Fly Ash-Based Structural Energy Storage for Sustainable and Resilient Buildings: Materials, Durability and Life-Cycle Perspectives

Abstract

Coal fly ash has emerged as a promising resource for developing multifunctional construction materials with integrated energy-storage capabilities. This structured narrative review examines fly ash-containing cementitious and geopolymer systems, fly ash-derived electroactive materials, and cenosphere-supported phase-change composites, with particular attention to their storage mechanisms, mechanical performance, durability, and potential building applications. By distinguishing electrode-level measurements from assembled-device performance and material-scale evidence from building-level functionality, the review identifies opportunities and limitations in integrating electrochemical and thermal storage into construction materials. Reported studies demonstrate laboratory-scale charge-storage functionality and measurable thermal-regulation effects, whereas long-term structural reliability, functional retention, safety, and building-scale operation remain insufficiently validated. An illustrative fixed-input screening compares a fly-ash-containing case with a no-fly-ash reference under an equal-mass cement-substitution assumption. Under the assigned material proxy coefficients, replacing 16 kg of OPC with 16 kg of fly ash reduces the A0 material-carbon proxy by 14.528 kg CO₂-eq (23.38%); this conditional result does not establish equivalent strength, storage service, or service life. The review therefore identifies feedstock characterization, coupled mechanical-storage durability, device packaging, and service-specific assessment as priorities for translating fly ash-based storage concepts from materials to building components.

1. Introduction

Building energy storage is shifting toward material and component integration as photovoltaics, electrified heating, and climate disruptions increase demands for buffering, thermal regulation and resilience [1,2,3,4,5,6]. Embodied carbon is also an important design constraint [7,8,9], motivating safe, durable, and scalable storage compatible with building structures.

Thermal energy storage (TES) in walls, roofs, floors, and façades can attenuate temperature fluctuations and shift heating and cooling loads [10,11,12]. Phase-change materials (PCMs) provide latent-heat storage, while conductive cementitious composites, geopolymers, and structural supercapacitors offer electrochemical routes. Both must meet mechanical, fire-safety, maintenance, scalability, and life-cycle requirements for building integration [1,2,3,4,10,11,12].

Supercapacitors offer high power and cycling stability, but active-material costs and manufacturing complicate construction-scale deployment [2,3,4]. Fly ash can reduce clinker demand while contributing aluminosilicate glass, residual carbon, iron-bearing phases, spherical particles, and cenospheres [13,14,15,16]. Depending on activation, conductive additives and PCM integration, these constituents influence pore transport, charge storage, and thermal buffering [10,11,12,17,18,19,20].

Fly ash–cement zinc-ion devices and conductive-polymer-coated fly ash geopolymers demonstrate laboratory storage functionality [19,20]. Related cementitious, electrode and TES studies use different feedstocks and test scales [1,2,3,4,21,22,23,24]; their results cannot automatically establish building performance.

Existing reviews [1,2,3,4,25] already examine cementitious electrical storage, PCM/TES, mechanical performance, durability, and aspects of implementation or environmental assessment. Table 1 compares their declared scopes with the present review without treating limited emphasis as absence. Here the distinctive organizing principle is how fly ash is retained, converted, extracted, or removed across electrochemical and thermal pathways, together with the strength of evidence linking each pathway to a building function.

Table 1. Scope comparison with related reviews and the present fly ash-focused synthesis.

Review	Materials and Fly Ash Role	Thermal Storage	Structural/Durability Evidence	Environmental Assessment/Emphasis
Dong et al. [1]	Broad cement-based harvesting/storage; fly ash may appear within the cementitious scope rather than as a single organizing variable.	Includes thermal-storage concrete/PCMs.	Addresses mechanical–functional balance, environmental durability and scalability.	Discusses sustainable infrastructure; not a fly-ash pathway-specific material-inventory comparison.
Barbhuiya et al. [2]	Cementitious chemical, electrical and thermal storage; fly ash is part of a broader material family.	Explicitly reviews PCMs and latent-heat approaches.	Covers performance, applications, challenges, and feasibility.	Discusses environmental and economic aspects in broad cementitious systems.
Feng et al. [3]	Cement-based structural supercapacitor electrodes/electrolytes; no exclusive fly ash focus.	Electrochemical rather than a systematic cenosphere/PCM comparison.	Examines electrochemical–mechanical design and application limitations.	Application prospects discussed; pathway-specific ash allocation is not its main objective.
Yousaf et al. [4]	Construction-compatible materials including OPC, geopolymers, MPC, bricks, wood, and polymers.	Focuses on structural supercapacitors rather than quantitative PCM storage.	Highlights mechanical performance, durability, pore design and fabrication.	Identifies implementation challenges without a matched-service FA substitution model.
Lyu et al. [25]	Cementitious structural supercapacitors across raw materials and fabrication routes.	Focus is electrochemical, not a quantitative cenosphere-PCM review.	Provides a microstructure–performance–durability framework and coupled-load discussion.	Highlights scaling/characterization needs; not centered on FA retained/extracted/template roles.
Present review	FA as binder/electrolyte, retained additive, extracted feedstock, removable template, and cenosphere carrier.	Compares PCM loading, latent heat, leakage, cycling and building-temperature evidence.	Separates binder, electrode, assembled device and structural/thermal service evidence.	Tests evidence boundaries and presents an illustrative OPC→FA input substitution, without claiming equivalent product service.

Figure 1 outlines these pathways. Section 2, Section 3 and Section 4 cover materials and storage mechanisms; Section 5, Section 6, Section 7 and Section 8 assess durability, environmental and economic implications, building deployment and research needs.

Figure 1. Author-composed schematic and conceptual renderings informed by [17,18,19,20,26,27]. Selected material textures, architectural views and illustrative elements were prepared with generative-AI assistance and then labelled, checked and assembled by the authors; they are not experimental images and have no measured scale. Lower panels show (a) the GeoAsh laboratory concept [19], (b) a non-fly-ash carbon-cement comparator [26], and (c) a cenosphere/PCM textile-reinforced concrete concept [27]. No third-party figure panel was reproduced.

Review Design and Literature Identification

Initial searches were conducted around April 2026 in Web of Science Core Collection and Google Scholar, followed by targeted literature updates and source checks through October 2026 using Crossref, OpenAlex, publisher pages and institutional repositories. No fixed earliest publication year was applied. The principal combinations joined (fly ash OR coal fly ash OR cenosphere) with (structural supercapacitor OR electrochemical energy storage OR geopolymer), (phase-change material OR thermal energy storage), (durability OR cracking OR moisture), and (life-cycle assessment OR embodied carbon OR building integration). The search supported a structured narrative review rather than an exhaustive systematic-review claim [28].

Titles and abstracts were screened for explicit fly ash involvement, a defined material role, a measured storage or supporting structural function, and sufficient test information for interpretation. Available primary articles and supplementary files of the reviewed studies were then checked for fly ash content or characteristics, active phases, cell and electrolyte geometry, voltage and current conditions, normalization basis, strength age, cycling protocol, PCM loading, thermal conditions, and evidence scale. Non-fly-ash cementitious and carbon-cement systems were retained only as labelled comparators.

Selection considered composition, test conditions, and available primary or supplementary evidence. Electrode, assembled-device, mechanical, and thermal-composite results were separated; incompatible test geometries or denominators were not pooled [29,30,31,32,33,34,35,36].

Unreported operating conditions were not inferred. Experimental measurements, proposed mechanisms, models, and building demonstrations were distinguished, particularly for durability and functional retention.

In Table 2, Table 3 and Table 4, NV means that a parameter could not be independently verified from the source material accessible for this review. NR is reserved for a parameter confirmed as not reported in inspected full text or supporting information of the reviewed studies. Missing operating conditions and normalization bases were not inferred or converted.

This review focuses on the retained or transformed roles of fly ash across electrical and thermal storage and the evidence needed for building-level claims, building upon cementitious-storage reviews [1,2,3,4,25].

2. Fly Ash as a Functional Precursor for Energy Storage

2.1. Chemical Composition and Phase Characteristics

Fly ash composition varies with coal source, combustion, and processing [13,37,38]. For energy-storage design, reactive aluminosilicate glass, residual carbon, iron-bearing phases, fineness, cenospheres, and accessible surface area can matter more than conventional calcium-based classification [13,38].

In cement, fly ash contributes pozzolanic reactions and microfilling; alkali activation forms geopolymer networks affecting pores and ionic transport [39,40,41]. The same ash can supply electroactive derivatives [19,20,22]. Its contribution therefore depends on the selected processing and device architecture.

Residual carbon can impair air entrainment and admixture demand in cement, yet contribute to conductive networks under controlled conditions [34]. Its benefit depends on distribution and connectivity; iron phases may also contribute composition-dependent redox or polarization responses.

Fineness, glass content, and activation affect binder pores [14,15,16], but pore refinement alone does not quantify device ion accessibility. Causal storage relationships require matched materials, curing, moisture, and electrochemical geometry.

The fly ash/slag study [42] describes binder pore regulation, not device retention; effects on durability and electrolyte access require device-specific testing.

Precursor selection should jointly consider phase composition, carbon, particle size, and accessible porosity, along with electrolyte and conductive-network continuity [13,14,15,16,17,18,19,20].

2.2. Particle Morphology and Cenosphere Characteristics

As illustrated in Figure 2, packing, connected pores, wettability, and added conductive phases influence transport. Spherical particles do not independently prove electroactive area or percolation; the dominant ions depend on the electrolyte.

Figure 2. Original schematic by the authors, informed by studies of fly ash pore regulation and cenosphere morphology [14,17,18,19]. (a) Conceptual comparison of particle packing without and with fly ash; (b) connected pore/electrolyte pathways and an added conductive network among fly ash particles; (c) particle–binder interfaces with conductive additives and electrolyte; (d) cenosphere shell structure and alternative uses for PCM or electrolyte confinement. Particle packing, pores, and interfaces are conceptual; conductive networks require added phases. PCM-filled shells and electrolyte-containing pores illustrate alternative uses. Ionic species and thermal response depend on the selected matrix, electrolyte and PCM.

Cenospheres introduce additional structural functionality. These hollow aluminosilicate microspheres have low density, internal voids, and ceramic-like thermal stability, making them attractive for lightweight multifunctional composites [17,18]. In structural energy-storage systems, their primary role is not to provide high intrinsic electrochemical activity, but to regulate the physical framework in which electrochemical and thermal-storage processes occur. Their shell-cavity structures add multiscale pore networks, internal free volume, thermal buffering, and interface-rich configurations whose transport or thermal contribution depends on how the cavities and interfaces are used. Their properties also vary substantially with fly ash source; differences in particle-size distribution, wall thickness, chemical composition, and thermal stability provide guidance for selecting cenospheres for applications such as lightweight structural electrodes and phase-change material carriers [18].

Carrier selection must account for recovery, shell integrity, and controlled mixing, curing and filling; morphology alone does not guarantee lower cost or improved transport.

2.3. Activation Pathways

Fly ash is activated either into a matrix or into electroactive derivatives. Alkali activation produces geopolymer networks with tunable pores for electrolyte or structural functions [1,21,43]; waterglass and NaOH activation produce different gel structures and transport properties [39]. Cement–fly ash mixtures retain conventional processing compatibility while modifying hydration and pores [20].

Other routes retain ash, extract feedstock, or remove ash templates to produce active phases [22,24,35,44,45,46,47]. Alloy-based high-temperature TES [23] and cenosphere-supported organic PCM [48,49,50] have distinct service temperatures and cannot alone establish a combined building component.

These routes span mineral matrices, geopolymer electrolytes, derived electrodes, and thermal composites. Each must be evaluated for its actual fly ash role and measured service.

2.4. Functional Implications for Structural Energy Storage

Fly ash’s engineering value depends on demonstrated material and device function; construction compatibility, durability, and life-cycle benefit remain formulation- and service-dependent.

3. Fly Ash-Based Energy-Storage Materials and Device Architectures

Figure 3 classifies four routes: fly ash–cement composites, geopolymer devices, fly ash-derived electrodes, and cenosphere/PCM thermal composites. Fly ash functions respectively as mineral matrix, activated framework, electroactive feedstock, or PCM carrier.

Figure 3. Original classification schematic by the authors, informed by [19,20,22,27,31,35,44,45,46,47,48,49,50]. (a) Conducting-network illustration, not a complete cell; separated electrodes and electrolyte/separator are required [20,31]. (b) Fly ash geopolymer example [19]; the montmorillonite system [21] is a non-fly-ash comparator. (c) Alternative retained-ash and derived-electrode routes [22,35,44,45,46,47], not sequential synthesis or load-bearing demonstrations. (d) Cenosphere/PCM concepts [17,27,48,49,50] require measured loading, confinement and cycling; the higher-temperature modified-fly-ash/alloy route [23] is a separate TES class. Rendered material textures are not experimental microscopy.

3.1. Fly Ash–Cement Structural Energy-Storage Composites

Fly ash–cement storage has been demonstrated at laboratory scale [20], where fly ash forms part of the ionic matrix and AC/Zn supplies the electrodes. In PEDOT-coated GeoAsh [19], the ash geopolymer is distinct from the active polymer. Neither shows intrinsic storage from untreated fly ash alone.

These composites require electrochemical performance alongside mechanical integrity and durability [20].

Ash effects on packing and pores vary with dosage, curing, and conductive additives; binder-strength improvements alone cannot establish storage retention. Electrode, electrolyte, and matrix contributions must be distinguished.

Activated structural carbon fibres can enhance electrochemical area [51], but benefits and tensile integrity must be verified separately in fly ash–cement devices.

Table 2 does not justify ranking fly ash–cement as the most practical option: moisture control, contacts, sealing, loading, and maintenance require component-scale qualification.

The non-fly-ash cementitious comparator [30] uses 30 mm cement/sand cubes cured for 28 d, approximately 20% porosity and 4.5 MPa matrix compressive strength. Its AC electrodes are approximately 2 mm thick, but their areal mass loading was not directly measured. CV uses −0.5 to 0.5 V at 0.02–0.10 V s⁻¹; the EIS volumetric normalization uses liquid held in the pores, not packaged-device volume. These distinctions preclude importing its numerical electrode or pore-volume results into a structural-component ranking.

Table 2 distinguishes fly ash systems from non-fly-ash comparators and preserves each study’s electrode, device and strength reporting basis; entries are not common-condition rankings.

Table 2 reporting note: capacitance (F) and capacity (Ah) are different quantities. For a constant-current discharge, C = IΔt/ΔV and Q = IΔt; dividing by mass, area or volume requires the same explicitly stated denominator. Capacity reported as mAh g⁻¹ is not relabelled F g⁻¹. Device energy depends on the actual voltage range and discharge curve. No conversion between electrode and device energy, or between mass, area, and volume, was made without a verified denominator and geometry. Cycling retention is not coulombic efficiency, and compressive strength at an early curing age is not a structural design value.

Table 2. Fly ash-containing structural storage systems and separately labelled mineral-matrix comparators.

System and Reference	FA Role or Comparator	Cell/Electrolyte	Capacitance or Capacity	Test Rate/Voltage	Energy and Basis	Compressive Strength	Cycling Result	Evidence and Comparability
Fly ash–cement zinc-ion hybrid structural capacitor [20]	FA mineral admixture; porous ionic matrix	Zn-ion hybrid device; AC/Zn; 2 M ZnSO4	Specific capacity 102.4 mAh g−1; normalized mass NV	0.1 A g−1; voltage window NV	73 Wh kg−1; normalized mass NV	5.5 MPa after 1 d	92.9% after 2000 cycles; cycling current and voltage window NV	FA dosage: 50 wt%; ionic conductivity: 25.6 mS cm−1. Energy normalization denominator and complete cycling protocol: NV.
PEDOT-coated fly ash geopolymer structural supercapacitor [19]	FA geopolymer matrix; PEDOT active coating	Two-electrode gel device; PVA/1 M H2SO4; PP separator	Areal capacitance 926 mF cm−2	GCD: 1 mA cm−2; device window NV	129 μWh cm−2; active/device footprint NV	~38 MPa; separate 10 mm cubes; 60 °C/72 h precursor cure	85.8% after 1500 GCD cycles at 5 mA cm−2	Electrochemical specimen 10 × 5 × 2.5 mm; compression on separate cubes.
Fly ash–cement composite structural supercapacitor [31]	FA partial cement replacement; CB/CF conductors	NV cell count; 1 M KCl solution	831 mF cm−2 (FA40 formulation); area basis NV	NV rate and voltage window	NV; mass/area basis NV	15 MPa; age NV	NV; cycling conditions not established	FA40; CB:CF = 5:4; not the Section 6 inventory.
Fly ash-slag geopolymer electrolyte asymmetric device [32]	FA/slag electrolyte; 1.0 wt% PEO; separate Cu–Co/rGO/Ni-foam electrodes	Asymmetric device: Cu–Co oxide/rGO/Ni-foam cathode; rGO/Ni-foam anode; FA/slag electrolyte separator	Device: 113.59 mF cm−2; electrode: 1003.14 mF cm−2 separately	Device: 0.5 mA cm−2; voltage NV; electrode: 1 mA cm−2	Device: 40.39 μWh cm−2; area denominator NV	NV	90.3% after 2000 GCD cycles; cycling current/window NV	Electrolyte conductivity 28.7 versus 13.7 mS cm−1.
3D-printed cement-based structural supercapacitor [52]	Non-FA comparator: OPC/sand/polymer mortar.	NV cell count; PVA–KOH modified cement mortar	46.95 mF cm−2; denominator NV	NV rate and voltage window	NV; mass/area basis NV	30.68 MPa; age NV	NV; cycling current/window NV	Also reports 13.37 MPa flexural strength.
Porous cement-based supercapacitor (activated carbon electrode) [30]	Non-FA comparator: OPC/sand porous matrix; no fly ash in formulation.	Two opposing AC/PTFE electrodes on nickel foam; NaOH 1.5–3.5 M or KOH 1–2 M; epoxy-sealed	EIS/CV response; no single common-condition capacitance extracted for ranking	CV: −0.5 to 0.5 V; 0.02–0.10 V s−1	Not compared: pore-liquid volume differs from complete device; gravimetric basis not harmonized	4.5 MPa; 28 d matrix cubes (30 mm)	No quantified long-cycle retention established for comparison	Contact area 9 cm2; AC loading unmeasured.
Redox-active cement-based asymmetric supercapacitor [53]	Comparator; no FA in listed matrix; formulation details NV	Asymmetric device; PAA/OPC/KOH/K3Fe(CN)6 electrolyte	166 mF cm−2; 1 cm2 device	1.5 V device; capacitance test current NV	51.87 μWh cm−2 at 7.5 mW cm−2; 1 cm2 device	NV	NV	100 cm2 device: 23.10 μWh cm−2.
Dual-carbon electrode cement-based structural supercapacitor [54]	Comparator; no FA in listed matrix; formulation details NV	Asymmetric device; PPy nanotube/CNT electrodes; PAM–OPC–Na2SO4	1180 mF cm−2; denominator NV	0.2 mA cm−2; voltage window NV	2.1 Wh kg−1; mass/area basis NV	41.3 MPa; age NV	97.8% (2000 cycles); cycling current/window NV	Energy mass basis NV.
Graphene/MPC separator [55]	Non-FA: magnesium phosphate cement	NV cell count; —	40.92 F g−1; M/P = 3; 28 d; mass denominator NV	NV rate and voltage window	NV; mass/area basis NV	24.59 MPa at 28 d; M/P = 3	NV; cycling current/window NV	1 d maximum 46.38 F g−1 (different curing age).
Porous CSA electrolyte [56]	Non-FA: CSA cement	CSA matrix; electrode/electrolyte details NV	540.44 mF cm−2; denominator NV	1 mA cm−2; voltage window NV	NV; mass/area basis NV	NV	92.43% (1000 cycles); cycling current/window NV	Electrolyte details NV.
Salt-modified OPC electrolyte [57]	Non-FA: OPC with sodium salts	NV cell count; Solid cement–Na2SO4 electrolyte	35.18 mF cm−2, reported maximum; area basis NV	NV rate and voltage window	NV; mass/area basis NV	32.67 MPa; age NV	NV	Maximum-value test conditions NV.
Polymer/OPC electrolyte [58]	Non-FA: PAA/OPC/LiOTf	NV cell count; PAA–cement–LiOTf polymer electrolyte	NV	4 V device; current for capacitance NV	0.65 mWh cm−2 at 0.58 mW cm−2	23.65 MPa; age NV	NV	Area/mass denominator details NV.
Steel slag/GGBS electrolyte [59]	Non-FA: steel slag/GGBS	NV cell count; Alkali-activated SS–GGBS electrolyte	131.16 mF cm−2; denominator NV	NV rate and voltage window	38.2 μWh cm−2; mass/area basis NV	NV	NV	Strength and cycling conditions NV.
Metakaolin geopolymer electrolyte [60]	Non-FA: metakaolin	NV cell count; Geopolymer solid electrolyte	603.93 mF cm−2; denominator NV	NV rate and voltage window	NV; mass/area basis NV	37.22 MPa at 7 d	NV	Cycling details NV.

Table 2. Fly ash-containing structural storage systems and separately labelled mineral-matrix comparators.

System and Reference	FA Role or Comparator	Cell/Electrolyte	Capacitance or Capacity	Test Rate/Voltage	Energy and Basis	Compressive Strength	Cycling Result	Evidence and Comparability
Fly ash–cement zinc-ion hybrid structural capacitor [20]	FA mineral admixture; porous ionic matrix	Zn-ion hybrid device; AC/Zn; 2 M ZnSO4	Specific capacity 102.4 mAh g−1; normalized mass NV	0.1 A g−1; voltage window NV	73 Wh kg−1; normalized mass NV	5.5 MPa after 1 d	92.9% after 2000 cycles; cycling current and voltage window NV	FA dosage: 50 wt%; ionic conductivity: 25.6 mS cm−1. Energy normalization denominator and complete cycling protocol: NV.
PEDOT-coated fly ash geopolymer structural supercapacitor [19]	FA geopolymer matrix; PEDOT active coating	Two-electrode gel device; PVA/1 M H2SO4; PP separator	Areal capacitance 926 mF cm−2	GCD: 1 mA cm−2; device window NV	129 μWh cm−2; active/device footprint NV	~38 MPa; separate 10 mm cubes; 60 °C/72 h precursor cure	85.8% after 1500 GCD cycles at 5 mA cm−2	Electrochemical specimen 10 × 5 × 2.5 mm; compression on separate cubes.
Fly ash–cement composite structural supercapacitor [31]	FA partial cement replacement; CB/CF conductors	NV cell count; 1 M KCl solution	831 mF cm−2 (FA40 formulation); area basis NV	NV rate and voltage window	NV; mass/area basis NV	15 MPa; age NV	NV; cycling conditions not established	FA40; CB:CF = 5:4; not the Section 6 inventory.
Fly ash-slag geopolymer electrolyte asymmetric device [32]	FA/slag electrolyte; 1.0 wt% PEO; separate Cu–Co/rGO/Ni-foam electrodes	Asymmetric device: Cu–Co oxide/rGO/Ni-foam cathode; rGO/Ni-foam anode; FA/slag electrolyte separator	Device: 113.59 mF cm−2; electrode: 1003.14 mF cm−2 separately	Device: 0.5 mA cm−2; voltage NV; electrode: 1 mA cm−2	Device: 40.39 μWh cm−2; area denominator NV	NV	90.3% after 2000 GCD cycles; cycling current/window NV	Electrolyte conductivity 28.7 versus 13.7 mS cm−1.
3D-printed cement-based structural supercapacitor [52]	Non-FA comparator: OPC/sand/polymer mortar.	NV cell count; PVA–KOH modified cement mortar	46.95 mF cm−2; denominator NV	NV rate and voltage window	NV; mass/area basis NV	30.68 MPa; age NV	NV; cycling current/window NV	Also reports 13.37 MPa flexural strength.
Porous cement-based supercapacitor (activated carbon electrode) [30]	Non-FA comparator: OPC/sand porous matrix; no fly ash in formulation.	Two opposing AC/PTFE electrodes on nickel foam; NaOH 1.5–3.5 M or KOH 1–2 M; epoxy-sealed	EIS/CV response; no single common-condition capacitance extracted for ranking	CV: −0.5 to 0.5 V; 0.02–0.10 V s−1	Not compared: pore-liquid volume differs from complete device; gravimetric basis not harmonized	4.5 MPa; 28 d matrix cubes (30 mm)	No quantified long-cycle retention established for comparison	Contact area 9 cm2; AC loading unmeasured.
Redox-active cement-based asymmetric supercapacitor [53]	Comparator; no FA in listed matrix; formulation details NV	Asymmetric device; PAA/OPC/KOH/K3Fe(CN)6 electrolyte	166 mF cm−2; 1 cm2 device	1.5 V device; capacitance test current NV	51.87 μWh cm−2 at 7.5 mW cm−2; 1 cm2 device	NV	NV	100 cm2 device: 23.10 μWh cm−2.
Dual-carbon electrode cement-based structural supercapacitor [54]	Comparator; no FA in listed matrix; formulation details NV	Asymmetric device; PPy nanotube/CNT electrodes; PAM–OPC–Na2SO4	1180 mF cm−2; denominator NV	0.2 mA cm−2; voltage window NV	2.1 Wh kg−1; mass/area basis NV	41.3 MPa; age NV	97.8% (2000 cycles); cycling current/window NV	Energy mass basis NV.
Graphene/MPC separator [55]	Non-FA: magnesium phosphate cement	NV cell count; —	40.92 F g−1; M/P = 3; 28 d; mass denominator NV	NV rate and voltage window	NV; mass/area basis NV	24.59 MPa at 28 d; M/P = 3	NV; cycling current/window NV	1 d maximum 46.38 F g−1 (different curing age).
Porous CSA electrolyte [56]	Non-FA: CSA cement	CSA matrix; electrode/electrolyte details NV	540.44 mF cm−2; denominator NV	1 mA cm−2; voltage window NV	NV; mass/area basis NV	NV	92.43% (1000 cycles); cycling current/window NV	Electrolyte details NV.
Salt-modified OPC electrolyte [57]	Non-FA: OPC with sodium salts	NV cell count; Solid cement–Na2SO4 electrolyte	35.18 mF cm−2, reported maximum; area basis NV	NV rate and voltage window	NV; mass/area basis NV	32.67 MPa; age NV	NV	Maximum-value test conditions NV.
Polymer/OPC electrolyte [58]	Non-FA: PAA/OPC/LiOTf	NV cell count; PAA–cement–LiOTf polymer electrolyte	NV	4 V device; current for capacitance NV	0.65 mWh cm−2 at 0.58 mW cm−2	23.65 MPa; age NV	NV	Area/mass denominator details NV.
Steel slag/GGBS electrolyte [59]	Non-FA: steel slag/GGBS	NV cell count; Alkali-activated SS–GGBS electrolyte	131.16 mF cm−2; denominator NV	NV rate and voltage window	38.2 μWh cm−2; mass/area basis NV	NV	NV	Strength and cycling conditions NV.
Metakaolin geopolymer electrolyte [60]	Non-FA: metakaolin	NV cell count; Geopolymer solid electrolyte	603.93 mF cm−2; denominator NV	NV rate and voltage window	NV; mass/area basis NV	37.22 MPa at 7 d	NV	Cycling details NV.

3.2. Geopolymer-Based Energy-Storage Systems

Geopolymers provide tunable aluminosilicate networks and pores for structural or quasi-solid electrolyte applications [1,21,40,61].

Geopolymer chemistry, pore control, and ion exchange support functional design [41], while scale-up and supply-chain stability remain significant barriers [62].

Activator chemistry, precursor ratios, curing, and moisture govern geopolymer transport behavior [1,21].

The montmorillonite membrane in [21] is a non-fly-ash comparator: its MnO₂/CNT electrode delivered 3310 mF cm⁻² at 1 mA cm⁻², while the assembled device gave 2094.4 mF cm⁻² under 15 MPa and 65.43% retention after 10,000 cycles.

The fly ash-slag electrolyte study [32] separates modifier and electrode roles. The study reports ionic conductivity of 28.7 mS cm⁻¹ with 1.0 wt% PEO, compared with 13.7 mS cm⁻¹ for its unmodified control. A Cu–Co oxide/rGO/nickel-foam electrode gives 1003.14 mF cm⁻² at 1 mA cm⁻², while the separately assembled asymmetric device gives 113.59 mF cm⁻² and 40.39 μWh cm⁻² at 0.5 mA cm⁻², with 90.3% retention after 2000 GCD cycles. The device voltage, normalization denominator, and cycling current are not specified in the comparative data. These are formulation-specific laboratory results, not a verified carbon footprint or construction route.

Studies of fly ash, slag, and metakaolin binders [63,64] provide mechanical context, not assembled-device performance.

Geopolymer tunability remains a design opportunity; printed or prefabricated devices still require coupled mechanical and environmental qualification.

3.3. Fly Ash-Derived Electroactive Materials

Fly ash may supply retained mineral phases, extracted feedstocks, or sacrificial templates for electroactive electrodes [22,24,35,44,45,46,47]. Performance belongs to the resulting electrode architecture, not ash origin; acid-treated electrodes [65] are not structural components.

The zeolite-derived electrode [66], retained PANI/FA composite [44] and carbon-microtube/FA/cement electrode [36] represent different material roles and test conditions. Reference [36] reports 669 F g⁻¹ at 1 A g⁻¹ in 3 M KOH for an electrode and 87 Wh kg⁻¹ for a separate symmetric device. Its supplementary information specifies a PVA/KOH gel and calls the device-energy denominator the overall device mass, but does not provide an itemized packaged-device inventory. Neither electrode performance nor the presence of cement establishes load-bearing component service.

Retained-ash, extracted-feedstock, and removed-template routes must remain distinct; an electrode made using a sacrificial ash template is not necessarily an ash-containing structural material.

In [35], CFA-MDC reached 306.59 F g⁻¹ in a three-electrode test; a CFA-FumMOF//CFA-MDC device delivered 4.0 mAh g⁻¹ at 0.5 A g⁻¹ and 2.48 Wh kg⁻¹ at 1 A g⁻¹. The 5.09 mAh g⁻¹ value belongs to a non-FA control, while 98.90% after 5000 cycles is coulombic efficiency.

Table 3 distinguishes retained ash composites, extracted-feedstock electrodes, and ash-template routes from structural storage devices. Electrode and full-device metrics are reported separately. For example, Ref. [22] reports 1419 F g⁻¹ for an electrode, whereas 26.6 Wh kg⁻¹ and 95.7% retention after 10,000 cycles refer to its hybrid device. In [47], 522.1 F g⁻¹ is a three-electrode result in 6 M KOH; the Na₂SO₄ and ionic-liquid device energies belong to separate configurations. No mass, area, or volume conversion is made without a verified denominator. The reported units are retained because cell architecture, voltage, and normalization differ; the entries are not a direct performance ranking. NV is used as defined under “Review Design and Literature Identification”.

Table 3. Fly ash-derived electroactive materials for supercapacitors.

Material and Source	FA Function and Scope	Processing Route	Cell/Electrolyte	Capacitance and Conditions	Cycling Definition	Energy and Basis	Evidence and Comparability
Fly ash/cement–assisted activated carbon microtube composite [36]	FA electrode additive; cement binder; not a structural-component test	Waste-mask carbonization at 700 °C followed by KOH activation at 900 °C	Electrode: 3 M KOH; separate symmetric two-electrode device; PVA/KOH gel (1:1 mass ratio; SI S1)	Electrode: 669 F g−1 at 1 A g−1; device capacitance NV	94.2% after 10,000 cycles	Device: 87 Wh kg−1 at 750.5 W kg−1; SI calls denominator overall device mass; inventory NV	Device voltage, cycling rate, and mass coverage NV.
Fly ash–derived NaP zeolite [66]	FA-derived zeolite electrode; structural service not tested	Calcination + acid washing + hydrothermal crystallization	Electrolyte and electrode count NV	25 F g−1 at 0.5 A g−1 (reported; mass basis NV)	—	NV; mass basis NV	2024 issue; online in 2025; configuration NV.
Fly ash–derived Co–Fe silicate sulfide composite (CNFs/FA-CoFeSiSx) [45]	FA-derived electrode; structural load-bearing not demonstrated	NaHCO3 roasting, hydrothermal conversion and sulfuration; CNF addition	Material electrode metric; separate solid asymmetric CNFs/FA-CoFeSiSx//AC device	Electrode: 493.33 F g−1 at 0.5 A g−1; mass denominator NV	79.5% after 1800 cycles	Asymmetric device: 25.43 Wh kg−1 at 359.07 W kg−1; mass denominator NV	Cycling configuration NV.
Fly ash–derived MOF-derived carbon (CFA-MDC) [35]	Al extracted from FA for MOF; MOF-derived carbon, not a retained-ash structural matrix	Hydrothermal MOF synthesis + carbonization	Three-electrode CFA-MDC; separate CFA-FumMOF//CFA-MDC coin cell; 6 M KOH	Electrode: 306.59 F g−1 at 0.5 A g−1; FA device: 4.0 mAh g−1 at 0.5 A g−1; 0–1.45 V	98.90% coulombic efficiency/5000 cycles; current NV; not capacity retention	FA device: 2.48 Wh kg−1 at 625 W kg−1 (1 A g−1); electrode mass, not packaged device	6.8 mg cm−2 loading; 5.09 mAh g−1 is non-FA control.
Fly ash–derived CoFeSi/NiCo-LDH heterostructure [22]	FA-derived electrode; structural load-bearing not demonstrated	Two-step solvothermal synthesis	Electrode metric; separate CoFeSi/NiCo-LDH-20//AC hybrid device	Electrode: 1419 F g−1 at 1 A g−1; device capacitance NV	Electrode: 89.35%; device: 95.7%, each after 10,000 cycles	Hybrid device: 26.6 Wh kg−1 at 400 W kg−1; mass denominator NV	Device capacitance NV.
Polyaniline/fly ash composite (PANI-FA) [44]	Retained FA (10 wt%) in PANI composite electrode; not a derived-carbon or structural device	In-situ oxidative polymerization	Electrolyte chemistry and electrode count NV	302 F g−1 at 0.75 A g−1 (reported graphical value); 208 F g−1 at 2.5 A g−1	>66% after 5000 GCD cycles at 2.5 A g−1; coulombic efficiency 99–100% separately	NV; device energy omitted	Voltage and cell basis NV.
Fly ash/3D graphene composite electrode [46]	Retained FA deposited on 3D graphene; electrode study, not a structural component	LPCVD graphene growth + solution deposition	Three-electrode CV; electrolyte details NV	25 mF cm−2 at 5 mV s−1; three-electrode CV	—	NV; mass basis NV	25 mF cm−2 = 0.025 F cm−2; area/electrolyte details NV.
Fly ash–templated N/P/O-doped hierarchical porous carbon [47]	FA used as template; final retained ash content NV	Hydrothermal pre-carbonization + KOH activation	Three-electrode: 6 M KOH; separate Na2SO4 and EMIMBF4 symmetric devices	Three-electrode: 522.1 F g−1 at 1 A g−1	NV	Na2SO4 device: 39.3 Wh kg−1; EMIMBF4 device: 97.9 Wh kg−1; mass bases NV	Ash-template residue NV.
Activated carbon/fly ash composite electrode (FA binder) [34]	Retained FA binder: 10 wt% FA/90 wt% AC; not a load-bearing device	Pressed electrode E1; laser-solidified electrode E2 (2 W, 445 nm, 1 mm min−1)	Three-electrode; E1/E2 working electrode; SCE reference; Pt mesh counter; 1 M NaNO3; room temperature	E2: 104.9 F g−1 by CV; 105.1 F g−1 by galvanostatic model at 0.1 A g−1; mass-component denominator not explicit	4.61% capacitance loss after 2000 CV cycles at 100 mV s−1; not GCD cycling	No verified device-level energy comparison	Full-device mass and structural service NV.

Table 3. Fly ash-derived electroactive materials for supercapacitors.

Material and Source	FA Function and Scope	Processing Route	Cell/Electrolyte	Capacitance and Conditions	Cycling Definition	Energy and Basis	Evidence and Comparability
Fly ash/cement–assisted activated carbon microtube composite [36]	FA electrode additive; cement binder; not a structural-component test	Waste-mask carbonization at 700 °C followed by KOH activation at 900 °C	Electrode: 3 M KOH; separate symmetric two-electrode device; PVA/KOH gel (1:1 mass ratio; SI S1)	Electrode: 669 F g−1 at 1 A g−1; device capacitance NV	94.2% after 10,000 cycles	Device: 87 Wh kg−1 at 750.5 W kg−1; SI calls denominator overall device mass; inventory NV	Device voltage, cycling rate, and mass coverage NV.
Fly ash–derived NaP zeolite [66]	FA-derived zeolite electrode; structural service not tested	Calcination + acid washing + hydrothermal crystallization	Electrolyte and electrode count NV	25 F g−1 at 0.5 A g−1 (reported; mass basis NV)	—	NV; mass basis NV	2024 issue; online in 2025; configuration NV.
Fly ash–derived Co–Fe silicate sulfide composite (CNFs/FA-CoFeSiSx) [45]	FA-derived electrode; structural load-bearing not demonstrated	NaHCO3 roasting, hydrothermal conversion and sulfuration; CNF addition	Material electrode metric; separate solid asymmetric CNFs/FA-CoFeSiSx//AC device	Electrode: 493.33 F g−1 at 0.5 A g−1; mass denominator NV	79.5% after 1800 cycles	Asymmetric device: 25.43 Wh kg−1 at 359.07 W kg−1; mass denominator NV	Cycling configuration NV.
Fly ash–derived MOF-derived carbon (CFA-MDC) [35]	Al extracted from FA for MOF; MOF-derived carbon, not a retained-ash structural matrix	Hydrothermal MOF synthesis + carbonization	Three-electrode CFA-MDC; separate CFA-FumMOF//CFA-MDC coin cell; 6 M KOH	Electrode: 306.59 F g−1 at 0.5 A g−1; FA device: 4.0 mAh g−1 at 0.5 A g−1; 0–1.45 V	98.90% coulombic efficiency/5000 cycles; current NV; not capacity retention	FA device: 2.48 Wh kg−1 at 625 W kg−1 (1 A g−1); electrode mass, not packaged device	6.8 mg cm−2 loading; 5.09 mAh g−1 is non-FA control.
Fly ash–derived CoFeSi/NiCo-LDH heterostructure [22]	FA-derived electrode; structural load-bearing not demonstrated	Two-step solvothermal synthesis	Electrode metric; separate CoFeSi/NiCo-LDH-20//AC hybrid device	Electrode: 1419 F g−1 at 1 A g−1; device capacitance NV	Electrode: 89.35%; device: 95.7%, each after 10,000 cycles	Hybrid device: 26.6 Wh kg−1 at 400 W kg−1; mass denominator NV	Device capacitance NV.
Polyaniline/fly ash composite (PANI-FA) [44]	Retained FA (10 wt%) in PANI composite electrode; not a derived-carbon or structural device	In-situ oxidative polymerization	Electrolyte chemistry and electrode count NV	302 F g−1 at 0.75 A g−1 (reported graphical value); 208 F g−1 at 2.5 A g−1	>66% after 5000 GCD cycles at 2.5 A g−1; coulombic efficiency 99–100% separately	NV; device energy omitted	Voltage and cell basis NV.
Fly ash/3D graphene composite electrode [46]	Retained FA deposited on 3D graphene; electrode study, not a structural component	LPCVD graphene growth + solution deposition	Three-electrode CV; electrolyte details NV	25 mF cm−2 at 5 mV s−1; three-electrode CV	—	NV; mass basis NV	25 mF cm−2 = 0.025 F cm−2; area/electrolyte details NV.
Fly ash–templated N/P/O-doped hierarchical porous carbon [47]	FA used as template; final retained ash content NV	Hydrothermal pre-carbonization + KOH activation	Three-electrode: 6 M KOH; separate Na2SO4 and EMIMBF4 symmetric devices	Three-electrode: 522.1 F g−1 at 1 A g−1	NV	Na2SO4 device: 39.3 Wh kg−1; EMIMBF4 device: 97.9 Wh kg−1; mass bases NV	Ash-template residue NV.
Activated carbon/fly ash composite electrode (FA binder) [34]	Retained FA binder: 10 wt% FA/90 wt% AC; not a load-bearing device	Pressed electrode E1; laser-solidified electrode E2 (2 W, 445 nm, 1 mm min−1)	Three-electrode; E1/E2 working electrode; SCE reference; Pt mesh counter; 1 M NaNO3; room temperature	E2: 104.9 F g−1 by CV; 105.1 F g−1 by galvanostatic model at 0.1 A g−1; mass-component denominator not explicit	4.61% capacitance loss after 2000 CV cycles at 100 mV s−1; not GCD cycling	No verified device-level energy comparison	Full-device mass and structural service NV.

A fair benchmark set includes the non-fly-ash cementitious devices in Table 2 and commercial storage matched to the intended duty. A sensor buffer requires suitable leakage, discharge rate, and usable voltage, whereas overnight backup requires delivered energy and autonomy. The A0–A substitution and A–B inventory contrasts in Section 6 address assigned material inputs only; none is a commercial-product or equal-storage-service benchmark. Powder-electrode values in Table 3 are therefore kept separate from packaged batteries and supercapacitors.

Processing burdens must be inventoried despite a low-cost feedstock. Reference [36] reports waste-mask carbonization at 700 °C and KOH activation at 900 °C; these temperatures do not provide measured process energy per usable device. Reference [45] uses roasting, ion exchange and subsequent functionalization. Activation reagents, furnace duty, yield, washing, waste treatment, and active phases belong inside a complete cost and environmental inventory. The generic material factors in Section 6 do not quantify these specific processing routes.

3.4. Fly Ash- and Cenosphere-Based Thermal Energy-Storage Composites

Cenosphere PCMs can buffer indoor temperatures [48,49,50,67], unlike the high-temperature Al–12Si/modified-fly-ash alloy route [23]. Insulation effects should not be equated with measured latent-heat storage.

Surface-engineered cenosphere pastes provide a verified example of the importance of normalization [49]. The capsules contained approximately 28.0 ± 1.0 wt% PCM with melamine-formaldehyde sealing or 27.0 ± 1.0 wt% with silica sealing according to DSC, while the pastes used 20 wt% functionalized cenospheres in the solids. Paste-level latent heats were 7.2 ± 0.2 and 6.7 ± 0.4 J g⁻¹, respectively. These are paste values, not capsule latent heats. After 50 cycles at 15–50 °C, the sealed particles changed in mass by less than 2%; this leakage test is not an independent measurement of retained latent heat.

The same study [49] reported approximately 30% lower thermal conductivity for the PCM-cenosphere pastes than the plain hydrated paste. Its 20 cm wall model used July weather near DTU and an indoor temperature of 22 °C. The integrated interior heat-flux demand was approximately 30% below plain paste but approximately 6% above the untreated-cenosphere mixture, with a simulated demand shift of up to 3 h. These model-specific comparisons are not measured annual whole-building HVAC savings. The stronger comparator performance also shows why thermal insulation and latent storage must be evaluated together.

A separate fly ash foam-concrete study [29] illustrates the need to preserve loading denominators. Its PCM18D-30% batch contained 2.25 kg cement, 0.45 kg fly ash, and 1.35 kg commercial PCM; the 30% label refers to the original reference cement dosage, not the final composite mass fraction. DSC at 10 °C min⁻¹ gave 44.73 J g⁻¹ melting and 41.76 J g⁻¹ freezing enthalpy for the selected composite. The study reports 4.5 MPa compressive strength at 28 d, compared with 13 MPa without PCM, and 55.3% lower conductivity. Laboratory prototype heating and cooling were delayed by approximately 3 and 8 h under its own protocol. It does not establish annual HVAC savings, long-term latent-heat retention, or a load-bearing storage wall. Inconsistent curing temperatures and cycle durations in the source prevent assigning a single verified exposure protocol.

Table 4 records thermal storage, leakage, and application evidence separately. Material or chamber results do not establish long-term wall, roof, or floor performance. Future component validation should measure usable heat within the operating temperature interval, leakage, and latent-heat retention after exposure, mechanical serviceability, fire response, and actual HVAC demand. A cradle-to-gate study of fly ash/nitrate/expanded-graphite PCM [68] concerns medium-temperature TES; its environmental result must not be transferred to an organic-PCM building envelope without equivalent temperature service and a new inventory.

Table 4. Thermal-storage evidence with explicit normalization and application boundaries.

Study and System	PCM Loading and Basis	Latent Heat/Transition	Thermal Conductivity	Cycling and Leakage	Building or Mechanical Evidence	Evidence and Comparability
[49] RT22HC in etched FA cenospheres; MF/silica seals	Capsule PCM: 28.0 ± 1.0/27.0 ± 1.0 wt% (DSC); capsules 20 wt% of paste solids	Paste latent heat: 7.2 ± 0.2/6.7 ± 0.4 J g−1; PCM range 20–23 °C	~30% below plain hydrated paste; absolute paste value NV	Sealed capsule mass change <2% after 50 cycles, 15–50 °C; latent-heat retention after these cycles NV	Silica-sealed paste >50% stronger than MF-sealed paste; 20 cm wall FEM, not field monitoring	20 cm wall FEM; post-cycling latent heat NV.
[50] Capric acid/cenosphere/silica composite	NV	Composite melting 27.60 °C, 69.07 J g−1; freezing 23.16 °C, 71.30 J g−1	NV	Cycling ΔT: 0.6 °C melting/0.1 °C freezing; cycle count and leakage NV	Material-level thermal characterization; load-bearing evidence NV	Cycle protocol NV.
[67] Hydrated-salt/FA-cenosphere with PEG/silica in mortar	20% sand replacement reported; mass/volume basis NV	Encapsulated-PCM and mortar bases must be distinguished; mortar latent heat NV	NV	Cycling count, latent-heat retention, and quantitative leakage NV	28 d compression/flexure: 36.5/6.2 MPa; chamber peak 3.1 °C lower; not building field savings	Long-term latent heat and leakage NV.
[23] Al–12Si/alkali-modified FA	65 wt% alloy optimum reported	Composite latent heat 124.10 J g−1; transition range NV	18.24 W m−1 K−1 at highest-latent-heat composition	After 100 cycles: latent heat 93.3%, conductivity 94.6% of initial; leakage NV	Metal-alloy TES; indoor-envelope service not established	Leakage protocol NV.
[68] CFA/nitrate eutectic/expanded graphite	50 wt% nitrate eutectic; optimized formulation includes 5 wt% EG (mix basis NV)	52.08 J g−1 initially; 51.19 J g−1 after cycles; medium-temperature	1.77 W m−1 K−1 for optimized sample	200 thermal cycles; 3.81% mass loss; detailed leakage separation NV	Comparative cradle-to-gate assessment; not equivalent room-temperature building service	Functional-unit details NV.
[48] CenoPCM in cementitious composites	NV	NV	Comparative conductivity discussed; numeric value NV	NV	Mechanical/thermal comparison against polymer capsules; no field demonstration established	No building-scale validation.
[29] FA foam concrete with commercial PCM18D	1.35 kg PCM; 0.45 kg FA; 2.25 kg cement per selected batch; 30% reference-cement basis	Composite melting/freezing: 44.73/41.76 J g−1; DSC 10 °C min−1	55.3% below no-PCM comparator; absolute value NV	No quantified long-term latent-heat retention or leakage verified	4.5 MPa at 28 d vs. 13 MPa; laboratory heating/cooling delay ~3/~8 h	Curing/cycling protocols vary.

NV denotes parameters that could not be independently verified from the available source material. MF denotes melamine-formaldehyde, and EG denotes expanded graphite. Cross-study comparisons depend on PCM chemistry, denominator and temperature range.

Table 4. Thermal-storage evidence with explicit normalization and application boundaries.

Study and System	PCM Loading and Basis	Latent Heat/Transition	Thermal Conductivity	Cycling and Leakage	Building or Mechanical Evidence	Evidence and Comparability
[49] RT22HC in etched FA cenospheres; MF/silica seals	Capsule PCM: 28.0 ± 1.0/27.0 ± 1.0 wt% (DSC); capsules 20 wt% of paste solids	Paste latent heat: 7.2 ± 0.2/6.7 ± 0.4 J g−1; PCM range 20–23 °C	~30% below plain hydrated paste; absolute paste value NV	Sealed capsule mass change <2% after 50 cycles, 15–50 °C; latent-heat retention after these cycles NV	Silica-sealed paste >50% stronger than MF-sealed paste; 20 cm wall FEM, not field monitoring	20 cm wall FEM; post-cycling latent heat NV.
[50] Capric acid/cenosphere/silica composite	NV	Composite melting 27.60 °C, 69.07 J g−1; freezing 23.16 °C, 71.30 J g−1	NV	Cycling ΔT: 0.6 °C melting/0.1 °C freezing; cycle count and leakage NV	Material-level thermal characterization; load-bearing evidence NV	Cycle protocol NV.
[67] Hydrated-salt/FA-cenosphere with PEG/silica in mortar	20% sand replacement reported; mass/volume basis NV	Encapsulated-PCM and mortar bases must be distinguished; mortar latent heat NV	NV	Cycling count, latent-heat retention, and quantitative leakage NV	28 d compression/flexure: 36.5/6.2 MPa; chamber peak 3.1 °C lower; not building field savings	Long-term latent heat and leakage NV.
[23] Al–12Si/alkali-modified FA	65 wt% alloy optimum reported	Composite latent heat 124.10 J g−1; transition range NV	18.24 W m−1 K−1 at highest-latent-heat composition	After 100 cycles: latent heat 93.3%, conductivity 94.6% of initial; leakage NV	Metal-alloy TES; indoor-envelope service not established	Leakage protocol NV.
[68] CFA/nitrate eutectic/expanded graphite	50 wt% nitrate eutectic; optimized formulation includes 5 wt% EG (mix basis NV)	52.08 J g−1 initially; 51.19 J g−1 after cycles; medium-temperature	1.77 W m−1 K−1 for optimized sample	200 thermal cycles; 3.81% mass loss; detailed leakage separation NV	Comparative cradle-to-gate assessment; not equivalent room-temperature building service	Functional-unit details NV.
[48] CenoPCM in cementitious composites	NV	NV	Comparative conductivity discussed; numeric value NV	NV	Mechanical/thermal comparison against polymer capsules; no field demonstration established	No building-scale validation.
[29] FA foam concrete with commercial PCM18D	1.35 kg PCM; 0.45 kg FA; 2.25 kg cement per selected batch; 30% reference-cement basis	Composite melting/freezing: 44.73/41.76 J g−1; DSC 10 °C min−1	55.3% below no-PCM comparator; absolute value NV	No quantified long-term latent-heat retention or leakage verified	4.5 MPa at 28 d vs. 13 MPa; laboratory heating/cooling delay ~3/~8 h	Curing/cycling protocols vary.

NV denotes parameters that could not be independently verified from the available source material. MF denotes melamine-formaldehyde, and EG denotes expanded graphite. Cross-study comparisons depend on PCM chemistry, denominator and temperature range.

The LC3/FAC-PCM study [33] found improved temperature regulation but poorer mechanical and transport durability associated with porous interfaces and fewer hydration products. Loading efficiency and ratio have different definitions and cannot be converted directly into final mortar PCM content or annual savings. See also [69].

4. Charge Storage Behavior and Structure–Property Relationships

Figure 4 illustrates the conceptual microstructure, candidate storage mechanisms, and structure–property relationships. Storage mechanisms are evaluated separately for electrodes, complete devices and thermal composites [70].

Figure 4. Author-composed microstructure and mechanism schematic informed by [20,31,70,71,72,73,74,75]. Generative-AI assistance was used for selected illustrative textures in panel (a), followed by author labelling and scientific review; panel (a) is not an experimental SEM image, measured pore geometry or spatial scale. Panel (b) presents candidate electrical-double-layer, redox and interfacial-polarization mechanisms that require separate confirmation in each system. Panel (c) is a conceptual trade-off map. No experimental dataset or third-party figure panel is reproduced.

Figure 4. Author-composed microstructure and mechanism schematic informed by [20,31,70,71,72,73,74,75]. Generative-AI assistance was used for selected illustrative textures in panel (a), followed by author labelling and scientific review; panel (a) is not an experimental SEM image, measured pore geometry or spatial scale. Panel (b) presents candidate electrical-double-layer, redox and interfacial-polarization mechanisms that require separate confirmation in each system. Panel (c) is a conceptual trade-off map. No experimental dataset or third-party figure panel is reproduced.

4.1. Heterogeneous Charge Storage in Fly Ash-Based Systems

Charge at conductive carbon/electrolyte interfaces can be stored in electrical double layers; active oxides and polymers can support redox storage [22,34,35,44,45,46,47,73,74,75]. Neither mechanism is universally intrinsic to fly ash, and interfacial polarization alone does not prove recoverable capacity.

CV peaks, curve shape, or equivalent circuits do not uniquely establish mechanisms in Figure 4b; phase-specific controls and electrochemical/chemical evidence are required.

4.2. Pore Structure, Ion Accessibility, and Interfacial Density

The accessible pore network depends on connectivity, tortuosity, wettability, retained liquid, and electronic percolation. Mineral characterization [17,18] supports morphology descriptions, whereas direct electrochemical measurements in the assembled device are needed to determine accessible charge-storage area and ion transport. Cenosphere cavity volume used for PCM loading must not be counted automatically as an electrochemical double-layer surface.

Pore connectivity, wettability, and retained electrolyte can favor ion access, while greater porosity may reduce load-bearing capacity. Densification can improve mechanical performance yet restrict accessible pathways, and additional conductive filler can alter workability, interfaces and environmental burden. These competing effects depend on composition and test state; the available studies do not establish a universal quantitative pore–transport–storage relationship for fly ash systems.

Reference [31] reports that increasing fly ash substitution improved electrochemical response while reducing compressive strength, with FA40 giving 831 mF cm⁻² and 15 MPa. The available results do not establish a quantitative pore-size/capacitance relationship. A separate ionic-conductivity change from 13.7 to 28.7 mS cm⁻¹ in [32] concerns PEO modification of a fly ash-slag electrolyte, not an ash-only effect. These within-study observations motivate matched pore/transport/mechanical testing; they cannot be pooled as a causal law or used to validate the A inventory recipe.

4.3. Building-Relevant Electrochemical Metrics, Conductive Networks, and Interfacial Effects

Electrochemical comparisons require the cell type, current or scan rate, voltage limits, discharge treatment, and mass, area, or volume denominator [1,2,3,4,70,73,74,75]. A three-electrode working-electrode value does not establish an assembled two-electrode device result. Areal and volumetric metrics aid geometrically constrained component design when electrode, electrolyte, and packaged-device dimensions are declared. Table 2 and Table 3 retain source-reported metrics: missing electrode area, volume, mass coverage, or cell conditions prevent defensible conversion to a common basis. Their entries describe the evidence rather than rank component performance.

Standby service depends on how much charge remains available between charging and use. Self-discharge should be assessed from open-circuit voltage decay after a stated charging and conditioning protocol; leakage current should be reported separately during a defined voltage hold. Initial voltage, hold time and temperature are needed to interpret either result. A fitted self-discharge resistance, such as the electrode-model parameter in [34], is not a measured building-device autonomy rating. These losses can erode low-power sensor buffering or outage reserves even when short-cycle coulombic efficiency is high [75].

The usable voltage window must be verified for the electrode–electrolyte pair, including water decomposition and other parasitic reactions in aqueous formulations; it is not a universal fixed ceiling [74,75]. Connecting matched cells in series raises the pack voltage while reducing the equivalent capacitance. Differences in capacitance or leakage can produce unequal cell voltages, requiring balancing, monitoring, insulation, and reliable interconnects. The series-connected GeoAsh demonstrations [19] establish laboratory connectivity, not a qualified building-voltage module.

Power capability requires complete-device discharge-rate, equivalent-series-resistance, and voltage-sag measurements within a declared usable voltage range [74,75]; low-rate capacitance alone cannot establish response to PV fluctuations or a short power demand. Temperature-dependent capacitance, resistance and leakage should be measured across the intended service range, because electrolyte transport and parasitic reactions affect available energy, power and aging. A room-temperature cycling result or an electrothermal heating demonstration does not establish retained storage under seasonal temperature changes. Geometry, load path, and electrical and fire safety must be assessed alongside these operating metrics.

Conductive-network formation governs this balance. Carbon black, carbon fibers, CNTs, graphene derivatives, conductive polymers, and residual carbon can provide electronic pathways, but excessive additions may reduce workability, increase cost, or weaken interfaces. Fly ash particles and cenospheres influence these networks through packing, pore connectivity, interface density, and electrolyte distribution.

Coulombic efficiency measures charge recovery under specified cycling conditions and differs from retained capacitance or energy [76]. Potential ion trapping and side reactions must be examined in the particular electrode-electrolyte system; the fly ash origin does not establish their magnitude.

4.4. Mechanical–Thermal–Electrochemical Trade-Offs for Building Integration

A material labelled structural must satisfy the relevant load path and serviceability requirements in addition to storing charge [70,77]. Compressive strength alone does not establish flexural or tensile capacity, fracture energy, reinforcement bond, creep, shrinkage, or cyclic-load resistance. For example, the 5.5 MPa result in [20] is a one-day material strength, not a design strength for a floor slab.

The cenosphere-mortar study [67] reports compression and flexure together, 36.5 and 6.2 MPa at 28 days for its reported 20% replacement mixture; this is more informative than compression alone, but still does not establish reinforced component performance. In [49], changing the sealing interface increased paste compressive strength by more than 50% relative to MF sealing, whereas the reported interface tensile values come from atomistic simulations and are not experimental structural tensile strengths.

A within-study mechanical comparison is available in [19]: uncoated GeoAsh gave 44.2 ± 9.4 MPa, the HCl-only control 31.8 ± 3.9 MPa, and PEDOT-coated material approximately 38.0 MPa. These compression results use separate 10 mm cubes, whereas electrochemical testing uses a different specimen geometry. They identify a treatment-related strength trade-off, not retained storage during structural loading, and do not establish a pore-controlled multifunctional optimum.

Coupled evaluation should measure residual load-bearing capacity and storage response during and after loading, cracking, and environmental exposure. Required records include specimen geometry, loading rate, test age, moisture condition, reinforcement, crack width, creep or shrinkage history, and device current/voltage limits. A matched-control design is needed to distinguish effects of the ash, conductive filler, and electrolyte.

Fibre and carbon-network studies [78,79] provide comparator evidence about multifunctional design. Their behavior in another binder, electrode or loading geometry is not transferred quantitatively to fly ash PCM or geopolymer systems. Two-electrode device measurements are reported separately from three-electrode working-electrode tests in Table 1 and Table 2.

Multifunctional balance requires direct evidence for each claimed service. The following durability and life-cycle sections therefore identify what is measured, what is contextual and what remains an assessment requirement.

5. Durability and Long-Term Serviceability of Building-Integrated Thermal–Electrical Storage Materials

Matrix durability, electrochemical device aging and PCM stability are distinct [80,81,82,83,84,85,86,87]. Binder survival or isolated laboratory cycling cannot by itself establish long-term storage function under building exposure.

Exposure to moisture, carbonation, chloride, freezing, cracking, or heat requires paired structural and storage measurements; binder microscopy or strength alone cannot quantify residual capacitance or latent heat.

Figure 5 proposes mechanisms and paired validation tests rather than experimental aging curves. SEM/EDS, FTIR, and XRD diagnose materials changes but cannot replace device-retention measurements.

Figure 5. Conceptual degradation and validation map informed by reported mechanisms in [80,81,82,83,84,85]. Selected state-change textures and icons were prepared with generative-AI assistance and then labelled, checked, and assembled by the authors. They are illustrative rather than experimental micrographs or measured aging data. The CV, GCD, and EIS sketches show proposed measurements only. Matrix-durability studies motivate exposure hypotheses but do not establish complete-device retention; [84] is a slag/metakaolin binder comparator. No third-party figure panel was reproduced.

Previous studies show that degradation mechanisms in alkali-activated systems differ fundamentally from those in conventional calcium silicate hydrate-based materials. Bernal and Provis noted that long-term deterioration is mainly governed by carbonation-induced pore-solution changes, alkali-related reactions, and drying-shrinkage cracking [80]. In fly ash-based geopolymers, these processes are further coupled with ongoing microstructural evolution, including continued geopolymerization and crystallization [82]. Magotra and Jee systematically reviewed the durability of fly ash-based geopolymer concrete under chloride exposure, sulfate attack, and high-temperature conditions, emphasizing the strong influence of activator chemistry and curing conditions on long-term performance [83].

5.1. Moisture Effects, Ionic Transport, and Thermal-Storage Stability

Moisture, carbonation, chloride, freeze–thaw, cracking, and temperature influence pores, ionic transport, conductive networks, and PCM confinement [1,2,3].

Drying may raise electrolyte resistance; excessive moisture may destabilize contacts or promote leakage and parasitic reactions. Moisture management is therefore crucial.

In [49], sealed cenospheres lost less than 2% mass after 50 cycles at 15–50 °C, but this does not establish latent-heat retention. The high-temperature alloy PCM in [23] retained 93.3% of latent heat after 100 cycles; service conditions differ.

Qualification should combine humidity-controlled EIS, drying–rewetting cycles, absorption and post-exposure leakage, and thermal-cycling tests.

In [33], PCM-related thermal benefits coincided with mechanical and transport-durability losses. Electrochemical retention after moisture, chloride, carbonation, or freezing requires separate paired measurements.

5.2. Carbonation, Shrinkage, Pore Evolution, and Storage Retention

Binder reactions and pore evolution can affect ion transport, but binder references [43,80,81,82,83,84,85,86,87,88] do not establish full-device storage lifetime.

Long-term tests should jointly track mechanical strength, electrochemical retention, latent heat, conductivity, pore evolution, and dimensional change.

Ten-year fly ash geopolymer results [82] show matrix evolution, not ten-year device retention; calcium-activated binder results [88] likewise cannot establish storage lifetime.

The slag/metakaolin study [84] is a non-fly-ash comparator and does not demonstrate fly ash device degradation.

5.3. Environmental and Mechanical Degradation

Cracks and environmental exposure can disrupt conductive contacts near percolation [1,2,3,4] and weaken PCM or matrix interfaces; coupled functional measurements are required.

Freeze–thaw and crack-exposure studies on ordinary binders [81] inform test design, but residual strength cannot substitute for capacitance or usable-energy retention. Published results reviewed here do not quantify complete-device storage retention after repeated freeze–thaw, carbonation, or chloride exposure. Short-term temperature operation and charge–discharge cycling address different endpoints. Paired measurements of strength, electrical continuity, capacitance, impedance, and leakage after a defined exposure remain necessary.

Mineral-matrix heat resistance [85] does not establish complete-device fire safety. PCM combustion, electrolyte leakage, gas, shorts and post-fire strength, and storage require assembly-level tests.

Service assessment should link mechanical damage and exposure to impedance, capacitance, PCM leakage, latent heat, and conductivity retention.

5.4. Durability Assessment

The component-level protocol should pair each conventional exposure test with measurements of storage-device function before and after exposure. Table 5 identifies representative single-domain standards and the corresponding device adaptations; none of these standards alone certifies a structural storage element. Temperature, humidity, state of charge, crack width, geometry, and aging history must be stated.

Table 5. Exposure-specific standards and additional tests needed for multifunctional storage components.

Exposure/Test Object	Existing Evidence Boundary	Reference Test Method	Device-Level Endpoint to Add
Moisture uptake/wet–dry	Binder porosity and absorption; device storage retention generally not quantified.	ASTM C1585-20 (sorptivity, concrete); define wet–dry conditioning separately. [89]	Mass/absorption, EIS/ESR, leakage, GCD and insulation before/after exposure.
Carbonation	Carbonated binder performance exists; paired storage measurements are sparse.	ISO 1920-12:2015 (accelerated carbonation of concrete). [90]	Depth/pH and strength plus device capacitance, resistance, and collector corrosion.
Chloride ingress	Binder transport/corrosion results, not automatic device lifetime.	ASTM C1202-25 (electrical indication, not a diffusion coefficient); ASTM C1556-25 (apparent chloride diffusion by bulk diffusion). [91,92]	Charge passed and/or chloride profile, apparent transport, reinforcement corrosion, leakage, and device storage retention.
Freeze–thaw	Concrete resistance known; coupled electrochemical retention rarely reported.	ASTM C666/C666M-26 (rapid freezing/thawing of concrete). [93]	Relative dynamic modulus, cracks, impedance, usable energy, and post-exposure strength.
Shrinkage/cracking	Binder length change and cracking have structural implications.	ASTM C157/C157M-24e1 (length change); controlled crack-width/load protocol required. [94]	Crack width, flexural/tensile response, contact resistance, and post-crack GCD/EIS.
Electrical cycling/self-discharge	Many laboratory devices report cycles under a single condition.	IEC 62391-1:2022; IEC 62391-2:2025 (EDLC component methods; adapt to structural cells). [95,96]	Capacitance/energy retention, leakage, self-discharge, ESR, temperature, and safe voltage.
PCM thermal cycling/leakage	Some capsule mass-loss and latent-heat data; methods vary.	ASTM E793-24 (DSC fusion/crystallization enthalpy); separate leakage conditioning. [97]	Latent-heat retention, transition range, mass loss, conductivity, and mechanical capacity.

Across these exposures, the deciding question is whether residual structural capacity and usable charge or heat remain sufficient for the intended component service. Measurements must be paired on the same or matched samples and interpreted against the intended use.

5.5. Electrical Safety and Reinforcement Corrosion

Electrical qualification requires insulation, leakage, and short-circuit testing under wet conditions, collector corrosion assessment, sealed penetrations, and safe disconnection. Alkaline/acidic electrolytes, zinc salts, polymers, and organic PCMs introduce hazards absent from the bare mineral matrix; matrix strength does not establish device fire safety.

Two distinct corrosion pathways must be considered. Carbonation can lower pore-solution alkalinity and destabilize steel passivation, while chloride ingress can induce localized depassivation and pitting. The response depends on precursor chemistry, alkalinity, redox environment, moisture, and steel type [80,98]. Recycled carbon-fibre or pore-refinement results do not establish universal steel passivation in fly ash-containing devices. Storage electrolytes and electrical connections introduce additional conditions that require direct corrosion testing rather than transfer of Portland-cement thresholds to alkali-activated systems.

6. Life-Cycle Assessment Considerations and Illustrative Material-Input Screening

Environmental performance depends on full material and process inputs, delivered service and life cycle. Fly ash savings may be offset by additives, activators, collectors, sealing, or replacements; Table 6 therefore compares assessment considerations, not universal rankings.

Table 6. Material-specific cost and environmental assessment considerations without universal ranking.

Material Family	Cost Assessment	Carbon Assessment	Resource Assessment	Durability Assessment	Relevant Use and Boundary
Fly ash-derived materials	Recovery, beneficiation, activator, and active-phase costs must be inventoried	Ash allocation and processing can change the result	Source-specific quality, yield, transport, and future supply	Binder durability is not device retention	Mineral matrices, derivative electrodes, or PCM shells; distinguish roles
Activated carbon	Grade, activation, washing, and collector costs	Precursor, activation energy, and yield	Commercial grade and regional supply	Electrolyte, voltage, packaging, and duty-specific	Conventional device benchmark only at matched service
Graphene-based materials	Grade and scale-dependent	Route, yield, drying, and energy mix-dependent	Qualified grade availability	Interface and device-specific	Conductive additive; no universal system advantage
Carbon nanotubes	Purity, dispersion, and dosage-dependent	Synthesis route and production energy-dependent	Qualified grade and production scale	Dispersion, cracking, and interfaces	Additive burden may dominate despite low mass
Metal oxides	Different metals are not one cost class	Mining, refining, and synthesis differ by oxide	Metal-specific supply and criticality	Dissolution and cycling depend on electrolyte	Active phase; distinguish from ash scaffold
Conducting polymers	Monomer, synthesis, and lifetime-dependent	Processing and replacement must be included	Chemistry and supplier-dependent	Swelling, redox window, and cycling matter	Coating or active electrode; not automatically fire-safe

Figure 6 maps proposed cradle-to-gate, use, and end-of-life assessment stages. Figure 7 quantifies listed material inputs only, not the entire framework.

Figure 6. Proposed life-cycle accounting framework. This diagram identifies stages that a future complete LCA could include. The numerical screening in Figure 7 includes only listed material-input proxies; separately inventoried manufacturing, transport, installation, use, maintenance, replacement, and end-of-life stages are not calculated [43,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117].

Figure 7. Illustrative fixed-input substitution between the no-fly-ash reference A0 and fly ash case A: (a) assigned material-carbon proxies; (b) relative changes in cost, material-carbon proxy, and embodied-energy proxy. A replaces 16 kg OPC with 16 kg fly ash and holds all other assigned inputs unchanged. These are screening assumptions, not harmonized cradle-to-gate results or evidence of equal mechanical, electrical, or lifetime service. The separate B-module scenario is described in Supplementary Section S1.

6.1. Embodied Carbon, Resource Utilization, and Building-Level Life-Cycle Value

As operational building energy falls, embodied impacts matter more [7,8,9,106]. Fly ash substitution, however, must be assessed alongside added chemicals and manufacturing burdens.

Fly ash can displace clinker, while cenosphere recovery and geopolymer activation add processing burdens [1,2,43]. Carbon and cost comparisons require explicit inputs and equivalent service.

A nominal 1 m² panel is a geometric unit, not an equivalent functional unit [99]. Comparisons must match delivered energy, usable voltage or thermal range, structural service, lifetime, and replacement rates.

6.2. Cost Perspective

Low ash price alone does not demonstrate low integrated cost; collectors, reinforcement, seals, controls, installation, inspection, and replacement contribute to project expense.

Cenosphere recovery incurs separation, grading, etching, sealing, and PCM filling; qualified yield and energy demand govern cost [17,48,49].

Costs for TES should be normalized to usable heat within the intended temperature window, and electrical storage to lifetime delivered energy for equivalent service. On a consistent batch basis, the cost per qualified carrier mass is C_qualified = (C_feed + C_separation + C_drying + C_treatment)/M_qualified, where M_qualified is the qualified output mass.

PCM loading ratio [33] is distinct from cenosphere recovery yield; cost comparisons require qualified output mass and compatible service.

6.3. Integrated Techno-Economic Evaluation Framework

Building integration requires joint electrochemical, thermal, mechanical, environmental, and economic assessment, including service life, geometry, and replacement.

Areal/volumetric capacitance and ESR support device comparison, but environmental assessment requires delivered energy and service lifetime; Section 6.3.2 presents only material-input screening.

6.3.1. Electrochemical Characterization and Reporting Requirements

Device geometry, test conditions, and structural qualification should accompany electrochemical metrics.

Areal/volumetric capacitance requires stated denominators. CV, GCD, and EIS complement device testing, but electrode metrics are not packaged-device results [70,73,74,75].

CV determines capacitance by integrating the current–voltage curve.

GCD provides capacitance, equivalent series resistance (ESR), and rate capability.

EIS probes ionic transport and interfaces; resistance depends on moisture, pores, and conductive continuity in fly ash composites.

6.3.2. Material-Inventory-Based Carbon and Cost Screening

The quantitative calculation is restricted to a nominal 1 m² A0–A input substitution, not a matched-service LCA. A0 substitutes 16 kg additional OPC for the 16 kg fly ash in A, while all other listed inputs are fixed. Equations (1), (3), and (4) calculate material-carbon, cost, and energy proxies; Equations (2) and (5) show service-normalized quantities that cannot yet be calculated. No equivalence in compression, cracking, usable electrical delivery, structural duty, or service life has been demonstrated. Upstream boundaries and incomplete factors differ, so neither a measured product advantage nor a verified carbon saving is claimed.

$$G_{\mathit{mat}}=\sum _i{m}_i {\mathit{EF}}_i$$

(1)

Here m_i is kg of listed material per nominal system, EF_i is kg CO₂-eq per kg material for the screening proxy, P_i is CNY kg⁻¹, and EE_i is MJ kg⁻¹. G_mat, C_mat and EE_mat are, respectively, kg CO₂-eq, CNY, and MJ per nominal system. Submitted per-tonne parameters are divided by 1000 before multiplication. The listed mass is not a measured finished-product yield or dry mass. Water, concentration, losses, encapsulation, and ancillary components require additional inventories; omitted processes are not assumed impact-free.

Service-normalized comparisons require matched delivered energy, lifetime, structural duty, and replacements; they are not computed without such data.

$$I_G={\displaystyle \frac{G_{\mathit{life}}}{{\sum }_k{E}_{\mathit{delivered},k}}}$$

(2)

Equation (2) uses lifetime carbon G_life (kg CO₂-eq) and usable delivered energy E_delivered,k (Wh), yielding I_G in kg CO₂-eq Wh⁻¹; Equation (5) defines the analogous cost intensity.

Fly ash is assigned a low-burden material proxy without an additional avoided-production credit; the cement-substitution difference is counted only through A0 versus A. All other processing burdens remain separate.

Table 7 lists the nine material factors used in the screening; they do not constitute a harmonized cradle-to-gate GWP dataset. In particular, the 2.62 kg CO₂ kg⁻¹ carbon-black value is an IPCC furnace-black process-CO₂ factor [112], not a complete product carbon footprint. The nickel study [116] concerns refined metal rather than foam conversion. Accordingly, the sum is an illustrative carbon proxy and its percentages must not be read as verified product-LCA reductions.

Table 7. Assumed material-input parameters and source limitations used in the screening.

Material	Price CNY kg−1	Carbon Proxy kg CO2-eq kg−1	Energy Proxy MJ kg−1	Source/Basis
Portland cement	0.35	0.912	5.5	Assumed; context [107]
Fly ash	0.1	0.004	0.1	Assumed allocation
Sand	0.13	0.0048	0.083	Assumed ICE proxy
Carbon black	23.04	2.62	70	Process CO2 [112]; other inputs assumed
Carbon fiber	187.2	19.85	318	Carbon/energy basis [113]; price assumed
Activated carbon	108	9.49	179.3	Carbon basis [102]; route context [101]; other inputs assumed
PVA	12	4	80	Assumed
KOH	8	1.48	15	Carbon basis [110]; other inputs assumed
Nickel foam	3000	8.5	160	Assumed; metal context [116]

Note: Values are scenario-specific screening assumptions, not harmonized cradle-to-gate LCA factors. Sources inform selected proxies, not all prices, material grades, or energy factors. Carbon black uses a process-CO₂ proxy; nickel foam uses assumed factors. These inputs do not establish verified product-level life-cycle impacts.

The assumed factors are deliberately heterogeneous: the 2.62 kg CO₂ kg⁻¹ carbon-black entry is a process CO₂ factor [112], not full product GWP; nickel foam factors are unverified; AC and PVA factors depend on production route [101,102,111]. The source-specific carbon-fibre inventory [113] approximates 19.85 kg CO₂ kg⁻¹ and 318 MJ kg⁻¹ on a stated boundary. The A0–A difference is driven by the assigned OPC and ash coefficients and does not validate other material factors or life-cycle equivalence.

The A0–A calculation also examines deliberately selected one-at-a-time ±20% perturbations, not measured uncertainty distributions or confidence intervals. The OPC and fly ash carbon proxies are varied separately with shared inputs held fixed. In the replacement-mass diagnostic, fly ash varies from 12.8 to 19.2 kg while OPC plus fly ash remains 32 kg and A0 retains 32 kg OPC. Common inputs are held fixed. These tests describe algebraic sensitivity of the A0−A material-carbon difference; they do not resolve the wider factor-boundary or performance uncertainty.

6.3.3. Illustrative A0–A Substitution Results

A contains 16 kg OPC, 16 kg fly ash, 4 kg sand, 2 kg carbon black, 0.8 kg carbon fibre, 2 kg PVA, 2 kg KOH, and 0.100 kg nickel foam. A0 substitutes 16 kg additional OPC for fly ash; both total 42.9 kg per nominal 1 m². Neither recipe claims to reproduce [19,20,31].

$$C_{\mathit{mat}}=\sum _i{m}_i P_i$$

(3)

$${\mathit{EE}}_{\mathit{mat}}=\sum _i{m}_i {\mathit{EE}}_i$$

(4)

$$I_{\mathit{cost}}={\displaystyle \frac{C_{\mathit{life}}}{{\sum }_k{E}_{\mathit{delivered},k}}}$$

(5)

An auxiliary conventional-panel plus activated-carbon-module scenario (B), including its assumed 4 kg AC input and 2–6 kg sensitivity, is documented separately in Supplementary Section S1; it is not used for the primary A0–A conclusion.

Table 8 summarizes the material-input screening totals. Under the specified input assumptions, A0 and A each contain 42.9 kg of listed materials per nominal 1 m². Replacing 16 kg of OPC with fly ash reduces the material-carbon proxy by 14.528 kg CO₂-eq (23.38% relative to A0), the assumed material cost by 4.000 CNY (0.73%), and the embodied-energy proxy by 86.400 MJ (11.12%). These are differences in prescribed input factors, not measured functional equivalence or lifetime carbon savings.

Table 8. Material-input screening totals for matched listed masses in A0 and A.

Scenario	Listed Mass kg	Cost CNY	Carbon Proxy kg CO2-eq	Energy Proxy MJ
A0	42.9	547.560	62.1332	776.732
A	42.9	543.560	47.6052	690.332

Table 8. Material-input screening totals for matched listed masses in A0 and A.

Scenario	Listed Mass kg	Cost CNY	Carbon Proxy kg CO2-eq	Energy Proxy MJ
A0	42.9	547.560	62.1332	776.732
A	42.9	543.560	47.6052	690.332

The A0−A carbon-proxy difference is 16 × (0.912 − 0.004) = 14.528 kg CO₂-eq. One-at-a-time ±20% changes in the assigned OPC factor produce an 11.6096–17.4464 kg difference; corresponding changes in the small fly ash factor produce 14.5152–14.5408 kg. These are algebraic diagnostics under assigned boundaries, not empirical uncertainty intervals.

7. Engineering Deployment in Sustainable and Energy-Resilient Buildings

Walls, roofs, floors, façades, and infrastructure are prospective applications; reviewed evidence is primarily from material tests, lab devices, chambers, and models, not occupied-building demonstrations.

7.1. Building Envelope and Prefabricated Component Integration

References [118,119,120] provide PV-storage integration, conventional elevator storage and cement microstructure context, respectively; none demonstrates a fly ash building component. The carbon–cement estimate [121] is also a non-fly-ash comparator.

(1) Walls and building envelopes

Wall candidates must meet thermal, moisture, fire, isolation, and structural requirements. The 20 cm model in [49] demonstrates conditional heat-flux response, not measured building operation.

GeoAsh [19] used 10 × 5 × 2.5 mm electrochemical specimens and separate 10 mm compression cubes. Three series-connected cells powered a watch for over 45 min; five solar-charged cells reached 4.9 V and operated an LED. These laboratory demonstrations require component-scale sealing, balancing and retained service validation. The 129 μWh cm⁻² reported in [19] corresponds arithmetically to 1.29 Wh m⁻² of the same area, not to demonstrated usable energy from a 1 m² building panel.

(2) Floors and slabs

Floors and slabs need post-cracking storage tests under service loading. The one-day strength and device energy in [20] do not establish structural design performance; [120] provides only cement-microstructure context.

Application thresholds should be derived from the intended service. For a sensor, E_required = P_load × t_autonomy/η gives a starting electrical budget; an illustrative 10 mW load for 24 h needs 0.24 Wh before efficiency losses and self-discharge. This arithmetic is a design example, not a measured bridge-sensor demand. A candidate must also provide the required instantaneous power, usable voltage, and safe reserve after aging.

(3) Prefabricated components

Non-FA benchmarks include 30.68 MPa compression and 13.37 MPa flexure for printed OPC mortar [52], and capacitance tested under 15 MPa for a montmorillonite device [21]. Neither qualifies fly ash beams or slabs for fatigue, cracking, or long-term retention.

Prefabrication enables controlled curing and connections, but qualification must advance from material tests through packaged cells and coupled component exposure to monitored field service. These are evidence stages, not numeric TRL ratings.

PEDOT-coated GeoAsh [19] still requires scalable coating, chemical handling, washing, adhesion, and quality-control validation.

7.2. Infrastructure-Integrated Multifunctional Systems

Road, bridge, and tunnel storage remains prospective until loading, exposure, connections, repair, and delivered-energy performance are demonstrated [1,2,3,4].

Application benchmarks should be service-specific: packaged batteries and conventional supercapacitors for sensor autonomy, leakage, and peak power; PCM boards and ordinary thermal-mass materials for matched heat capacity, temperature range, cycling, and HVAC effects. These benchmarks are reference duties, not direct claims of electrical/structural equivalence. Building deployment requires packaged-cell and component tests before monitored occupancy-scale demonstrations.

The carbon-cement study [121] presents a scale-up estimate in which approximately 45 m³ of material could correspond to around 10 kWh. This is a projection based on the reported carbon-cement system, not a full-scale operating demonstration and not a fly ash result. The later architectural-storage study [26] is also retained as a non-fly-ash comparator. Neither establishes the energy capacity, fire response, or resilience of a fly ash building component.

7.3. Hybrid Thermal–Electrical Energy-Management Systems

Thermal–electrical integration remains prospective: cenosphere/textile concrete [27] and wall modeling [49] support thermal functions, while geopolymer insulation [122] does not imply PCM storage without measured latent heat.

Hybrid service requires compatible thermal/electrical design, insulation, and moisture management and a validated control strategy; separate material demonstrations do not establish PV smoothing, backup, or resilience.

Deployment also requires reproducible feedstock specifications for reactive glass, residual carbon, particle size, contaminant content, and cenosphere yield. Leachability must be tested after activation, cycling, and damage under the relevant disposal or building exposure conditions. Coal-plant changes and transport distances can alter future supply and burdens; current fly ash availability is not a universal long-term supply guarantee.

Connections, electrode isolation, encapsulation, replaceable modules, inspection access, moisture management, collector corrosion, sensor/control power, and end-of-life separation all belong to the component design and inventory. Maintenance intervals and replacement fractions are not assigned without evidence. Embedding storage in a long-life structure may complicate repair, so component replaceability must be evaluated alongside any space-saving benefit.

8. Research Gaps and Future Perspectives

Research priorities are feedstock characterization, phase-specific attribution, normalized device tests, coupled exposure/structural retention and verified component inventories. Evidence should progress from material to packaged device, coupled component and monitored field service without claiming an unsupported numeric TRL.

TES qualification requires declared PCM fraction, temperature range, latent heat, conductivity, leakage, and post-cycling retention. Building studies must state climate and HVAC baselines; chamber or model heat reductions are not annual energy savings.

Structural electrochemical qualification needs cell geometry, voltage/current, self-discharge, and efficiency alongside flexure, cracking, creep, and post-exposure capacitance or EIS. Binder durability alone cannot establish device lifetime.

A0−A is an assumed cement-substitution calculation; B uses an unverified 4 kg AC module. Their mass and capacity changes do not establish matched electrical or structural service.

Future comparisons need harmonized factors, production and transport inventories, verified component performance, lifetime, and end-of-life data. Nominal area or mass cannot substitute for delivered functional equivalence.

9. Conclusions

Fly ash provides distinct pathways: laboratory fly ash–cement and geopolymer devices [19,20,31], derived electrodes with added active phases, and cenospheres carrying PCM. Non-FA cement, slag, metakaolin, and carbon–cement results remain comparators.

Cenosphere/PCM research provides material and wall-model thermal evidence [48,49,50,67]. In [49], sealed capsules lost less than 2% mass over 50 cycles; [29] reports measurable latent heat and thermal delays alongside reduced strength. These do not establish annual HVAC savings or transfer high-temperature TES results [23,68] to indoor service.

Electrode/device energy, binder strength, and cycle retention do not alone qualify structural storage. Comparable measurements need voltage, normalization, loading, cracking, creep, exposure, and electrical-isolation conditions.

PV smoothing, backup, and thermal–electrical building resilience remain prospective until tested for defined services.

Within the illustrative A0–A substitution, replacing 16 kg OPC with fly ash while holding all other inputs unchanged yields a 14.528 kg CO₂-eq (23.38%) material-carbon proxy difference under assigned coefficients. It does not establish equal strength, energy service or lifetime, or a verified complete life-cycle saving. The main contribution of this review is to distinguish fly ash pathways and the evidence required to test their material-to-building translation.

The practical research priority is to connect reproducible fly ash feedstocks with packaged devices and thermally appropriate PCM components, then verify coupled structural, storage, safety, and environmental performance at the component and building scales. Current evidence identifies promising routes and substantial qualification needs, rather than a demonstrated replacement for conventional building storage systems.