Radioactivity of Five Typical General Industrial Solid Wastes and its Influence in Solid Waste Recycling

The level of radionuclides is an important index for the preparation of building materials from industrial solid waste. In order to investigate the radiological hazard of five kinds of typical general industrial solid wastes in Guizhou, China, including fly ash (FA), red mud (RM), phosphorus slag (PS), phosphogypsum (PG), and electrolytic manganese residue (EMR), the radiation intensity and associated radiological impact were studied. The results show that concentrations of 238U, 235U, 232Th, 226Ra, 210Pb, and 40K for different samples vary widely. The concentration of 238U was both positively correlated with 235U and 226Ra, and the uranium contents in the measured samples were all of natural origin. The radiation levels of PG, EMR, EMR-Na (EMR activated by NaOH), and EMR-Ca (EMR activated by Ca(OH)2) were all lower than the Chinese and the world’s recommended highest levels for materials allowed to be directly used as building materials. The values of the internal and external illumination index (IRa and Iγ, respectively) for FA and RM were higher (IRa > 1.0 and Iγ > 1.3 for FA, IRa > 2.0 and Iγ > 2.0 for RM). The radium equivalent activity (Raeq), indoor and outdoor absorbed dose (Din and Dout, respectively), and corresponding annual effective dose rate (Ein and Eout) of RM, PS, and FA were higher than the recommended limit values (i.e., 370 Bq/kg, 84 nGy/h, 59 nGy/h, 0.4 mSv/y, and 0.07 mSv/y, respectively), resulting from the higher relative contribution of 226Ra and 232Th. The portion of RM, FA, and PS in building materials should be less than 75.44%, 29.72%, and 66.01%, respectively. This study provides quantitative analysis for the safe utilization of FA, RM, PS, PG, and EMR in Guizhou building materials.


Introduction
General industrial solid waste refers to waste discharged in various industrial production processes, such as the waste of the electric power industry, aluminum industry, phosphorus chemical industry, coal chemical industry, metallurgy, and so forth.This includes fly ash (FA), red mud (RM), phosphogypsum (PG), phosphorus slag (PS), and electrolytic manganese residue (EMR).At present, most general industrial solid waste is mainly used as raw materials for construction, such as cement and concrete, as well as environmental functional materials and a source of valuable elements [1].In 2016, the comprehensive utilization of general industrial solid waste in China accounted for 48.0% of Minerals 2019, 9, 168; doi:10.3390/min9030168www.mdpi.com/journal/minerals the country's total utilization and disposal, and disposal and storage accounted for 21.2% and 30.7% [2], respectively.As mineral resources usually contain natural radionuclides, the activity of radionuclides, such as FA, PS, PG, RM, and EMR, in some industrial solid wastes tends to increase after roasting or electrolyzation.When those general industrial solid wastes are used to prepare building materials, the gamma rays released by radionuclides can pose a radiation hazard to the living environment.Natural radionuclides, ubiquitously spread in the natural environment [3,4], consist of three well-known radioactive series, that is, the actinium series originating from 235 U, the uranium series originating from 238 U, and the thorium series originating from 232 Th.In addition, there are several isolated radionuclides, such as 40 K [5,6].The radioactive decay chains of 232 Th, 238 U, 235 U, and 40 K are the main contributors to the dose of natural radiation [7].When the ratio of 235 U/ 238 U is less than 1%, the contribution of 235 U to the environmental dose is very small [8].Since 98.5% of the radiological effects from uranium series nuclides are produced by 226 Ra and its decay products, the radiation from 238 U and the other 226 Ra precursors are negligible [9].When industrial solid waste is used to prepare building materials, radionuclides in the environment can be determined by the natural radioactivity level of building materials or the industrial solid waste.Thus, radiation hazards can be assessed.At present, the internal and external exposure index, radium equivalent activity, and indoor and outdoor absorbed dose rate are commonly used indicators in evaluating radionuclide radiation hazards.
The average activity concentrations of 226 Ra, 232 Th, and 40 K in FA, RM, PG, and EMR samples from similar studies in different parts of the world are illustrated in Table 1.As shown in Table 1, it is suggested that the mean activity concentrations of 226 Ra for FA in Turkey, Greece, and China (Xiangyang) are 360, 815, and 441 Bq/kg, respectively, higher than those in other countries.Concentrations of 40 K in EU countries are generally higher than in others.In terms of RM, the average activity concentrations of 226 Ra, 232 Th, and 40 K in Australia and Jamaica are notably higher than those in other countries.In addition, 226 Ra is the main nuclide for PG in Israel, Spain, Korea, Egypt, and Turkey, while the concentrations of 226 Ra, 232 Th, and 40 K for PG in South Africa are relatively balanced.It can be concluded from Table 1 that radionuclide activity concentrations differ from one location to another.Additionally, many studies have analyzed the radiation hazards of industrial solid wastes.Mamta Gupta et al. [10] reported that the radium equivalent activity of the FA from a thermal power plant in India was 353.9 Bq/kg, which is close to the maximum upper limit of 370 Bq/kg.L. Taoufiq et al. [28] characterized the radioactivity of FA from a thermal power plant in Morocco, and found that the radium equivalent activity was 241-298 Bq/kg, which is lower than 370 Bq/kg.In summary, the radionuclide activity concentrations and associated radiation hazards differ from one location to another.Different isotopes concentration in ores is a result of different conditions, including the metallogenetic body, formation age, epigenetic transformation, and so forth, during deposit formation in different regions.Therefore, the radioactive level of general industrial solid wastes in other regions cannot be used as the reference for Guizhou, China.
The dominant mineral resources, such as coal, phosphorite, bauxite, and manganese, are found in Guizhou.During the development and utilization process for these resources, a large amount of general solid waste including FA, PS, PG, RM, and EMR is accumulated.According to the data from the China statistical yearbook as shown in Table 2, from 2012 to 2017, the national generation volume of general industrial solid wastes varied from 3.09 to 3.32 billion tons, while the generation of Guizhou province varied from 0.071 to 0.094 billion tons.The substantial discharge and stockpiling of those aforementioned industrial solid wastes result in serious environmental pollution.It is vital to find an optimal solution for applying the solid wastes.The study of natural radionuclides and their radiation hazards is of great significance for the comprehensive utilization of industrial solid waste resources in the field of building materials.In this study, the radionuclide activity of five typical general industrial solid wastes including FA, PS, RM, PG, and EMR in Guizhou, China, was measured using a gamma spectrometry technique.The radioactivity level and associated radiation hazard of these industrial solid wastes were evaluated using indicators such as the internal and external irradiation index (I Ra and I γ , respectively), radium equivalent ratio (Ra eq ), indoor and outdoor external dose (D in and D out , respectively), and indoor and outdoor annual effective dose rate (E in and E out , respectively).The maximum dosages of solid wastes in building materials were calculated.This study, through radiation evaluation, hopes to provide a mixing amount reference for the aforementioned solid wastes for building materials that meet the radiation limitation requirements.

Samples
The samples in this study include: PS, FA, RM, PG, EMR, electrolytic manganese slag activated by NaOH (EMR-Na), and electrolytic manganese slag activated by Ca(OH) 2 (EMR-Ca).Sample PS taken from a building material company in Guizhou, China, was produced using the electric furnace process of preparing yellow phosphorus, which is grayish white or white and partially agglomerated.Sample FA obtained from a coal-burning power plant in Guizhou was black.Sample RM produced by Bayer process from an aluminum company in Guizhou was light red.Sample PG obtained from a phosphorus chemical company in Guizhou was gray and very agglomerated.Sample EMR obtained from an electrolytic manganese plant in Guizhou was a black, fresh slurry.EMR has a certain potential gelling activity with a small amount of silicon and aluminum.The alkaline substance can better excite the potential activity and form a hydrated silicate and aluminate product with hydration characteristics, resulting in the gelling properties of EMR.The alkali-activated EMR can replace part of the cement used to prepare building materials.Previous studies showed that NaOH and Ca(OH) 2 have better activation effects on EMR.Therefore, EMR-Na used in this study was obtained by mixing 75 g of fresh EMR, 15 wt.%NaOH (accounting for EMR), and 100 mL of tap water in a 500 mL stirred tank and stirring this mixture for 20 min.EMR-Ca was obtained by mixing 100 g of fresh EMR, 20 wt.% Ca(OH) 2 (accounting for EMR), and 100 mL of tap water in a 500 mL stirred tank and stirring this mixture for 15 min.

Radioactivity Measurement
All samples were aggregated, identified, and oven-dried to constant weight in the laboratory, then grounded to a particle size of less than 0.075 mm.Each sample was homogenized and dried in an oven at 105 • C for 3 h to remove moisture.Then, 200 g of each sample were weighed and placed into a cylinder measuring sample box with a diameter of 35 mm and a height of 20 mm, then sealed for eight weeks to achieve radioactive secular equilibrium between 226 Ra and its daughters.The measurements of activity concentrations were carried out at the Institute of Geochemistry, Chinese Academy of Sciences, using a vertical closed coaxial HPGe detector (GX6020, CANBERRA, Oak Ridge, TN, USA).The detector has an energy range of 3 keV to 10 MeV, with an energy resolution of 2.0 keV.The relative efficiency is 60% at 1332 keV γ-ray, and the peak-to-Compton ratio is 66:1.The actual energy range used to test the samples was 35-3000 keV.The measurement time for each sample was set as 180,000 s.The test data was collated and analyzed using management software OpenEMS with a data management system (RDBMS).To reduce the gamma ray background, a cylindrical lead shield detector was used to absorb X-rays, which contains two inner concentric cylinders of copper and aluminum.The calibration sources with an energy range covering nuclides 238 U, 226 Ra, 232 Th, 40 K, 241 Am, 137 Cs, and 60 Co were used for the determination of the detection efficiency of the measurement system, and the typically obtained values were within 6% of certified values.
The γ-ray lines that were used to measure the activities for nuclides were represented mainly by gamma-ray-emitting nuclei in the decay series of 232 Th, 226 Ra, and 40 K.The 40 K and 210 Pb were determined from their single photo peaks of 1460 keV (1.2%) and 46.5 keV (4.26%), respectively.The 238 U and 232 Th are not gamma ray emitters.However, it is possible to measure the gamma rays of their decay products.The decay product taken for 238 U was 234 Th (63.3 keV (3.81%)).The intensity of gamma rays emitted by 232 Th is very weak, and its decay product 228 Ra has a long half-life without gamma rays.The half-life of 228 Ac, the daughter of 228 Ra, is 6.13 h.According to the literature, Th and Ra both have affinity for silicates [37].Moreover, some studies have shown that the activities of 232 Th and 228 Ra in solid wastes such as red mud, phosphogypsum, and fly ash are approximately balanced [38][39][40].Therefore, the equilibrium between 232 Th and 228 Ra was assumed, and the 232 Th was determined using 208 Tl (583.2 keV (30.78%) and 2614.5 keV (35.7%)) and 228 Ac (911.2 keV (26.6%)).In addition, the 214 Pb (351.9 keV (35.8%)] and 214 Bi [609.3 keV (45%)) were used to determine the 226 Ra, and the decay product taken for 235 U was 235 U (185.7 keV (57.5%)).As there is an interference between 185.7 keV from 235 U and 186.2 keV from 226 Ra, the 235 U activity can be deduced from the 186 keV multiplet after removal of the 226 Ra contribution.Thus, the activity concentration of 235 U radionuclide is given by Equation (1) [41]: where CR and CR total(186 keV) are calculated by using Equations ( 2) and (3): where CR is the counting rate in full-energy peak in count/s, CR total(186 keV) is the counting rate for the 186 keV multiple, A is the activity of radionuclide in Bq/kg, BR is the branching ratio or the gamma-ray emission rate, ε is the detection efficiency. 226Ra and 235 U contribute about 58% and 42% of the total count rate of the 186 keV peak with the existence of equilibrium, respectively [42].

Internal and External Illumination Index
The internal exposure index (I Ra ) refers to the specific activity ratio of 226 Ra in the building materials to the 226 Ra limit specified in the national standard (GB6566-2010) [43].The external radiation index (I γ ) refers to the sum of the specific activity ratio of 226 Ra, 232 Th, and 40 K in building materials to their respective standard limits.The I Ra and I γ were calculated by using Equations ( 4) and ( 5): where C Ra , C Th , and C K are the mean radioactivity concentrations of 226 Ra, 232 Th, and 40 K (Bq/kg), respectively.The specific activity limit of 226 Ra in building materials specified in the GB6566-2010 is 200 Bq/kg, considering only the internal irradiation conditions.The prescribed limits were 370, 260, and 4200 Bq/kg for 226 Ra, 232 Th, and 40 K, respectively, in building materials when they exist separately under the external irradiation condition.

Radium Equivalent Activity
The distribution of 226 Ra, 232 Th, and 40 K in building materials is not uniform [44].The radium equivalent activity (Ra eq ) [45] was used to compare the relative gamma radioactivity of 226 Ra, 232 Th, and 40 K in building materials.It has been estimated that 1 Bq/kg of 226 Ra, 0.7 Bq/kg of 232 Th, and 13 Bq/kg of 40 K produce the same gamma ray dose [46,47].Thus, the radium equivalent activities (Ra eq ) can be estimated using Equation ( 6) [48][49][50]: An irradiation scenario is required to evaluate the 226 Ra, 232 Th, and 40 K absorbed doses produced by the building materials.The European Commission proposed that the length, width, and height of the concrete room are 4, 5, and 2.8 m, respectively.The thickness and density of the wall are 20 cm and 2350 kg•m −3 , respectively [51].Then, the indoor external dose D in could be calculated by using Equation (7): To assess the dose of radiation from building materials in a room, the portion from natural radiation needs to be subtracted.According to the survey results of the Chinese National Environmental Protection Department, the weighted mean by area and population are 62.8 and 62.1 nGy•h −1 , respectively.Taking 62.1 nGy•h −1 as the natural radiation background value, the calculated absorbed dose rate should be reduced by 62.1 [52].
The outdoor external dose (D out ) assessed from 226 Ra, 232 Th, and 40 K was supposed to be equally distributed at 1 m above the ground surface.Therefore, the D out was calculated using Equation ( 8) [53]:

Annual Effective Dose Rate
Buildings are the main places for daily activities of human beings, and the indoor and outdoor occupancy factors are 0.8 and 0.2, respectively (i.e., 80% and 20% of the time they are occupied indoors and outdoors, respectively) [54,55].The conversion factor from the absorbed dose in the air to the effective dose received by the individual is 0.7 [56].The annual indoor effective dose rate (E in ) and annual outdoor effective dose rate (E out ) can be calculated using Equations ( 9) and (10), respectively [55,57]:

Maximum Dosage of Solid Waste in Building Materials
The maximum dosage of solid waste in building materials f s can be calculated by using Equations ( 11) and (12): where C Ra , C Th , and C K are the mean radioactivity concentrations of 226 Ra, 232 Th, and 40 K (in Bq/kg) for other components in building materials, respectively.When the ratio of C Ra , C Th , and C K are all meant to be zero, the ratio of f s calculated using Equations ( 11) and ( 12) is the maximum dosage of solid waste in building materials.

Activity Concentration
The average values of the activity concentration of six nuclides for seven samples were calculated and are illustrated in Table 4.It can be concluded that the seven industrial solid wastes all contained 238 U, 235 U, 232 Th, 226 Ra, 210 Pb, and 40 K.The activity concentrations of 40 K were relatively high in EMR, EMR-Na, PS, and FA, with values of 443.8, 423.9, 529.4,and 461.0 Bq/kg, respectively.In FA, RM, and PS, the activity concentrations of 226 Ra were relatively higher than those of the other samples, at 208.2, 462.7, and 187.4 Bq/kg, respectively.The activity concentrations of 232 Th in FA and RM were as high as 165.6 and 457.7 Bq/kg, respectively.In addition, activity concentrations of 238 U in FA, RM, and PS were higher than those in other solid wastes, which were 234.9, 513.0, and 199.8 Bq/kg, respectively.On the contrary, the contents of 210 Pb and 235 U were very low in all samples.Furthermore, when NaOH and Ca(OH) 2 were used to activate EMR, the activity concentrations of the six nuclides in EMR-Na and EMR-Ca decreased, indicating that the addition of NaOH and Ca(OH) 2 can weaken the radioactivity of EMR.

The Source Analysis of Uranium
The analysis of the relationship between 238 U and 235 U and 226 Ra can be used to trace the source of the radioactive contamination by uranium in the environment.The activity concentrations of 238 U were plotted against the activity concentrations of 235 U and 226 Ra, as shown in Figure 1.The ratios of 238 U/ 235 U and 238 U/ 226 Ra were calculated and are shown in Table 4.As shown in Figure 1a, the linear fitting of the graph shows a good correlation between 238 U and 235 U (R 2 = 0.990).From the natural isotope abundance of uranium isotopes ( 238 U is 99.2%, 235 U is 0.72%) and its half-life, it is well known that naturally occurring uranium has a constant 238 U/ 235 U activity ratio of 21.7 [58].The results given in Table 4 indicate that the ratios of 238 U/ 235 U for the seven samples vary from 10.37 to 30.62, and the 238 U/ 235 U of PG is the lowest at 10.37.The ratio of PS, RM, and FA is close to the constant value, while the 238 U/ 235 U of EMR, EMR-Na, and EMR-Ca are all higher than 21.7.The reason for the deviation may be the higher uncertainty values caused by the self-absorption effect.Additionally, the emanation of radon from the sealed samples may also cause an underestimation of uranium activity concentrations [59].As CaO, SiO 2 , Al 2 O 3 are the major components in PS, RM, FA, and PG, according to the literature, the accumulations of U are often associated with clays because the clay fraction has an affinity for absorbing U; 238 U may be easily enriched in clay minerals from the perspective of adsorption [60].PS, RM, FA, and PG are obtained in the process of calcination, coal alumina, dissolution combustion, and phosphoric acid production, thus the adsorption of clay minerals may have little effect on uranium migration.
As shown in Table 5, the ratios of 238 U/ 226 Ra for the seven solid wastes varied from 0.46 to 1.88, and the concentrations of 238 U were commonly lower than 226 Ra.The reason for this may be that 238 U/ 226 Ra was disturbed in favor of 226 Ra [59].As shown in Figure 1b, the linear fitting of the graph shows a good correlation (R 2 = 0.985), and the slope of the line has a value (1.09) close to the average value of 1.29 for 238 U/ 226 Ra activity ratios.This indicates that there may exist a radioactive balance between 238 U and 226 Ra.According to previous literature reports, there is depleted uranium pollution in addition to natural uranium in the sample when the 238 U/ 226 Ra ratio is greater than 5 [61].Based on this information, it may be concluded that the uranium contents in the measured seven samples are all of natural origin.an affinity for absorbing U; 238 U may be easily enriched in clay minerals from the perspective of adsorption [60].PS, RM, FA, and PG are obtained in the process of calcination, coal alumina, dissolution combustion, and phosphoric acid production, thus the adsorption of clay minerals may have little effect on uranium migration.As shown in Table 5, the ratios of 238 U/ 226 Ra for the seven solid wastes varied from 0.46 to 1.88, and the concentrations of 238 U were commonly lower than 226 Ra.The reason for this may be that 238 U/ 226 Ra was disturbed in favor of 226 Ra [59].As shown in Figure 1b, the linear fitting of the graph shows a good correlation (R 2 = 0.985), and the slope of the line has a value (1.09) close to the average value of 1.29 for 238 U/ 226 Ra activity ratios.This indicates that there may exist a radioactive balance between 238 U and 226 Ra.According to previous literature reports, there is depleted uranium pollution in addition to natural uranium in the sample when the 238 U/ 226 Ra ratio is greater than 5 [61].Based on this information, it may be concluded that the uranium contents in the measured seven samples are all of natural origin.The IRa, Iγ, Raeq, Din, Dout, Ein, and Eout were calculated according to the activity concentrations of the radionuclides of the seven solid wastes and Equations ( 1)- (7).As illustrated in Table 6, the IRa and Iγ of the seven industrial solid wastes were 0.11-2.31and 0.17-3.07Bq/kg, respectively, of which PS, PG, EMR, EMR-Na, and EMR-Ca were all less than 1.The IRa was greater than 1 and the Iγ was greater  The I Ra , I γ , Ra eq , D in , D out , E in , and E out were calculated according to the activity concentrations of the radionuclides of the seven solid wastes and Equations ( 1)- (7).As illustrated in Table 6, the I Ra and I γ of the seven industrial solid wastes were 0.11-2.31and 0.17-3.07Bq/kg, respectively, of which PS, PG, EMR, EMR-Na, and EMR-Ca were all less than 1.The I Ra was greater than 1 and the I γ was greater than 1.3 for FA.The I Ra and I γ of the RM were both greater than 2. According to Table 7, "Limited Standards for Radionuclide of Building Materials", PS, PG, EMR, EMR-Na, and EMR-Ca can be directly used as building materials and decorative materials of class A, B, and C. FA can be used as decoration materials of class B and class C.This means that almost all samples are safe for use as they meet the PRC National Standard.Adding FA and RM in building materials should be considered.
According to Table 6, the Ra eq values of the seven samples were between 64.54 and 1137.18Bq/kg, among which the Ra eq of PG, EMR, EMR-Na, and EMR-Ca were lower than the world's recommended limit (370 Bq/kg) for building materials [56].The Ra eq values of the RM, PS, and FA were as high as 1137.18,557.03, and 485.78 Bq/kg, respectively.Therefore, the dosage of RM, PS, and FA should be considered when used as the source for building materials.
It also can be seen from Table 6 that the D in and D out of the seven industrial solid wastes were 6.38-888.07 and 29.70-501.03nGy/h, respectively.Specifically, the D in of PS, RM, and FA were as high as 404.67, 888.07, and 354.49nGy/h, respectively, while their D out values were 246.93, 501.03, and 218.29 nGy/h, respectively.These values are higher than the world's average values (i.e., 84 nGy/h for D in and 59 nGy/h for D out [56,62]).The results show that the values of the annual effective dose rate for the seven samples were 0.03-4.36mSv/y for E in and 0.04-0.61mSv/y for E out .The values of E in and E out for PS, RM, and FA were all higher than the world's recommended values (i.e., 0.4 mSv/y for E in and 0.07 mSv/y for E out [56]).
In summary, the radioactive levels of RM, FA, and PS exceed the "Limited Standards for the Radionuclide of Building Materials", and they cannot be directly used for building materials, while the PG, EMR, EMR-Na, and EMR-Ca could be directly used for building materials within the recommended levels.

Contribution Analysis of Nuclides to Radiation
According to Equations ( 1)-( 7), the contribution of radionuclides 26 Ra, 232 Th, and 40 K to radiation hazard indexes varies in different solid wastes.The contributions of 26 Ra, 232 Th, and 40 K to I γ , Ra eq , D in , E in , D out , and E out were calculated and plotted in Figure 2. As shown in Figure 2, 226 Ra was the main contributor to I γ , Ra eq , D in , E in , D out , and E out in RM, FA, and PG, and the relative contributions of 226 Ra were in the range of 40.69-44.80%for RM, 42.44-45.97%for FA, and 92.44-95.19%for PG.Similarly, 232 Th was the main contributor to I γ , Ra eq , D in , E in , D out , and E out in PS, RM, and FA, and the relative contributions of 232 Th varied from 55.07 to 59.99% for PS, 52.98 to 57.55% for RM, and 43.73 to 48.76% for FA.However, the relative contributions of 40 K to I γ , Ra eq , D in , E in , D out , and E out in EMR, EMR-Na, and EMR-Ca were in the range of 35.51-41.00%,36.68-42.19%,and 32.66-38.02%,respectively.This data shows that 226 Ra and 232 Th were the main contributors to radiation hazard indexes in PS, RM, FA, and PG, while the contributions of 226 Ra, 232 Th, and 40 K in EMR, EMR-Na, and EMR-Ca were relatively balanced.However, the phase analysis of known nuclides in those aforementioned solid wastes has not been covered in this study, and future research should focus on the removal or decrease of the nuclides in different phases.

Limitation Analysis of Solid Wastes in Building Materials
As the solid wastes RM, FA, and PS cannot be directly used for building materials, the maximum dosages (fs) of solid wastes FA, RM, and PS in building materials were calculated using Equations (11) and (12) with results of 75.44%, 29.72%, and 66.01%, respectively.According to the different related research reviews related, increasing the addition of FA in concrete can result in a decrease in the compressive strength of concrete.The optimum FA percentages were found to be 10%, 20%, 22%, and 35% in various studies [63][64][65][66].Similarly, previous research showed that the corrosion rate of concrete is the lowest between 20 wt.% and 30 wt. % of added RM content [67].Other studies showed that the addition of RM in amounts greater than 20% causes a decrease in the hydration temperature

Limitation Analysis of Solid Wastes in Building Materials
As the solid wastes RM, FA, and PS cannot be directly used for building materials, the maximum dosages (f s ) of solid wastes FA, RM, and PS in building materials were calculated using Equations (11) and (12) with results of 75.44%, 29.72%, and 66.01%, respectively.According to the different related research reviews related, increasing the addition of FA in concrete can result in a decrease in the compressive strength of concrete.The optimum FA percentages were found to be 10%, 20%, 22%, and 35% in various studies [63][64][65][66].Similarly, previous research showed that the corrosion rate of concrete is the lowest between 20 wt.% and 30 wt. % of added RM content [67].Other studies showed that the addition of RM in amounts greater than 20% causes a decrease in the hydration temperature and results in a decrease in the compressive strength [68].Therefore, in terms of the mechanical properties of building materials, the optimum addition of FA, RM, and PS in building materials will not exceed the maximum addition allowed by their radioactivity level.That is, FA, RM, and PS can be used even if their radioactivity levels are above the standard limits.

. 2 . 4 .
Indoor External Dose (D in ) and Outdoor External Dose (D out )

Figure 1 .
Figure 1.Variation of 238 U activity concentration versus (a) 235 U activity and (b) 226 Ra activity.

Figure 1 .
Figure 1.Variation of 238 U activity concentration versus (a) 235 U activity and (b) 226 Ra activity.

Figure 2 .
Figure 2. The relative concentrations of 226 Ra, 232 Th, and 40 K in (a) I γ , (b) Ra eq , (c) D in and E in , (d) D out and E out in industrial solid wastes.

Table 1 .
The obtained average activity concentrations of 226 Ra,232Th, and40K in FA, RM, PG, and EMR samples from similar studies in different parts of the world (unit: Bq/kg).

Table 3 .
Major components in seven solid wastes.

Table 4 .
The activity concentrations (Bq/kg) of six nuclides for the seven solid wastes.

Table 5 .
The activity ratio for different samples under investigation.

Table 5 .
The activity ratio for different samples under investigation.

Table 6 .
The I Ra , I γ , Ra eq , D in , D out , E in , and E out for industrial solid wastes.

Wastes I Ra (Bq/kg) I γ (Bq/kg) Ra eq (Bq/kg) D in (nGy/h) D out (nGy/h) E in (mSv/y) E out (mSv/y)
The minus sign represents that the D in of PG is lower than the natural radiation background value. *