From Cementitious Systems to Crushed Construction and Demolition Waste-Derived Geopolymers: Advanced Matrices for Radionuclide Immobilization

1. Introduction and Motivation

The continuously increasing global demand for energy, driven by industrial expansion, urbanization, and population growth, has intensified the reliance on large-scale energy production systems worldwide. Despite the global aspiration to transition toward low-carbon energy sources, such as renewable energy sources (RES), progress to date has remained limited. This can be attributed to the widespread availability of fossil fuel reserves and their deeply embedded role in current energy infrastructures, which hinders a rapid transition, as well as to the inherent intermittency of RES. According to the latest available data, in 2024 fossil fuels accounted for approximately 59.6% of global electricity generation, marking the first recorded decline of this share below the 60% [1]. Within this fossil-based energy structure, coal remains the leading source of electricity generation, accounting for approximately 35% of global electricity production in 2024, although its share has begun to gradually decline in recent years [1]. As a consequence, energy-related carbon dioxide emissions continue to represent the largest share of anthropogenic greenhouse gas emissions, substantially contributing to global warming and climate change [2]. The energy sector is therefore under growing pressure to transition toward low-carbon and sustainable alternatives capable of ensuring both environmental protection and long-term energy security. In this context, greenhouse gas emissions can be substantially mitigated by optimizing energy consumption, improving energy efficiency, and increasing the share of RES and nuclear power within the energy mix.

Nuclear energy enables large-scale electricity generation with minimal direct CO₂ emissions, making it an important contributor to reducing dependence on fossil fuels. On a global scale, nuclear power plants account for approximately 9–10% of total electricity generation, while exhibiting a very low carbon footprint comparable to RES [3,4]. Therefore, nuclear energy is often considered a transitional technology in the decarbonization of the energy sector. Total CO₂ emissions associated with nuclear energy are estimated at approximately 5–20 g CO₂-eq/kWh, which is significantly lower compared to fossil fuels [4,5]. In contrast to these advantages, nuclear energy inevitably generates radioactive waste (RW), with a single nuclear power plant typically producing about 20–50 m³ of high-level waste and approximately 200–1000 m³ of low- and intermediate-level waste annually, depending on reactor type and operational conditions [6,7,8,9]. In addition, RW is also generated in a wide range of medical, industrial, and research activities, which significantly contribute to the overall inventory of low- and intermediate-level RW. According to the International Atomic Energy Agency, more than 90% of the global RW volume is classified as low- and intermediate-level waste [8]. Among these waste streams, liquid RW represents a significant fraction, accounting for approximately 10–30% of the total RW generated, and reaching up to about 40% in specific facilities such as spent fuel reprocessing plants, depending on operational conditions and facility type [9]. Through appropriate treatment processes, such as evaporation and ion exchange, the volume of liquid RW can be reduced by approximately 90–99%, thereby significantly decreasing its disposal requirements [9,10]. Further conditioning of liquid RW requires immobilization matrices capable of preventing radionuclide migration and preventing their leaching into the environment during long-term storage [11,12,13]. Stabilization and long-term isolation of radioactive ions are achieved through solidification into durable matrices in which radioactive species are physically encapsulated and/or chemically bound [12,13,14]. Among the available technologies, cementitious materials, particularly systems based on Ordinary Portland Cement (OPC), are widely used for the conditioning of low- and intermediate-level liquid RW [15,16], primarily due to their low cost, simple processing, chemical compatibility, and well-established application in waste management systems [17,18].

Nevertheless, the widespread use of cementitious materials for radionuclide immobilization raises significant sustainability concerns, particularly given that cement is the most widely produced and consumed construction material worldwide and is associated with a substantial environmental burden during production [19,20]. Global cement production reached approximately 4.4 billion tons in 2024 and is projected to surpass 5 billion tons by 2030, corresponding to nearly 0.6 tons per capita per year [19,21,22]. Cement production is an energy-intensive process involving the calcination and sintering of raw materials at temperatures approaching 1450–1500 °C, requiring substantial thermal and electrical energy inputs [19]. On average, the production of 1 t of cement consumes approximately 3.3 GJ of thermal energy and around 110 kWh of electricity, with the clinker sintering stage representing the most energy-demanding step of the process [23]. Depending on the fuel source and production technology, manufacturing 1 t of cement may generate approximately 0.9–1.0 t of CO₂ eq [24]. As a result, the cement industry is responsible for approximately 7–8% of total anthropogenic carbon dioxide emissions, while simultaneously accounting for a considerable share (5%) of industrial energy consumption [19,25]. With an estimated annual growth of 4% in cement production [26], carbon dioxide emissions will increase and cause additional environmental burdens. In addition to its carbon and energy footprint, cement production relies heavily on virgin raw materials such as limestone and clay, contributing to the progressive depletion of natural mineral resources. Accordingly, current research on binder systems has increasingly focused on the principles of sustainable development. This is primarily reflected in reducing the consumption of natural resources and promoting the use of secondary raw materials, including industrial by-products and waste-derived materials, in order to decrease environmental impacts and improve resource efficiency [27,28]. In this context, the development of alternative low-carbon binders has gained significant attention, with geopolymers widely recognized as promising alternatives to OPC due to their reduced carbon footprint and strong potential for waste valorization [29,30]. Geopolymers exhibit a combination of advantageous properties, including high mechanical strength, low permeability, and excellent resistance to chemically aggressive environments. Their three-dimensional aluminosilicate network enables the incorporation of various precursors during the geopolymerization process, providing considerable flexibility in raw material selection [31,32]. Moreover, they facilitate the utilization of diverse waste streams, such as blast furnace slag, fly ash, different types of clay, and agricultural residues, thereby contributing to resource conservation and the advancement of a Circular Economy (CE) [33,34].

Furthermore, from a sustainability and CE perspective, C&DW is recognized as a significant waste stream whose valorization is increasingly investigated through its use in alternative binder systems such as geopolymers [35]. It is particularly important that C&DW is produced and disposed of in large quantities and therefore represents an economically advantageous raw material [36]. Owing to its high availability and heterogeneous mineralogical composition, often rich in aluminosilicate phases, C&DW is considered a promising secondary raw material and precursor for geopolymer production for their production [37,38,39,40,41]. In addition, materials obtained from construction sources show compatibility with conventional matrices for immobilization (mortar, concrete, glass and bitumen) of liquid RW and sorbents for wastewater treatment [13,42,43]. Consequently, the sorption properties of C&DW components (concrete, bricks, ceramic residues, and asphalt) have been widely investigated for radionuclide ions present in liquid RW, demonstrating significant sorption capacity for metal ions such as Co, Ni, Sr, and Cs, which are relevant contaminants in liquid RW systems [14,17,18,36,40,41,44,45,46].

This review provides an overview of research on the transition from conventional cementitious systems to C&DW-derived geopolymers for radionuclide immobilization, with a focus on integrating two complementary research directions: the sorption properties of C&DW and the structural and environmental advantages of geopolymer matrices. Although waste-based geopolymers have been widely investigated as sustainable construction materials [39,47,48,49], their application in radionuclide or toxic metal ion encapsulation remains relatively underexplored [50,51]. These findings indicate that C&DW can simultaneously support both sorption and encapsulation mechanisms, thereby facilitating the immobilization of contaminants within a stable matrix [50,52]. Accordingly, this review aims to encourage further experimental research on C&DW-based geopolymer systems for radionuclide immobilization, while highlighting their potential to contribute to reduced energy consumption and emissions, mitigation of climate change, and valorization of C&DW through decreased disposal and conservation of natural resources. C&DW has emerged as a particularly promising secondary resource, not only due to its abundance but also owing to its physicochemical characteristics [53]. Therefore, understanding its generation, composition, and functional properties is essential for evaluating its potential in advanced applications such as geopolymer production and radionuclide immobilization.

2. Construction and Demolition Waste as a Secondary Resource for Advanced Applications

2.1. Generation and Environmental Impact of C&DW

Rapid urbanization, infrastructure renewal, and demolition activities have made construction and demolition waste (C&DW) one of the largest waste streams globally, accounting for approximately 30–40% of total solid waste generation worldwide [54,55]. In the European Union (EU), C&DW represents the largest waste stream, accounting for approximately 33–35% of total waste generation [56,57]. In the United States of America (USA), C&DW accounts for approximately 40% of total solid waste [58], while in China it contributes approximately 30–40% of total urban solid waste, representing the dominant waste stream in urban areas [59].

Recycling rates of C&DW vary significantly across regions, reflecting differences in waste management systems, regulatory frameworks, and available technologies. The EU reports highly heterogeneous performance, ranging from below 10% to above 90% depending on the member state [60,61,62], whereas in the USA recovery levels typically range between 30% and 70% depending on material type [63]. In China, reported recycling and reuse rates are generally lower and more variable, typically ranging from approximately 50% to 70% in urban regions [64,65].

From a practical perspective, C&DW is suitable for recycling and is commonly used as recycled aggregate in non-structural applications such as road base layers, embankments, sub-base systems, backfilling, and other infrastructural works [66,67]. Concrete, asphalt, and bricks are among the most recoverable fractions, with reported recovery rates typically ranging from approximately 80–95% for asphalt, 70–90% for concrete, and 60–85% for masonry materials, depending on local processing technologies and waste management systems [68,69,70]. For example, the EU Waste Framework Directive 2008/98/EC sets a target of at least 70% recycling and recovery of C&DW, explicitly promoting improvements in waste management systems [71]. However, achieving this target requires not only optimization of conventional recycling technologies but also the development of alternative high-value valorization pathways that extend beyond traditional construction applications.

Despite these efforts, current practices indicate a limited level of material circularity, resulting in continued dependence on primary raw materials. This issue is further exacerbated by the significant environmental footprint of the construction sector, which contributes approximately 50% of climate change impacts, 40% of energy consumption, and 50% of landfill waste generation, while also causing air and water pollution, ecosystem degradation, and adverse effects on human health [72,73,74].

Accordingly, C&DW should not be regarded solely as a waste stream, but rather as a significant environmental and economic challenge, associated with substantial costs related to its collection, transport, processing, and disposal. These challenges highlight the urgent need for innovative, economically viable, and environmentally sustainable solutions for its management and valorization.

2.2. Composition and Physicochemical Properties of C&DW

C&DW includes a wide range of materials of different origin and composition [41]. Following selective demolition, reconstruction, renovation, and removal of non-mineral components (e.g., metals, glass, wood, plastic, and plaster), a predominantly mineral fraction, referred to as crushed C&DW (stony waste), is obtained. This fraction is highly heterogeneous, reflecting the diversity of source materials and processing conditions.

Crushed C&DW mainly consists of cementitious and ceramic-derived materials, including concrete, mortar, bricks, and ceramics, with smaller contributions from aged asphalt originating from building access roads. The mineralogical evolution of these constituents is governed by long-term environmental exposure, including relative humidity, precipitation, wet–dry cycles, atmospheric CO₂, and temperature fluctuations, which induce decalcification and secondary transformations [75,76,77,78,79]. Concrete and mortar represent the dominant fraction and are composite materials consisting of water, cement, and silica-rich aggregates [80]. The cement component, typically OPC is produced by high-temperature processing of limestone and aluminosilicate clays [19,81], yielding calcium silicate phases that, upon hydration, form calcium silicate hydrate (C–S–H) as the main binding phase, along with calcium aluminate hydrates (C–A–H) and calcium aluminosilicate hydrates (C–A–S–H) [75,76]. Under the influence of long-term environmental exposure, these hydration products undergo progressive transformation due to decalcification, silicate polymerization, and carbonation of portlandite [82,83], leading to the formation of a Si–Ca–O matrix with reduced porosity observed by SEM–EDS [14]. As a result, in aged concrete, quartz (SiO₂) and calcite (CaCO₃) become the dominant crystalline phases [14,17,44].

Brick materials originate from clay-based raw mixtures subjected to thermal treatment, which induces mineralogical transformations depending on firing temperature [84]. During firing, kaolinite transforms into metakaolinite, while higher temperatures promote the formation of quartz, mullite, anorthite, and sanidine [84,85]. Bricks are typically composed of quartz and calcite with additional silicate and aluminosilicate phases reflecting raw material variability [44], while higher firing temperatures lead to simplified mineral assemblages due to progressive phase transformations [84,85,86,87,88].

Minor constituents of crushed C&DW, including ceramic and roof tiles as well as asphalt materials, also exhibit distinct mineralogical characteristics. Ceramic and roof tiles, similarly to clay-based materials, contain quartz, albite, and calcite as dominant phases, with additional minor metallic compounds associated with pigments and opacifiers [18]. Fresh asphalt consists of mineral aggregates bound by a bituminous organic phase, while aging is characterized by oxidation and the loss of lighter organic fractions, accompanied by the formation or enrichment of secondary mineral phases such as quartz, calcite, and dolomite [17,44,89].

From a chemical perspective, literature data from concrete, brick, ceramic, and asphalt materials collectively indicate that crushed C&DW is predominantly composed of SiO₂, Al₂O₃, and CaO [14,17,18,44,90,91]. Minor contributions of Fe₂O₃, MgO, and alkali oxides are also typically reported in C&DW systems [90,92]. FT-IR analysis confirms the ubiquitous presence of Si–O and carbonate groups across C&DW fractions, while asphalt additionally exhibits alkyl C–H vibrations associated with bituminous binders [14,18,44]. In general, cement-based materials behave as silicate–carbonate systems, where carbonation of portlandite leads to CaCO₃ formation and progressive transformation of hydration products [93,94], whereas asphalt undergoes oxidation-driven aging and degradation of its organic phase [95,96].

Beyond bulk composition, the reactivity of C&DW is strongly governed by the presence of amorphous and poorly crystalline phases originating from both cementitious and ceramic components, including decalcified C–S–H and thermally transformed aluminosilicates. These phases exhibit higher structural disorder and free energy, making them significantly more reactive under alkaline conditions [94,97]. Their dissolution represents the rate-controlling step of geopolymerization [96,98], releasing silicate and aluminate species that subsequently form three-dimensional aluminosilicate networks such as N–A–S–H gels or calcium-modified C–(A)–S–H structures [99,100]. Consequently, the coexistence of crystalline and amorphous phases, together with variable calcium content, governs dissolution kinetics, gel formation, and final physicochemical performance, highlighting the strong potential of properly processed C&DW as a precursor for alkali-activated and geopolymer materials.

2.3. Sorption and Immobilization Performance of Crushed C&DW

The composition and physicochemical properties of crushed C&DW fundamentally govern its interaction with dissolved species. The presence of reactive mineral phases, surface functional groups, and developed porosity provides a variety of active sites for contaminant binding [14,17,18,36,44]. Consequently, crushed C&DW should be regarded as a heterogeneous system with essential functional properties relevant to environmental applications. This heterogeneity arises from the coexistence of cementitious, ceramic, and mineral phases, each contributing differently to sorption behavior [101,102].

Sorption in such systems is a multi-mechanistic process involving surface adsorption driven by attractive molecular forces, ion exchange, formation of ion pairs, precipitation, specific sorption (often combining adsorption and precipitation), hydrogen bonding, and complexation, as well as their combined effects [36,41]. Sorption mechanisms mostly depend on the physico-chemical properties of the sorbent material and the target species. They are further influenced by operational parameters such as pH, contact time, temperature, and the presence of competing ions [11,12,13,41,103,104]. In C&DW systems, elevated pH values induced by cementitious phases additionally enhance metal ion immobilization through precipitation and surface complexation mechanisms [14,18,44].

Construction industry by-products and C&DW-derived materials have consistently demonstrated strong potential as low-cost sorbents for inorganic contaminants. Brick-derived materials, particularly brick dust, exhibit high affinity toward metal ions such as Pb²⁺, Cd²⁺, and Cs⁺ [105,106,107], and have been widely reported as efficient aluminosilicate-based sorbents in aqueous systems, primarily due to ion exchange and surface complexation mechanisms. In addition, ceramic-derived materials have demonstrated the capability for interaction with radionuclide-relevant oxyanions As(V), Cr(VI) and U(VI), under solution conditions governed by their speciation, surface reactivity, and pH-dependent charge characteristics [107,108]. Comparative studies further indicate that thermally treated brick powders outperform raw clays in the removal of Pb²⁺, Cd²⁺, and Zn²⁺, due to firing-induced structural modifications that enhance surface reactivity and the density of active sites [109]. Similarly, crushed concrete fines derived from recycled aggregates demonstrate efficient immobilization of Cu²⁺, Zn²⁺, and Pb²⁺ ions, with performance comparable to other established low-cost sorbents, confirming the functional relevance of cementitious waste fractions beyond their structural reuse [110]. Subsequent studies further expanded the application of crushed concrete fines toward the immobilization of Cr³⁺, Ni²⁺, Sr²⁺, Cd²⁺, Co²⁺, and molybdate species, emphasizing the important role of cement-rich fine fractions in the retention of metal ions and radionuclide-relevant contaminants [111,112].

In addition to these general trends, more detailed investigations into specific C&DW fractions reveal pronounced differences in the uptake of transition metal ions. Cementitious materials exhibit the highest sorption capacities for Co²⁺ and Ni²⁺, significantly outperforming ceramic-, brick-, and asphalt-derived fractions. Reported Sr²⁺ uptake values reach up to 0.25 mmol/g, while Co²⁺ and Ni²⁺ sorption can exceed 0.32 and 0.55 mmol/g in cement-based samples, respectively, whereas other fractions showed lower capacities [14,44]. This behavior is consistent with the known affinity of calcium silicate hydrate (C–S–H) phases toward divalent cations, confirming their central role in immobilization processes. In addition to adsorption and ion exchange, precipitation of metal hydroxides may contribute to Co²⁺ and Ni²⁺ removal under near-neutral to alkaline conditions, where the formation of Co(OH)₂ and Ni(OH)₂ becomes thermodynamically favorable [14,44]. In multi-component systems, competitive interactions between Co²⁺ and Ni²⁺ may occur, although cementitious phases maintain relatively stable sorption performance even under equimolar conditions [14,36,41,44]. Additionally, significant amounts of Ca²⁺ ions contained in the C&DW itself can easily be transferred into solution and assume the role of a competing cation [41]. Sorption efficiency is strongly influenced by process parameters such as particle size, solid-to-liquid ratio, and surface accessibility, with finer fractions exhibiting enhanced reactivity due to increased specific surface area [41,113].

From a mechanistic perspective, sorption behavior is primarily governed by the coexistence of calcium-rich phases and aluminosilicate structures, which provide reactive surfaces for interaction with aqueous species. These interactions proceed through ion exchange (particularly involving Ca²⁺ and Na⁺), electrostatic attraction, and surface precipitation under alkaline conditions. The role of Ca-bearing phases has been consistently highlighted in studies on recycled concrete, where Ca²⁺ release and subsequent exchange reactions significantly influence sorption efficiency [112,114].

3. C&DW-Derived Geopolymers as Sustainable Cementitious Alternatives

Geopolymers are alkali-activated aluminosilicate binders that enable the incorporation of industrial by-products and secondary raw materials into structurally stable networks [34,115]. The integration of C&DW into such systems has attracted increasing research attention as a strategy to reduce the environmental impact of conventional cement production while promoting material circularity [30,31,116].

A Scopus-based analysis of geopolymer research incorporating C&DW identified 221 publications for the period 2016–2026 when the search was restricted to studies involving reactive fractions such as powders, fines, precursors, and binder-related systems. This indicates that only a subset of the broader C&DW–geopolymer literature explicitly addresses the chemically active role of waste-derived materials in geopolymerization processes. Qualitative screening of the retrieved publications, combined with commonly used terminology, shows that recycled concrete-derived materials—typically in the form of recycled concrete powder or fines—represent the most frequently studied C&DW fraction as a geopolymer precursor. This is consistent with their wide availability and Ca-rich composition, which supports the formation of hybrid C-(A)-S-H and (N,C)-A-S-H binding phases. Brick-derived waste follows as the second most commonly investigated precursor due to its aluminosilicate-rich composition and increased reactivity after mechanical activation. Ceramic waste-based systems are also reported, although less frequently, and are typically characterized by lower intrinsic reactivity, which limits their role to supplementary or partial precursors. In contrast, mixed C&DW systems remain less explored due to their heterogeneous composition and associated variability in performance. Fine and dust fractions generated during C&DW processing are the least studied category, despite their high specific surface area and potential for enhanced dissolution kinetics. The observed distribution is derived from a qualitative assessment of publication focus and keyword usage. No strict bibliometric quantification per fraction was performed due to overlapping terminology and inconsistent descriptor usage across studies, which limits direct numerical comparison between individual C&DW categories. The identified trend nevertheless points to a clear research preference for well-defined, single-source precursors, while more complex or highly processed fractions remain underrepresented.

C&DW can be incorporated into geopolymer systems through different pathways depending on its processing and particle size. Its function may range from that of an inert aggregate to a reactive aluminosilicate precursor capable of participating in geopolymerization reactions. When used as recycled aggregate, C&D waste primarily acts as a filler or aggregate phase, rather than actively participating in geopolymerization reactions [66,67]. Experimental studies on unseparated C&DW streams confirm that coarse fractions mainly influence packing density, while fine fractions control the variability of mechanical performance [117]. In this case, its role is predominantly physical, with limited contribution to binding phase formation, and is typically associated with low- to medium-strength construction materials. This is consistent with the reported compressive strength range of approximately 25–35 MPa for such systems, depending on precursor composition and curing conditions [117,118,119]. Nevertheless, higher strengths may be achieved through optimized mixture design and incorporation of ceramic-rich fractions, with values exceeding 55 MPa and, in some cases, approaching ~70 MPa [120,121]. At the same time, finely ground C&DW fractions can exhibit significant chemical reactivity, particularly when enriched in brick- and ceramic-derived materials [122,123]. Such fractions may contain partially amorphous aluminosilicate phases capable of dissolving under alkaline conditions and contributing to geopolymer network formation [41,124,125,126,127]. The extent of this reactivity is largely governed by the amorphous phase content and particle fineness, both of which influence dissolution kinetics and the availability of reactive Si and Al species. Enhanced dissolution following mechanical milling has been reported for brick- and ceramic-rich fractions, facilitating partial geopolymerization under highly alkaline conditions [31,53,125,126,127,128]. However, certain ceramic fractions (e.g., sanitary ware, porcelain, and roof tiles) contain a relatively high proportion of crystalline phases such as quartz and feldspars, and therefore often exhibit lower reactivity compared to brick-derived materials.

The reaction products formed during alkali activation depend strongly on the chemical composition of the precursor, particularly its calcium content. In low-calcium systems, N-A-S-H-type gel is generally the dominant binding phase, whereas calcium-rich fractions may promote the formation of C-(A)-S-H gels or hybrid gel structures resulting from the coexistence of sodium- and calcium-bearing aluminosilicate networks [31,53,125,126,127,128]. Calcium present in C&DW, primarily originating from hydrated cement phases and carbonate-containing fractions, therefore plays a key role in directing the development of calcium-bearing reaction products during alkali activation.

Since mixed C&DW streams contain varying proportions of concrete, brick, ceramic, and mortar components, their alkali activation commonly results in complex multi-gel systems. The fine dust fraction, in particular, exhibits variable reactivity due to its heterogeneous composition, mixed amorphous–crystalline character, and fluctuating calcium content. Consequently, alkali-activated C&DW frequently contains a combination of N-A-S-H, C-(A)-S-H, and hybrid (N,C)-A-S-H gel phases, whose relative abundance depends on the precursor composition and processing conditions. In contrast, crystalline phases such as quartz and calcite remain largely inert under alkaline conditions and primarily serve as microstructural fillers [30,31,32].

An overview of the principal C&DW fractions employed in geopolymer systems, together with their composition, role, reactivity, and dominant binding products, is provided in Table 1.

Pre-treatment strategies, including mechanical and thermal activation, significantly influence the reactivity of C&DW by increasing structural disorder and accelerating dissolution kinetics, thereby facilitating more efficient geopolymerization [132]. Thermomechanical processing, in particular, has been reported to enhance the amorphization of precursor phases and improve early-stage geopolymer gel formation by increasing the availability of reactive aluminosilicate species [53,132].

Despite these advantages, the application of C&DW in geopolymer systems is constrained by its inherently heterogeneous composition, which typically includes varying proportions of concrete, brick, ceramic, mortar, and natural stone fractions [41,124]. This variability directly affects dissolution behavior, ionic release, and subsequent gel formation, resulting in fluctuations in microstructure and mechanical performance. Additionally, the presence of hydrated cement phases and carbonation products can influence alkalinity consumption and alter reaction pathways during alkali activation [30,33,99]. Reviews consistently identify heterogeneity as a key barrier to large-scale application, as it leads to inconsistent Na–Al–Si gel formation across different waste sources [53,133,134]. At the same time, enhanced dissolution of aluminosilicate phases has been linked to the formation of more homogeneous binding gels and refined pore structures, which contribute to improved mechanical performance [132].

To overcome limitations associated with low and variable reactivity, hybrid geopolymer systems incorporating highly reactive aluminosilicate precursors such as fly ash, blast furnace slag, and metakaolin have been extensively investigated [30,31,67,135,136,137,138]. These additions enhance dissolution kinetics, increase the availability of reactive species, and promote the formation of more homogeneous and mechanically stable binding gels. Hybrid systems incorporating C&DW with fly ash or slag have been widely reported to enhance compressive strength and durability relative to single-source C&DW-based binders, primarily due to improved gel formation and microstructural densification [51,139,140,141]. Additionally, binary systems incorporating OPC have also been proposed to improve early-age strength, particularly in low-reactivity C&DW-based binders [34,142]. This approach is especially relevant for concrete-derived waste, which is typically rich in calcium-containing phases but relatively poor in reactive aluminosilicates. In such systems, OPC contributes to the formation of calcium–aluminosilicate hydrate (C-(A)-S-H) phases alongside geopolymeric N-A-S-H gels, resulting in hybrid binding mechanisms with improved mechanical performance [30,143]. The relative contribution of these phases depends on calcium availability and activator composition, which together control gel chemistry and microstructural evolution [99,144]. Within this framework of composition–structure–property relationships, it has been widely reported that optimized C&DW-based alkali-activated systems can achieve mechanical performance comparable to conventional cementitious binders [53,117]. However, reproducible performance requires careful control of precursor composition, particle fineness, and activation parameters.

The ability of geopolymer systems to accommodate diverse C&DW-derived precursors while forming stable aluminosilicate networks has established them as promising multifunctional materials [53]. Their flexibility in composition and durability support applications beyond conventional construction materials, particularly in environmentally oriented fields [145]. Such developments reinforce their relevance to circular economy strategies [116], while highlighting the need for further investigation of long-term performance under application-specific conditions [76,146].

4. Geopolymer Matrices from C&DW for Radionuclide Immobilization

In this review, radionuclide immobilization is considered as a combined process involving encapsulation and sorption. Encapsulation refers to the physical and chemical incorporation of radionuclides into the geopolymer matrix, where metal ions are either trapped within pores or integrated into the aluminosilicate structure. Sorption, on the other hand, involves interactions occurring at the material surface and within its pore network [147,148,149,150].

The high immobilization efficiency of geopolymers is largely attributed to their well-developed three-dimensional aluminosilicate network, which forms a negatively charged and highly reactive framework [31,32,99,100]. This structure provides energetically favorable sites for binding various metal ions and supports multiple retention pathways [151,152,153]. In addition, the dense geopolymer gel contributes to immobilization by physically restricting mass transport, thereby enhancing long-term stability of incorporated species [53,128]. Radionuclide retention in geopolymer systems is governed by several concurrent mechanisms, including physical entrapment, structural incorporation into the aluminosilicate network, ion exchange with alkali and alkaline-earth cations, and binding to silicate and aluminate groups [99,100]. Functional groups such as silanol (–Si–OH) and aluminol (–Al–OH) play a key role by facilitating ion exchange and surface complexation, leading to the formation of stable metal–hydroxyl bonds (–Si–O–M and –Al–O–M). These interactions promote the incorporation of metal ions into the geopolymer framework and contribute to their effective encapsulation [148,149,150]. The relative importance of these mechanisms depends strongly on the chemical environment, particularly the high alkalinity and the composition of the activating solution [148,149,150].

Sorption processes are driven by surface functional groups and involve a combination of physical and chemical interactions, including surface adsorption, ion exchange, ion pair formation, and precipitation occurring both at the solid–liquid interface and within the pore structure under highly alkaline conditions [11,12,13,36,41,103,104]. In many cases, sorption proceeds through coupled adsorption–precipitation mechanisms, often accompanied by the formation of hydrogen bonds or inner-sphere complexes [18,36,41]. Highly alkaline conditions further promote the formation of low-solubility metal phases, which additionally enhances immobilization efficiency [18,44,154].

The combined action of encapsulation and sorption significantly reduces radionuclide mobility and leaching, ensuring long-term physicochemical stability under disposal conditions [18,36,41,44,53,128]. Accordingly, immobilization in geopolymer systems can be described as a multi-pathway process involving precipitation, ion exchange, adsorption, and physical encapsulation occurring simultaneously within a structurally and chemically evolving matrix.

4.1. Waste-Derived Geopolymers for Radionuclide Immobilization

A wide range of waste-derived precursor materials, including fly ash, blast furnace slag, and other industrial by-products, have been successfully utilized in geopolymerization processes due to their partial solubility in alkaline activating solutions and their ability to form stable aluminosilicate frameworks [52]. Owing to these properties, such systems have been extensively investigated for both hazardous waste stabilization and RW management, demonstrating the ability to immobilize a broad spectrum of contaminants, including metal ions (Ni(II), Pb(II), Cu(II), Cd(II), Cr(III/VI), Mn(II)) and radionuclides or activation products such as ⁶³Ni, ¹³⁷Cs, ⁹⁰Sr, and ⁶⁰Co, as well as surrogate ions including U(VI) and Th(IV) [155,156,157,158]. Among radionuclides of major environmental concern, cesium and strontium are the most extensively studied due to their high mobility and long half-lives, with numerous studies confirming their effective incorporation and strong leaching resistance in geopolymer matrices [159,160,161,162,163]. In addition to these, other radionuclides such as technetium, iodine, and europium-based actinide surrogates have also been investigated. From a mechanistic perspective, radionuclide immobilization in these systems is primarily governed by ion exchange processes involving Na⁺ and Ca²⁺ species within the geopolymer structure [155], as well as direct chemical interactions with Si–O– and Al–O– functional groups. Such interactions have been reported for various ions, including Pb(II), Cd(II), Cu(II), U(VI), and Th(IV), contributing to their stable incorporation within the geopolymer matrix.

Within this broader context, C&DW-based geopolymer systems offer particularly attractive prospects by coupling radionuclide immobilization performance with the valorization of C&DW [11,14,16,17,18,36,40,44,45,47,48,49].

4.2. C&DW-Derived Geopolymers for Radionuclide Immobilization

In systems where C&DW is used as the primary precursor, often combined with industrial or secondary aluminosilicate sources, the formation of a stable geopolymer network has been consistently observed. This network enables effective immobilization of metal ions such as Pb, Ni, Co, Ba, Cd, Cr, Cu, and Zn through a combination of mechanisms. These ions interact electrostatically with negatively charged AlO^4- tetrahedra and become physically entrapped within the gel pore structure, resulting in reduced mobility and leachability [164,165,166,167,168].

C&DW-based geopolymers, particularly when blended with aluminosilicate-rich materials such as fly ash, slag, or ceramic fractions, form heterogeneous gel systems characterized by the coexistence of N-A-S-H and C-A-S-H phases. This effect is especially pronounced in blended systems where C&DW is combined with additional waste streams, such as incineration ash or industrial by-products, leading to improved microstructural densification and reduced metal leaching. The formation of dense C-(A)-S-H and N-A-S-H gels plays a key role in the long-term stabilization of contaminants [166,169]. Furthermore, the formation of hydration products such as C–S–H and ettringite phases has been shown to significantly reduce the leaching of Zn, As, and Ni, highlighting the important role of secondary mineral phases in binding and retaining metal species within the solid matrix [170]. These phases can act as additional sinks for metal ions through incorporation, surface complexation, or precipitation mechanisms, further enhancing overall immobilization efficiency.

Although studies explicitly addressing radionuclide immobilization in such systems remain limited, existing reviews suggest that similar binding mechanisms can be extended to radionuclides such as Co, Cs, and Sr, due to their comparable ionic characteristics and their ability to be incorporated into aluminosilicate frameworks [47]. In particular, monovalent and divalent radionuclides may readily participate in ion-exchange processes or occupy charge-balancing sites within the geopolymer structure, supporting their effective retention.

4.3. Quantitative Overview and Research Trends

To assess the current state of research on radionuclide immobilization using C&DW-based geopolymers, a series of targeted Scopus searches was conducted employing combinations of terms related to C&DW, geopolymer technology, radionuclides, RW, immobilization processes, and contaminant release. The results revealed a very limited number of publications directly addressing radionuclide-related applications. Searches explicitly combining C&DW-derived geopolymers with radionuclide immobilization or RW management returned only 2–3 publications. Broader searches involving radionuclide-relevant elements such as Cs, Sr, Co, and U yielded 24 records, while the inclusion of general immobilization-related terms (immobilization, encapsulation, stabilization, solidification, and fixation) resulted in 16 publications.

In contrast, a larger body of literature (18 publications) was identified when focusing on the leaching behavior of C&DW-based geopolymer systems. These studies do not target intentional immobilization of contaminants, but rather evaluate the release of elements originally present in the raw C&DW-derived precursors under different environmental conditions. As such, leaching tests provide indirect insight into the retention capacity and structural stability of geopolymer matrices. An even broader set of studies (284 publications) addresses the behavior of heavy metals and metal-containing species in C&DW-based geopolymers, primarily through leaching and durability assessments. Although these investigations are not designed as radionuclide immobilization studies, they offer relevant insight into the ability of geopolymer networks to incorporate and retain metal species within their aluminosilicate structure.

These observations highlight the need for a more comprehensive and systematic evaluation of radionuclide immobilization in C&DW-based geopolymer systems, particularly in relation to the existing body of knowledge on metal retention and leaching behavior, which is further discussed in the following section.

5. Knowledge Gaps and Future Perspectives

Despite the demonstrated potential of geopolymer systems for the immobilization of hazardous and radioactive species, their practical application remains constrained by several unresolved scientific and technological challenges. These limitations are particularly pronounced in C&DW-based systems, where compositional heterogeneity and multi-phase complexity introduce additional uncertainties.

An important aspect in this context is the limited integration between two parallel but related research domains: sorption (immobilization) studies performed on raw C&DW materials [14,17,18,36,40,41,43,44,45,105,106,107,108,109,110,111,112] and investigation of geopolymer systems derived from C&DW [38,39,49,51,53,67,117,121,125,126,127,128,132,133,136,137,138,139,140,142]. The existing studies have demonstrated that C&DW fractions can effectively sorb a wide range of metal ions and radionuclides, revealing their intrinsic retention capacity. However, these findings are typically considered independently from studies on C&DW-based geopolymer. As a result, the potential relationships between sorption behavior of raw precursors and immobilization performance of the resulting geopolymer matrices remain insufficiently explored, limiting the ability to directly relate these two aspects.

Although research on the immobilization of metal ions in C&DW-derived geopolymers remains limited, a growing body of literature indicates that geopolymer matrices can effectively incorporate a wide range of metal ions and radionuclides, including ⁶³Ni, ¹³⁷Cs, ⁹⁰Sr, and ⁶⁰Co, as well as species such as U(VI) and Th(IV) [155,156,157,158,159,160,161,162,163]. However, immobilization performance has been shown to depend strongly on system composition and redox conditions [171], as well as on the precursor composition and the resulting matrix structure [160]. These dependencies highlight the need for a more systematic understanding of structure–performance relationships in geopolymer systems.

In the specific case of C&DW-derived geopolymers, the heterogeneous mineral composition, the presence of residual cementitious phases, and the coexistence of calcium-rich and aluminosilicate-rich domains promote diversified retention mechanisms [31,32,37,38,39,40,41,43,53,67,76,99,100,125,126,127,128,163]. While such complexity may enable the simultaneous immobilization of radionuclides with different chemical behaviors, it also introduces significant uncertainty in predicting radionuclide partitioning and long-term stability.

Hybrid gel systems characterized by the coexistence of N-A-S-H and C-(A)-S-H phases further contribute to this complexity. These structures facilitate the incorporation of metal ions and may exhibit behavior analogous to radionuclide immobilization through sorption, physical encapsulation, and chemical stabilization mechanisms [169]. However, the relative contribution of these mechanisms remains insufficiently quantified, particularly in multi-phase systems.

A critical limitation in the current body of research is the scarcity of studies explicitly addressing radionuclide immobilization in C&DW-based geopolymer systems. Although existing reviews suggest that similar binding mechanisms may apply to radionuclides such as Cs, Sr, and Co [47], direct experimental validation under realistic conditions remains limited.

This gap is further supported by a quantitative analysis of the available literature. Despite increasing research activity in the field of C&DW-based geopolymers, only a very limited number of studies directly address radionuclide immobilization or RW management. In contrast, a significantly larger body of work focuses on leaching behavior and the retention of metal species. These studies provide valuable, but largely indirect, evidence of immobilization performance. The lack of systematic translation of these findings to radionuclide systems represents a major research gap and suggests that the application of C&DW-derived geopolymers for targeted radionuclide immobilization is still in its early stage of development. The most relevant knowledge gaps and future research needs are summarized in Table 2.

Bridging the gap between the sorption behavior of raw C&DW materials and the immobilization performance of geopolymer matrices represents a key research priority. Establishing quantitative relationships between precursor characteristics and final waste-form performance could enable the transition from empirical formulations toward predictive, performance-driven design of geopolymer-based immobilization systems. Such an approach is essential for advancing these materials from laboratory-scale studies to practical applications in RW management.

6. Conclusions

This review highlights the potential of C&DW-derived geopolymer systems as advanced matrices for radionuclide immobilization by integrating knowledge from two closely related but largely separated research directions: sorption studies on crushed C&DW and the development and characterization of C&DW-derived geopolymers. The available literature demonstrates that C&DW materials possess intrinsic retention capacity toward a range of metal ions and radionuclides, including Co²⁺, Ni²⁺, Sr²⁺, Cs⁺, Pb²⁺, Cd²⁺, Zn²⁺, As(V), Cr(VI), and U(VI). At the same time, C&DW-derived geopolymers have been extensively investigated as sustainable cementitious alternatives, with particular emphasis on their mechanical properties and phase evolution.

However, the translation of these findings into radionuclide immobilization performance of C&DW-derived geopolymer matrices remains insufficiently developed. Direct studies addressing radionuclide retention in C&DW-based geopolymers are still limited, and existing knowledge is often derived indirectly from other waste-based geopolymer systems. Establishing clear relationships between precursor composition, sorption behavior, geopolymer structure, and radionuclide retention represents a key research priority.

Further progress requires standardized approaches for heterogeneous C&DW feedstocks, comprehensive assessment under realistic multi-ion conditions, and long-term evaluation beyond conventional short-term leaching tests. Integrating advanced characterization, experimental validation, and predictive modelling will be essential for transforming C&DW-derived geopolymers from promising materials into reliable matrices for RW immobilization.