidimensional approach that emerges from this analysis demonstrates that geopolymerization applied to 3D printing offers a proposal to address both technical and environmental challenges, thereby consolidating its role as a key solution for mining waste management and innovation in construction materials.

To show the evolution of research into the use of mine tailings in the manufacture of geopolymer materials through to 3D printing, a diagram has been created with key sections in the timeframe 2010–2025 (Figure 4) and highlighting the most significant developments at each stage. In the period of 2010–2015, research was conducted in the stabilization and application of tailings as geopolymeric materials, in the improvement of mechanical strength, chemical stability and durability through alkaline activation, as well as in the development of construction materials and their ability to immobilize heavy metals [16,34,35,36,37].

The period of 2015–2020 was characterized by research on tailings-based geopolymers, their feasibility and optimization of mechanical, thermal, and durability properties through improved formulations and alkaline activation that allowed progress towards applicability in 3D printing, on the use of sulfide tailings, on the improvement in mechanical strength, on the ability to immobilize heavy metals, as well as on the influence of microstructure on the mechanical properties of these geopolymers [38,39,40,41,42,43,44,45].

In the period of 2020–2025, research focused on two aspects. On the one hand, it explored the use of geopolymerization to produce high quality building materials from industrial waste, reducing environmental impact and dependence on conventional materials such as Portland cement and in optimizing physico-chemical properties for various applications [46,47,48,49,50,51]. On the other hand, research on the use of tailings-derived geopolymers for 3D printing has advanced through the improvement and optimization of rheological and mechanical properties [26,27,52,53,54,55,56,57], the specific formulation of alkaline activators [57,58], and in the capabilities for encapsulation of hazardous waste [13,59] and resistance to aggressive chemical environments [32,33,60,61,62,63].

Current trends are towards optimizing their physical and chemical properties, integrating them with advanced technologies and deploying them on a large scale in strategic sectors such as construction and energy, and also as a way to decarbonize the construction sector. These trends and perspectives focus on seven main lines of action: (1) Improving and optimizing the properties of geopolymers for 3D printing [27,30,53,64], with the challenge of industrial scalability; (2) as materials to improve energy efficiency, towards advanced thermal insulation materials [65,66,67], with the challenge of compatibility of geopolymers with other insulation materials and long-term performance; (3) materials with the potential to sequester CO₂ within their mineralogical structure [68,69]; (4) integration of new wastes or materials such as fly ash, metallurgical slags and other industrial wastes into blends to improve the sustainability of geopolymers [48,49,70]; (5) the incorporation of nanomaterials and their effects on the properties of geopolymers [71,72,73] and the development of self-repairing geopolymers for smart infrastructure and advanced construction, which can significantly reduce maintenance costs and improve structural safety [74,75]; and (6) functionalized geopolymers for energy storage applications with applications such as supercapacitors, sodium ion baths, and thermal storage [76,77].

3. Challenges in Reusing Tailings

Mining tailings represent a significant environmental concern within the extractive industry, primarily due to the substantial volume of waste generated and its complex chemical composition [1]. These waste materials pose a dual threat to both the environment and to human health. However, if the technical, economic, and regulatory challenges are adequately addressed, opportunities for valorization emerge.

The process of extracting heavy metals and sulfates from tailings can result in the contamination of groundwater and surface water, as well as the acidification of soils through acid mine drainage (AMD). This phenomenon is attributed to the oxidation of sulfide minerals, such as pyrite, which has been observed to generate significant impacts on surrounding ecosystems and nearby communities [78]. This highlights the necessity for effective strategies to encapsulate metals and minimize their mobility into the environment [41].

The complexity of standardization in reuse processes is attributable to the significant variability in the mineralogical and chemical composition of the tailings. This necessitates the development of customized formulations tailored to distinct categories of waste, particularly in contexts such as geopolymerization, where precise chemical composition is paramount to ensure the attainment of optimal mechanical properties [78,79]. For instance, tailings with high crystallinity or a low proportion of reactive aluminosilicates are limited in their direct application in alkaline activation processes. Consequently, they necessitate pretreatments such as thermal or mechanical activation to enhance their reactivity and enable their incorporation into geopolymeric mixtures [3,80]. The integration of tailings with reactive materials, such as metakaolin or blast furnace slag, necessitates precise adjustments in the proportions of alkaline activators to ensure the reactivity and dimensional stability of the materials produced [41,81].

The treatment and valorization of tailings through geopolymerization has shown promising results on a small scale. However, at an industrial level, it entails high initial investment costs in specialized facilities and advanced technologies [4,5]. Consequently, they still currently face technical, logistical, and economic barriers. Additionally, the substantial energy demands, and the logistics involved in transporting large volumes of tailings impede their widespread adoption [79,82]. Another salient factor pertains to the formulation of explicit regulations that facilitate the utilization and valorization of these materials. This aspect is identified as a pivotal impediment to the advancement of industrial development in countries such as Peru and Chile [5,83]. However, it is noteworthy that these limitations are also observed in various mining waste contexts in Europe.

Geopolymers are inorganic materials that are synthesized through the alkaline activation of aluminosilicate precursors. This process, which occurs in three stages (dissolution, orientation, and polycondensation), generates three-dimensional networks of aluminosilicates characterized by covalent bonds of the type –Si–O–Al–O–. These bonds are stabilized by alkali ions, such as sodium (Na⁺), and coordinated water molecules. These networks exhibit properties such as high thermal, chemical, and structural resistance, rendering them well-suited for demanding industrial applications [84,85,86].

Aluminosilicate precursors, including metakaolin, fly ash, and mine tailings, undergo a reaction with alkaline solutions (NaOH and sodium silicates), resulting in the release of reactive monomers such as [Si(OH)₄] and [Al(OH)₄]⁻. This initial stage establishes the chemical basis for the formation of structural bonds [21,85,87]. The monomers undergo a process of condensation, resulting in the formation of sialate oligomers (–Si–O–Al–O–) and sialate–siloxo (–Si–O–Al–O–Si–O–) structures, which collectively serve as the fundamental framework for geopolymeric gels [85,88]. Depending on the composition of the system, N–A–S–H (sodium–aluminum–silicate hydrate) gels can be formed in calcium-poor systems, or C–A–S–H (calcium–aluminum–silicate hydrate) gels in calcium-rich systems. These gels are primarily responsible for the advanced properties of geopolymers [13,89,90].

3.1. Determining Factors in the Chemistry of Geopolymers

The characteristics of geopolymers are determined by a variety of factors, among which the most relevant are the Si/Al molar ratio, the concentration of the alkaline activator, the curing conditions, and the molecular and chemical properties.

The Si/Al molar ratio is key to the reactivity and mechanical properties of geopolymers. Values between 1.5 and 2.0 favor initial reactivity and early strength due to the high density of reactive sites [35,91]. In contrast, ratios between 3.0 and 4.5 promote more polymerized three-dimensional networks, improving chemical stability and resistance to aggressive environments [50,70]. Values above 4.5 reduce the density of reactive sites, affecting workability and structural cohesion [26,68].

The concentration and type of activator affect both the solubility of the precursors and the reaction rate. These activators are solutions of alkaline earth elements, such as calcium (Ca) and alkaline elements such as sodium (Na) and potassium (K). Excessive concentrations of these solutions can induce efflorescence due to excess alkalis [21,90,92].

The curing conditions are pivotal in determining the compaction and final density of the material. The optimal temperature range is between 40 and 90 °C, and the relative humidity should be maintained at 95%. It has been demonstrated that elevated temperatures enhance structural stability [88,89].

Finally, the molecular and chemical properties, as evidenced by the Si–O–Si and Si–O–Al covalent bonds, are essential in determining the thermal and mechanical properties. Molecular studies have identified average bond lengths of 1.60–1.65 Å for Si–O and 1.74–1.80 Å for Al–O, highlighting their robustness [86,90]. Moreover, the presence of alkali ions, such as Na⁺, has been observed to enhance the chemical stability of the network, thereby facilitating the formation of homogeneous and resilient structures [13,84].

3.2. Sorption Mechanisms of Geopolymers

The sorption of heavy metals onto geopolymer matrices is a complex process involving various chemical and physical mechanisms that contribute to the efficiency of these materials in immobilizing and removing metal contaminants from wastewater and other contaminated environments [93,94]. Specifically, the ion exchange mechanisms between the cations present in the geopolymer structure (e.g., Na⁺ or K⁺) that are replaced by metallic cations (e.g., Pb²⁺, Cu²⁺, or Zn²⁺) are of particular significance [25,95,96]. Additionally, the interactions with the functional groups on the geopolymer surface, such as the silanol (Si–OH) and aluminol (Al–OH) groups, are noteworthy [90], and so is the formation of precipitates. In adequate pH environments, these precipitates allow the generation of insoluble compounds, such as hydroxides, carbonates, or sulfates, which precipitate and remain trapped in the geopolymer matrix [13,95]. Conversely, physical mechanisms also exert an influence, wherein the porous structure of the geopolymer generates cavities and pores, thereby increasing the specific surface and favoring the physical interaction between the metals and the surface of the adsorbent [23,97].

3.3. Geopolymerization of Mining Tailings: Potential and Challenges

Tailings are a byproduct of mining that present a significant environmental concern due to their metal and metalloid content, including elements such as Pb, Cs, and As. However, their composition, which is rich in silica (SiO₂) and alumina (Al₂O₃), also makes them well-suited for the synthesis of geopolymers through alkaline activation [41,80,82,92]. The three-dimensional structure of geopolymers offers potential applications in construction and contaminant containment [22,89,98]. These materials possess advantageous mechanical, thermal, and chemical properties, thus offering a sustainable solution for their management and valorization [19,92]. The formation of three-dimensional gels, contingent on the composition of the tailings, gives rise to N–A–S–H or C–A–S–H phases. These phases exhibit the capacity to encapsulate contaminants through diverse sorption mechanisms that curtail the mobility and leaching of metals. Moreover, these phases comply with environmental stabilization requirements [97,99].

The Si/Al ratio is a crucial parameter in the reactivity and properties of geopolymers, with a ratio between 1.5 and 2.0 favoring the formation of dense and resistant structures, while higher ratios, higher than 4.5, decrease the density of reactive sites [92,100]. Furthermore, the curing conditions (temperature and humidity) and the type of alkaline activator (e.g., NaOH, sodium silicates) are essential to control the reaction kinetics and final density of the material [85,92,100]. While hydroxides and silicates enhance reactivity, their high alkalinity can present environmental and operational challenges [10,95,99].

Geopolymers derived from tailings have been shown to possess remarkable properties, including high mechanical strength (20–80 MPa) [41,82,99], notable thermal stability, and chemical durability in aggressive environments [59,93]. However, their performance may be subject to variation due to the heterogeneity of the mineralogical composition of the tailings and the extreme pH conditions, which affect the chemical stability of the matrix [80,87,92]. These challenges necessitate constant optimization to ensure uniformity and safety in critical applications [13,41,90,100].

Geopolymers have also been shown to be an alternative for the immobilization of heavy metals in mining waste, reducing mobility and environmental toxicity [27,60]. Depending on the type of metal and the composition of the geopolymer, stabilization mechanisms vary from adsorption to precipitation and chemical reduction, with varying effects on the mechanical properties of the material. Table 1 shows the main effects of different metals on geopolymers, highlighting the most effective immobilization efficiency and stabilization strategy.

Hg immobilization has been shown to be highly effective, achieving a 90% reduction in soils using steel slag, fly ash, and metakaolin [60]. Cu-containing geopolymers have shown immobilizations of over 98% without compromising mechanical strength or even increasing mechanical stability [27,101]. Pb and Zn, which pose a high environmental risk in mine tailings, can be effectively immobilized by fly ash geopolymers that stabilize Pb-Zn by forming hydroxycarbonates, resulting in 80% less leaching and increased material durability [51,91,102,103]. As has been immobilized to 95.4% in mining tailings using alkali-activated geopolymers and Fe–As and Ca–As immobilization mechanisms [91,104]. Cr(VI), which is particularly difficult to immobilize due to its high mobility in soils and waters, can be reduced to Cr(IIII) by geopolymers [103] or by the formation of hydrotalcites in ash geopolymers [91].

Table 1. Effects of metals/metalloids on the performance of geopolymers.

Element	Immobilization Method	Immobilization Efficiency (%)	Impact on Mechanical Properties	References
Hg	Adsorption on aluminosilicate matrix	90	No significant effect	[60]
Cu	Encapsulation in geopolymeric matrix	98	Improves mechanical stability	[27,101]
Pb	Formation of stable hydroxycarbonates	95	Increased durability	[51,91]
Zn	Encapsulation in geopolymeric structure	80	Minimal reduction in strength	[51,102]
As	Precipitation of Fe–As and Ca–As	95.4	No significant adverse effects	[91,104]
Cr	Reduction of Cr(VI) to Cr(III) and formation of hydrotalcites	Variable	Depends on the type of activator	[103]

Table 1. Effects of metals/metalloids on the performance of geopolymers.

Element	Immobilization Method	Immobilization Efficiency (%)	Impact on Mechanical Properties	References
Hg	Adsorption on aluminosilicate matrix	90	No significant effect	[60]
Cu	Encapsulation in geopolymeric matrix	98	Improves mechanical stability	[27,101]
Pb	Formation of stable hydroxycarbonates	95	Increased durability	[51,91]
Zn	Encapsulation in geopolymeric structure	80	Minimal reduction in strength	[51,102]
As	Precipitation of Fe–As and Ca–As	95.4	No significant adverse effects	[91,104]
Cr	Reduction of Cr(VI) to Cr(III) and formation of hydrotalcites	Variable	Depends on the type of activator	[103]

4. Three-Dimensional Printing of Construction Materials

Additive manufacturing, also known as 3D printing, is a disruptive innovation in the construction industry. This technology combines the principles of additive manufacturing, technological efficiency, and sustainability. This technology facilitates the fabrication of intricate structures through a layer-by-layer extrusion process, thereby eliminating the requirement for formwork and substantially reducing material wastage [105,106,107]. The underlying principle of additive manufacturing, in which a digital model guides the controlled deposition of materials, has been instrumental in this technological advancement. The advent of crane-based systems has precipitated a paradigm shift in the realm of architectural design and construction, giving rise to a plethora of 3D printers that have the potential to curtail construction times and costs [68,108]. Despite its considerable potential, the technology faces significant challenges, including technical, regulatory, and economic limitations, which currently hinder its widespread adoption [109,110].

The manufacturing process comprises three primary stages: digital design, layer-by-layer extrusion, and post-processing. Each stage of the process necessitates the integration of specialized systems that oversee the amalgamation, transmission, and extrusion of materials. These systems ensure the seamless flow of materials, extrudability, and stability [17,54,110]. Furthermore, critical parameters such as printing speed, line spacing, and setting time influence the quality of the structures and their thermal and mechanical properties [111,112]. The technology necessitates the utilization of materials that meet specific criteria, including adequate flowability to prevent blockages in the pipes, mechanical stability during and after printing, and compressive and flexural strength [112,113]. However, these properties vary significantly depending on the loading direction, creating anisotropies that affect structural integrity [110,113]. Research has demonstrated that rheological parameters, including viscosity and setting time, can be adjusted through the incorporation of additives such as polymers and fibers. This adjustment serves to enhance the stability of the printed layers. For instance, it has been determined that maintaining a maximum nozzle diameter to aggregate size ratio of 1:10 is optimal to prevent blockages and ensure material extrudability [112,114,115].

The primary benefits of 3D printing include operational cost reduction, sustainability, efficiency enhancement, and customization. This technology has been shown to decrease waste by 30–60%, reduce labor costs by 50–80%, and shorten project completion time by 70%, thereby enhancing safety and efficiency. However, it is important to note that 3D printing also poses challenges related to training, regulatory standards, and labor restructuring [105,110,116].

In terms of sustainability, the technology allows for the use of recycled materials, such as fly ash and granulated blast furnace slag, reducing CO₂ emissions and promoting the circular economy [82,111,114]. Additionally, the elimination of formwork and the reduction in waste contribute to a decrease in the environmental impact of the construction process [113,117]. Another significant advantage of customization is that it allows for the creation of complex and adaptive architectural designs without increasing costs. Practical examples include the Yhnova project in France, where Batiprint3D™ technology was used to construct sustainable homes with high standards of thermal efficiency [118]. Despite the numerous benefits offered by 3D printing, it faces considerable challenges, including technical and regulatory issues, which hinder its large-scale implementation [68,116]. The absence of definitive standards and specific guidelines hinders the assurance of structural integrity and durability [68,116]. Additionally, material-reinforcement bonding represents a technical challenge, as the adhesion between stamped concrete and steel or fiber reinforcements is not yet optimized [113,116].

Another significant challenge is mechanical anisotropy, which affects the strength of structures depending on the orientation of the printed layers [113,119]. Addressing this issue necessitates further research to enhance interlaminar cohesion and ensure the durability of structures [110,115]. The future of 3D printing lies in the development of larger printers and integrated transportation systems to enhance on-site construction processes [107,108]. Additionally, research into advanced materials, such as geopolymers and coarse aggregate concretes, holds promise for enhancing the mechanical properties and sustainability of structures [55,114,120]. The integration of advanced materials with digital technologies, such as Building Information Modeling (BIM) modeling, is imperative for the efficient planning and monitoring of construction projects [110,121,122]. Additionally, the optimization of printing parameters, such as speed and line spacing, has been shown to enhance thermal properties and reduce energy consumption of buildings [112,121].

5. Comparative Analysis of Tailings Geopolymers and Cements in 3D Printing

The advent of 3D printing has precipitated a paradigm shift within the construction industry, facilitating the fabrication of intricate structures with unparalleled precision and efficiency [105,106,107]. In this context, geopolymers emerge as a sustainable alternative to Portland cement, given their capacity to incorporate recycled materials such as fly ash (FA), blast furnace slag (GGBFS), and mine tailings, while substantially reducing CO₂ emissions associated with conventional construction methods [27,55,100]. The reaction of these materials under alkaline activators, such as Na₂SiO₃ and NaOH, results in the formation of solid and durable matrices. These matrices possess properties that are adaptable to the demands of 3D printing applications [26,123].

Mine tailings, like fly ash, contain high concentrations of SiO₂ and Al₂O₃, which renders them promising candidates for utilization as geopolymeric precursors [59,92,105]. However, their reactivity is limited by the presence of inactive phases. This limitation can be overcome by grinding and thermal pretreatments, which increase the capacity to form N–A–S–H and C–A–S–H gels [27,55,58]. Conversely, GGBFS, characterized by its high CaO content, exhibits rapid strength gain through C–A–S–H gels. However, its accelerated reaction rate can concurrently diminish the workability of the material, a pivotal consideration in the context of 3D printing applications [26,123].

Conversely, Portland cement has been adapted for 3D printing through the incorporation of additives that enhance rheological properties, dimensional stability, and setting time. Nevertheless, its high environmental impact, which is associated with the release of CO₂ during clinker production, poses limitations to its sustainability [27,57]. Notwithstanding these limitations, Portland cement’s notable versatility in construction and its initial mechanical properties have enabled its integration into additive printing technologies [57,105,123].

This comparative analysis aims to explore the similarities and differences between geopolymers and Portland cement in 3D printing, with a particular focus on the integration of mine tailings as a raw material in geopolymers. Key factors such as chemical composition, activators, chemical reactions, mechanical properties, and rheological behavior will be evaluated through comparative tables, based on the results of multiple recent investigations [18,27,52,54].

For instance, alkaline activators have been demonstrated to play a pivotal role in the regulation of parameters such as initial viscosity and interparticle cohesion, thereby facilitating the attainment of compressive strengths that can reach up to 34 MPa in optimized geopolymer formulations [27,55]. Conversely, Portland cement necessitates the incorporation of additives, such as superplasticizers, to ensure sufficient fluidity, thereby mitigating the occurrence of segregation and cracking during the layer stacking process in three-dimensional printing [113,116,118]. Rheological properties also play a crucial role in extrudability and dimensional stability. Additives such as nanocellulose (NFC) and expansive magnesia (MgO) have enhanced shape retention and diminished plastic shrinkage in geopolymers, bestowing considerable advantages over conventional cement in 3D printing applications [26,58,124]. Furthermore, the control of thixotropy in geopolymers has enabled the construction of collapse-free multilayer structures, highlighting their capability for advanced architectural applications [27,52,123].

5.1. Geopolymers with Tailings in 3D Printing Applications

Table 2. Conditions of raw materials, activators, and additives of mine tailings-based geopolymer formulations.

Type of Tailings	Raw Material/Average Particle Size	Relationships Between Reactants and Reagents and Water (pH)	Temperature (Ta), Activation and Curing Time (CT)	References
Ilmenite (TiO2) Titanium mine (Engebø, Naustdal, Norway)	MT/Slag-0.5: 40%; 16 µm GGBFS: 60%, 6–10 µm, BF: 10–20 µm	Sodium Silicate (SiO2/Na2O):1.5 pH: 12.5 S/L: 0.34/NaOH 10 M, Na2SiO3 (35% solids), Superplasticizer FLUBE OS 39 (1.5%)	Tª: 25 °C CT: 28 days	[56]
	MT/Slag-0.5: 40%, 16 µm GGBFS: 60%, 6–10 µm BF: 0.5%, 10–20 µm	Sodium Silicate (SiO2/Na2O):1.5 pH: 12.5 S/L: 0.34/NaOH 10 M, Na2SiO3 (35% solids), Superplasticizer FLUBE OS 39 (1%)
Iron tailings (SIT) hematite (Fe2O3) Maanshan Nanshan Mining Co., Ltd. (Maanshan, China)	CS-3DPG (sin CSA) SIT 46.7%, 40 µm, CFA: 22.94%, 50 µm	S/L: 38% Alkaline activator module (SiO2/Na2O): 1 Na2SiO3 (30%), NaOH (6%) SC (0.4%) as a retarder	Tª: 25 ± 1 °C HR: 50% ± 5% CT: 28 days	[53]
	SIT: 50%, 40 µm CFA: 44%, 50 µm CSA: 6%	S/L: 38% Alkaline activator module (SiO2/Na2O): 1 Na2SiO3 (30%), NaOH (6%) SC (0.4%), CSA (6%) as an accelerator
Iron tailings (SIT) hematite (Fe2O3) Maanshan Nanshan Mining Co., Ltd. (Maanshan, China)	SC Retarder-Free Mixture SIT 80%: 19.8 µm, CFA 20%: 30 µm	S/L: 38% Alkaline activator module (SiO2/Na2O): 1 NaOH Not specified Na2SiO3 (6%); SC (0.4%)	Tª: 25 °C ± 1 °C, HR: 50% ± 5%	[125]
	Mix with 0.4% SC SIT 80%: 19.8 µm CFA 20%: 30 µm	S/L: 38% Alkaline activator module (SiO2/Na2O): 1 NaOH Not specified Na2SiO3 (6%); SC (0.4%)

MT: mining tailing; BF: basalt fiber; GGBFS: blast furnace slag, SIT: iron tailings, CFA: fly ash; CSA: calcium carbonate; SC: sodium citrate; HR: relative humidity.

and

Table 3. Properties of geopolymers based on mining tailings.

System	Mechanical and Setting Properties			Microstructure and Chemical Composition Properties			Rheological and 3D Printing Properties
	Compressive Strength (UCS)	Flexural Strength	Setting Time	SEM/EDS	XRD	FTIR	Rheology/Thixotropy	Form Stability	Workability	Extrudability	Buildability	Vol. Pores	References
	MPa		min							%		%
MT/Slag-0.5	31	4.8	120	Dense, without fibers	ND	ND	1000 Pa·s (Viscosity at 0.1 s−1)	4	155 (0% fiber)	5	10	12	[56]
MT/Slag-0.5 with BF	28	5.5	170	Porous microstructure with matrix-fiber separation	ND	ND	700 Pa·s (Viscosity at 0.1 s−1)	Lower stability with fibers, deformation > 18%	145 mm (0.5% fiber)	30	8	16
CS-3DPG (without CSA)	21.5 (3 days) 28.3 (7 days) 35.8 (28 days)	4.2	15	Dense texture 10% porosity	Gelatinous phases such as N–A–S–H and C–(A)–S–H	More compact gel with a higher proportion of Si-O bonds.	Yield stress of 284.5 Pa and plastic viscosity of 18.8 Pa·s	Good stability, no deformation	Good (flow-through) 187 mm	30	Stable formation, layer height 1 cm	35	[53]
CS-3DPG (with CSA)	24.18 (3 days) 28.00 (28 days)	5	9	C–(A)–S–H gel near calcium carbonate.	C–(A)–S–H and N–A–S–H phases. A higher density was observed in the samples with 6% CSA.	C–(A)–S–H and N–A–S–H gels	Yield stress of 778.5 Pa and plastic viscosity of 38.5 Pa·s after the addition of CSA	Ability to maintain up to 11 layers without deformation, indicating high stability in 3D construction	Very good (high fluidity) 173 mm for mixing with CSA	30	High buildability and adequate structural support	30
without Retardants (SC)	22.29 (3 days) 33.07 (28 days)	ND	514	C–(A)–S–H and N–A–S–H gels were observed	C–(A)–S–H and N–A–S–H gels	Formation of C–(A)–S–H and N–A–S–H gels	Evaluation of plastic viscosity and shear stress.284 Pa (stress), 18.8 Pa⋅s	Stable form	Good	30	Adequate	ND	[125]
with Retarders (SC)	19.80 (3 days) 32.14 (28 days)	ND	698	C–(A)–S–H and N–A–S–H gels and unreacted components were observed.	Less gel formation	A decrease in the intensity of the Si–O–T peak was observed compared to CS-3DPG.	Evaluation of plastic viscosity and shear stress.248.2 Pa (stress), 16.43 Pa·s	Stable form	Enhanced with SC	30	Enhanced with SC	ND

ND: no data; MT: mining tailing; BF: basalt fiber; CFA: fly ash; CSA: calcium carbonate; SC: sodium citrate; HR: relative humidity.

present the properties and characteristics of geopolymers made from mine tailings using 3D printing techniques. These tables demonstrate how incorporating mine tailings as a raw material in the synthesis of geopolymers, combined with specific activators and additives, plays a key role in improving mechanical, rheological, and microstructural properties. This improvement opens new opportunities for sustainable applications, such as 3D printing [53,56,125]. These mining by-products, which are abundant in silicates, aluminates, and metal oxides (iron and calcium), are conducive to the formation of N–A–S–H (sodium silicate–aluminate–hydrated) gels and, in calcium-rich systems, C–(A)–S–H (calcium silicate–aluminate–hydrated) hybrid phases through alkaline activation [59,125]. These phases contribute to the cohesion and density of the matrix, significantly improving the mechanical strength [62,82,93].

The utilization of mine tailings in geopolymeric systems has been a subject of extensive research, with notable variations in their composition and mechanical performance. This has led to the identification of the importance of adjusting the mixing proportions and conditions to optimize mechanical and chemical properties, as highlighted by Shoaei et al. [56] and Lu et al. [53]. In contrast, Lu et al. [125] have focused on the workability of these systems in advanced applications, such as 3D printing.

In the study by Shoaei et al. [56], the MT/Slag-0.5 system was examined, comprising 40% titanium mine tailings (MT) with a particle size of 16 µm and a 60% granulated blast furnace slag (GGBFS) ranging from 6 to 10 µm. The fine granulometry of GGBFS has been shown to favor the formation of N–A–S–H gels, resulting in a dense matrix with low porosity and a compressive strength of 31 MPa at 28 days. This system is regarded as optimal for structural applications. However, incorporating basalt fibers (BF) at a rate of 0.5% has been shown to enhance flexural strength from 4.8 MPa to 5.5 MPa. This enhancement, however, is accompanied by an increase in porosity, resulting from the separation of the matrix and the fibers. This observation underscores the necessity for optimizing the mixing conditions to achieve a balance between strength and structural cohesion. In a related study, Lu et al. [53] examined the CS-3DPG system, which is composed of 46.7% iron tailings (ITS) and 22.94% fly ash (FASH) with relatively large particles (40–50 µm). The system displays a porous microstructure, characterized by 8.8% porosity, which impedes cohesion and delays strength development, reaching 33.07 MPa after 28 days. Notwithstanding these constraints, it is regarded as suitable for applications where the primary concern is the delayed strength development. In contrast, incorporating 6% CSA (calcium carbonate) in a mixture of 50% iron tailings and 44% fly ash resulted in enhanced microstructure density and expedited setting to a mere 9 min. This system demonstrated an early strength of 24.18 MPa after three days, which is a promising development for 3D printing applications where rapid setting is essential.

In their study, Lu et al. [125] investigated formulations devoid of retarders or additives, comprising 80% iron tailings with 19.8 µm particles and 20% 30 µm fly ash. This composition enhanced chemical reactivity, exhibiting high viscosity and suboptimal cohesion, which restricts its application in 3D printing technologies. To address these limitations, the incorporation of 0.4% sodium citrate (SC) as a retarder has been demonstrated to reduce shear stress (12.6%) and plastic viscosity (18.3%), thereby enhancing extrudability and shape stability. Despite its initial strength (19.8 MPa at 3 days) being lower compared to systems with CSA, its processability renders it competitive for applications in layered construction.

Shoaei et al. [56] evaluated the Engebø tailings activated with an alkaline solution of Na₂SiO₃ (SiO₂/Na₂O = 1.5) and NaOH at 10 M. This system favored the predominant formation of N-A-S-H gels, which provide initial cohesion and densification of the matrix, highlighting its structural stability and positioning it as an efficient option for conventional structural applications. Conversely, Lu et al. [53] underscored the efficacy of the SIT tailings-based system through the incorporation of 6% CSA (calcium carbonate) in conjunction with NaOH (6%) and Na₂SiO₃ (30%). The incorporation of CSA has been shown to induce the formation of C–(A)–S–H gels, which significantly densify the microstructure, reducing porosity to 7% and accelerating the initial strength to 24.18 MPa within a mere three days. This behavior renders it highly competitive for 3D printing applications, where rapid setting is paramount.

The incorporation of additives profoundly altered the workability of geopolymeric systems. Shoaei et al. [56] examined a system comprising 1% FLUBE OS 39 superplasticizer, which facilitates the effective formation of N–A–S–H gels while enhancing fluidity (155 mm) and viscosity (1000 Pa·s). These properties render this system suitable for applications where stability and workability are equally important, such as the design of 3D printed materials. In contrast, the study by Lu et al. [125] evaluated the addition of 0.4% sodium citrate (SC) as a retarder in SIT systems with a SiO₂/Na₂O ratio set to 1. This additive reduced plastic viscosity (16.43 Pa·s) and shear stress, improving extrudability and control during 3D printing. However, the addition of SC resulted in a delay in gel formation, thereby restricting the initial strength (19.8 MPa at 3 days) and its application in accelerated construction processes. The SiO₂/Na₂O ratio is a critical parameter in the optimization of geopolymers. Studies by Shoaei et al. [56] with Engebø tailings demonstrated that a ratio of 1.5 promoted the formation of N-A-S-H gels and adequate initial cohesion. Conversely, Lu et al. [53] observed that adjusting this ratio to 1, in conjunction with CSA, expedites the formation of C–(A)–S–H gels, enhancing density and reducing porosity, a property that is well-suited for structural and 3D printing applications.

All the analyzed systems were cured at room temperature (25 °C), a practice that is conducive to energy sustainability. However, performance exhibited variability depending on the additives. Systems incorporating CSA demonstrated rapid setting times (9 min) and high initial strength (24.18 MPa after 3 days), making them well-suited for accelerated 3D printing applications [83]. Conversely, systems with SC exhibited prolonged setting times (514 min), enhancing workability while constraining initial load capacity [125]. This renders them particularly well-suited for applications that demand precision and control. Systems with CSA have been shown to exhibit rapid strength gain, low porosity, and the capacity to build consecutive layers without deformation. These characteristics position them as a prime choice for complex and fast-loading structures. Conversely, SC systems demonstrate efficacy in projects necessitating prolonged handling times and controlled flow, though their initial mechanical performance may be inadequate for accelerated construction applications. Systems incorporating superplasticizers, such as FLUBE OS 39, achieve a balance between flow and stability, rendering them well-suited for customized designs in additive printing [125].

Shoaei et al. [56] and Lu et al. [53] underscored the significance of precursor physical properties and chemical composition on the reactivity and strength of the systems. While systems with GGBFS achieved denser and stronger matrices, systems with coarser particles, such as in CS-3DPG, exhibited elevated porosities that limit the initial strength development. Conversely, Lu et al. [125] underscored the significance of regulating workability through the incorporation of additives such as SC, a method that enhances not only extrudability but also stability in three-dimensional printing applications. The employment of diversified activators and additives enables the customization of geopolymeric systems to a broad spectrum of technical requirements. Systems with CSA demonstrate proficiency in applications requiring early strength and stability, while systems with SC exhibit flexibility in workability and precision. These findings underscore the necessity of customizing formulations to optimize performance based on the specific needs of each project.

5.2. Other Components for 3D Printing of Geopolymers

The utilization of 3D printing in the fabrication of geopolymers has emerged as a pioneering and eco-friendly technology, demonstrating the potential for the reuse of construction waste materials, thereby mitigating environmental impact and enhancing process efficiency. Consequently, this section will address the most recent scientific contributions associated with the behavior of materials commonly used in geopolymerization [18,52,54,112,126,127,128,129], such as fly ash, granulated blast furnace slag, and construction waste, in the context of 3D printing (

Table 4. Conditions of raw materials, activators, and additives for alternative geopolymeric components.

Type of Raw Material	Raw Material/ Average Particle Size	Relationships Between Reactants and Reagents and Water (pH)	Temperature (Tª), Humidity (H), Activation Time (AT) and Curing Time (CT)	References
GGBS and FA supplied by Independent Cement and Lime Pty Ltd., (Melbourne, Australia)	GGBS: 50%, 14 µm FA: 50%, 10 µm	(Na2SiO3): 10% w/w, (MAS—Magnesio Aluminio Silicato): 0.75% w/w, Retarder (Sucrose): 1.5% w/w S/L: 0.31, SiO2/Al2O3 = 4.42, H2O/Na2O = 22.38	Tª: 23.5 °C H: 40% CT:24 h	[52]
FA: Changsha (China). GGBFS KSC and KF: Changsha, (China).	FA: 78.5%, 50 µm GGBFS: 19.5%, 45 µm KSC: 1.5%, 1000 µm KF: 0.2%, 15,000 µm	Na2SiO3 (42%) + NaOH (96%) 10 M: 20% S/L: 2.5:1 Retarder (Sodium Gluconate) C₆H₁₁NaO₇): 1% SiO2/Na2O: 1	Tª: 25 °C AT: 54 min CT: 28 days	[18]
FA: Ningbo (China); GGBS: Ningbo (China); RS: Construction site in Ningbo City; Superplasticizer: Zhejiang Province	FA: 64%, 16.90 µm GGBS: 16% RS: 20%	Na2SiO3 (17.74%) 12 M NaOH 8 M (2.35%) Superplasticizer Polycarboxylate (0.67%)	Tª: 20 ± 2 °C AT: 24 min CT: 28 days	[127]
Ash from paper mill boiler (AP), construction and demolition wasten (C&D), metakaolin, sand (coarse and fine), recycled glass wool (Finland)	AP: 7%, 50–100 µm C&D: 10%, 4000 µm, metakaolin: 13%, 2–10 µm, Course MT: 30%, 1000–2000 µm Fine MT: 9%, 100–500 µm Glass wool: 2%, 3 mm	S/L: 3:1 (Na2SiO3): 10 M	Tª: 60 °C by 24 h in oven, followed by 48 h at room temperature CT: 28 days	[54]
BFS: Capital Iron and Steel, Beijing (China) Steel Slag (SS) Beijing, (China)	BFS: 81%, ~10 µm SS: 9%, ~20 µm	S/L: 0.35 Na2SiO3 10 M: 8.1% NaOH 8 M: 0.9% Defoamer (0.5%) Superplasticizer (1%) Redispersible latex (1%) 0.5 Si/Na ratio (best performance), 0.6, 0.7, 0.8, 0.9, 1.0	Tª: 25 °C CT: 7 days	[126]

MT: mining tailing; GGBS: ground granulated blast furnace slag, GGBFS: blast furnace slag; KSC: kenaf core; KF: kenaf fiber; FA: Fly ash.

and

Table 5. Properties of geopolymers with auxiliary materials (fly ash, slag, construction residues, and nanomaterials).

Compressive Strength (UCS)	Flexural Strength	Setting Time	Rheology and Thixotropy	Form Stability	Workability	Extrudability	Buildability	References
MPa	MPa	min			Fluency	Layer height	Nº of layers
At 28 days: Perpendicular: 37 Lateral: 35 Longitudinal: 32	At 28 days: Perpendicular: 6 Lateral: 5.5 Longitudinal: 5	40	Yield Stress: Increases with time, 4500 Pa at 60 min (10ACT) Apparent Viscosity: Decreases with shear rate; initial values close to 10,000 Pa·s for 10ACT	70% after extrusion; good recovery to ensure shape stability	decreases with increasing activator content.	Good Easily extruded without blockages during the printing process	Stable; >120 layers without collapsing (height 1.4 m)	[52]
ND	10.56	57	Initial viscosity of 0.75 × 106 mPa·s viscosity after extrusion of 8.24 × 106 mPa·s	SRRt (shape retention in thickness): 96%, SRRw (shape retention in width): 96%	Excellent extrudability and shape retention in multi-layer prints	The mixture was extruded smoothly through the nozzle, showing good fluidity.	6 layers	[18]
29	N/D	Initial: 42 End: 58	Plastic Viscosity 7.98 (Pa·s) Apparent Viscosity 1500 (Pa·s) Static Yield Stress 2800 (Pa)	Layer Height 160 mm	fluidity in range 155–160 mm	Layer Height 24 mm	47 layers without deformation	[127]
12.02 (28 days)	6.7	A: 38–44 (20 °C) + 23 (60 °C) B: shorter setting time as tª increases	Initial viscosity: 1200–1500 Pa·s; viscosity increase to 2200 Pa·s after 10 min standing; Reflow time: 5–8 s after 10 min standing Thixotropy improved with heating to 60 °C; Flow index: 0.87 at 20 °C (indicates pseudoplastic behavior)	A: deformation 4.1 mm at 20 °C 1.6 mm at 100 °C B: deformation 2.5 mm at 20 °C 1.8 mm at 100 °C	Adjustable with water/silicate ratios optimal with 28–31% liquid volume	Stable layer height after 10 min of mixing; maximum 20 min of extrudability without loss of shape	Up to 20 layers with good shape retention in extrusion tests	[54]
Max. 53.04 for Si/Na = 0.9 37.90 for Si/Na = 0.5	ND	ND	Plastic Viscosity (η) 0.78 Pa·s for Si/Na = 0.5; decreases with increasing Si/Na, 0.42 Pa·s for Si/Na = 1.0 Yield Stress (s₀)—20 min 5.30 Pa for Si/Na = 1.0; increases with decreasing Si/Na to 3.439 Pa (Si/Na = 0.5)	Structural Recovery (SRE) The best mix has a SRE of 132 J/s m2 for Si/Na = 0.5 after 20 min,	Stress Development increase from 1.71 Pa (1 min) to 5.3 Pa (20 min), depending on rest time and Si/Na	Layer Evaluated by printing in 10 mm layers	Maximum stability with Si/Na = 0.5, supporting 20 layers	[126]

). A reference framework was established by consolidating and evaluating key factors such as chemical composition, particle size, alkaline activators, and curing conditions.

The chemical composition of the base materials is a critical factor in determining their reactivity and final properties. According to Kong et al. [18], fly ash (FA) is distinguished by its elevated content of amorphous silica and alumina, which renders it a highly reactive precursor when activated with alkalis. This characteristic is particularly advantageous for its application in 3D printing, as it leads to the formation of dense and resilient matrices. Conversely, Muthukrishnan et al. [52] highlighted that granulated blast furnace slag (GGBS), due to its elevated calcium content, expedites alkaline reactions, resulting in dense networks that enhance mechanical strength. The works of Zhang et al. [126] and Zhou et al. [127] highlighted that the amalgamation of FA and GGBS can optimize the initial workability and long-term mechanical strength, a crucial consideration in materials with compositional variability, such as mine tailings. Conversely, Munir et al. [54] examined construction and demolition waste, emphasizing that, while these materials constitute a viable alternative, their heterogeneity necessitates precise adjustments in the proportions of activators. These strategies can be directly adapted to the use of mine tailings, which require a detailed analysis of their composition to design balanced mixtures.

The role of particle size in the reactivity and mechanical properties of geopolymers is critical. The works of Kong et al. [18] and Muthukrishnan et al. [52] concluded that the specific surface area available for alkaline reactions is maximized by fine particles with diameters less than 50 µm, thereby increasing chemical reactivity and mechanical strength. However, Munir et al. [54] cautioned that excessive reliance on larger particles, ranging from 50 to 100 µm, during 3D printing can result in flow and segregation issues, potentially compromising the structural homogeneity. Conversely, Zhang et al. [126] underscored the significance of reducing particle size through additional grinding, particularly in heterogeneous materials such as mine tailings, to ensure uniform extrusion and meet structural precision demands.

The relationship between alkaline and reactive activators is another fundamental aspect in the quality of geopolymers, as demonstrated by the works of Kong et al. [18] and Zhou et al. [127]. In the context of geopolymerization, alkaline activators are compounds that increase the pH of the medium, facilitating the dissolution of alumino-silicate precursors and promoting the polymerization reaction [130]. However, not all alkaline activators contribute directly to the formation of the geopolymeric structure. Reactive activators, in addition to providing the necessary alkalinity, contribute essential chemical species that favor the formation of the geopolymeric matrix, thereby improving the stability and mechanical properties of the material [131].

These studies identified that a combination of Na₂SiO₃ (10–42%) and NaOH (8–12 M) ensures optimal levels of alkalinity, promoting high reactivity and improved mechanical strength. For instance, sodium hydroxide (NaOH) functions as an alkaline activator, thereby increasing the alkalinity of the system and facilitating the dissolution of the aluminosilicate-cate components. Conversely, sodium silicate (Na₂SiO₃) functions as a reactive activator, contributing to an increase in pH and supplying silicate ions that play an active role in the formation of gels, such as N-A-S-H (sodium alumino silicate hydrate) and C-A-S-H (calcium alumino silicate hydrate). These gels are responsible for the cohesion, chemical stability, and mechanical strength of the final geopolymer [131].

Zhang et al. [126] underscored that maintaining an optimal balance between liquid and solid activators not only enhances the workability of the mixture but also prevents the formation of cracks, particularly in heterogeneous materials such as tailings. However, Munir et al. [54] emphasized that the proportions of activators should be customized based on the chemical composition of the waste used. This is particularly relevant for mine tailings, whose variable oxide content may require specific adjustments to ensure the formation of stable matrices and avoid uncontrolled reactions [130].

Another crucial factor pertains to the curing conditions, which are indispensable for delineating the dimensional stability and ultimate mechanical properties of the geopolymers. Muthukrishnan et al. [52] have indicated that an initial curing at room temperature, approximately 23.5 °C for 24 h, is sufficient for reactive materials such as GGBS. However, alternative approaches have been proposed. For instance, Kong et al. [18] and Zhou et al. [127] have suggested initial curing at elevated temperatures, ranging from 40 to 60 °C, to expedite the formation of geopolymeric matrices, particularly in mixtures with low calcium content. These recommendations are supported by Zhang et al. [126], who have proposed extending the curing process up to 28 days to optimize compressive strength and shape stability, paramount factors for 3D printing applications. In the case of mine tailings, a hybrid approach could be ideal: an initial cure at 60 °C for 24 h to enhance reactivity, followed by an environmental cure that reduces energy consumption without compromising the mechanical properties of the material.

The ratio between alkaline activators, such as Na₂SiO₃ and NaOH, and base materials is a key determinant in the reactivity and formation of a stable aluminosilicate matrix. According to Zhang et al. [126] an optimal Si/Na ratio of 0.9 results in a compressive strength of up to 53.04 MPa at 28 days, standing out as one of the highest reported. Conversely, Muthukrishnan et al. [52] have indicated that lower ratios promote dimensional stability, thereby reducing critical deformations during multilayer printing. These findings underscore the significance of precise modulation of this ratio to prioritize strength or stability, depending on the specific requirements of the application. Another salient aspect pertains to the equilibrium between liquid and solid activators. Zhou et al. [127] observed that a formulation with 8 M NaOH, combined with superplasticizers, achieved uniform extrusion and controlled viscosity, despite incorporating materials such as soil residues, which tend to be more heterogeneous. However, these formulations presented cohesion challenges in upper layers, underscoring the need for carefully calibrated proportions for recycled materials.

The mechanical properties of the mixture are indicative of the interaction between its composition, the presence of activators, and the addition of additives. Muthukrishnan et al. [52] reported that mixtures with 10% Na₂SiO₃ and 0.75% MAS reached a compressive strength of 37 MPa after 28 days, attributed to the formation of homogeneous matrices with low porosity. Conversely, Kong et al. [18] observed that the incorporation of 0.2% kenaf fibers enhanced the flexural strength to 10.56 MPa, thereby demonstrating the reinforcing effect of these fibers, which is paramount for stability in multilayer applications.

In the case of more heterogeneous mixtures, the use of soil residues reduced the compressive strength to 29 MPa [127]. While this figure is lower than that of homogeneous formulations, it remains adequate for non-structural applications, underscoring the viability of reusing residual materials. These findings are particularly relevant for exploring the incorporation of mine tailings, whose mechanical performance could be optimized by appropriate adjustment of activators and additives.

The setting time of the mixture directly influences its workability and stability during the printing process. According to the findings of Muthukrishnan et al. [52], a mixture activated with Na₂SiO₃ and sucrose attained an initial setting time of 40 min, enabling adequate extrudability without compromising structural stability. Conversely, the study by Kong et al. [18] demonstrated that the incorporation of kenaf fibers led to a reduction in setting time to 57 min. This phenomenon was attributed to the acceleration of geopolymerization, a process facilitated by the chemical interaction between fibers and activators.

Furthermore, Munir et al. [54] demonstrated that curing at 60 °C can reduce the setting time to 23 min, thereby enhancing printing speed in high-productivity processes. However, this approach has the potential to increase energy costs. Therefore, a hybrid approach, combining accelerated initial curing and ambient conditions, could be more sustainable. Rheology and shape stability are critical to ensure that geopolymers can be accurately extruded and maintain their shape during layer stacking. The findings of Muthukrishnan et al. [52] demonstrated that the incorporation of MAS enhances the initial viscosity and expedites structural recovery post-extrusion, enabling the construction of up to 120 layers with heights surpassing 1.4 m. These findings underscore the significance of thixotropic additives in the construction of intricate structures.

As demonstrated in the study by Kong et al. [18], kenaf fibers not only reinforced the matrix but also increased dimensional retention to 96%, thereby significantly reducing deformations in the lower layers. In contrast, the work of Zhou et al. [127] observed that mixtures with high soil residue contents presented structural buckling after 47 layers, due to insufficient cohesion in the matrix. These findings underscore the necessity of calibrating the proportion of activators and additives in accordance with the material’s composition.

The incorporation of mine tailings as a raw material in geopolymers for 3D printing has been identified as a promising avenue for advancing towards more sustainable construction practices. Nevertheless, the effective integration of mine tailings into geopolymers for 3D printing is hindered by challenges related to chemical variability and particle size. According to Zhang et al. [126], a reduction in particle size to less than 50 µm can lead to a substantial enhancement in reactivity, thereby promoting improved homogeneity and cohesion within the matrix. Furthermore, the incorporation of additives such as MAS and recycled fibers could optimize rheological properties and dimensional stability, thereby adapting the mixtures to the requirements of 3D printing.

5.3. Mining Tailings and Cementitious Mixtures for 3D Printing

The utilization of mine tailings as a primary constituent in cementitious and geopolymeric mixtures constitutes an innovative solution to the problem of industrial waste accumulation, while concurrently enhancing the mechanical and rheological properties of the resulting materials. This analysis focuses on comparing different formulations that include mine tailings, additives, and specific proportions, evaluating their performance in terms of physical, chemical, and mechanical properties, and highlighting their applicability in 3D printing (

Table 6. Composition and mechanical properties of cementitious mixtures and mining tailings.

Raw Material	S/L	Temperature, Activation Time and Curing	SEM and EDS	XRD	Compressive Strength (UCS)	Flexural Strength	References
					MPa	MPa
Red Mud: 15% FA: 9% Iron MT: 45% PC: 24% BSC: 0% FDN: 1%	0.183	Tª: 20 °C CT: 7–28 days	C–S–H, ettringite, Ca(OH)2 observed; compact microstructure with aggregates wrapped in C–S–H gel	Quartz, calcite, hematite and gibbsite in red mud; calcite, clinochlore, and magnesiohornblende in iron MT	39 (28 days)	11.91 (28 days)	[128]
Sand: 65% Bauxite MT: 35% PC: 42.5% SAC: 0.125 SP: 0.03 HPMC: 0.002 Water: 0.31	3.2:1	Tª: 20 °C CT: 60 days	ND	ND	Max. 48 (60 days) 24 (3 days) 32 (7 days)	Max. 8 (60 days) 4 (3 days) 5 (7 days)	[129]
PC: 69.96% FA: 19.99% SF: 9.99% Cu MT: 29.98% PP: 0.055%	0.26	Tª: 20 °C CT: 28 days	ND	ND	53.2 (28 days)	Molded: 7.0 Printed: 4.8	[132]
Fe MT: (40%) Cu MT: (10%) FA (19%) BC (30%) FDN (1%)	0.20	Tª: 20 °C CT: 7–28 days	Three-dimensional structure of C–S–H and ettringite crystals, forming a dense matrix	Quartz, ettringite, calcite, albite, perovskite, Ca2SiO4, and C-S-H gel	45.2	8.2	[42]
PC: 40.63% SF: 4.06% Si sand: 11.30% Sb MT: 29.32%	0.35 PVA: 0.12% HPMC: 0.04% SP: 0.29% Na-G: 1 g Nanoclay: 3 g water: 14.22%	Tª: 20 °C CT: 7–28 days	Improved microstructure with reduced porosity and cracks	Detection of C-S-H phases and pozzolanic reactions	105	7.0	[133]
Sand: 24% Limestone powder: 16% PC: 23.3% FA: 10% Water: 10% PCE: 0.083%	0.45	Tª: 20 °C CT: 7–28 days	dense structures with minimal transition zone	ND	50.39	ND	[134]
MT 20% PC: 20% MasterRheobuild-1000: (1.0%) MasterSet AC 100: (1.0%)	4	Tª: ~25 °C CT: 28 days	ND	ND	26.6 (28 days)	4	[135]

MT: mining tailing; BC: belite cement; FDN: sulfonated naphthalene formaldehyde water reducer; FA: fly ash; PC: Portland cement; SAC: calcium sulfoaluminate clinker; HPMC: hydroxypropyl methylcellulose; PP: polypropylene fiber; PCE: polycarboxylate superplasticizer; SF: silica fume; PVA: polyacrylamide fiber; SP: superplasticizer; Na-G: sodium glutamate; ND: no data.

and

Table 7. Properties for 3D printing of cement mixtures and mining tailings.

Setting Time	Hydration Kinetics Analysis	Rheology and Thixotropy	Form Stability	Workability	Extrudability	Buildability	Density	Environmental Toxicity Testing	References
Min							g/cm3
Initial: 50 End: 85	ND	Fluidity adjusted to 205 mm	ND	Good	3 cm in diameter	ND	ND	Values below the limits established in GB 8978-1996	[128]
44	Greater hydration delay with higher glue content	12,000 Pa·s	Deformación estructural: 1.4%	Good	Good	Good	2.43	ND	[129]
ND	ND	Fluency—V-Funnel Test (s) 22.1 (R0) to 26.4 s (R50)	40–50 min of rest	Good	High: 5.4 L/min, continuous up to 80 min	138 mm	ND	ND	[132]
Initial: 50 End: 82	ND	Flow: 197.5 mm, suitable for 3D extrusion	High, allows maintaining the integrity of the layers during printing	Suitable for continuous extrusion; good interlayer adhesion and shape stability	High; material is extruded evenly without obstructions in the system	High; supports prints up to 1.8 m × 1.8 m × 1.8 m	2.03	Values below the limits established in GB 8978-1996	[42]
ND	ND	HPMC controlled and super-suitable for 3D printing	High, suitable for layer-by-layer extrusion	Improved with SP and other optimal 3D printing	High, with speed of 8830 mm3/s	Maximum printing height: 480 mm	2.25	In progress; preliminary results within safe limits	[133]
Initial: 75 End: 120	ND	Slump 8.18 cm at 20 min (within optimal range of 3.5–8 cm for printability) Extension Diameter 20.26 cm on jump table test at 20 min (within range of 17–20.5 cm)	Good (4%)	Aspect Ratio Aℎw 3.5 (optimal), indicating high structural stability in layers	Volumetric Contraction (%) 0.6% at 180 min, showing moderate stability	Good, with stable shape maintenance during layer-by-layer printing	ND	ND	[134]
60	ND	Dimensional Stability in Fluidity (cm) At 1 min: 19.0 × 19.0 At 10 min: 20.0 × 21.0 At 20 min: 19.5 × 20.0	2.5 cm of bearable height every 7 min	(Mini Abrams Cone) Extension 77, 73 × 75	Joint Adhesion Strength (MPa) 1.67	At 7 min per 2.5 cm layer: construction of up to 1 m in 4.6 h	ND	ND	[135]

, and Figure 5 and Figure 6).

The study by Zhang et al. [128] evaluated a mixture composed of 15% Red mud, 45% iron tailings, and 24% Portland cement, enhanced with a naphthalene-based superplasticizer. This formulation yielded a compressive strength of 39 MPa and a flexural strength of 11.91 MPa after 28 days of curing. The low water-to-solid ratio (0.183) facilitated the development of a compact microstructure, as evidenced by SEM analysis, wherein the particles were encircled by C-S-H gels. XRD analysis confirmed the presence of ettringite and Ca(OH)₂, and the distinctive reddish color of red mud added aesthetic value, making it suitable for architectural applications. In contrast, Zhou et al. [129] examined mixtures comprising 35% bauxite tailings and 65% natural sand. These formulations demonstrated a compressive strength of 48 MPa after a 60-day curing period. Notably, the workability was found to be influenced by the high water-to-solid ratio (3.2:1), while the ultrafine particle size (5.12 µm) was observed to promote more intensive pozzolanic reactions, as confirmed by thermal analysis. These analyses indicated a heightened conversion of free water into reactive hydration products, underscoring their suitability for structural applications.

An alternative approach was examined by Ma et al. [132], who integrated 29.98% copper tailings, polypropylene fibers, and fly ash in a formulation that reached 53.2 MPa in compression at 28 days. While polypropylene fibers enhanced internal cohesion, tests demonstrated that they did not fully compensate for interlaminar weaknesses, resulting in reduced flexural strength in 3D printing applications (4.8 MPa) compared to molded samples (7 MPa). However, the incorporation of silica fume led to enhanced matrix densification, thereby augmenting the overall material properties.

Concurrently, Li et al. [42] investigated a mixture comprising 40% iron tailings and 10% copper tailings, blended with belite cement and fly ash. This formulation demonstrated compressive strengths of 45.2 MPa and flexural strengths of 8.2 MPa after a curing period of 28 days. The water-to-solid ratio of 0.20 was identified as a critical parameter for optimizing the extrudability of the material, thereby facilitating its application in 3D printing processes. The incorporation of a naphthalene-based superplasticizer led to a substantial reduction in viscosity.

Singh et al. [133] investigated the utilization of 29.32% antimony tailings in conjunction with nanoclay, PVA fibers, and superplasticizers. This formulation yielded high performance, with a compression strength of 105 MPa after 28 days. FTIR analyses identified highly reactive C–S–H gels, while extremely low porosity (<0.3%) guaranteed a dense and stable matrix, ideal for structural applications requiring high load-bearing capacity.

Furthermore, Zhang et al. [134] evaluated a mixture replacing natural sand with 40% limestone powder and 10% fly ash, achieving 50.39 MPa in compression. The initial flowability of 8.18 cm and the satisfactory dimensional stability of the mixture rendered it a compelling option for 3D printing applications. Scanning electron microscopy (SEM) analyses revealed well-compacted bonds between particles, ensuring optimal internal cohesion.

Comparative data showed that the OPC cement + silica fume formulation presented the highest compressive strength (105 MPa), attributed to the high reactivity of silica and the formation of a denser C–S–H gel with lower porosity. In contrast, the mixture of mine tailings + silica sand + cement exhibits the lowest strength (26.6 MPa), possibly due to a low reactivity of the components and lower degree of compaction (Figure 5). In terms of flexural strength, the red mud + iron tailings formulation achieved the best performance (11.91 MPa), suggesting a more interconnected microstructure, while the mine tailings + silica sand + cement mixture showed lower flexural strength (4 MPa), hence higher brittleness. In terms of 3D printing performance, only the cement + copper tailings formulation reported results with a strength of 4.8 MPa, a balance between flowability and early strength, fundamental for stability in additive construction (Figure 6), suggesting that while cement with silica fume is ideal for high load structures, materials with red mud can improve ductility in pavements and thin structures, and copper tailings have potential in 3D printing.

Finally, Álvarez-Fernández et al. [135] analyzed a mixture containing 20% mine tailings, superplasticizers, and setting accelerators. Despite its mechanical inferiority (26.6 MPa in compression), this formulation exhibited a rapid setting time (60 min), rendering it suitable for expeditious construction using multilayer stacking. Interlaminar cohesion analyses indicated sufficient bond strength for structures up to one meter high over a 5 h period.

A comprehensive array of chemical and microstructural analyses (FTIR, XRD, SEM) was conducted on these formulations, unveiling substantial disparities in reactivity and performance [128,134,135]. Mixtures with bauxite and antimony tailings exhibited elevated levels of reactive gels, thereby justifying their high strength values. Conversely, less reactive copper and iron tailings demonstrated a reliance on fibers and superplasticizers to enhance workability and cohesion, particularly in three-dimensional printing applications.

The setting time in 3D printing mixtures is a pivotal factor influencing both continuous extrusion and adhesion between printed layers. The extant literature documents a range of initial setting times, from 44 min [92] to 75 min [134], with final times ranging from 82 to 120 min [42,134]. The incorporation of additives, such as polycarboxylate superplasticizers and hydroxypropyl methylcellulose (HPMC), has enabled enhanced precision in the modulation of hydration, thereby mitigating exothermic peaks and reducing volumetric shrinkage. This process extends to the induction stage, thereby preventing premature hardening that would impede the continuity of the printing process [129]. In the context of copper and iron tailings mixtures, it has been observed that an elevated tailings content accelerates the setting time, a phenomenon that is advantageous to the initial stages of construction. However, this behavior may compromise structural cohesion due to faster hydration, which requires a careful balance between setting speed and mechanical stability [42].

Rheological properties, including flowability and thixotropy, are critical for the printability of materials in three-dimensional (3D) printing applications. The most effective blends have demonstrated flowability ranges between 197.5 mm and 205 mm, values that ensure unclogging extrusion and uniform deposition [42,134]. Conversely, Zhou et al. [129] reported a thixotropic stress of 12,000 Pa·s, which reduced structural deformation to 1.4%, enabling the material to maintain its shape and stability during the printing process. Further validation was provided by additional tests, such as the V-Funnel method, which yielded flow times between 22.1 and 26.4 s [134]. These findings collectively substantiate the viability of these blends for uninterrupted, continuous printing. Concurrently, Zhang et al. [134] underscored the significance of preserving a uniform extrusion rate in conjunction with optimal aspect ratios, thereby ensuring the stability of successive layers and averting their collapse during the construction process.

Shape stability is an essential requirement for printed layers to retain their structural integrity throughout the construction process. In this regard, formulations incorporating copper and iron tailings have demonstrated remarkable stability, enabling the construction of structures with up to 1.8 m³ of volume without substantial deformations [42]. Moreover, a mixture containing 75% antimony tailings, as reported by Singh et al. [133], attained a maximum printing height of 480 mm, attributable to a dense microstructure formed through pozzolanic reactions between the tailings and the cement. Conversely, Zhang et al. [134] highlighted that structural stability can be enhanced by maintaining moderate volumetric shrinkage levels, approaching 0.6% after 180 min. This approach fosters adhesion between printed layers, thereby mitigating warping and ensuring the efficacy of complex architectural applications.

The bulk density of the mixtures is a critical factor in determining their mechanical properties. The formulations evaluated presented densities between 2.03 and 2.43 g/cm³, which directly correlates with the compressive strengths achieved. In a related study, Zhou et al. [129] reported a density of 2.43 g/cm³ associated with a compressive strength of 48 MPa, while Singh et al. [133] achieved strengths of up to 105 MPa using mixtures with antimony tailings.

However, the flexural strengths of the printed samples were found to be lower compared to the molded ones, with losses of up to 31.4% attributed to the formation of weak interfaces between layers [132]. This phenomenon underscores the necessity to optimize additives and deposition methods to enhance interlaminar cohesion and final mechanical properties. From an environmental perspective, optimized mixtures have been shown to comply with international regulations, such as GB 8978-1996, maintaining heavy metal concentrations (As, Pb, Hg) below the permitted limits [128,134]. This enhanced environmental performance can be attributed to the encapsulation of metals in hydration products, such as C–S–H gels and ettringite, which impede their release and mobility. Moreover, the incorporation of tailings and fly ash into the formulations led to a significant 8.63% decrease in greenhouse gas emissions, thereby fostering more sustainable practices within the construction industry [133].

Optimized mixtures for 3D printing not only meet the necessary technical and mechanical requirements but also offer significant advantages in terms of environmental sustainability by reusing industrial tailings. However, there are still areas for improvement that require further research. Among these areas is the need to evaluate the durability of materials under extreme conditions, such as freeze–thaw cycles, to ensure their long-term stability [42,132]. The development of new additives that enhance interlayer cohesion and reduce volumetric shrinkage during the curing process is also recommended [128,135]. The validation of the technical and economic viability of these formulations in real construction environments is paramount, particularly in contexts where tailings quality may vary [128,135]. These initiatives have the potential to consolidate 3D printing with mine tailings as an innovative tool for sustainable construction.

Environmental toxicity data on cement and mine tailings mixtures for 3D printing show that, in the formulations tested, the values are below the limits established in GB 8978-1996, indicating a low potential for contaminant leaching [42,97]. However, the lack of detailed information on the specific elements evaluated and the absence of studies on several formulations generate uncertainty about their long-term stability. Compared to geopolymers, which have been widely studied for their ability to immobilize heavy metals due to their amorphous structure and high chemical reactivity, cement mixtures with tailings could present a higher risk of releasing toxic elements under adverse environmental conditions, especially in the presence of water or in structures exposed to weathering. Unlike geopolymers, which can be designed to reduce contaminant mobility through alkaline activation and the formation of highly stable aluminosilicate gels, cement blends with mine tailings rely heavily on cement hydration and the interaction between mineral residues and hydrated compounds in Portland cement [42,97]. In recent studies, the use of iron and copper tailings in 3D printing formulations has been shown to improve the structural stability of the mortar without generating significant leaching of hazardous contaminants [97]. However, although the tests performed indicate values within the permissible limits, the long-term stability of these materials still requires further studies, especially in scenarios of prolonged exposure to water and pH variations, which may affect the mobility of trace metals [133]. Thus, while initial results indicate controlled toxicity in some tailings-based cement formulations, their environmental performance should be further evaluated to ensure their safety in sustainable construction applications [133].

6. The Sustainability Perspective in Tailings Geopolymerization

The circular economy is predicated on the principles of closing material cycles through the implementation of recycling and reuse of resources [120,136,137]. Mining tailings, which are conventionally regarded as waste, contain chemical components that can be utilized to develop geopolymers, thereby reducing the extraction of virgin raw materials [82,132,138,139]. These tailings have demonstrated their applicability and integration with three-dimensional (3D) printing technologies in road infrastructure, as well as in other construction sectors [59,139,140]. This approach has been demonstrated to contribute to the mitigation of the environmental impact of mining, as well as to the recovery of economic value in tailings and the reduction in the environmental footprint [29,36,100,105,140]. The findings of these studies demonstrated that geopolymers exhibit a reduced carbon footprint and are more socially acceptable, particularly in communities impacted by mining, thereby reinforcing the correlation between sustainability and local economic development.

The utilization of tailings-derived geopolymers in road subgrade construction has been demonstrated to be an effective solution. A study by Singh et al. [59] demonstrated that the incorporation of bauxite, iron, and zinc tailings led to substantial enhancements in compressive strength (UCS), durability against wetting and drying cycles, and reduced hydraulic conductivity. These properties are crucial for ensuring the durability of pavements, even under challenging conditions. Furthermore, the research by Lu et al. [139] expanded this perspective by addressing the design of specific geopolymers for mining roads, with a focus on efflorescence control and mechanical performance under heavy traffic. This highlights the versatility of geopolymers and their ability to meet the demands of different operating environments.

Additive printing technologies facilitate the fabrication of pavements with customized properties, enhancing precision and efficiency. According to the findings of Kodikara et al. [141], the implementation of parametric designs and digital twins facilitates the creation of adaptive structures that optimize load distribution, thereby prolonging the service life of pavements [57,68]. Additionally, 3D printing offers the advantage of adjusting the rheological properties of mixtures in real time during the manufacturing process, ensuring uniformity and stability. Innovations in the use of mineral waste, such as limestone powder, have been proposed. This low-cost material can be integrated with geopolymeric technologies to improve geometric stability and adhesion between layers [125,134]. These combinations have been shown to enhance the mechanical and thermal properties of pavements, while concomitantly contributing to the reduction in maintenance costs and associated carbon emissions [55,106,107].

In general, although geopolymers derived from tailings have been demonstrated to attain properties analogous to or superior to conventional Portland cement, such as compressive strength or durability [93,133,142]. The primary challenge resides in the chemical variability intrinsic to tailings, which hinders the standardization of processes and the assurance of the technology’s sustainability. In order to surmount the aforementioned challenges, a number of strategies have been employed, including pre-classification of tailings, amalgamation with auxiliary materials such as fly ash or metakaolin, and precise adjustment of the proportions of alkaline activators. These solutions have been shown to optimize resource utilization while ensuring the economic viability and adaptability of the technology to industrial-scale applications [134]. As Araya et al. [78] rightly argue, the establishment of clear regulations and access to financing for innovation projects are essential to overcome these barriers, mainly in search of the sustainability of the processes, profitability, and applicability of the technology, obtaining regulatory frameworks that also allow establishing the valorization of these mining wastes and generating value in the environment.

7. Final Remarks and Future Directions

This review has addressed the technical aspects and sustainability of mine tailings geopolymerization, its versatility and applicability for 3D printing, and its status as an innovative and sustainable solution for the transformation of waste into high-performance materials. This approach has not yet taken into account the cost-effectiveness and associated impacts through a life cycle analysis, which are relevant to consider in future studies. Despite the ongoing challenges related to the chemical stability and variability of tailings, technological advancements, such as molecular simulations, hold promise in overcoming these obstacles. This approach not only mitigates the environmental impacts of tailings, but also drives the transition towards a circular economy, contributing to the sustainable development of the construction industry and decarbonization strategies, with potential reductions of up to 80% of emissions compared to traditional Portland cement.

The integration of mine tailings into the material cycle through geopolymerization and its subsequent utilization in 3D printing exemplifies a paradigm of circular economy, addressing two key concerns: (1) the management of waste with reduced environmental risk and (2) the transformation of waste into a valuable raw material that promotes enhanced resource management. This transformation has the potential to yield high-value products, such as paving or bricks, which not only reduce the volume of industrial waste requiring management but also establish a new revenue stream for extractive industries and surrounding communities.

The utilization and applicability of three-dimensional printing (3DP) facilitate enhanced flexibility and customization in the fabrication of structures, thereby reducing material waste and enhancing production efficiency. This technology offers significant advantages in terms of cost and resource reduction, driving the development of more sustainable and efficient infrastructures. The integration of geopolymerization with advanced technologies, such as 3D printing, facilitates the development of innovative materials with optimized mechanical properties for specific applications. The capacity to calibrate geopolymer blends in accordance with the requirements of each project enhances both the structural integrity and the adaptability of the material to diverse conditions.

Although the reuse of mine tailings by geopolymerization has demonstrated significant potential, there remain numerous challenges to be addressed and research directions to achieve adequate industrial applicability. One of the main challenges in formulating geopolymers with tailings lies in balancing reactivity and workability, while the challenge to a leap in industrial applicability is conditioned by the high heterogeneity of chemical composition, which affects reactivity and mechanical properties. Current solutions to minimize these effects are based on the use of activators, although with many limitations in the operability of the processes and in the acceleration of the setting processes, generating limitations for some applications.

The limitation of homogeneous conditions in the processes and origins of tailings for the elaboration of geopolymers compromises their application on an industrial scale, as well as the lack of standardization processes in the characterization of materials, production, and evaluation criteria. Establish well-standardized processes that allow their applicability in situ, contributing to an improvement of the environmental footprint and economic viability.

Current research addresses the versatility of geopolymers in different strategic sectors with the capacity to be integrated into advanced technologies that allow addressing challenges in sustainable construction, energy efficiency, CO₂ capture, waste valorization, nanotechnology, and energy storage, which are shown below as future directions:

(1) At the level of sustainable construction and additive manufacturing, the trends are focused on the combination of nanoparticles and fibers for the improvement of mechanical and thermal strength of structures, as well as improving the durability, structural efficiency, and industrial scalability of geopolymers in advanced construction.

(2) At the level of energy efficiency and thermal storage, research is focused on optimizing thermal inertia and reducing conductivity, properties that will position geopolymers as efficient alternatives in the construction of low-energy buildings, as well as on the integration of phase change materials (PCM) in printed geopolymers for thermal storage.

(3) The use of geopolymers as a potential matrix for capturing and storing atmospheric CO₂, through their highly reactive mineral structure by means of carbonation processes, which makes them a viable alternative for decarbonization. Also in this regard, research avenues are being addressed to fabricate environmental containment barriers with alkali-activated geopolymers for soil and water remediation. Many challenges remain to demonstrate the long-term stability and risk of the materials to contaminant exposure in strongly acidic and oxidizing environments.

(4) Optimization of geopolymers using nanomaterials is key to improving mechanical strength, chemical stability, and durability properties. Current trends show that the incorporation of materials into the structure reduces porosity and improves structural performance, although long-term behavior is unknown and should focus on research focused on improving self-healing, chemical activation of nanostructures and industrial scalability.

(5) Research on functionalized geopolymers has shown great potential, positioning them as promising materials for the application of supercapacitors and solid-state batteries. For their adoption on an industrial scale, it will be essential to optimize formulations, standardize processes, and develop regulations to ensure their viability in the energy sector. Their effective implementation could mark a turning point in the sustainability of materials for infrastructure and energy storage, consolidating their role in the transition to a circular economy.

In all the different lines of research that are being developed, the focus should be not only on technical aspects but also on the economic perspective and the adaptability of conditions that allow to obtain collaborative processing conditions and that allow these innovative solutions to reach competences at the industrial level.