Radium content and radon exhalation rate from sulfur polymer composites (SPC) based on mineral fillers

Construction and Building Materials 198 (2019) 390–398

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Radium content and radon exhalation rate from sulfur polymer composites (SPC) based on mineral fillers Piotr Szajerski a,⇑, Joanna Celinska a, Henryk Bem b, Andrzej Gasiorowski a, Rafal Anyszka c, Piotr Dziugan d a

Institute of Applied Radiation Chemistry, Lodz University of Technology, Wroblewskiego 15, 93-590 Lodz, Poland The President Stanislaw Wojciechowski State University of Applied Sciences in Kalisz, Nowy Swiat 4, 62-800 Kalisz, Poland c Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland d Institute of Fermentation Technology and Microbiology, Lodz University of Technology, Wolczanska 171/173, 90-924 Lodz, Poland b

h i g h l i g h t s
Efficient method for incorporation of industrial waste into polymeric matrix.
Ra-226 concentration in NORM based sulfur polymer concrete (SPC) measured.
Radon exhalation rate for NORM-SPC composites determined and analyzed.
Reduction of radon exhalation rate/emanation coefficient from SPC composites.
Correlation of binder composition and Rn-222 exhalation rate/emanation coefficient.

a r t i c l e

i n f o

Article history: Received 21 May 2018 Received in revised form 7 September 2018 Accepted 27 November 2018

Keywords: Radon exhalation rate Sulfur polymer concrete Building materials radioactivity Phosphogypsum Fly ash Slag NORM TENORM

a b s t r a c t Several samples of materials of potential interest as component in building materials, especially in sulfur polymer concrete (SPC) composites, were checked for their radium content and radon exhalation rate. However, the radiation hazard strongly depends on radon exhalation rate from these materials, the influence of SPC composition was investigated concerning radon exhalation and emanation from the composite matrix. The accumulation method using RAD7 device was used for specific exhalation rate determinations. The activity concentrations of Ra-226 in the man-made tiles were in the range of 6.1–593 Bqkg1. The Rn-222 exhalation rates from these materials were in the range of 103 mBqm2 h1–9.22 Bqm2 h1, whereas from the SPC composites from 40.2 to 438 mBqm2 h1. SPC manufacture technology and sulfur polymers applied for composites preparation allow for nearly 50-fold decrease of radon emanation coefficient. Different factors (e.g.: physical form of additives or polymer composition and its weight fraction) affecting radon emanation coefficient and its exhalation rate from examined raw materials and composites were discussed. Using a standard room parameters, the Rn-222 concentration resulting from its exhalation from the walls and annual internal doses were estimated indicating for satisfactory results even for raw materials of the highest Ra-226 concentration. Ó 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction There is increasing demand for introduction of innovative schemes (zero-waste technologies) for wastes produced in the multitude industrial processes such as: phosphogypsum, mine tailings, metallurgical slugs, coal ash or water purification residues. Very often naturally occurring radioactive materials (NORM) dur⇑ Corresponding author. E-mail addresses: [email protected] (P. Szajerski), [email protected] (J. Celinska), [email protected] (H. Bem), [email protected] (A. Gasiorowski), [email protected] (R. Anyszka), [email protected]. pl (P. Dziugan).

ing industrial activities are concentrated in the wastes and these so-called TENORM (Technologically Enhanced Naturally Occurring Radioactive Materials) wastes, e.g. coal ash, red mud and phosphogypsum are extensively used in the construction industry. Radiation exposure due to building materials can be divided into external and internal exposure. External c-ray exposure from natural radionuclides is caused mainly of the uranium (U-238) and thorium (Th-232) series, and the radioactive isotope of potassium (K-40) present in building materials. Radiological protection principles concerning the natural radioactivity of building materials, including derivation of activity concentration indices for such byproducts have been recommended in EU guidances [1] and in the recent EU directive [2]. However, a part of the total radiological

https://doi.org/10.1016/j.conbuildmat.2018.11.262 0950-0618/Ó 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

P. Szajerski et al. / Construction and Building Materials 198 (2019) 390–398

hazard may come from inhalation of Ra-226 decay product – gaseous radionuclide Rn-222 and its a-particle emitting short-lived daughters, because it is well known that building materials can generally contribute to radon indoors in a limited way. So, the one of the main parameters influencing the indoor radon concentration is its exhalation from the building materials. In practical conditions the Rn exhalation rate is a composed function of Ra226 content in material and Rn-222 diffusion length, which in turn, depends on several physical parameters of the material (humidity, porosity and geometry). Several authors have described the determination of the exhalation rate and emanation factor of materials [3–11]. A special attention has been paid to investigation of the possibility of using phosphate industry byproduct called phosphogypsum (PG) as a component of the building materials. Large quantities of PG are produced worldwide, and the world production is estimated at 200–280106 t per year [11]. However, the phosphogypsum byproduct contains elevated specific activity of Ra-226, usually between one and two orders of magnitude higher than the natural gypsum, depending on the uranium content of the phosphorite ore [12]. This fact is a serious restriction for the wide uses of this material, which is accumulated in regulated stock-piles and occupied vast areas of land, located in the town’s surroundings with the consequent environmental problems. Therefore, Rn-222 exhalation rates from materials containing PG are widely studied. A review discussing potential use of PG including Rn-222 exhalation problems has been published [13]. Regarding the application of industrial byproducts in a construction sector, many new promising technologies have been evaluated during last years. One of them is production of the sulfur polymer composites (SPC) based on mineral fillers – waste and byproducts from chemical and extraction industry, as well as from energetic sector. The aim of this study was to evaluate Rn-222 exhalation rate from raw materials with limited application in a traditional building industry, as well as from sulfur polymer concrete samples based on these fillers.

2. Materials and methods 2.1. Samples preparation A few compositions of sulfur polymer binders were prepared for the purpose of SPC radon exhalation rate studies. In all instances, the basic component of the polymer matrix was pure sulfur (S; KiZPS Siarkopol, Poland), and as co-monomers modifying polymer phase properties, styrene (STY; Brenntag Polska Sp. z o.o.), turpentine (TRP; Avantor Performance Materials Poland S.A.), dicyclopentadiene (DCPD; Brenntag Polska Sp. z o.o.) and furfural (FUR; Sigma-Aldrich Polska Sp. z. o.o) were used. Details of the sulfur polymers used are presented in Table 1. Raw waste materials used as mineral fillers for production of SPC samples were lignite fly ash, lignite slag and phosphogypsum. All fillers were provided by Polish suppliers. Lignite fly ash and slag were kindly provided by PGE Gornictwo i Energetyka Konwencjonalna SA Oddzial Elektrownia Belchatow, and phosphogypsum by Grupa Azoty Zakłady Chemiczne ‘‘Police” SA. Phosphogypsum was produced from phosphorites of African origin. Composite samples were prepared by sulfur polymer synthesis followed by introduction of mineral fillers. Basically, three or four component samples were prepared, where the first component was continuous phase of sulfur polymer, second and third strength providing mineral sand (2 mm) and coarse Table 1 The binders (SP1-5) used for SPC samples preparation. Component

Sulfur (S) Styrene (STY) Turpentine (TRP) Dicyclopentadiene (DCPD) Furfural (FUR)

Fraction in (w/w): SP1

SP2

SP3

SP4

SP5

1.000

0.900 0.100

0.950 0.025 0.025

0.950

0.900

0.050

0.050 0.050

391

gravel aggregates (2 cm) fractions, and finally fourth, fine fraction of waste filler. SPC samples were prepared by hot mixing of sulfur polymer with gradually added fillers. Detailed samples composition is presented in Table 2. Two stage process was applied for samples preparation. At the first step, pure sulfur polymer was synthesized by melting sulfur (ca. 120 °C) followed by progressive introduction of comonomer. Then, the reaction mixture was continuously stirred and maintained at a temperature range 130–140 °C for about 6 h. After that, polymeric product was poured into flat metal forms. The second phase of samples preparation was melting of the prepared previously sulfur polymer at 130 °C, and after reaching liquid state, preheated to ca. 140 °C mineral fillers were progressively introduced to the hot liquid polymer. In order to provide good homogenization of the composite after complete introduction of all components, hot blend was additionally mixed for ten minutes. Samples were casted in a preheated to ca. 140 °C steel molds (100  100  100 mm3). Samples were unmolded after 24 h conditioning time. Sulfur polymer synthesis as well as SPC samples preparation were carried out in a commercially available 50 dm3 capacity, stainless steel closed blender equipped with oil heating system (type VSM-C, Glass GmbH, Germany). 2.2. Ra-226 determination The Ra-226 activity concentration in samples was determined both in the raw materials used for samples preparation, as well as in the final SPC samples. Samples for analysis were crushed (particle size <2 mm), dried overnight at 60 °C, packed into 1 dm3 Marinelli beakers and sealed with silicone to prevent radon loss. Samples were left for at least 30 days to reach radioactive equilibrium between Ra226 and its progeny Pb-214 and Bi-214. For c-ray spectrometric measurements the reversed germanium coaxial detector was used, type GX2018 from Canberra Industries (Meriden CT, USA) equipped with In-Spector gamma spectrometry system with 4096 channels MCA. Detector resolution was 0.9 keV for the 122 keV peak and relative efficiency was 20% for the 1.33 MeV Co-60 peak. All gamma-ray spectra were collected during 80,000 s inside the 10 cm thick wall lead shield lined with the 2 mm copper cylinder inside sample chamber. Some additional information for radiation measuring system are included elsewhere [14–16]. The Ra-226 activity concentration was determined taking into account its 186.2 keV photons, as well as photon emissions from its daughters in radioactive equilibrium: Pb-214 (242.0, 295.2 keV) and Bi-214 (609.3, 1120.3 and 1764.5 keV). Collected spectra were analyzed using Geenie2k software. Each time correction for the background radiation (background spectrum collected during 500,000 s) was performed. Absolute photon detection efficiency for the gamma spectrometry system used for a given conditions (1 dm3 Marinelli geometry) was determined using the set of sealed artificial standards, prepared with representative samples (low background fine aggregate) spiked with known activities of Ra-226 and Th-228 standards (based on CRM 101-A, pitchblende ore and CRM 106-A, monazite sand standards, New Brunswick Laboratory, USA). Ra-226, Pb-214 and Bi-214 photons emission rates from standards were used for calculation of the total efficiency determination, according to dependence edet = Inet/(Asxem), where edet – detection efficiency for a given gamma photon energy line, Inet – background corrected count rate in cps, As – Ra-226 or Th228 activity concentration in standard in Bq, xem – gamma photon emission probability. Rearranging and taking into account net photons emission rates from investigated samples allowed for determination of Ra-226 activity concentration in raw materials and sulfur polymer composites, according to the dependence As = Inet/(medetxem). Ra-226 activity concentration was calculated as an average of Ra-226, Pb-214 and Bi-214 concentrations. 2.3. Rn-222 exhalation rate measurements The Rn-222 exhalation rate measurements from investigated samples were performed by means of closed chamber method using Durridge RAD7 radon detector. Raw materials were dried at 105 °C overnight and used in their native form (powders and granulates without previous mechanical processing), whereas for measurements of SPC samples solid samples were used in the form of flat plates cut from the composite block. The same form of raw materials were used for preparation of the SPC samples. Choice of different form of fillers and composite product was due to the fact, that verification of radon exhalation rate should be done for the most realistic scenario, as it would be done in real application. Both raw materials as well as SPC samples were closed in a sealed steel container in a central position of the chamber. Measurements were carried out in a constant geometry of cylindrical source of about 10 mm thickness and 90 mm diameter. Accumulation chamber was equipped with a small fan in the lower part of the container providing good mixing of the gas inside container volume. RAD7 detector was connected through drying tube filled with Drierite (CaSO4 + 2% CoCl2), and relative humidity during experiment did not exceed 6%. Scheme of the experimental setup is presented in Fig. 1. 2.4. Rn-222 exhalation rate model The most common method for studying radon exhalation phenomenon is the closed chamber method based on the Rn-222 accumulation kinetics. The proper analysis of the kinetic data reflecting radon concentration buildup in time should

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Table 2 Raw materials and composition of SPC samples used for Rn-222 radon exhalation rate measurements. Sample

Fraction in (w/w): SPC01

SP1 SP2 SP3 SP4 SP5 Fine aggregate Coarse aggregate Lignite fly ash Lignite slag Dolomite PG-S1 (phosphogypsum) PG-S2 (phosphogypsum) Rubber aggregate GraphitePMM-11/99.5

SPC02

SPC03

SPC04

SPC05

SPC06

SPC07

SPC08

SPC09

0.200

0.250

0.200

0.222

0.258

SPC10

SPC11

0.300 0.250

0.250

0.250

0.625

0.625

0.610

0.600

0.125

0.1175

0.125

0.100

0.200 0.600

0.625

0.200

0.125

0.156 0.544 0.078

0.064 0.587

0.258 0.258 0.131 0.520

0.131 0.520 0.091

0.600 0.091 0.091 0.015 0.0075

experimental system [19]. This contribution is represented by the simple exponential decay kinetic component involving combined Rn-222 decay-leakage-backdiffusion rate, resulting in Eq. (3):



 P  P Eo S kL C B  P þP 1  e ki t þ C o e ki t V ki ki

C ðt Þ ¼

ð3Þ

where Co – the initial Rn-222 concentration in the experimental setup in Bqm3. All presented data, and results were calculated taking into account model represented by Eq. (3). The equilibrium concentration (C1) of Rn-222 inside experimental loop is obtained for t approaching infinity, and is given by Eq. (4):

C1 ¼

V

Eo S kL C B P þP ki ki

ð4Þ

and from this dependence one can easily calculate radon exhalation rate, Eo in Bqm2 h1, given by Eq. (5).

 V X Eo ¼ C 1 ki  kL C B S

ð5Þ

For a given experimental setup one has to determine radon leakage rate from the measurement loop, what can be easily achieved by the activity decay measurement of previously injected portion of Rn-222 to the experimental setup. For investigated setup, measured leakage rate was kL = 2.70103 h1. For calculation of the back diffusion rate (kB) one can apply Eq. (6), estimation given in work, where authors studied model for Rn-222 release from phosphogypsum boards [20]:

kB ¼ kRn Fig. 1. Experimental setup for Rn-222 exhalation rate measurement.

ð1Þ

where C – Rn-222 concentration in the measurement chamber in Bqm3 at time t, Eo – areal radon exhalation rate in Bqm2 h1, S – surface area of the sample in m2, V – volume of the experimental setup in m3, kRn, kL and kB – Rn-222 decay constant, leakage rate and back diffusion rate respectively in h1; CB – is the Rn-222 background concentration in the laboratory room (ca. 15 Bqm3 in this case). The solution of the presented above 1st order linear differential equation is well known and can be presented by Eq. (2).

 C ðt Þ ¼

V

 P  Eo S kL C B  P þP 1  e ki t ki ki

ð6Þ 3

include such components as Rn-222 radioactive decay, leakage rate from the experimental setup and back diffusion process. Buildup of the Rn-222 activity is dependent on the exhalation rate from the sample and radon background inside laboratory room. The basic differential equation involving all these parameters can be presented as follows by Eq. (1):

dC Eo S ¼ þ kL C B  kRn C  kL C  kB C dt V

VS V

ð2Þ

where Rki = kRn + kL + kB is the combined radon decay-leakage-back diffusion rate. Presented model has been extensively studied and found to be effective in many experimental setups [17,18], however, for small exhalation rates, low radium content and low measured Rn-222 concentration, kinetic model should involve also radon contribution from initial Rn-222 level inside accumulation chamber. For that purpose model applied for fitting experimental data was modified by the component related to the Rn-222 fraction unbound with the exhalation process in the

where VS is radon source volume in m . In case of sources used for measurements, back diffusion rates were within the range of 3.06105–4.45104 h1. The mass exhalation rate, Em in Bqkg1 h1, for investigated samples can be calculated taking into account weight of the sample m and its surface area S, according to Eq. (7) [21].

Em ¼ Eo

S m

ð7Þ

To describe in a quantitative way the potential radon risk due to material, so called radon potential, XRn in Bqkg1, can be calculated using Eq. (8). By definition, the radon potential, called also effective radium concentration, reflects the fraction of the total radium within the material contributing to radon exhalation [19,22]. Dependence between radon potential and total radium concentration can be presented as Eq. (8):

XRn ¼ eARa

ð8Þ

where e is a dimensionless radon emanation factor, independent of experimental conditions and representing fraction of radium, which effectively produces radon reaching pores of material and can be exhaled outside the material boundaries, and ARa is Ra-226 activity concentration in material, in Bqkg1. Radon emanation fraction, e, can be calculated taking into account relation between areal radon exhalation rate and Ra-226 concentration, Eo = kRneARam/S (symbols as previously). 2.5. Dose assessment from Rn-222 Evaluation of the radiological hazard from Rn-222 and its progeny requires calculation model, which allows for estimation of radiation dose for people from a radiation. Among various models available [23], one of the most frequently used

P. Szajerski et al. / Construction and Building Materials 198 (2019) 390–398 is based on the recommendations included in the publication of United Nations Scientific Committee’s on the Effects of Atomic Radiation (UNSCEAR) [24–27]. This model allows for calculation of the annual effective dose from exposure to radon, according to Eq. (9):

Eeff ¼ C Rn  F  T  DCF

ð9Þ 1

where Eeff – is the effective indoor dose rate in mSvy , CRn – time averaged radon concentration in Bqm3, F – a dimensionless equilibrium factor between Rn-222 and its progeny, typical recommended value F = 0.4 [28,29], T – indoor occupancy time of 7000 h, and DCF – recommended dose conversion factor of 16.75 nSv m3 Bq1 h1 [30]. Time averaged radon concentration was estimated by Eq. (10), which takes into account indoor air exchange rate, kV.

C Rn ¼

Eo S VkV

ð10Þ

3. Results and discussion 3.1. Ra-226 activity concentrations in raw materials and SPC samples International [1,2,31] and national [32–36] regulations regarding natural radioactivity of building materials take into account three main natural radionuclides: K-40, Ra-226 and Th-228. All of them are important due to their significance in c-ray exposure, however Ra-226 concentration is under special supervision because of a-ray exposure from Rn-222 and its possible release into indoor atmosphere. From this reason, for Ra-226 concentration independent concentrations limits are established in order to quantify and control a-ray exposure of Rn-222 and its progeny Po-218 and Po-214. Currently, utilization of many industrial residues with potential application in building industry is limited due to increased Ra-226 concentration and excessive Rn-222 exhalation rate contributing to above reference level concentration (typically 100-300 Bqm-3) of indoor radon. These limitations do not apply to radium bearing waste directly, in the sense of their quantity, but arise indirectly from established in most countries annual dose reference levels, that is total annual dose must not exceed reference level, regardless of the dose contribution from each radionuclide taken into consideration. In order to apply such materials in building industry, only solutions resulting in reduction of radium concentration allow for real adjustment to the regulations in force. In practice this would mean, in most cases, necessity of introduction costly chemical modification of the waste products. For many reasons, among which financial aspects are the most important, these solutions are not being introduced in technological practice. But for waste products, for which incorporation into building materials is limited due to increased Ra-226 content, one can imagine solutions based on such reprocessing technologies, which allow for materials properties modifications resulting in lower radon exhalation rate from waste based product, in comparison with raw materials. Potential methods should provide such modification of material, that resulting product would have also lower radon emanation coefficient comparing with raw material. As Rn-222 originates from Ra-226 radioactive decay within the mineral matrix, the key parameter for the analysis of the exhalation process is the total concentration of Ra-226 isotope in a material. For the raw materials studied in this work, gamma spectrometric measurements revealed Ra-226 activity concentrations within the range of few Bqkg1 up to ca. 600 Bqkg1. For verification, Ra-226 activity concentrations in SPC samples (ARa-226) were also calculated taking into account activity of the particular component (Ai) used for composite samples preparation and its weight fraction (wi) according to Eq. (11). No contribution from sulfur polymer fraction to the total Ra-226 activity in SPC samples was assumed.

ARa226 ¼

Xn

i¼1

wi Ai

ð11Þ

393

Ra-226 activity concentrations obtained from gamma spectrometric measurements correspond well to these obtained from calculation model applied and are presented in Table 3. Discrepancies observed between the calculation model based results and experimental data, in most cases do not exceed 15% and should be attributed to slightly different measurement conditions when shifting from raw materials measurements to SPC samples analysis: increasing samples density, decreasing samples porosity and possible influence of these factors on detection efficiency coefficients. Measured specific activity varies from 6.1 Bqkg1 (for fine aggregate filler) up to 593 Bqkg1 for phosphogypsum (PG-S1). It is useful to divide these materials into two categories, where to the first group fine and coarse aggregates, as well as natural stones and lignite combustion wastes should be classified due to common values of Ra-226 activity concentration, met frequently for other building materials and raw materials used for their production [12,37]. As a second group of materials phosphogypsum samples should be indicated, where Ra-226 concentration is much higher and in consequence, application of this material in building industry is much more limited due to potential excessive radiation exposure. Measured Ra-226 activity concentration, 593 and 476 Bqkg1for PGS1 and PG-S2 samples respectively, are typical for phosphogypsum [11,38]. 3.2. Rn-222 exhalation rates from raw materials and SPC samples Currently, when building materials industry is far dependent on waste fly ash and slags, special attention should be paid on verification if and in what amount Rn-222 is exhaled from particular material of interest. For that purpose many solutions focused on the radon exhalation rate measurements were introduced to laboratory practice [39–45]. Next to Rn-222 buildup kinetics, radon exhalation rate can be also measured using gamma spectrometry technique [43,46,47], comparing Rn-222 progeny Pb-214 and Bi-214 concentration in sample in open and closed geometry conditions. Procedure requires difficult and time consuming standardization procedure. Radon exhalation rate measurements performed by means of the closed chamber method, provides accurate Rn-222 activity concentration accumulation data and allows for analysis of the material properties concerning Rn-222 exhalation rate. This is especially important for waste based building materials, where Ra-226 containing components may be present. Rn-222 exhalation measurements were carried out for several weeks (typically 7–21 days) for each sample. Taking into account calculation model represented by Eqs. (3)–(8), values obtained from radon accumulation data allowed for determination of areal and mass exhalation coefficients, as well as effective radium concentration. These data are presented in Table 3. Sample Rn-222 accumulation curves for pure phosphogypsum (PG-S1) and phosphogypsum based SPC sample (SPC09) are presented in Fig. 2. In ideal conditions, with zero leakage rate and negligible back-diffusion contribution, radioactive equilibrium should be reached after ca. 4–5 weeks. In practice, for any measuring system, leakage phenomenon cannot be avoided, what results in faster approaching to equilibrium state. Presented in Fig. 2 data clearly indicate that for observed accumulation curves radioactive equilibrium is reached after ca. 360 h, that is about 2 weeks. Similar results were observed also for other samples measured. Experimentally determined, according to Eq. (5), areal radon exhalation coefficients of raw materials used for SPC samples production varied from 103 mBqm2 h1 for lignite slag up to as high as 9.22 Bqm2 h1 for phosphogypsum. For SPC samples, corresponding values were within the range of 40.2–438 mBqm2 h1, and are comparable with traditional building materials [5,7,48,49]. All these data are presented in Table 3. Obtained values confirm, that

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Table 3 Ra-226 activity concentrations in raw materials and SPC samples measured by gamma spectrometry and calculated according to Eq. (4), areal – Eo and mass – Em exhalation coefficients, e – radon emanation factor (errors < 15%) and radon potential – XRn; in parentheses main mineral components in a sample, na – not applicable. Sample

Density kg/m3

Fine aggregate (KD) Coarse aggregate (KG) Lignite fly ash (FA) Lignite slag (SL) Dolomite aggregate (DM) PG-S1 (phosphogypsum) PG-S2 (phosphogypsum) SPC01 (KD, FA) SPC02 (KD, FA) SPC03 (KD, FA) SPC04 (KD, FA) SPC05 (KG, FA) SPC06 (KD, FA) SPC07 (KD, DM) SPC08 (KD, KG, FA) SPC09 (KD, KG, PG-S1) SPC10 (KD, KG, PG-S2) SPC11 (KD, KG, SL)

1812 1313 1122 672 1607 807 875 2029 2153 2053 2285 2256 2160 2387 2235 2203 2256 2256

Activity of Ra-226, Bq/kg Measured

Calculated

6.1 ± 0.6 44.4 ± 3.3 112 ± 7 54.3 ± 2.4 10.2 ± 0.7 593 ± 34 476 ± 28 20.2 ± 5.7 19.9 ± 6.5 21.3 ± 6.8 18.8 ± 6.3 8.8 ± 2.8 13.2 ± 3.3 7.2 ± 3.4 31.4 ± 2.2 74.1 ± 9.4 61.5 ± 9.6 15.9 ± 4.9

na na na na na na na 17.8 16.9 17.7 14.8 27.8 17.8 7.3 33.8 80.4 67.1 28.8

Fig. 2. Rn-222 exhalation kinetics from raw phosphogypsum (PG-S1) and SPC09 samples containing 0.091 wt fraction of PG-S1 filler.

significant modification of the material physical form, here addition of the continuous phase of the sulfur polymer binder to aggregates and fillers, results in essential modification of the radon exhalation rate, which would be very difficult to predict using simple calculation models. However, there are many publications concerning measurement of radon exhalation from building materials, there is only limited number of works referring directly to Rn-222 exhalation from sulfur polymer concrete composites. For comparison it is worth to invoke values of areal radon exhalation rate for phosphogypsum based SPC composites, where results presented in several papers indicate on about 2-, 3-fold higher values than presented here, in the range of 1.0–1.5 Bqm2 h1 [9,50,51]. Simultaneously calculated values for mass exhalation rate coefficients for raw materials, according to Eq. (6), were in the range of 12.0–1071 mBqkg1 h1. Similar values for SPC samples were from 2.5 up to 54.1 Bqkg1 h1. In order to make proper quantification of the relation of radon exhalation rates from raw materials with these obtained for SPC samples, one should take into consideration possible effects of sulfur polymers used as a continuous phase in investigated composites. First of all, in almost all cases one observes lower radon exhalation rate for SPC samples in

Eo mBqm2 h1

Em mBqkg1 h1

e

Xrn Bqkg1

323 ± 24 726 ± 13 198 ± 37 103 ± 13 366 ± 35 9222 ± 38 7731 ± 98 315 ± 51 251 ± 29 358 ± 33 132 ± 8 330 ± 56 354 ± 25 290 ± 19 97.8 ± 4.7 438 ± 12 232 ± 9 40.2 ± 8.6

37.5 ± 4.4 84.3 ± 9.8 23.0 ± 2.7 12.0 ± 1.4 42.6 ± 4.9 1071 ± 125 898 ± 104 36.6 ± 4.3 29.1 ± 3.4 41.6 ± 4.8 16.1 ± 2.0 38.3 ± 4.4 41.1 ± 4.8 33.7 ± 3.9 12.4 ± 1.6 54.1 ± 6.7 25.0 ± 2.7 2.5 ± 0.1

0.817 0.252 0.027 0.029 0.554 0.239 0.250 0.239 0.193 0.259 0.113 0.576 0.413 0.623 0.052 0.097 0.054 0.020

5.0 ± 0.5 11.2 ± 0.8 3.0 ± 0.2 1.6 ± 0.1 5.6 ± 0.4 142 ± 8 119 ± 7 4.8 ± 1.4 3.9 ± 1.3 5.5 ± 1.8 2.1 ± 0.7 5.1 ± 1.6 5.4 ± 1.4 4.5 ± 2.1 1.6 ± 0.1 7.2 ± 0.9 3.3 ± 0.5 0.3 ± 0.1

comparison with the raw component with the highest fraction in a given composite. This clearly indicates that SPC manufacturing process significantly affects physical properties of the raw materials, as expected. As the first reason, for elucidation of this effect, decrease of the material porosity should be considered. In case of raw materials in powder form with loose grains, porosity of these materials were determined to be in the range of 35.3–70.7%. For the investigated SPC samples observed porosity was only between 0.9 and 7.6%, and these values are in good agreement with those presented by Lopez and coworkers [51]. This results in a dramatic decrease of a material specific surface. Additionally, for materials in a powder form, pores structure can be considered as open, whereas for SPC composites one has to deal rather with closed pores, where only a fraction of total internal (pore) volume is able to exchange air with external atmosphere. All these factors greatly affect radon transport and lead to reduction of its efficiency, and in consequence to decrease of observed areal radon exhalation rate coefficients. Next to these observations, one should take into consideration also possible sulfur polymer effect on the areal radon exhalation rate. SPC samples were processed by means of the hot mixing process, where sulfur polymer is a continuous phase agglutinating mineral fillers used for composites production. During mixing process, the sulfur polymer acts as an encapsulating agent by forming thin polymeric layer around fillers grains embedded within composite matrix. Such behavior of sulfur polymer phase modified by DCPD or olefin hydrocarbon polymers was already confirmed in a several previously published works, where different fillers including phosphogypsum, cement kiln dust or class C fly ash were used for SPC manufacturing [51–53]. Exposure for a-radiation from Rn-222 and its progeny is directly related to Ra-226 activity concentration in a material from which radon exhale, but also strongly depends on other parameters. All factors as porosity, specific surface, Ra-226 distribution within the material grains, grain coverage degree by polymer film, etc. influence Ra-226 availability for emanation and exhalation processes. For all these reasons, only a fraction of total Ra-226 concentration can be considered as a real contribution for radon exhaled outside material. Instead of specific activity of radium, good measure of radon exhalation abilities is radon potential reflecting Ra-226 corresponding to effective radon concentration. Fig. 3 depicts this dependence for investigated SPC samples, where very good linearity of areal radon exhalation rate vs. radon potential index is observed.

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3.3. Factors affecting radon emanation coefficient In order to graduate, however only in a semi-quantitative way, the potential abilities of sulfur polymers used in this study to reduce Ra-226 emanating fraction, it is necessary to have closer insight into relation between emanation factors for mineral fillers and SPC composites. Taking into account experimental data, calculated radon emanation coefficients vary from 0.027 up to 0.817 for the raw materials, and in the range 0.020–0.623 for SPC composites. Surprisingly, for raw materials, the highest emanation coefficient was observed for fine aggregate filler (e = 0.817) with simultaneous the lowest Ra-226 concentration of 6.1 Bqkg1. Possible and reasonable explanation for that can be surface deposition of Ra-226 atoms on the material grains. Radon emanation factors for other SPC composites based on phosphogypsum, given by Lopez and coworkers [51], indicate for comparable values between 0.084 and 0.164 for phosphogypsum content from 10 to 50%. Contrary to results presented in this study, authors did not observe reduction of emanation factor when moving from pure phosphogypsum (e = 0.116) towards SPC composites [51]. Our results clearly suggest for significant decrease of emanation factor. For instance, the most illustrative examples could be that for phosphogypsum, where emanation coefficient was reduced from 0.239 to 0.097 or from 0.250 to 0.054 respectively for PG-S1 and PG-S2 phosphogypsum samples and their composites SPC09 and SPC10. Good visualization of this dependence is presented in Fig. 4, where for all investigated SPC samples, their emanation coefficients and sulfur polymer content, as well as mineral component and type of polymer used are indicated. However, sulfur polymer weight fraction within the SPC composites was studied in a narrow range from 0.20 to 0.30, the general observation is that increasing sulfur polymer content results in decreasing of radon emanation coefficient. Emanation coefficient can be the measure of a radium reduction factor, f, which could be defined as the ratio of raw material activity, Ao, and effective radium concentration XRn. Then f = Ao/XRn = e1. For investigated raw materials and SPC composites radium reduction factor would be within the range between 1.6 and 49.0. To recognize if the decrease of emanation coefficient is related to application of polymer binder, or also in part to sample physical form used during measurement, two series of measurements were performed for PG-S1 and PG-S2 phosphogypsum samples.

Fig. 4. Correlation of radon emanation coefficient vs. weight fraction and composition of sulfur polymer used for SPC composites preparation.

Results of these experiments are presented in Table 4 and indicate only for minor decrease of emanation coefficient related to physical form of the sample. For the same geometry of powdered samples and phosphogypsum plates of ca. 90 mm diameter and 10 mm thickness, emanation coefficients were 0.239 and 0.164 for PG-S1, and 0.250 and 0.153 for PG-S2 samples. The most frequent polymer (SP4) used is this work was based on sulfur (95%) and dicyclopentadiene (DCPD, 5%). It should be noted, that when SP4 polymer based set of samples is considered, for all 6 composites emanation coefficient decreasing tendency is very clear, when moving from 0.200 to 0.258 polymer weight fraction. Among samples with the same polymer weight fraction, better performance was observed for SP2 polymer (S 90%, STY 10%) before SP3 (S 90%, STY 5%, TRP 5%) and SP4 binders for 0.250 wt fraction, and SP4 polymer before SP5 (S 90%, DCPD 5%, FUR 5%) for 0.258 polymer weight fraction. It is important to emphasize, that above considerations are rather more quantitative than qualitative, but give an idea about direction which possibly allows for further improvement of the sulfur polymers properties in the context of reduction of radon emanation coefficient. Until now, there was no information in literature regarding effect of polymer composition on the observed radon emanation coefficients. 3.4. Density of sulfur polymer composites and its influence on Rn-222 exhalation rate One additional parameter of the material to be considered as a construction material is its density. When considering concrete and concrete-making materials, both can be classified due to their density for lightweight, normal weight and heavyweight [54]. Table 3 contains density data for all investigated SPC and raw

Table 4 Comparison of radon emanation coefficients for different physical forms of the samples. Sample form

Fig. 3. Dependence of areal radon exhalation coefficient vs. effective radium concentration for investigated SPC composites.

Powder, cylinder, d = 90, h = 10 Plate, cylinder, d = 90, h = 10 Plate, SPC composites with PG-S1or PG-S2

Emanation coefficient PG-S1

PG-S2

0.239 0.164 0.097

0.250 0.153 0.054

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Table 5 Effective dose – Eeff estimation due to Rn-222 exhalation from SPC composites without and for 0.5 and 2.0 h1 ventilation rates. Sample

Eo, mBqm2 h1

CRn, Bqm3 no vent.

Eeff, lSvy1

CRn, Bqm3 kV = 0.5 h1

Eeff, lSvy1

CRn, Bqm3 kV = 2.0 h1

Eeff, lSvy1

SPC11 SPC04 SPC02 SPC03 SPC09

40.2 132 251 358 438

8.5 28.2 53.6 76.5 93.5

403 1322 2514 3586 4387

0.13 0.43 0.81 1.16 1.41

6.1 20.0 38.0 54.2 66.3

0.03 0.11 0.20 0.29 0.35

1.5 5.0 9.5 13.6 16.6

materials used. According to ASTM standards (C 33 and C 29), in case of aggregates, lightweight materials are these for which loose bulk density does not exceed 1120 kgm3, for normal weight aggregates is between 1200 and 1760 kgm3, and for heavyweight aggregates within the range of 1760–4640 kgm3 [55,56]. Taking into account this classification, all mineral fillers and aggregates used for preparation of SPC samples can be classified either as lightweight (lignite slag, phosphogypsum), normal weight (coarse aggregate, fly ash, dolomite) or even heavyweight (fine aggregate). Density of the investigated SPC samples vary between 2029 and 2387 kgm3 and is generally lower, or sometimes comparable (SPC07, SPC04) with normal weight Portland cement concrete composites (PCC), for which bulk density usually is close to 2400 kgm3. Higher density of all SPC in comparison with aggregates and fillers used is a result of good filling of dead loads between higher fractions, which is additionally intensified by application of the continuous phase of binder (liquid during composite production process) able to penetrate air filled volumes. Due to the same reason, mentioned previously porosity of the investigated SPC samples is significantly lower comparing with PCC, however clear correlation between this parameter and radon exhalation rate, here cannot be found. Density and exhalation rate data of SPC presented in Table 3 do not exhibit any correlation (R2 = 0.14). It is important to point out, that within all investigated SPC samples, variation in density is only ca. ±8% (Table 3, average density of SPC composites q = 2208 kgm3), what greatly limits range of analysis. Simultaneous variation of other parameters (aggregate and fillers contribution, sulfur polymer binder composition) additionally makes this analysis more complex. However aerial and mass exhalation rate can be expressed as a function of the sample density (Em = EoS/m = Eo/qh), here density should be considered only as a geometry related parameter being consequence of the sample thickness (h), which does not contribute significantly to the resulting radon exhalation rate. Many authors indicate density as the crucial parameter influencing the radon exhalation process, but one should remember, that this influence should be considered only within the material with homogeneous properties and structure, like in case of single grain of material. Increasing the scale and including additional variables (porosity, composition), direct influence of the density is not obvious. 3.5. Radiation risk and dose estimation from Rn-222 Usually, the dose contribution from indoor radon exhaled from building materials can be assumed taking into account model room of dimensions 5.0  5.0  2.5 m3. In this case, essential parameters for dose estimation model are: total surface area of the walls, floor and ceiling S = 100 m2 and total air volume inside room V = 62.5 m3. Additionally, three scenarios assuming no ventilation case, as well as kV = 0.5 and 2.0 h1 room air ventilation rate, were taken into consideration. For analysis five SPC composites covering whole range of observed areal radon exhalation rate were chosen: SPC11, SPC04, SPC02, SPC03 and SPC09, for which Eo coefficients varied from 40.2 to 438 mBqm2 h1. Results of simulation are presented in Table 5.

As it is clearly indicated, only for scenario with no active ventilation, when indoor radon is able to accumulate (then kV = kRn), one has to deal with significant radiation hazard due to Rn-222 and its progeny. Calculated average radon concentration was in the range 8.5–93.5 Bqm3, respectively for room built of investigated SPC composites, what directly converts to annual effective equivalent dose in the range of 0.4–4.39 mSvy1. For both scenarios with forced mechanical ventilation and highest areal radon exhalation rate 438 mBqm2 h1, maximum Rn-222 concentration of 1.4 and 0.35 Bqm3 were found for 0.5 and 2.0 h1 ventilation rates respectively. These averaged Rn-222 concentration contributes only in a minor extent to the total dose at levels of 66.3 and 16.6 lSvy1 respectively. DCF value of 16.75 nSvm3 Bq1 h1, according to [30] and recommended since 2017 is almost twice as large as the previously recommended value of 9 nSvm3 Bq1h1 [24]. The DCF = 9 nSvm3 Bq1 h1 was the most commonly used value in similar calculation models before introduction new recommendation. This means, the radiological hazard due to radon exposure is also doubled. Moreover, current discussion concerning necessity of modification of DCF coefficient is still open and directs into further possible increase of this parameter [26,27]. The new epidemiological and dosimetric data suggest on necessity to increase value of DCF to even 20 nSvm3 Bq1 h1 [57] or higher. In such case, calculated previously and given in Table 5 effective equivalent dose values should be recalculated. 4. Conclusions Results presented in this work focus on the Rn-222 exhalation phenomenon from raw materials, mostly industrial waste, and sulfur polymer concrete composites based on these waste materials. Seven raw materials and eleven SPC composites were investigated toward determination of their radon exhalation rate and radon emanation coefficient. Materials of the highest radiological hazard among tested was phosphogypsum, with Ra-226 concentration reaching 600 Bqkg1 and exhalation rate over 9 Bqm2 h1. For lignite fly ash and slag, as well as, for other fillers typical Ra-226 concentrations and radon exhalation rates were observed up to ca. 110 Bqkg1 and 370 mBqm2 h1. Data analysis proved significant decrease of radon exhalation rates and radon emanation coefficients from sulfur polymer concrete composites based on tested waste materials as fillers. The highest radon exhalation rates from SPC composites were observed for phosphogypsum based materials reaching ca. 440 mBqm2 h1, what is comparable with traditional Portland cement based and ceramic building materials. Radon emanation coefficient in SPC composites was reduced even 50-fold toward raw materials. Radon exhalation rate measurements carried out for raw materials in a powder and solid form geometry, as well as, for solid SPC composites proved, that exhalation rate as well emanation coefficient reduction is related directly to application of sulfur polymer. Variation of sulfur polymer compositions indicate on possible orientation of final SPC properties depending on selection of organic sulfur modifiers. Potentially, further optimization of sulfur polymer composition is possible toward obtaining SPC composites of even better parameters.

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Conflicts of interest None. Acknowledgments This research project was supported by the Polish National Center for Research and Development (NCBR) under the Grant No.: GEKON1/O5/213122/26/2015 titled ‘‘Development and preparation for implementation of sulfur-polymer concrete manufacturing technology based on waste products from the energy and petrochemical sectors”. References [1]

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