Solidification/stabilization of fly ash contaminated with radiocaesium into geopolymers

A three-component geopolymer mixture commercially available under the name Geocem, produced by GEOFIX, s.r.o. (Trnava, Slovak Republic), was used in the preparation of the geopolymer matrices. This mixture consists of three basic components, namely Geosil A consisting of sodium polysilicate, sodium hydroxide and potassium hydroxide; Geosil B containing calcium oxide, silicon oxide, aluminum oxide and magnesium oxide; and Geosil C including calcium oxide, silicon oxide and aluminum oxide.

The fly ash as waste studied came from the Vojany TPP (Slovak Republic). The sample was obtained at the end of 2020, as fly ash K5 (annual sample collection). The physico-chemical analysis of this fly ash was carried out by Eurofins Environment Testing Slovakia, s.r.o. (Slovak Republic), which included the determination of selected metals and semi-metals (Sb, As, Ba, Be, Bi, B, Sn, Al, Mg, Cr, Cd, Cu, Ni, Pb, Hg, Se, Tl, Te, V, Ca, Fe, and Zn) using different analytical methods (HG-AAS, ICP-OES, F-AAS, AAS-AMA, or UV-Vis), oxides (K₂O, P₂O₅, Al₂O₃, MgO, SiO₂, MnO, Na₂O, TiO₂, CaO, and Fe₂O₃), specific weight, solids content, loss of organic matter by annealing at 550°C, total sulfur, pyrite sulfur, sulfate sulfur, combustible matter at 830°C, loss by drying, dry matter at 105°C, bulk density, and determination of total chlorine. In addition, grain size testing, according to ČSN EN ISO 17892-4 (Česká agentura pro standardizaci [ČSN], 2017), and at the Výskumný ústav pre hnedé uhlie (Research Institute for Brown Coal) in Most (VUHU, a.s., Czech Republic), determination of the percentage of water, and the pH of the fly ash reaction according to STN ISO 10390 (Slovenski inštitut za standardizacijo [STN], 2005) were carried out.

Before the artificial contamination of the fly ash with the radioisotope ¹³⁷Cs, the possible level of this radionuclide was analyzed by scintillation gamma-spectrometry in a sample of original fly ash (1 kg) placed in a Marinelli-type beaker. An accurately weighed quantity of fly ash was transferred into an Erlenmeyer flask containing a CsCl solution in deionized water spiked with ¹³⁷CsCl of known activity, chemical concentration, and pH value. The pH of the solution was adjusted by the addition of 1 mol⋅dm⁻³ NaOH solution to a final value of pH 5.5. The exposure was carried out under stirring on a rotary shaker (150 min⁻¹) at 25°C and the risk of evaporation of the water from the solution was prevented by covering the neck of the flask with parafilm. After 48 h of exposure, fly ash with bound ¹³⁷Cs activity was collected on filter paper. Subsequently, an aliquot of the solution was removed and filtered through a syringe filter (13 mm diameter; 0.45 μm permeability) to remove any residual fly ash, and the residual ¹³⁷Cs activity in the solution was determined using a scintillation gamma-spectrometer. The captured fly ash was allowed to dry in a laboratory oven at 40°C for several days to a constant weight.

Plastic samplers with a circular base with a diameter of 35 mm and a height of 50 mm were designed and fabricated to prepare the cylindrical geopolymer waste forms. Before loading the geopolymer matrix, these plastic samplers were covered with parafilm on the base to prevent the unsolidified geopolymer matrix from leaking out of the samplers.

The preparation of the geopolymer mass containing different percentages by weight of fly ash (5%, 10%, 20%, and 40%) artificially contaminated with ¹³⁷Cs followed the proposed recipes shown in Table 1. From each variant, four samples were prepared in the form of cylinders of the geopolymer waste forms. The samples were allowed to cure for 45 days under laboratory conditions (25°C).

In the first step, the initial ¹³⁷Cs activity bounds in a given formed geopolymer waste form was determined based on the known specific activity of ¹³⁷Cs (Bq⋅g⁻¹; d.w.) in the used, artificially contaminated fly ash sample, the percentage by weight of this fly ash in the final product, and the total dry weight of the formed geopolymer waste forms.

The leachability test was performed according to ANSI/ANS-16.1 (American National Standard [ANSI], 1986). The test itself was carried out in sealed plastic beakers with a total volume of 1 dm³, to which was added an exact volume of deionized water (0.05 μS⋅cm⁻¹) calculated as ten times the surface area of the sample of the geopolymer waste form. The geopolymer waste form sample was then immersed in the deionized water and exposed under laboratory conditions at 25°C. After 2 h, the sample was removed from the water and immersed in a new plastic beaker of fresh deionized water of identical volume as above. The water sample obtained after exposure was analyzed for pH, conductivity, and ¹³⁷Cs activity released from the geopolymer waste form using Marinelli-type beakers and scintillation gamma-spectrometry. Similar procedures were subsequently followed for exposure times of 4 h, 7 h, 24 h, 48 h, 72 h, 96 h, and 120 h.

As the fly ash was artificially contaminated with radiocesium ¹³⁷Cs, its long half-life of T_1/2 = 30.2 y allows us to ignore its radioactive decay during the testing in terms of the activity determined in the leachates. Under such conditions, the effective diffusivity can be calculated using the following mathematical relationship (Eq. 1): (1) D=AnA01Δtn2VS2T, D = {\left( {{{{A_n}} \over {{A_0}}}{1 \over {\Delta {t_n}}}} \right)^2}{\left( {{V \over S}} \right)^2}T, where: D – effective diffusivity [cm²⋅s⁻¹], V – volume of the geopolymer waste form sample [cm³], S – geometric surface area of the geopolymer waste form sample [cm²], A_n – activity released from the geopolymer waste form sample during the leaching interval n [Bq], A₀ – total activity solidified in the geopolymer waste form sample at the beginning of the first leaching interval [Bq], Δt_n = t_n – t_n−1 = duration of the n^th interval [s], T – mean time of the leaching interval [s], which is calculated using relation (Eq. 2): (2) T=12tn12+tn−1122. T = {\left\{ {{1 \over 2}\left( {t_n^{{1 \over 2}} + t_{n - 1}^{{1 \over 2}}} \right)} \right\}^2}.

The leachability index L_i is then calculated using the following equation (Eq. 3): (3) Li=1n∑1nβDn, {L_i} = {1 \over n}\sum\nolimits_1^n {{{\left( {{\beta \over D}} \right)}_n},} where: L_i – leachability index, n – number of leaching intervals, β – constant (β = 1) [cm²⋅s⁻¹], D – effective diffusivity [cm²⋅s⁻¹].

The compressive strength test of the formed geopolymer waste form was carried out using a prototype of a piece of compressive strength measurement equipment (GEOFIX, s.r.o.). It was a manual hydraulic press with an integrated manometer and a maximum pressure of 20 t. The compressive strength measurement was preceded by the pressing of the geopolymer waste form samples into perfectly flat surfaces. The prepared sample was inserted between the contact surfaces of the hydraulic press and the pressure on the sample was increased by stepwise pumping. The pressure at which sample destruction occurred was then read on the manometer.

A scintillation gamma-spectrometer with a 3″ diameter NaI(Tl) well-type crystal 76BP76/3 (Envinet, 2024) coupled with the evaluation program ScintiVision-32 (Ortec, USA) was used to determine the initial and residual ¹³⁷Cs activity in the solutions from the experiments aimed at artificial contamination of fly ash with radionuclide ¹³⁷Cs, and to determine the bound ¹³⁷Cs activity directly in the fly ash. The detector was placed in a cylindrical lead shield with a height of 250 mm and a wall thickness of 35 mm of Pb to minimize the background effect. For the leachate samples obtained from the leachability test, the ¹³⁷Cs activity was measured on an identical scintillation detector, but with the difference that the leachate samples with a volume of 600‒800 cm³ were analyzed in Marinelli-type beakers (type 1040G; max. volume 1,000 cm³) and on a detector placed in a lead shield designed for high-resolution gamma-spectrometry (Envinet, Czech Republic; height 400 mm, wall thickness 100 mm Pb). The scintillation gamma-spectrometers were calibrated in terms of γ-photon energies and measurement efficiencies using the above evaluation program and the built library of analyzed radioisotopes: ¹⁰⁹Cd (E_γ = 88.04 keV), ¹³⁷Cs (E_γ = 661.66 keV), and ⁶⁰Co (E_γ = 1,173.24 keV), using standard solutions of the given radioisotopes. The calibration considered the geometry of the measurement, either in the form of vials inserted into the detector well or in the form of Marinelli-type beakers placed on the detector. The duration of the sample analysis was 600 s, which ensured a relative measurement uncertainty of up to 2%.

The obtained data were statistically evaluated and processed into graphical dependencies using Origin Pro 2016 (OriginLab Corporation, 2016) and MS Excel 2016 (Microsoft Corporation, 2016).

In the first step, we characterized in detail the fly ash sample studied. Based on the results obtained, we can say that the studied fly ash sample did not contain heavy metals (Cu, Zn, Hg, Cr, Cd, or Pb) in concentrations that would be abnormal or unnatural for the given sample type. We also found that the fly ash sample studied contained naturally high concentrations of the elements Al, Mg, and Ca, which also form an important component within the geopolymers. Al was present in the fly ash sample at a concentration of 107 g⋅kg⁻¹ (10.7%), Ca at 101 g⋅kg⁻¹ (10.1%), and Mg at 10.5 gkg⁻¹ (1.05%).

Other important chemical components of geopolymers in terms of their final structure and polymerization include oxides, in particular aluminum oxide, magnesium oxide, silicon oxide, and calcium oxide. For this reason, analyses were also carried out to determine the abundance of oxides in the studied fly ash sample. We found that the fly ash sample contained silicon oxide at a percentage by weight of 39.5%, aluminum oxide at 20.2%, calcium oxide at 14.1% and magnesium oxide at a percentage of 1.74% (Table 2). It should be noted at this point that the mechanism of the effect of some oxides, such as CaO and SO₃, on the mechanical properties and dimensional stability of fly ash-based geopolymers is not clear. Chen et al. (2022) observed that the mechanical properties of fly ash-based geopolymers initially increase and then decrease with the increasing CaO and SO₃ content.

In the final analyses, we focused on the abundance of basic elements such as sulfur in its various chemical forms, and total chlorine, as well as on the solids content, loss of organic matter by annealing at 550°C, combustible matter at 830°C, loss by drying, dry matter determined at 105°C, bulk density, and specific weight. These analyses were supplemented by the determination of the pH reaction of the fly ash in suspension with water (weight ratio of fly ash to deionized water equals 1 : 5). We found that the fly ash showed a strongly basic character with a pH of 12.2.

Based on these facts, it can be assumed that the studied fly ash represents a suitable material that chemically complements the basic components of the geopolymer mixture, which implies that the evaluated fly ash can be applied to this mixture in significant weight proportions, representing tens of percent. This assumption is also the basis of the extensive review work by Li et al. (2022), which deals with the preparation, properties, and application possibilities of fly ash-based geopolymers.

For the artificial contamination of a fly ash sample with ¹³⁷Cs, we found that with the increasing CsCl concentration in deionized water, the percentage of ¹³⁷Cs bound activity by the studied fly ash sample decreased significantly. The conducted experiments also showed that the amount of initial ¹³⁷Cs activity in the deionized water had a significant effect on the ¹³⁷Cs specific binding values obtained in Bq⋅g⁻¹ d.w. The specific ¹³⁷Cs activity bound in the individual fly ash samples ranged from 420 Bq⋅g⁻¹ (d.w.) to 969 Bq⋅g⁻¹ (d.w.). The fly ash samples thus artificially contaminated with radiocesium ¹³⁷Cs were subsequently applied for the preparation of geopolymer waste forms containing different percentages by weight of fly ash (5%, 10%, 20%, and 40%), whose physico-chemical and geometric characteristics are presented in Table 3.

Many authors point out that several factors will play an important role in the preparation of fly ash-based geopolymers for the successful and highly efficient solidification or immobilization of Cs, as well as the microstructure of the formed geopolymers. Jain et al. (2022a; 2022b) found that the critical process parameters are the curing time and curing temperature, followed by the interaction between the curing temperature and the Cs dosage. They also explained (Jain et al., 2022a) that the Cs dosage affected the resulting phase compositions and pore structure of fly ash-based geopolymers, eventually influencing the Cs immobilization in these matrices. In our case, we worked with the radioisotope ¹³⁷Cs and simulated radioactive wastes originating from NPP operations, which are characterized by very low Cs concentrations but not negligible radiochemical characteristics. In addition, Tian et al. (2019) showed that a high Si/Al ratio could contribute to a more compact of geopolymer, as confirmed by the decrease in the pore volume with the increasing Si/Al ratio. In contrast, the geopolymer with a low Si/Al ratio had a higher Cs⁺ immobilization efficiency than the geopolymer with a high Si/Al ratio due to the high adsorption density.

The leachability test performed according to the ANSI/ANS-16.1 standard is implemented in the operations of the National Radioactive Waste Repository (NRWR) in Mochovce (Slovak Republic) as an important parameter determining the suitability of the solidified RAWs for their storage.

While performing the leachability test and at the specified times of 2 h, 4 h, 7 h, 24 h, 48 h, 72 h, 96 h, and 120 h, the ¹³⁷Cs activity in the whole volume of deionized water was determined, and the pH and conductivity of the water were measured. Figure 1 shows graphically the changes in the pH, conductivity, and ¹³⁷Cs activity values in the deionized water released from the prepared geopolymer waste forms with different fly ash contents. As can be seen, there was a gradual decrease in the above measured values at short time periods, i.e. after 2 h, 4 h, and 7 h of exposure. A similar dependence was observed for measurements that proceeded at daily intervals, i.e., after one day, two days, three days, four days, and five days of exposure. In the case of conductivity, we found that the highest values of conductivity, in mS⋅cm⁻¹, probably originating from the release of chemical components of the prepared geopolymer waste forms, were observed for the samples exhibiting the lowest fly ash proportions. The conductivity values ranged from approx. 1 mS⋅cm⁻¹ to 5 mS⋅cm⁻¹. This was not clearly demonstrated for the pH values of the water; the pH of the deionized water increased from an initial pH 5.5 to pH values of 10.6‒11.8 within the individual times of measurement. Regarding the ¹³⁷Cs activity released into the water, the opposite phenomenon was observed, namely that the highest values of the measured activities were observed for the geopolymer waste form samples containing the highest applied content of fly ash (40 wt.%).

FIGURE 1.

Changes in pH (A), conductivity (B), and ¹³⁷Cs activity (C) values in deionized water released from prepared geopolymer waste forms with different fly ash contents (5 wt.%, 10 wt.%, 20 wt.%, and 40 wt.%). After each measurement, the deionized water was replaced with fresh water and the duration of exposure of the geopolymer waste form sample from the beginning of the leachability test was as follows: 1^st measurement – after 2 h of exposure; 2^nd measurement – after 4 h; 3^rd measurement – after 7 h; 4^th measurement – after 24 h; 5^th measurement – after 48 h; 6^th measurement – after 72 h; 7^th measurement – after 96 h; 8^th measurement – after 120 h of exposure. The values represent the arithmetic means and the error bars describe the standard deviation of three independent experiments

Source: own work.

We also evaluated the ¹³⁷Cs leachability within individual samples and measurements in the context of the initial ¹³⁷Cs activity determined in a given cylinder of the geopolymer waste forms through the applied fly ash contaminated with ¹³⁷Cs. We found that the highest percentage releases of ¹³⁷Cs activity within the individual measurements were observed for the geopolymer waste form samples containing fly ash at 40% by weight. At the same time, these results showed that we confirmed a slightly higher percentage release of ¹³⁷Cs activity than 1% in only one case of exposure time.

Within this leachability test and in accordance with the ANSI/ANS-16.1 standard, parameters such as the effective diffusivity (D) and, in particular, the dimensionless leachability index (L_i) were evaluated, based on which solidified RAWs are determined to be suitable or unsuitable for storage in RAW repositories. This standard defines that if the L_i value is greater than 6, the solidified RAWs are suitable for storage in an RAW repository. Conversely, if the L_i value is less than 6, the solidified product does not meet the criteria for storage in a repository, e.g. the NRWR in Mochovce. Based on the calculated L_i values, we found that the prepared geopolymer waste form samples with 5%, 10%, and 20% fly ash content by weight showed L_i values ranging from 6.34 to 6.54, and thus met this criterion. For the sample with 40% fly ash content, we obtained a value of L_i equal to 5.58, from which we can conclude that this solidified RAWs did not pass the leachability test and therefore cannot be stored in a RAW repository.

Jain et al. (2022b) found that for prepared fly ash-based geopolymers and Cs immobilization, the leachability index L_i ranged from 7.4 to 14.6 (determined according to the ANSI/ANS-16.1-1 standard), which significantly exceeded the minimum acceptable value of 6. They explained these very high L_i values via the combination of the chemical (within the amorphous gel and zeolitic phase) and physical (adsorbed on surfaces and trapped within porosity) immobilization of Cs in fly ash-based geopolymers. They also emphasized that the remarkable improvement in the performance is due to the formation of the Cs-bearing zeolitic phase (pollucite) found in properly processed fly ash-based geopolymers.

As in the leachability test, the compressive strength test of solidified RAWs is considered a critical parameter determining their suitability or unsuitability for storage in RAW repositories planned for hundreds of years. In each country, the value of the pressure at which the solidified RAW is destroyed may vary according to the standards adopted for this purpose. In the conditions of the Slovak Republic, a minimum pressure on the wall of a solidified RAW of at least 5 MPa has been set for a given solidified RAW to be able to resist pressure. In Figure 2, we present the results of the material compressive strength tests for the individual prepared geopolymer waste form samples. As can be seen, the samples with fly ash contents ranging from 0 wt.% to 35 wt.% met the above limit. At the same time, we found that the value of the pressure at which the destruction of a given sample occurred decreased from a value of approx. 12 MPa determined for the sample without fly ash addition to a value of 8 MPa for the geopolymer waste form sample containing 35 wt.% of fly ash. As with the leachability test, the sample containing 40 wt.% fly ash proved to be unsuitable for storage in the RAW repository, with the destruction occurring at a pressure of only 4 MPa.

From the point of view of comparing our results concerning the effect of fly ash content in geopolymer waste forms on their compressive strength with the results reported in scientific databases, there are papers that show a different pattern of this dependence. For example, Carrillo-Beltran et al. (2021), when studying the effect of the percentage amount of fly ash originating from the combustion of olive tree biomass in prepared geopolymers (from 10 wt.% to 40 wt.%) on their compressive strength, found that this parameter increased with the increasing fly ash percentage up to 30 wt.% and then started to decrease slightly, reaching compressive strength values in the tens of MPa in all the cases analyzed. Also, Teixeira et al. (2022) confirmed in their work the synergistic effect of adding fly ash derived from biomass combustion on improving the characteristics of concrete containing fly ash derived from coal combustion.

FIGURE 2.

Compressive strength measured for prepared geopolymer waste forms with different fly ash contents. The values represent the arithmetic means and the error bars describe the standard deviation of three independent experiments

Source: own work.