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

Abstract

Construction and demolition waste (C&DW) has been investigated both for radionuclide sorption and as a precursor for geopolymer materials. However, research on C&DW-derived geopolymers has primarily focused on synthesis and characterization, while their role in radionuclide immobilization remains insufficiently explored. This review evaluates C&DW-derived geopolymers as promising matrices for radionuclide immobilization, focusing on retention capabilities and factors influencing immobilization performance. Available studies indicate that waste-derived geopolymer systems can limit radionuclide mobility and leaching, but their performance strongly depends on precursor composition, phase assemblage, and matrix structure. The mineral complexity of C&DW-derived matrices may provide diverse retention pathways, supporting their consideration as immobilization materials. Linking the documented sorption capacity of C&DW materials with the immobilization potential and favorable characteristics of C&DW-derived geopolymer matrices represents a promising approach for developing advanced systems for radionuclide solidification. Due to the limited number of studies directly addressing radionuclide immobilization in C&DW-derived geopolymer systems, evidence from related studies is considered to support assessment of their immobilization capacity. Nevertheless, further clarification is required regarding the integration of existing research findings, precursor heterogeneity, multi-ion interactions, and long-term performance under realistic conditions. Addressing these limitations through systematic investigations is essential to support their application in radioactive waste (RW) management.

1. Introduction

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 (RESs), 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 RESs. 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 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 RESs 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 RESs [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 by 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 (IAEA), 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 and other alkali-activated materials (AAMs) widely recognized as promising alternatives to OPC due to their potential for lower carbon emissions compared with conventional cement-based systems and strong potential for waste valorization [29,30]. In this review, the term “geopolymer” is used following the broad terminology commonly adopted in the literature for AAMs and geopolymer-based binders, while recognizing that the resulting reaction products may vary depending on precursor composition, particularly calcium availability. 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]. Depending on the chemical composition of the precursors, particularly their calcium content, these systems may involve the formation of different binding phases, including N-A-S-H, C-(A)-S-H, C-S-H, or hybrid assemblages. 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 [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. In this context, this paper offers an integrated perspective that seeks to link these approaches. It highlights the need to bridge the gap between them in order to better understand and advance the application of C&DW-derived geopolymers for radionuclide immobilization. The integration of these approaches may be achieved through different conditioning concepts, including sequential and combined strategies, which require further investigation to determine their suitability for effective radionuclide immobilization in C&DW-derived geopolymer systems.

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 suggest that C&DW-derived systems may benefit from this combination of sorption and subsequent matrix encapsulation mechanisms [50,52]. However, the extent to which precursor sorption properties contribute to radionuclide retention after geopolymerization remains to be fully established. 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 hydrate (C–A–H) and calcium aluminosilicate hydrate (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, as 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 for 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 different aluminosilicate-based binding structures, 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

Crushed C&DW represents 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]. Furthermore, the composition and physicochemical properties of crushed C&DW fundamentally govern its interactions with dissolved species. The presence of reactive mineral phases, surface functional groups, and developed porosity provides a variety of active sites for ion retention [14,17,18,36,44]. Consequently, most C&DW sorption studies focus on cationic species, reflecting the generally higher affinity of C&DW-derived materials toward positively charged species under alkaline conditions, while anionic species have been considerably less investigated due to their different retention behavior [13,44,103,104,105,106,107,108,109,110,111,112]. Reported target species commonly include Co²⁺, Ni²⁺, Sr²⁺, Cs⁺, Pb²⁺, Cd²⁺, Mn²⁺, etc., corresponding to elements with radioactive isotopes of interest for radionuclide immobilization studies [13]. Such cationic radionuclide species may be present in different waste streams, including activation and corrosion products from metallic components and cooling systems, fission products, and naturally occurring radionuclides associated with decay series. Additionally, since sorption behavior is governed by the chemical properties of the elements rather than their nuclear characteristics, stable isotopes (non-radioactive analogs) of relevant elements are frequently used in radionuclide immobilization studies due to practical and safety considerations [14,44,46].

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, etc., 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].

3.1. Literature Search and Overview of Research Trends

To assess the current state of research on C&DW-derived geopolymers, a literature search was conducted using the Scopus database, selected due to its extensive coverage of peer-reviewed publications in the fields of materials science, civil engineering, and environmental applications. The search was performed using the Advanced Search option, with the TITLE-ABS-KEY field applied for all queries. Publications published between 2016 and 2026 were considered (PUBYEAR > 2015). The search was conducted on 5 August 2026.

The literature search followed a stepwise approach, beginning with a broad search to capture the overall research landscape related to C&DW-derived geopolymer systems, followed by a refined search focusing on studies involving reactive C&DW-derived fractions, including powders, fines, precursors, and binder-related materials. To distinguish original research output from secondary literature, the number of retrieved records is reported both as the total number of publications and as the number obtained after excluding review articles (DOCTYPE(re)). The search equations and the corresponding numbers of retrieved records are summarized in

Table 1. Scopus search strategy used to identify studies on C&DW-derived geopolymer systems.

Search Step	Scopus Search Equation	Purpose	Number of Records (Total/Excluding Reviews)
1	TITLE-ABS-KEY((geopolymer* OR “alkali activated material*” OR AAM*) AND (“construction and demolition waste” OR CDW OR “C&DW” OR “construction waste” OR “demolition waste”)) AND PUBYEAR > 2015	Identification of the overall research landscape related to C&DW-based geopolymer and AAM systems	528/486
2	TITLE-ABS-KEY((geopolymer* OR “alkali activated material*” OR AAM*) AND (“construction and demolition waste” OR CDW OR “C&DW”) AND (powder OR fines OR precursor OR binder)) AND PUBYEAR > 2015	Refinement of the search results to identify studies involving C&DW-derived fractions as potential geopolymer precursors or binder components	233/215

.

The overall workflow used for the identification and qualitative assessment of C&DW-derived geopolymer studies is presented in Figure 1.

The initial search identified 528 publications representing the overall research landscape related to C&DW-based geopolymer and alkali-activated material (AAM) systems. Since this broad dataset also included studies where C&DW may have been considered as an aggregate or filler or mentioned only in the broader context of sustainable construction materials, an additional refinement step was applied. The refined search resulted in 233 publications containing terms associated with potentially reactive C&DW fractions, such as powders, fines, precursors, or binder components. After excluding review articles, 486 and 215 original research publications remained from the initial and refined searches, respectively. These publications represented the literature base considered for the subsequent qualitative analysis of C&DW-derived geopolymer precursor 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 analysis of the retrieved publications, supported by commonly used terminology in the field, indicates that recycled concrete-derived materials—most commonly reported as recycled concrete powder or fine fractions—represent the most extensively investigated C&DW source for geopolymer precursor applications. Their widespread availability and Ca-rich composition favor the formation of calcium-containing binding phases, including hybrid C-(A)-S-H- and (N,C)-A-S-H-type gels. Brick-derived waste is also frequently investigated due to its aluminosilicate-rich composition and enhanced reactivity following mechanical activation. Ceramic waste-based systems have been reported as well, although generally with lower intrinsic reactivity, which often results in their use as supplementary or partial precursors. In comparison, mixed C&DW systems remain less consistently investigated because of their heterogeneous composition and the resulting variability in material performance. Fine and dust fractions generated during C&DW processing represent a less explored category, despite their potentially beneficial characteristics, including increased specific surface area and enhanced dissolution kinetics.

The observed trend is based on qualitative assessment of publication focus and terminology usage rather than on quantitative bibliometric classification. Due to overlapping terminology and inconsistent descriptors used for C&DW-derived materials across the literature, direct numerical comparison among individual waste categories was not performed. Nevertheless, the identified trend indicates a clear research preference for well-defined, single-source precursors, whereas more complex or highly processed C&DW fractions remain comparatively underrepresented.

3.2. Role of C&DW Fractions in Geopolymer Systems

Based on the literature overview presented in Section 3.1, the following section discusses the role of different C&DW fractions in geopolymer systems, with emphasis on their composition, reactivity, and contribution to geopolymer network formation.

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 and geopolymerization processes 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 2. Main C&DW fractions and their role in geopolymer systems.

C&DW Fraction	Main Composition	Typical Geopolymer Role	Reactivity	Main Binding Products	References
Recycled concrete fines	Hydrated cement phases, calcite, silica-rich phases	Hybrid precursor/filler	Medium	C-(A)-S-H and N-(C)-A-S-H	[129,130,131]
Brick waste	Amorphous aluminosilicates	Primary precursor	Medium–high	N-A-S-H	[40,109,125,126,127]
Ceramic waste	Aluminosilicates, quartz, feldspars	Primary/supplementary precursor	Medium	N-A-S-H	[31,40,109,125,126,127]
Mixed C&DW	Mixed mineral phases	Variable precursor	Low–medium	Mixed gel systems	[23,53,121]
C&DW fine dust fraction	Fine amorphous and crystalline phases	Reactive precursor/filler	Variable	N-A-S-H and hybrid gels	[122,123]

.

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 N-A-S-H-type gel structures, 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]. These characteristics are particularly relevant for environmental applications requiring both structural stability and efficient retention of hazardous species, which is further discussed in the following section focusing on radionuclide immobilization using C&DW-derived geopolymer matrices.

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 interplay between these processes, together with precursor dissolution and gel formation, is schematically illustrated in Figure 2.

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 the metal ions (Ni(II), Pb(II), Cu(II), Cd(II), Cr(III/VI), and 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. Scopus-Based Quantitative Overview of C&DW-Derived Geopolymers for Immobilization of Radionuclides and Their Stable Isotope Analogs

To assess the current research landscape on C&DW-derived geopolymers for radionuclide and stable isotope analog immobilization, targeted literature searches were performed using the Scopus database on 5 August 2026. The Advanced Search option was applied using the TITLE-ABS-KEY field, without applying a publication year restriction; the retrieved records included publications indexed in Scopus from 2014 onward. A stepwise search strategy was adopted, initially focusing on direct evidence of radionuclide immobilization in C&DW-derived geopolymer systems and subsequently expanding the scope to include C&DW fractions, stable isotope analogs, and related contaminant retention studies. To distinguish original research output from secondary literature, review articles were excluded using the DOCTYPE(re) filter. Conference papers and records not directly related to experimentally investigated ion retention or immobilization in C&DW-derived geopolymer systems were subsequently excluded during manual screening. The final selection was based on the relevance of the investigated C&DW precursors, geopolymer matrices, and demonstrated retention or immobilization behavior of radionuclides or relevant stable isotope analogs. The applied search equations and the corresponding number of identified records are summarized in

Table 3. Scopus search strategy used to evaluate research on radionuclide and stable isotope analog immobilization in C&DW-derived geopolymer systems.

Search Step	Scopus Search Equation	Purpose	Number of Records (Retrieved/Excluding Reviews/Included After Screening)
1	TITLE-ABS-KEY((“construction and demolition waste” OR “C&DW” OR CDW OR “construction waste”) AND (geopolymer* OR “alkali-activated material*” OR AAM OR “alkali activated”) AND (radionuclide* OR “radioactive waste” OR cesium OR strontium OR cobalt OR nickel OR Cs OR Sr OR Co OR Ni) AND (immobilization OR immobilization OR solidification OR encapsulation))	Identification of studies on radionuclide immobilization in C&DW-derived geopolymers	3/1/0
2	TITLE-ABS-KEY((“construction and demolition waste” OR “C&DW” OR CDW OR “recycled concrete” OR “recycled concrete powder” OR “waste concrete” OR “concrete waste” OR “construction waste” OR “waste brick” OR “recycled brick” OR “brick waste” OR “waste masonry” OR “recycled masonry” OR “masonry waste”) AND (geopolymer* OR “alkali-activated material*” OR AAM OR “alkali activated”) AND (radionuclide* OR “radioactive waste” OR cesium OR strontium OR cobalt OR nickel OR Cs OR Sr OR Co OR Ni) AND (immobilization OR immobilization OR solidification OR encapsulation OR adsorption OR sorption))	Expansion to C&DW fractions and radionuclide analog studies	9/7/0
3	TITLE-ABS-KEY((“construction and demolition waste” OR “C&DW” OR CDW OR “recycled concrete” OR “recycled concrete powder” OR “waste concrete” OR “concrete waste” OR “construction waste” OR “waste brick” OR “recycled brick” OR “brick waste” OR “waste masonry” OR “recycled masonry” OR “masonry waste”) AND (geopolymer* OR “alkali-activated material*” OR AAM OR “alkali activated”) AND (“Co2+” OR “Ni2+” OR “Sr2+” OR “Cs+” OR “Pb2+” OR “Cd2+” OR “Mn2+” OR “Cu2+” OR “Zn2+” OR cobalt OR nickel OR cesium OR strontium OR lead OR cadmium OR manganese OR copper OR zinc) AND (immobilization OR immobilization OR solidification OR encapsulation OR adsorption OR sorption))	Inclusion of stable isotope analogs (Co, Ni, Sr, Cs) and related contaminant studies	18/16/2
4	TITLE-ABS-KEY((“construction and demolition waste” OR “C&DW” OR CDW OR “recycled concrete” OR “recycled concrete powder” OR “waste concrete” OR “concrete waste” OR “construction waste” OR “waste brick” OR “recycled brick” OR “brick waste” OR “waste masonry” OR “recycled masonry” OR “masonry waste”) AND (geopolymer* OR “alkali-activated material*” OR AAM OR “alkali activated”) AND (“Co2+” OR “Ni2+” OR “Sr2+” OR “Cs+” OR “Pb2+” OR “Cd2+” OR “Mn2+” OR “Cu2+” OR “Zn2+” OR cobalt OR nickel OR cesium OR strontium OR lead OR cadmium OR manganese OR copper OR zinc) AND (retention OR uptake OR incorporation OR “ion exchange” OR leaching OR migration OR adsorption OR sorption OR immobilization OR immobilization OR solidification OR encapsulation))	Comprehensive identification of retention and immobilization studies in C&DW-derived geopolymers	53/49/4

.

The results revealed a very limited number of publications directly addressing radionuclide-related applications in C&DW-derived geopolymer systems. The initial search targeting direct evidence of radionuclide immobilization identified only a small number of studies, confirming the scarcity of research specifically combining C&DW-derived geopolymers with radionuclides or radioactive waste-related applications. When the search scope was expanded to include C&DW fractions and stable isotope analogs relevant to radionuclide behavior (Cs, Sr, Co, and Ni), a limited number of additional studies were identified, mainly addressing analogous retention processes rather than direct radionuclide immobilization.

A broader search including relevant metal species and retention-related terms resulted in a higher number of publications, reflecting the larger body of knowledge on the incorporation, immobilization, adsorption, and release behavior of metal ions in geopolymer matrices. These studies were not considered as direct radionuclide immobilization investigations; however, they provide valuable information on the retention capacity, chemical interactions, and structural stability of C&DW-derived geopolymer systems.

In particular, such studies contribute to understanding mechanisms potentially relevant for radionuclide immobilization, including ion exchange, incorporation into aluminosilicate networks, physical encapsulation, and resistance to element release under different environmental conditions.

These observations highlight the need for further systematic investigation of radionuclide immobilization in C&DW-derived geopolymer systems, while utilizing the existing knowledge on stable isotope analogs and metal retention behavior to support the interpretation of immobilization mechanisms and long-term matrix performance.

4.3. 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 Ni, Co, Pb, Cd, Cr, Cu, and Zn, etc., through a combination of mechanisms including electrostatic interactions, physical entrapment, and incorporation into newly formed binding phases. 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].

The integration of C&DW sorption capacity with geopolymerization processes may be considered through different conditioning concepts, reflecting possible pathways for combining radionuclide uptake and immobilization within the same treatment framework. The first involves the preliminary use of raw C&DW as a sorbent for radionuclide capture from liquid waste, followed by the incorporation of the radionuclide-loaded C&DW together with additional C&DW-derived material during geopolymerization to form a consolidated immobilization matrix [159,161]. The second represents an integrated approach in which C&DW-derived precursor material is used directly for geopolymer formation while potentially contributing to radionuclide retention through precursor-derived and newly formed phases, allowing sorption and encapsulation processes to contribute within the same system [169,170,171,172]. The feasibility, optimization, and long-term immobilization performance of such conditioning approaches remain subjects for further investigation (Figure 3).

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 [166,172]. Furthermore, the formation of hydration products such as C–S–H and ettringite phases has been shown to contribute to the immobilization of metal species through sorption, substitution, and incorporation mechanisms within the solid matrix [173]. 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 may provide relevant insights into the retention of radionuclides such as Co, Cs, and Sr, due to their comparable ionic characteristics and their ability to be incorporated into aluminosilicate frameworks [47]. However, it is important to distinguish between immobilization mechanisms inferred from chemical similarity and experimentally demonstrated radionuclide retention. To date, only a limited number of studies have directly investigated radionuclides or radionuclide-relevant stable isotope analogs in C&DW-derived geopolymer systems. Due to the scarcity of studies involving radioactive species, investigations using chemically relevant stable metal ions, such as Pb and Cu, were also considered, as they provide experimental evidence on ion retention and immobilization behavior within geopolymer matrices. Recent experimental studies on C&DW-derived alkali-activated systems have demonstrated the retention of metal ions through sorption-related mechanisms, including adsorption and ion exchange, as well as incorporation into newly formed binding phases and stabilization within aluminosilicate matrices [170,171,174,175]. These studies are summarized in

Table 4. Reported studies on immobilization and retention of radionuclide-relevant metal ions in C&DW-derived geopolymer systems.

Reference	C&DW Precursor	Investigated Radionuclide-Relevant ion/Analog	Geopolymer Formulation	Main Immobilization Findings
[170]	Waste brick	Cu(II), Pb(II)	Porous geopolymer foams synthesized using waste brick, phosphogypsum, and silica fume under alkaline activation	Waste brick-derived geopolymer foams showed adsorption capacity toward Cu(II) and Pb(II) ions from aqueous solutions. Metal uptake followed adsorption models, demonstrating retention of dissolved metal species within the geopolymer structure.
[171]	Red-clay brick waste (RCBW)	Pb (from lead-bearing sludge)	Alkali-activated/sintered porous materials based on waste glass–red-clay brick waste systems incorporating lead-bearing sludge	Leaching tests confirmed effective Pb stabilization within the alkali-activated/sintered matrix. The process reduced Pb release and demonstrated the potential of brick waste-containing matrices for hazardous metal immobilization.
[174]	Waste concrete powder (CoW)	Pb (from lead-bearing sludge)	Alkali-activated concrete waste/lead-bearing sludge composites prepared using NaOH and Na2SiO3 activation	The alkali-activated waste concrete composite exhibited high Pb immobilization efficiency and reduced Pb leaching. Pb retention was attributed to incorporation within reaction products and formation of stable binding phases.
[175]	Uncalcined waste concrete powder (WCP)	Pb(II) (simulated Pb-contaminated soil)	WCP–GGBS alkali-activated solidification/stabilization system	The system effectively reduced Pb migration through formation of C-(A)-S-H and C-S-H gels. Pb immobilization was associated with encapsulation, ion exchange and adsorption mechanisms within the reaction products.

, providing an overview of the investigated C&DW precursors, geopolymer formulation conditions, target ion/analog species, and the main experimentally observed immobilization or retention outcomes, including the reported mechanisms responsible for ion binding within the geopolymer matrices.

The studies summarized in Table 4 demonstrate that, although direct investigations remain scarce, C&DW-derived geopolymer systems have shown potential for the retention of radionuclide-relevant stable isotope analogs through a combination of ion exchange, incorporation into aluminosilicate structures, precipitation of secondary phases, and physical encapsulation. The investigated systems represent different conditioning approaches, including direct incorporation of metal-containing species during alkali activation of C&DW-derived precursors [170,171,174] and stabilization of pre-existing contaminated waste components within alkali-activated concrete waste matrices [175]. Nevertheless, the available studies differ considerably in terms of precursor composition, investigated ion species or analogs, waste loading, and leaching methodologies, limiting direct comparison of immobilization performance among different systems.

These findings suggest that monovalent and divalent ions relevant to radionuclide immobilization may participate in ion exchange processes or occupy charge-balancing sites within the geopolymer structure, contributing to their retention. However, the potential application of C&DW-derived geopolymer systems as liquid RW immobilization matrices requires evaluation beyond retention mechanisms alone. Although sorption capacity and mechanical strength are important indicators of material performance, they are insufficient by themselves to qualify a material as a suitable matrix for RW immobilization. A reliable immobilization matrix should additionally demonstrate low radionuclide release under relevant leaching conditions, chemical and radiological stability, low permeability, dimensional stability, compatibility with the incorporated waste, adequate waste loading capacity, and reproducible performance. Therefore, future evaluations of C&DW-derived geopolymer matrices should extend beyond laboratory-scale retention studies and include long-term durability assessments under conditions representative of storage and disposal environments. As these materials are still at an early research stage, the development of standardized qualification criteria and assessment approaches will be essential for evaluating their potential applicability in RW management.

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]. 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 geopolymers. As a result, the potential relationships between the sorption behavior of raw precursors and the 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 [176], 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 and alkali-activated systems, the heterogeneous mineral composition, which may vary depending on waste origin, composition, and processing history, 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. Furthermore, the influence of C&DW incorporation level on geopolymerization efficiency, matrix development, and potential radionuclide retention performance remains insufficiently understood, as variations in precursor content may alter the balance among reactive aluminosilicate phases, calcium-containing components, and the resulting binding environment. In addition to performance-related aspects, the environmental sustainability of C&DW-derived geopolymer systems requires comprehensive evaluation, as the overall environmental benefits depend not only on waste valorization, but also on factors such as activator production, processing requirements, and life-cycle impacts [53,167,177].

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 [172]. 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. A quantitative analysis of the available literature further confirms this gap. Despite the 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 larger number of works focus on the binding behavior of stable metal ions or non-radioactive analogs. As mentioned, these studies provide valuable data on the sorption capacity of C&DW-derived materials and contribute to the assessment of their retention potential. However, leaching results require careful interpretation, as the reported values depend largely on the applied testing methodology and are not directly comparable when different protocols are used. For radionuclide systems, assessment approaches referenced by the IAEA, such as the HESPE leaching method for evaluating immobilized RW forms, provide a basis for comparing the immobilization performance of different materials [15,178]. Although immobilization behavior at substantially higher radionuclide concentrations may differ due to increased loading and its potential influence on retention mechanisms and matrix properties, such conditions are generally not representative of low- and intermediate-level liquid RW systems. Prior to treatment, these effluents typically exhibit radionuclide molar concentrations in the sub-picomolar to micromolar range (10⁻¹³ to 10⁻⁶ M) [179,180]. Selective sorption is therefore required to capture these trace radiocontaminants from complex aqueous streams before final immobilization. Notably, the concentrations used in lab-scale sorption studies involving target ions or stable analogs frequently encompass or exceed this practical range. Such experimental setups enable a conservative assessment of material performance over a broader chemical spectrum, while providing valuable insights into maximum sorption capacity. Through this process, the radionuclides are concentrated onto solid matrices, which are subsequently encapsulated into geopolymers for long-term storage. However, the extent to which precursor sorption properties contribute to radionuclide retention after the destructive dissolution and reconstitution of the precursor during geopolymerization remains to be fully established.

The lack of systematic translation of these findings to radionuclide systems represents a 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 5. Key knowledge gaps and future research directions regarding C&DW-based geopolymer systems for radionuclide immobilization.

Aspect	Identified Gap	Implications/Future Needs
Immobilization in C&DW-derived geopolymers	Despite existing studies on radionuclide sorption by raw C&DW and on the characteristics of C&DW-derived geopolymers, direct evaluation of radionuclide immobilization in these systems remains limited	Direct immobilization studies in C&DW-derived geopolymers, supported by prior sorption knowledge to interpret retention mechanisms and predict performance, together with the development of standardized approaches enabling comparison between precursor sorption behavior and radionuclide retention in final geopolymer matrices.
Sorption vs. encapsulation	Strong evidence exists for radionuclide sorption on raw C&DW materials [14,36,41,44,105,106,107,108,109,110,111,112,113,114], but direct correlations between precursor sorption behavior and radionuclide retention in corresponding geopolymer systems remain limited. Specifically, the influence of sorption processes occurring prior to geopolymerization, as well as the simultaneous contribution of precursor-derived sorption sites and newly formed geopolymer phases to radionuclide retention, remains insufficiently understood	Studies directly linking precursor sorption properties with radionuclide retention performance of final geopolymer matrices are needed, including evaluation of whether retained ions are incorporated into the new 3D gel network or remain associated with residual precursor phases. Comparative assessment of sequential sorption–geopolymerization and integrated sorption–geopolymer systems is required to determine the most suitable strategies for radionuclide immobilization.
Mechanisms of immobilization	Insufficient quantification of contributions from encapsulation, ion exchange, and chemical bonding, especially in multi-phase systems	Advanced spectroscopic and microstructural studies to resolve radionuclide binding environments.
Multi-phase complexity (C&DW systems)	Coexistence of N-A-S-H and C-(A)-S-H gels introduces uncertainty in radionuclide partitioning and retention behavior	Systematic studies linking phase assemblage with radionuclide speciation and retention.
Structure–performance relationships	Limited understanding of how precursor composition, geopolymerization-induced structural changes, and phase evolution influence final immobilization efficiency, including the relationship between microstructure, pore characteristics, transport properties, and leaching behavior	Development of predictive models for rational material design integrating gel composition, pore structure, diffusion/transport parameters, and radionuclide retention performance.
Waste-derived solid fractions loading effects	Systematic evaluation of the effect of increasing incorporation levels of C&DW fractions in C&DW-derived geopolymer systems is largely lacking. Most studies investigate immobilization under limited waste loading conditions, leaving the influence of higher macro-scale C&DW incorporation levels on geopolymerization processes, matrix formation, and long-term performance insufficiently understood. Furthermore, a clear distinction is often omitted between the high mass/volume loading of the solid feedstock and the trace chemical concentrations of the immobilized radionuclides	Investigation of the influence of increasing C&DW fraction loading (wt.% or vol.%) on geopolymerization processes (precursor dissolution, gel formation), as well as on mechanical strength, porosity, dimensional stability (drying shrinkage, cracking), setting kinetics, heat evolution, and leaching behavior, in order to define operational limits and ensure matrix integrity under application-relevant conditions.
Feedstock variability	High heterogeneity of C&DW affects precursor reactivity, phase evolution, and reproducibility	Development of standardized precursor classification and characterization protocols, including chemical, mineralogical, textural, and thermal analyses, to enable reliable comparison between different C&DW-derived systems.
Multi-ion systems	Most studies focus on single-ion systems (Cs+, Sr2+, Co2+), while real RW contains complex mixtures	Investigation of competitive interactions and multi-component systems.
Radionuclide scope	Limited data for environmentally relevant radionuclides (e.g., 137Cs, 90Sr, 60Co, 63Ni) under realistic conditions	Expanded studies with representative waste compositions.
Cross-waste insights	Limited integration of findings from other waste systems (e.g., mining residues)	Comparative studies to identify transferable immobilization mechanisms (often similar across waste systems), currently underutilized for predicting system behavior.
Methodological limitations	Lack of standardized assessment protocols and qualification criteria specifically developed for evaluating radionuclide immobilization in C&DW-derived geopolymer systems limits the comparability of reported results. Differences in experimental design, testing procedures, and performance evaluation criteria contribute to variability among studies	Development of validated and harmonized evaluation frameworks, including consistent performance criteria and complementary characterization approaches, to enable reliable comparison of immobilization efficiency across different C&DW-derived geopolymer systems.
Mechanical and chemical durability	Long-term integrity under environmental exposure is not fully verified for recycled material-based systems [11,146]	Coupled mechanical–chemical durability assessment under realistic conditions.
Long-term performance	Limited understanding of stability under repository-relevant conditions (carbonation, wet–dry cycles, radiation, groundwater interaction) [146]	Long-term experiments and predictive modelling for multi-decadal to geological timescales
Environmental sustainability assessment	Limited comprehensive evaluation of the environmental performance of C&DW-derived geopolymer systems, considering the combined effects of waste valorization, activator production, processing requirements, and curing conditions	Life-cycle assessment and comparative studies with conventional immobilization matrices are needed to quantify environmental benefits while considering functional performance, including radionuclide retention, durability, and leaching behavior.

.

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 of 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 among precursor composition, sorption behavior, geopolymer structure, and radionuclide retention represents a key research priority.

The current state of research in this emerging field is characterized by a limited number of studies directly addressing radionuclide ion immobilization in C&DW-derived geopolymer systems. The available literature remains relatively scarce and fragmented, which highlights the need for further systematic investigations to establish a more comprehensive understanding of the performance of these materials under different conditions.

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. In particular, systematic studies that link precursor variability, geopolymerization processes, and resulting structural features with radionuclide retention mechanisms are still lacking and should be addressed in future research. Moreover, the integration of advanced characterization techniques with experimental validation and predictive modelling will be essential for improving the understanding of immobilization mechanisms and ensuring the reliability of these materials under realistic disposal conditions. Addressing these research needs is crucial for bridging the gap between laboratory-scale investigations and practical applications, ultimately enabling the transition of C&DW-derived geopolymers from promising materials to robust and dependable for RW immobilization.