Journal of Nuclear and Radiochemical Sciences, Vol. 22, pp. 7-13, 2022

ArticlesPhysical and adsorption characteristics of geopolymers prepared using 1–5 M NaOH solution
for immobilization of radioactive wastes
Norikazu Kinoshitaa,*, Yuya Yodaa, Hitoshi Nakashimaa, Motoyuki Asadaa, Shunsuke Kiyomuraa,
Yuki Sasakib, Kazuyuki Toriib, Keisuke SuekicaInstitute of Technology, Shimizu Corporation, 3-4-17 Etchujima Koto-ku, 135-8520 Tokyo, Japan.
Nuclear Projects Division, Building Construction Headquarters, Shimizu Corporation, 2-16-1 Kyobashi
Chuo-ku, Tokyo 104-0031, Japan.
cFaculty of Pure and Applied Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan.
bReceived September 26, 2022; Accepted December 6, 2022; Published online December 12, 2022
The temporal variations of compressive strength and flowability (slump-flow), and the adsorption behaviors of
Co, Sr, and Cs for fly ash-based geopolymers, were investigated for immobilization of radioactive wastes. The
geopolymer samples were prepared by using 1–5 M NaOH solution and cured at ambient temperature. The
strength and flow were related to the amount and concentration of NaOH solution. The trends were discussedwith the amount of heat generated at an early stage of curation. In addition, the geopolymers adsorbed Co equivalent to cements, Sr and Cs more than cements. The trends of absorption were related to the pH of leachates and themagnitude of x-ray diffraction (XRD) peaks of materials which possess larger distribution coefficients. We considered the possibilities to improve performance and to resolve problems in workability for actual use for immobilization of radioactive wastes according to the results of the present work.

1. Introduction
Low-level radioactive wastes such as filters, metals, spentresin, sludge, etc., are continuously discharged from nuclear
power plants. The filters and metals contain radioactive isotopes of 14C, 60Co, 63Ni, 129I, etc. The resin and sludge contain90
Sr, 137Cs, etc1. In Japan, these wastes are immobilized withcement in metal drums or containers in order to prevent elution of the radioisotopes if rainwater and groundwater reach
the waste. The immobilized wastes are eventually buried for
disposal. In the immobilization, the void space in the container
is filled with cement for large-volume wastes such as filters,
metals, etc. Several drums with a volume of 250 L each inwhich the wastes are immobilized are accumulated for storing.
The immobilized wastes should have a compressive strength
enough to stand for the load, although the standard for thestrength has not been instituted. Wastes in the form of sludge,
spent resin, etc., are uniformly immobilized with cement. A
compressive strength higher than 1.47 MPa is required for theimmobilized sludge and resin waste. Low elution rates for theradionuclides are essential for the immobilization of theseradioactive wastes. Flowability of the filler is an importantparameter for filling void space in the container and considering detailed working process. The immobilized wastes which
possess lower elution rates for the relevant radioisotopes, i.e.
higher distribution coefficients, Kd, are preferred to minimize
pollution when water reaches the wastes during long term storage. The standards for the flowability and Kd have not been
instituted.
Apart from immobilization using cement, attention has
recently focused on geopolymers for the immobilization material of the radioactive wastes instead of cement because of its
lower leaching rates for Cs and Sr than cement2. Geopolymers
are inorganic polymers formed by polymerization of aluminosilicates by activation using an alkaline solution such as*Corresponding author. E-mail: norikazu.kinoshita@shimz.co.jp
Tel: +81-90-2673-7056

NaOH3–5. The polymerization occurs by a chemical reaction
between Si-O-Al-O polymer chains and Al-Si oxides. Main
structure has been reported by Davidovits6, etc. The structureand detailed chemical process depend on a Si/Al ratio7–10.
Numerous studies on geopolymers have been conductedworldwide for the past 100 years to understand their characteristics and utilize them for building materials.
Wang et al.11 and Somna et al.12 investigated the compressive
strength of metakaolin-based and fly ash-based geopolymer
cured at ambient temperature. They reported compressive
strengths for geopolymers prepared using NaOH solutions
with concentrations higher than 4 M. Strength was greater as
the concentration increased and showed a maximum at 10–14
M. The strengths were related to curing temperature and Si/Al
ratio. The strength increased with curing temperature andreached a maximum at 60–80 °C. The strength then decreased
as the temperature rose13–15. Duxson et al.16 and Lee et al.17
reported that geopolymer strength varied by Si/Al ratio. The
strength increased with increasing Si/Al ratio, showed a maximum at Si/Al = 2.0–3.5, then quickly decreased above 4.0.
Generally, higher curing temperature and alkaline solutions
with higher concentrations are required to improve the compressive strength of the geopolymer.
An NaOH solution with a concentration higher than 1.25 M
is classified as a deleterious substance under Japan’s Poisonousand Deleterious Substances Control Law. Special heaters are
required for preparation of general geopolymers to cure athigh temperatures. Therefore, the workability at sites isreduced by the use of alkaline solutions with higher concentrations and curing at higher temperatures. There are few reportson the physical characteristics of geopolymers prepared usingalkaline solutions with low concentration and cured at ambient
temperature. The Kd values for the relevant nuclides on geopolymers have been poorly understood in terms of propertiesof the ingredients. We focused on a fly ash-based geopolymer
which comprises industrial wastes excepting the alkaline solu-© 2022 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/12/2022

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Kinoshita

J. Nucl. Radiochem. Sci., Vol. 22, 2022

tion. Utilization of lower concentration of NaOH solution provides lower cost for preparation of geopolymer. Blast-furnace
slag was also used with the f ly ash to ensure a certain
strength18.
The compressive strength, flowability, and adsorption performance related with the ingredient are important for theimmobilization of radioactive wastes in terms of long-timestorage. In the present work, we showed the compressive
strength and slump-flow, which is one of the indicators forflowability, as well as the Kd values of Co, Sr, and Cs, which
are representative isotopes in the wastes, on the geopolymers
prepared using 1–5 M NaOH solution which have not beenreported. These trends were related with the amount of heatgeneration during curation, pH of leachates, and products
observed by an x-ray diffraction (XRD), to understand thephenomena more deeply. Furthermore, we considered the possibilities to improve performance and resolve problems for
actual use of these geopolymers according to the results in thepresent work.

BFS, silicate sand, and 1–5 M NaOH solution were mixed for 2
minutes. The composition of each sample is shown in Table 1.
The chemical composition of major elements, Co, Sr, and Cs ineach ingredient are shown in Table 2 (see section 2.3.4 foranalysis method). The slump-flow was measured at a certain
time after mixing according to the procedure described in section 2.2.2. Eventually, the samples were transferred into plastic
molds of 50 mm in diameter and 100 mm in length. The samples were cured at 20 °C and their compressive strength wasmeasured as described in section 2.2.3.
2.2.2. Slump-flow test. A slump-flow test was carried out
at 0 h, 1 h, and 2 h after mixing based on ISO 9597:200819 andISO 679:200920. Briefly, samples transferred on a stage using aflow cone (upper diameter 70 mm, lower diameter 100 mm,
height 60 mm) slumped 15 times from a height of 10 mm using
a flow testing machine (MARUTO). Shorter and longer diameters of the samples spread on the stage were measured.
2.2.3. Compressive strength measurement. Compressive
strengths were measured using a compression testing machine(UH-500 kN XR, SHIMADZU) at 7 d, 28 d, 91 d, and 1 year
after curing, based on ISO 679:200920.
2.3. Chemical characteristics
2. 3.1. Geopolymer and cement paste preparation.
Silicate sand was not mixed into the geopolymers for measurements of chemical characteristics to eliminate its effect.
Ingredients of FA, BFS, and NaOH solution with compositions
shown in Table 1 were mixed for 2 minutes. Then, the samples
were transferred into a plastic container followed by curing at
20 °C. Cement pastes of ordinary Portland cement (OPC) andblast-furnace slag cement type B (BB) were prepared with thecompositions shown in Table 1 using the same method usedwith the geopolymers. Analyses of Kd, pH, and XRD described

2. Experimental
2.1. Materials
The coal fly ash which fulfills a standard of JIS A 6201 wasused for the fly ash (hereafter FA). CERAMENT A, the granulated blast-furnace slag which fulfills a standard of JIS A
6206, purchased from DC Co., Ltd. was used for the blast-furnace slag (hereafter BFS). The slag contained gypsum. The
commercial NaOH solutions for the volumetric analysis wereused for 1–5 M NaOH solution.
2.2. Physical characteristics
2.2.1. Geopolymer preparation. Known amounts of FA,

TABLE 1: Weight Ratios Used for Preparation of the Geopolymer and Cement Samples. The Compositions for Measurements of
the Physical Characteristics are Outside Parentheses, and Those of the Chemical Characteristics are in Parentheses. The Sample
Names Indicate NaOH Concentration or Species of Cement, and Weight Percentage of the NaOH Solution or Water to Sum of FA
and BFS.
Sample name

FA

BFS

NaOH

Silicate sand

1M-30%

0.7 (0.7)

0.3 (0.3)

0.3 (0.3)

3.0 (0)

1M-35%

0.7 (0.7)

0.3 (0.3)

0.35 (0.35)

3.0 (0)

1M-40%

0.7 (0.7)

0.3 (0.3)

0.4 (0.4)

3.0 (0)

1M-50%

0.7 (0.7)

0.3 (0.3)

0.5 (0.5)

3.0 (0)2M-50%

0.7 (0.7)

0.3 (0.3)

0.5 (0.5)

3.0 (0)3M-50%

0.7 (0.7)

0.3 (0.3)

0.5 (0.5)

3.0 (0)

4M-50%

0.7 (0.7)

0.3 (0.3)

0.5 (0.5)

3.0 (0)5M-50%

0.7 (0.7)

0.3 (0.3)

0.5 (0.5)

3.0 (0)Sample Name

CementWater

Silicate sandOPC-50%

(1.0)

(0.5)

(0)

BB-50%

(1.0)

(0.5)

(0)

TABLE 2: Elemental Compositions of the Major Elements, Co, Sr, and Cs in the Ingredients
Ingredients

Ca
/%

Si
/%

Al
/%

Fe
/%

S
/%

Mg
/%

K
/%

Ti
/%

Co
/ ppm

Sr
/ ppm

Cs
/ ppm

FA

6.0

26.0

10.5

5.8

0.3

0.5

1.7

1.1

54.5

589

0.969

BFS

39.9

11.6

5.1

0.4

1.9

1.6

0.3

0.4

0.164

311

0.176

OPC

52.5

6.7

1.9

2.8

1.2

0.3

0.3

0.2

15.8

313

0.649

BB

46.9

9.2

3.0

1.7

1.4

0.9

0.3

0.4

9.94

344

0.317

J. Nucl. Radiochem. Sci., Vol. 22, 2022

Physical and adsorption characteristics of geopolymers prepared using

below were carried out 3 months after the curing started.
2.3.2. Kd measurement. The K d values were analyzed
based on the manner described by Toyoshima et al. 21. Twograms of the samples pulverized to less than 1 mm were mixed
with 20 mL of water including approximately 1 kBq each of60
Co, 85Sr and 137Cs activities. The samples were shaken for 6 h
on the basis of JTL-13 for the leaching test in Japan 22. The
experiment was carried out at ambient temperature of approximately 20 °C. Known amounts of the filtrates, obtained bycentrifugation and filtration using a membrane filter with 0.22
μm pore size, were analyzed using a high purity germanium
detector (GEM, ORTEC). The amount of each radionuclide
was determined by subtracting the amount of the radionuclide
in the solution after the treatment from the amount before thetreatment. The Kd values were calculated using Eq. (1).

Kd =Cafter
CbeforeMbeforeMsolid
MafterMsolid
Cafter
MafterVliquid(1)

where Cbefore and Cafter are count rates of gamma ray peaks foreach radioisotope in the solution before and after the treatment. Mbefore and Mafter are weights of the measured solutions
before and after treatment, respectively. Vliquid is the volume of
the aqueous phase (20 mL in the present work), and Msolid is theweight of the solid phase (2 g) used in the experiment.
2.3.3. pH measurement. Four grams of each pulverized
sample was stirred with 40 mL of deionized water for 6 h. The
pH values of the leachates were analyzed using a pH meter(D-72, HORIBA).
2.3.4. Chemical composition and XRD measurements.
Approximately 0.2 g of each ingredient was digested with
HNO3 + HF + HClO4 using a microwave oven. Contents of Co,
Sr and Cs in these solutions were analyzed using an inductively coupled plasma mass spectrometry (ICP-MS) (iCAP Q,
Thermo SCIENTIFIC). In addition, major elements thatexceed approximately 0.1% were analyzed using an X-ray fluorescence (XRF) spectrometry (JSX-1000S, JEOL). The XRD
spectra (SmartLab, RIGAKU) of the pulverized geopolymer
and cement samples were analyzed as well.
2.3.5. Heat generation measurement. The amounts ofheat generation during curation at 20 °C from 20 g of 1M-50%
through 5M-50% in Table 1 were recorded every 30 minutes
for 1 month using a 6-point type conduction calorimeter(TOKYO RIKO).

3. Results and discussion
3.1. Physical characteristics affected by NaOH concentration and composition. Fig. 1 shows the temporal variations of the slump-f lows for the samples prepared using
different amounts and different concentrations of NaOH solution. The flows of 1M-30% and 5M-50% at 2 h could not bemeasured because of the rapid solidification. The flow at eachtime increased with increase of the amount of NaOH solution,
among the geopolymers prepared using 1 M NaOH solution(Fig. 1(a)). The flow at each time decreased with increase of
the NaOH concentration comparing among the geopolymers
which 50% weight of the NaOH solution to FA and BFS wasadded (Fig. 1(b)). The temporal variations of the compressive
strengths are shown in Fig. 2. The strength decreases withincrease of the amount of NaOH solution (Fig 2(a)). The
strengths of geopolymers prepared using 2–5 M NaOH solution were higher than those using 1 M NaOH solution, but
comparable to each other at each curing time (Fig. 2(b)).
Fig. 3 shows the temporal variations of heat generated during curing in the geopolymer samples prepared using 1–5 M
NaOH solutions. The cumulative heat showed steep increases
at an early stage, and the slopes became gradually gentler withtime. The heat flows, which the cumulative heat is differentiated with time, formed peaks earlier in the samples preparedusing higher-concentration NaOH solutions than in those withlower concentration. After peaking, the heat flows decreased
with time. Generally, the cumulative heat and heat flow are
related to the amount of reaction product and its reaction rate,
respectively. Comparing the f low of 1M-50% with that of
2M-50%, approximately 5% difference was observed at 0 h
and 1 h, but the difference increased to 20% at 2 h (Fig. 1(b)).
Ten percent difference was observed in the heat flows of thetwo geopolymers until 1.5 h, but the difference graduallybecame larger after 1.5 h. These heat flows indicated that thechemical reaction between the ingredients in 2M-50% was
more active than that in 1M-50% after 1.5 h. The flows of thesamples prepared using 3–5 M NaOH solution decreased more
steeply than those using the 1–2 M solutions because of more
active chemical reaction, as seen in the heat flows.
The cumulative heat was comparable between 20 h and 200
h among the samples prepared using 2–5 M NaOH solutions aswell as the strengths as shown in Fig. 2(b) and Fig. 3(a). The
sample prepared using 1 M NaOH solution showed the lowest
cumulative heat at the time, as well as the lowest strength. The
cumulative heat indicates qualitatively that the chemical reac-

250

(a)

200

200

150

150

Flow / mm

Flow / mm

250100
1M-30%
1M-35%
1M-40%
1M-50%

50
0
0

0.5

1.0
1.5
Time / h(b)100

1M-50%
2M-50%
3M-50%
4M-50%
5M-50%

50
2.0

9

0
0

0.5

1.0
1.5
Time / h

2.0

Figure 1. Temporal changes of the slump-flows for samples prepared using various amount of 1 M NaOH solution (a)and using constant amount of 1–5 M NaOH solution (b).

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J. Nucl. Radiochem. Sci., Vol. 22, 2022

(a)

30

Compressive strength / MPa

Compressive strength / MPa

1020

1M-30%
1M-35%
1M-40%
1M-50%

10

0
0

100

200
Time / d

300

(b)

30

20

0
0

400

1M-50%
2M-50%
3M-50%
4M-50%
5M-50%

10

100

200
Time / d

300

400

(b)
100
-1

10

(a)

2

101

100

10-1
100

1M-50%
2M-50%
3M-50%
4M-50%
5M-50%
101
102
Time / hHeat flow / mW g

Cumulative heat / J g-1

Figure 2. Temporal changes of the compressive strengths for samples prepared using various amount of 1 M NaOH solution (a) and using constant amount of 1–5 M NaOH solution (b).

10-1
10-2
10-3
100

1M-50%
2M-50%
3M-50%
4M-50%
5M-50%
101
102
Time / hFigure 3. Temporal variations of cumulative heats (a) and heat flows (b).

tion that occurs at 20–200 h dominates the strength. We shouldnote that the strength is dominated by not only the chemical
reaction observed as the cumulative heat, but also by theporosity, etc. in the sample.
The cumulative heat increased slightly after around 200 h in
the samples prepared using higher-concentration NaOH solutions. The heat f low showed a more prominent peak in thesample with higher alkaline concentrations after 200 h, andthen the heat flows decreased with time. Therefore, the cumulative heat reached almost saturation after 1 month (= 700 h).
The heat generated at 1 month is not related to the strength
because a significant difference was not observed in the trendof their strengths at 7–30 d.
The Si/Al ratio of the geopolymers prepared in this work is
estimated to be 2.4 on the basis of the elemental compositionsof FA and BFS (Table 2). The Si/Al ratios in general FA andBFS vary between approximately 2 and 3, including the materials used in the present work 23–27. The ratio varies dependingon the region and factory, etc. where the materials are manufactured. The strength should vary by the ratio17, 18. However,the present work shows standard results because the ingredients possess standard chemical composition.
3.2. Adsorption performance characterized by NaOH

concentration and composition. The Kd of Co, Sr, and Cs foreach sample are shown in Fig. 4 together with pH of leachate.
The Kd values of Co were on the order of 103 in all samples
including cements. Generally, Co forms precipitate of hydroxide in alkaline solution, including pH range in the present
work, because of its low solubility28. Larger Kd results fromadsorption of the hydroxide of Co on bodies of geopolymer
and cement rather than from a structural mechanism.
The Kd values of Sr and Cs were comparable to each otheramong the samples prepared using different amounts of 1 M
NaOH solution. The amount of the NaOH solution was not
related to the K d . Both cement samples of OPC-50% andBB-50% showed 1 order lower Kd than the geopolymers using
1 M NaOH solution. Comparing samples of 1M-50% through
5M-50%, the Kd for both elements increased with the concentration of NaOH solution. The Kd of the geopolymers increased
as NaOH concentration increased. The solubility of CsOH andthe solubility product Sr(OH)2 in water are known to be 395
g/100 mL at 15 °C and 3.2×10 – 4 mol 3/L 3 at 25 °C, respectively29, 30. Approximately 2.2 g of Sr2+ can be dissolved in 1
liter at pH = 13.05, equal to OPC-50%. More Sr(OH)2 can be
dissolved in a solution with lower pH than that. The samples of
1M-30% contained 390 ppm of Sr and 0.56 ppm of Cs, based13.5

pH (R-scale)
Co
Sr
Cs

5

10

4

[105]

E

10

12.5

1

10

0

10

-1

P
P

E

M M M

Intensity

2

10

E, P

1.5

13.0

3

E

pH

Kd / mL g

-1

10

1.0

CZ M

M

BB-50%
OPC-50%

Q

M
M M

M Q
Z MM

M

12.0

-5
0%

-5
0%

BB

O
PC

-5
0%

-5
0%

5M

-5
0%

4M

-5
0%

3M

-5
0%

2M

1M

5%

-4
0%

1M

1M

1M
-3

-3
0%

10

0.5

Figure 4. The pH of leachates (right (R) scale) and Kd values (leftscale) of Co, Sr, and Cs for the geopolymer and cement samples.

Eon the chemical composition of the ingredients shown in
Table 2. The concentrations in the cements and the other geopolymers were lower than 1M-30%. Even if all of the Sr andCs in the samples is dissolved during the Kd measurement, theSr and Cs concentrations are lower than the solubility. The pH
does not contribute to the Kd variations for Sr and Cs. Trendsof the Kd for Sr and Cs are discussed below with products
observed by the XRD.
Fig. 5 shows the XRD spectra of the geopolymer andcement samples. Most of the peaks observed in OPC and BB
were assigned to be ettringite and portlandite, which are typical products of cement. The spectra show that less reactive
quartz and mullite presented in the FA remained in all the geopolymer samples. Ettringite was detected in the samples prepared using 1 M NaOH solution. The peaks decrease withincrease of the amount of the NaOH solution. On the other
hand, peaks of chabazite and p-type zeolite were greater withincrease of the concentration. The chabazite has been observedin geopolymers investigated by other several groups as
well31–34. Naghizadeh et al.33 investigated the products in thegeopolymers prepared using 1–3 M NaOH solution and cured
at 80 °C. The chabazite did not form in the samples using the 1
M s ol u t i o n b u t d i d fo r m u s i n g t h e 3 M s ol u t i o n .
Suksiripattanapong et al.34 reported the temporal variation of
XRD spectra in the geopolymers cured at different temperatures. Comparing the geopolymers cured at 65–85 °C, a larger
peak of chabazite was observed in the sample cured at higher
temperature at an early stage of curation. Takeda et al.35 investigated the formation of p-type zeolite in the mixture of FA
and 2–5 M NaOH solutions. The XRD peaks of p-type zeolite
at a certain time increased with increasing NaOH concentrations, but the peaks were not observed in the sample treatedwith the 5 M NaOH solution. The magnitude of the XRD
peaks increased with time and reached a constant after a few
days.
The crystal structures of chabazite and p-type zeolite arereported by Yakubovich et al.36 and Chen et al.37. Yang et al.38
investigated the Kd of Sr and Cs on the chabazite in the solutions containing K, Cs, Ca, and Sr with different ratios. WhenK and Ca did not present in the solution, the Kd was on theorder of 10 6 for Cs and 105 for Sr. The Kd of both Sr and Cs
decreased with increasing concentrations of K and Ca because
of interference. In addition, Mimura and Akiba39 investigatedthe Kd on p-type zeolite varied by Na concentration and pH in
solution. The Kd of Sr and Cs showed approximately 10 4 insolution with Na concentration lower than 10 –3 M, but
decreased with increasing Na concentrations in the solutions.

11

J. Nucl. Radiochem. Sci., Vol. 22, 2022

Physical and adsorption characteristics of geopolymers prepared using

0

10

E

E

20

30
40
2 (Cu)FA
BFS
5M-50%
4M-50%
3M-50%
2M-50%
1M-50%
1M-40%
1M-35%
1M-30%

50

60

Figure 5. XRD spectra of the geopolymers, ingredients, andcements. C: chabazite, E: ettringite, M: mullite, P: portlandite, Q:
quartz, Z: p-type zeolite.

The Kd for both elements were larger in an alkaline solution
than in an acid solution. The chabazite and p-type zeolite are
related to the adsorption of Sr and Cs on the geopolymer. The
poor adsorption on the cements results from less formation ofthese products, as seen in the XRD spectra. In the present
work, Sr showed the Kd 1–2 orders of magnitude greater thanCs in the geopolymer samples, in contrast to these investigations. The magnitude of Kd is affected by the concentrations ofelements eluted from the geopolymers that interfere with theadsorption of Sr and Cs.
3.3. Possibilities of further improvement for actual use.
A geopolymer with a larger flow is preferable because void
spaces between wastes can be easily filled due to higher flowability, when used for the immobilization of large-volume
radioactive wastes as a filler. The geopolymer prepared using
more volume of 1 M NaOH solution resulted in improvement
for the flow but degradation for the strength. The same trend
should be observed in the geopolymers prepared using 2–5 M
NaOH solutions according to basic concept in the concrete 40.
A mortar prepared with a weight ratio of cement : fine aggregate : water for 10 : 30 : 5, which is the same composition as
the present work, showed a compressive strength of 22 MPa at
an age of 28 days40. The strengths of geopolymers in the present work are lower than this mortar by more than 30%. The
mortars prepared using more water resulted in degradation forthe strength as well as the geopolymers. Wastes which sludge
and resin, etc. are immobilized with cement showed thestrengths equal or less than the cement mortars41. The strength
would be degraded for the geopolymers which these wastes
are immobilized as well as the cementation.
On the other hand, the geopolymers prepared using NaOH
solution with a higher concentration would be suitable forimmobilizing radioactive wastes to suppress leaching.
However, the geopolymer prepared using 1 M NaOH solution,
which is not classified as a deleterious substance, provided
better adsorption than cement. For actual use as a filler, fineaggregates are mixed to increase the volume of a geopolymer
at low cost. The Kd of aggregates is reported to be 2.0–1.0×105

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J. Nucl. Radiochem. Sci., Vol. 22, 2022

(mean: 640) for Co, 0.4–2400 (mean: 22) for Sr, and 9.6–
3.5×104 (mean: 530) for Cs42. In particular, the adsorption of Cs
can be expected to be improved by mixing in fine aggregates.
Co and Sr would adsorb to the geopolymer part rather than theaggregates based on the Kd.
We reported the physical and adsorption properties of thegeopolymer prepared using NaOH solutions with different
concentrations in the present paper. Use of lower concentrations and less amounts of NaOH solution enables to decrease
the cost for the geopolymers. Adsorption performance for Cs
would be greater than the data in the present work for actual
use as f ine aggregates are mixed in the geopolymers.
Flowability and strength are affected by the amount of NaOH
solution. In contrast, adsorption performance is not varied
regardless of the amount of the solution. Concrete admixtures
are frequently used to improve flowability and delay time toset. All the commercial admixtures are not necessarily effective with geopolymers. If suitable admixtures are mixed in thegeopolymer, the flowability of the one prepared using higher
concentration of NaOH solution would be improved43. In addition, a certain flowability would be maintained even though
the amount of NaOH solution is reduced. As mentioned above,the strength can be improved by using less amount of NaOH
solution. Both strength and adsorption of the geopolymers
using 1 M NaOH may surpass those of the cements by further
investigation. The present work will serve as basic data forfurther improvement in workability and strength while maintaining a certain adsorption performance depending on theconcentration of NaOH solution.
4. Conclusion
The temporal variations of the compressive strengths andslump-flows, and distribution coefficients for fly ash-based
geopolymers were investigated for possible use as filler for theimmobilization of large-volume radioactive wastes. All thegeopolymers prepared using different amounts of 1 M NaOH
solution achieved strength of approximately 20 MPa at 1 year.
The geopolymers using more NaOH solution showed greaterf lowability. When 2–5 M NaOH solutions were used, thef lows decreased as the concentration of NaOH solution
increased, but the strengths were comparable with each other.
The strength and temporal change of the flow were explainedby the amount of heat generated during an early stage of curation. On the other hand, distribution coefficients, Kd, of Sr andCs, one of the indicators to evaluate the adsorption behaviorsof these elements, improved with increase in concentrations of
NaOH solution. Geopolymers showed greater distribution
coefficients than cement by a factor of more than 10. Co
showed comparable Kd regardless of the concentration andcomposition of the ingredients. The trends of Sr and Cs wererelated to the amount of p-type zeolite and chabazite observed
in XRD spectra. Adsorption performance for Cs would begreater than data in the present work for actual use as fineaggregates are mixed. The present work will serve as basic
data to improve the strength and flowability using concrete
admixtures. We need further investigation to immobilize
radioactive wastes by the geopolymer prepared using lower
concentration and less amount of NaOH solution.
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