Optimization of humidification stabilization for SRF co-firing ash: Balancing expansion inhibition and drying shrinkage

Origin of SRF-coal co-fired fly ash

2.1.1

The raw co-fired fly ash utilized in this study was sourced from the electrostatic precipitator hoppers of a 32 MW CFB boiler in Taiwan (Mailiao). The plant utilizes a fuel blend consisting of 90% bituminous coal and 10% industrial-grade solid recovered fuel. The alternative fuel feedstock was derived primarily from processed petrochemical textile waste and plastic residues, with a net calorific value of approximately 22 MJ/kg.

Sampling and bulk homogenization process

2.1.2

To ensure representativeness and mitigate temporal fluctuations arising from the heterogeneous solid recovered fuel input, sampling was conducted continuously over a 30-day window in accordance with ISO 21645:2021 protocols. To ensure bulk homogeneity, the collected co-fired ash underwent a standardized homogenization process where 500 kg of bulk material was mixed in an industrial rotary drum mixer for 24 h. Subsequently, the homogenized ash was dried at 105 ± 5°C for 24 h in a forced-air oven, and sieved through a No. 200 mesh (75 μm) to eliminate unburnt carbon agglomerates. Physical properties, including a specific gravity of 2.58 and a median particle size (d ₅₀) of 18.4 μm, were determined via laser diffraction analysis. Bulk oxide composition was determined via X-ray fluorescence (XRF) using a Shimadzu XRF-1800 spectrometer (Table 1).

The raw morphology consists of irregular, porous shards that differ from those of spherical coal fly ash. The micro-morphology of the solid recovered fuel ash was initially scrutinized using SEM (Figure 1), and its mineralogical assemblage was identified using the XRD atlas (Figure 2).

Figure 1

Figure 2

Microwave-assisted acid digestion

2.2.1

Before trace metal analysis, a 0.1 g ash sample was digested in a closed-vessel microwave system using a high-purity acid mixture of 6 mL HNO₃, 2 mL HCl, and 1 mL HF (analytical grade, Merck).

ICP-OES spectrometric calibration

2.2.2

Trace heavy metal concentrations were determined via ICP-OES using a PerkinElmer Avio 500 spectrometer (Table 2). Calibration standard curves (R ² > 0.999) were prepared using multi-element standard solutions (Merck, analytical grade) over a concentration range of 0.1–10 mg/L.

XRD protocol

2.3.1

Mineralogical evolution was examined using a Shimadzu XRD-6000 diffractometer. Before scanning, samples were dried and ground to below 45 μm. Measurements were taken from 3° to 90° 2θ at a scan rate of 2°/min using Cu-Kα radiation (40 kV/30 mA).

TGA/DTG conditions

2.3.2

Thermal evolution of hydrates was performed using a Hitachi STA-200 thermal analyzer. Approximately 15 ± 0.5 mg of dry powder sample was placed in an alumina crucible and heated from room temperature (25°C) to 1,000°C at a constant heating rate of 10°C/min under a high-purity nitrogen flow of 50 mL/min.

High-resolution SEM-EDS microstructural analysis

2.3.3

Micro-morphology was scrutinized using a Hitachi S-3000N SEM-EDS. To ensure representativeness, microstructural observations were conducted on at least five distinct fields of view per specimen. EDS spot analysis was performed in triplicate at different locations, and the reported elemental weight percentages represent the average values of these localized measurements.

Mortar mix proportions and pre-hydration curing

2.4.1

Mortar specimens (1:3 binder/sand ratio using standard quartz sand) were prepared at a fixed water/binder ratio of 0.40. Four humidification ratios were investigated (W _water/ W _ash = 0, 30, 50, 100%), with the 0% sample serving as the control group. Deionized water was introduced via an atomizing spray and mixed for 15 min. Humidified samples were pre-cured at 23 ± 2°C for 1 day to achieve pre-hydration and facilitate mortar fabrication. The standardized processing flowchart for material pre-processing and humidification curing is sequentially illustrated in Figure 3.

Figure 3

Accelerated linear expansion testing (CNS 15311)

2.4.2

Accelerated linear expansion was evaluated in accordance with CNS 15311 (the Taiwanese standard for testing the potential expansion of aggregates and binders). Specimens were immersed in a 70°C water bath, and length changes were measured over 7 days.

28-day drying shrinkage evaluation (CNS 14603)

2.4.3

Drying shrinkage was determined in accordance with the CNS 14603 protocol (equivalent to ASTM C157). All expansion and shrinkage measurements were performed on triplicate specimens (n = 3) to guarantee data reliability and assess statistical significance.

The testing standards for evaluation are consolidated in Table 3.

Crystalline assembly of raw and hydrated residues

3.1.1

The mineralogical profile of the co-fired fly ash, as determined by XRD, reveals a complex crystalline assemblage. Specifically, the presence of alite (C₃S) at the diffraction peaks of 29.50° and 32.30° 2θ confirms the latent hydraulic potential of this atypical residue. Mechanistically, as analyzed in the foundational work of Basu [7], the CFB combustion environment maintains a bulk temperature of approximately 850–900°C. However, the localized combustion of high-calorific textile and plastic solid recovered fuel components induces lateral thermal peaks that locally exceed the clinkering threshold. This enables the solid-state synthesis of alite and belite (C₂S), a phenomenon also observed by Kuhn et al. [9] in their pilot-plant trials. These hydraulic clinker phases, as analyzed by Bullard et al. [10], are essential for the subsequent formation of the self-cementing matrix, providing the binding calcium silicate hydrate gels that bind the irregular ash shards together.

Humidification activation and precursor dissolution

3.1.2

The Weight Change Chart further elucidates the impact of stabilization on mineral stability (Figure 4). The effects of different humidification ratios on mineralogical patterns are illustrated in Figures 5–7.

Figure 4

Figure 5

Figure 6

Figure 7

As moisture content increases, a systematic reduction in anhydrite intensity is observed across the XRD patterns. Mechanistically, the introduction of liquid moisture serves as an essential medium for ion diffusion, overcoming the kinetic barrier to free calcium oxide hydration. In the untreated dry state, the reactive minerals are encapsulated within dense crystalline shards, preventing interaction with ambient humidity. The humidification process facilitates the dissolution-precipitation cycle, where calcium and sulfate ions are released into the aqueous phase. As demonstrated by Yuan et al. [21], the combined presence of Anhydrite and limestone provides additional nucleation sites for AFt crystallization, thereby accelerating the degree of hydration in aluminate-rich systems. The saturation of the liquid phase, as described in the kinetic models of Tang et al. [22], provides the necessary environment for the pre-consumption of precursor expansion energy. This pre-emptive formation of stable hydrates consumes the chemical potential that would otherwise trigger destructive crystallization pressure in the hardened state, as Collepardi [14] warned.

Carbonate-sulfate interactions and AFt stability

3.1.3

According to the thermodynamic investigation of phase interactions established by Long et al. [19], the stability of AFt is significantly enhanced by the mutual influence of calcium sulfate and calcium carbonate. The presence of calcite (29.40°), identified in the XRD spectra and listed in Table 4 as a carbonation product, acts as a thermodynamic stabilizer for carbo-aluminate phases during the stabilization sink. The relative thermodynamic stability of these phases at 25°C is detailed in Table 5, confirming that AFt serves as the primary chemical sink for expansion energy.

Thermal behavior of unreacted latent precursors

3.2.1

The quantitative kinetics of the stabilization process were monitored via synchronized TGA/DTG (Figures 8–11) and summarized in Table 6. This thermodynamic approach provides a precise measurement of the “chemical energy sink” achieved through humidification. In the raw co-fired ash (0% group, Figure 8), the weight loss profile is largely dominated by the decarbonation of limestone residues in the 600–800°C range (11.45% mass loss). Crucially, the mass loss in the hydrate region (50–200°C for AFt and 400–500°C for Portlandite) is negligible, confirming that the expansion drivers remain in their high-energy, unreacted crystalline states. Under real-world ambient conditions, the solid-state hydration of anhydrous anhydrite and crystalline-free calcium oxide is extremely slow, requiring months to manifest. Elevating the temperature to 70°C in accelerated testing significantly overcomes the solid-state kinetic barriers of these sluggish precursors, enabling evaluation of the potential volumetric swelling pressure within a feasible experimental timeframe (7 days), as standardized by ASTM D4792. This is consistent with the temperature-dependent pore-solution and phase-equilibrium models discussed by Lothenbach et al. [23].

Figure 8

Figure 9

Figure 10

Figure 11

Pre-hydration energy pre-consumption kinetics

3.2.2

Upon the introduction of moisture at the 30% optimized ratio (Figure 9), a dramatic transformation in the thermal decomposition kinetics is observed. The appearance of distinct endothermic DTG peaks at 120 and 450°C indicates the emergence of a hydrated structure. The mass loss in the AFt region increases from 1.12 to 3.45%, representing a pre-consumption of the expansion energy. Mechanistically, this process allows the immense crystallization pressure (P _c) associated with the conversion of free calcium oxide to calcium hydroxide to dissipate while the ash is in a non-rigid bulk state. According to the hydration kinetics discussed by Tang et al. [22], the availability of liquid water is the rate-limiting step for these reactions. In our system, the 1-day stabilization sink provided by the humidification protocol ensures that most of the chemical “swelling work” occurs outside the hardened binder matrix. The heat of hydration is safely dissipated during the mixing and pre-curing stages, preventing the buildup of internal thermal stresses that could lead to micro-cracking in the final mortar specimens.

At the 100% humidification level (Figure 11), the “chemical sink” reaches its absolute capacity. The mass loss associated with AFt needles increases by a staggering 514% compared to the control group, reaching 6.88% (Table 6). Mechanistically, this exhaustive conversion channels all available chemical energy into needle-like products, thereby refining the pore. While this consumes expansion precursors, it establishes the physical foundation for extreme capillary tension during evaporation, which is governed by the multi-ion competition discussed by Lu et al. [20]. As observed by Gu et al. [24] in their study on seashell hydration, the excessive growth of AFt needles radically refines the micro-morphology, effectively converting a porous ash network into a rigid, congested matrix prone to physical contraction. Furthermore, as established by Muhammad et al. [25] in their comprehensive assessment of municipal solid waste incineration fly ash, using such residues can enhance heavy metal immobilization through the joint precipitation of aluminosilicate gels.

Surface morphologies of encapsulated raw shards

3.3.1

The micro-morphological transition and localized elemental evolution, as evidenced by high-resolution SEM images in Figures 12–15 and EDS weight percentages in Table 7, provide direct evidence of the pore refinement mechanism. In the raw ash (Figure 12), the morphology consists primarily of irregular, porous shards and spherical particles with a high carbon content (16.23 wt%). These porous structures are characteristic of CFB residues, where the lower combustion temperatures prevent complete melting and vitrification. The localized energy dispersive spectroscopy analysis of these shards reveals a silicon-rich matrix (26.61 wt%) with relatively low calcium (3.04 wt%) at the surface, indicating that the reactive free calcium oxide is largely sequestered within the crystalline lattice of the shards, as described by Götze et al. [6] in their chemical characterization of household waste fractions. This crystalline encapsulation explains the delayed nature of the expansion drivers, as moisture must first penetrate the rigid mineral shell before hydration can occur.

Figure 12

Figure 13

Figure 14

Figure 15

AFt proliferation and pore refinement

3.3.2

Upon humidification to 30% (Figure 13), the particles begin to aggregate, and gelatinous hydration products appear. This transition signals the onset of the self-cementing reaction, in which the alite and belite phases, as identified by X-ray diffraction, begin to form a binding matrix. However, the most significant morphological shift occurs at 100% humidification (Figure 15), where SEM images reveal a matrix completely congested with clusters of needle-like crystals ranging from 2 to 10 μm in length. These needles are the morphological manifestation of AFt. The localized energy dispersive spectroscopy fingerprinting confirms this identification, showing a dramatic enrichment of sulfur (increasing to 4.16 wt%) and calcium (increasing to 21.43 wt%) at these needle clusters.

Mechanistically, this massive growth of AFt needles is the direct cause of pore refinement. As the needles proliferate, they bridge the gaps between the original ash particles and fill the macro-pore voids. According to the microstructural guides provided by Scrivener et al. [17] on cement hydrate analysis, such hydrate congestion effectively divides larger capillary pores into a higher population of much smaller pores, typically with radii below 50 nm. While enhancing ion resistance, the system becomes extremely vulnerable to drying-induced volume loss, as described in the chloride diffusion studies of Zhang et al. [12]. The transition from a “macro-pore dominant” system to a “capillary-pore dominant” system, as established by Liu et al. [18], is the physical tipping point for the shrinkage paradox. The EDS data in Table 7 provide elemental evidence that the chemical potential of the solid recovered fuel ash is so high that excessive moisture forces the formation of a dense, stressed microstructure prone to physical contraction upon drying. The 30% optimized ratio creates a balanced morphology in which hydration products stabilize the particles without forcing the entire system into a needle-congested state, thereby maintaining a relatively larger pore radius that mitigates capillary stress.

Expansion control via pre-hardening hydration sink

3.4.1

The macroscopic performance data summarized in Tables 8 and 9 quantify the “competitive volume dynamics” – the central mechanistic trade-off of this study. The raw SRF ash (Control Group) exhibits a destructive 7-day linear expansion of 1.62% (±0.12%), which is more than three times the engineering safety threshold defined by CNS 15311. This swelling is the macroscopic manifestation of the internal crystallization pressure (P _c) generated by the unreacted free calcium oxide and anhydrite. Humidification stabilization at 30% for 1 day effectively “mitigates expansion,” reducing the expansion to a safe 0.39% (±0.04%). This reduction is achieved because the chemical expansion work is performed during the stabilization sink period, before the binder reaches its hardened state. As established by Taylor et al. [16] in their research on delayed AFt formation, addressing these precursors early is the only way to prevent delayed cracking.

Indirect evidence of pore refinement and term moderation

3.4.2

However, the paradox emerges when the humidification ratio is increased to 100%. While this group achieves the minimum expansion (0.18%), it triggers a substantial 1.21% (±0.08%) drying shrinkage. The paradox is visually confirmed in the 28-day rate chart in Figure 16. The 28-day data points in Figure 16 clearly show a nonlinear relationship between water content and dimensional loss, with the 100% group crossing the structural safety limit after only 14 days of drying. This level of contraction is nearly five times higher than that of the control group and exceeds the typical limit for high-value binders. Mechanistically, this paradox is governed by the Kelvin–Laplace equation, which describes the pressure difference (ΔP) across the meniscus of a liquid in a pore:

Figure 16

The identification of this “suitable balance” is the key scientific contribution of this study. As established in related research by Fang et al. [15], expansive agents and shrinkage-reducing admixtures must be carefully balanced to prevent structural failure. In our atypical ash system, the AFt needles act as both the solution (expansion inhibition) and the problem (shrinkage induction). The 30% humidification ratio is the “Golden Threshold” because it provides sufficient moisture to consume the reactive free calcium oxide and anhydrite, ensuring safe expansion while maintaining a sufficiently large average pore radius ${\tau }_{\mathrm{P}}$ to avoid the extreme capillary tension associated with excessive refinement. The 28-day shrinkage for the 30% group remains at a manageable 0.52% (±0.04%). This balance ensures that the binder remains dimensionally stable under varying environmental conditions.

Multi-factor contraction: Carbonation, kinetics, and C–S–H densification

3.4.3

Furthermore, drying shrinkage is a complex multi-factor process. In addition to capillary tension, several other physical and chemical factors contribute to this contraction:

1. Carbonation shrinkage: The carbonation of Portlandite into calcite (confirmed by our calcite peaks) can release bound water and cause local microstructural rearrangement, increasing macroscopic contraction.

2. Calcium silicate hydrate (C–S–H) densification: The structural densification and polymerization of hydraulic C–S–H gel during drying induce significant irreversible drying shrinkage.

3. Drying kinetics: Differences in evaporation rates and internal humidity gradients under RH 50% curing drive differential early-age shrinkage.

By balancing these mechanisms, we establish that 30% humidification for 1 day is the optimal pre-hydration protocol. It successfully mitigates expansion (0.39% < 0.50% limit) while maintaining a moderate capillary structure, thereby limiting drying shrinkage to 0.52%.