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

Abstract

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 (I_Ra and I_γ, respectively) for FA and RM were higher (I_Ra > 1.0 and I_γ > 1.3 for FA, I_Ra > 2.0 and I_γ > 2.0 for RM). The radium equivalent activity (Ra_eq), indoor and outdoor absorbed dose (D_in and D_out, respectively), and corresponding annual effective dose rate (E_in and E_out) 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.

1. 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 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 ²³⁵U, the uranium series originating from ²³⁸U, and the thorium series originating from ²³²Th. In addition, there are several isolated radionuclides, such as ⁴⁰K [5,6]. The radioactive decay chains of ²³²Th, ²³⁸U, ²³⁵U, and ⁴⁰K are the main contributors to the dose of natural radiation [7]. When the ratio of ²³⁵U/²³⁸U is less than 1%, the contribution of ²³⁵U to the environmental dose is very small [8]. Since 98.5% of the radiological effects from uranium series nuclides are produced by ²²⁶Ra and its decay products, the radiation from ²³⁸U and the other ²²⁶Ra 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 ²²⁶Ra, ²³²Th, and ⁴⁰K in FA, RM, PG, and EMR samples from similar studies in different parts of the world are illustrated in

Table 1. The obtained average activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in FA, RM, PG, and EMR samples from similar studies in different parts of the world (unit: Bq/kg).

Country	Sample Type	Activity Concentration (Bq/kg)			Reference
		226Ra	232Th	40K
India	FA	119	147	352	[10]
Turkey		360	102	517	[11]
Hungary		178	55	387	[12]
Greece		815	56	400	[13]
Czech Rep.		146	86	669	[13]
Germany		164	94	517	[13]
Italy		170	140	400	[13]
Poland		200	118	798	[13]
Romania		219	116	595	[13]
China (Baoji)		112	148	386	[14]
China (Xiangyang)		441	110	510	[15]
Turkey	RM	210	539	112	[13]
Hungary		301	295	50	[13]
Greece		244	364	57	[13]
Germany		171	318	215	[13]
Italy		97	118	115	[13]
Australia		310	1350	350	[16]
Jamaica		1047	350	335	[17]
Turkey	PG	436	9	13	[18]
Bangladesh		234	21	108	[19]
Egypt		596	6	2	[20]
Jordan		376	4	40	[21]
Korea		618	9	24	[22]
Israel		747	14	63	[23]
Spain		647	8	33	[24]
South Africa		109	189	>100	[25]
Hungary	EMR	52	40	607	[26]
China (Chongqing)		37	58	631	[27]

. As shown in Table 1, it is suggested that the mean activity concentrations of ²²⁶Ra for FA in Turkey, Greece, and China (Xiangyang) are 360, 815, and 441 Bq/kg, respectively, higher than those in other countries. Concentrations of ⁴⁰K in EU countries are generally higher than in others. In terms of RM, the average activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in Australia and Jamaica are notably higher than those in other countries. In addition, ²²⁶Ra is the main nuclide for PG in Israel, Spain, Korea, Egypt, and Turkey, while the concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K 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. Generation of general industrial solid waste during 2012–2017, in Guizhou, China (unit: billion tons).

Year	China	Guizhou	Reference
2012	3.290	0.078	[29]
2013	3.277	0.082	[30]
2014	3.256	0.074	[31]
2015	3.271	0.071	[32]
2016	3.092	0.078	[33]
2017	3.316	0.094	[34]

, 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.

2. Materials and Methods

2.1. 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.

The moisture contents of PS, FA, RM, PG, and EMR were 4.29%, 4.13%, 5.01%, 20.73%, and 15.7%, respectively. The densities of PS, FA, RM, PG, EMR, EMR-Na, and EMR-Ca were 3.06, 2.33, 3.01, 2.57, 2.31, 2.35, and 2.48 g/cm³, respectively. Major components in seven solid wastes were analyzed by using X-ray fluorescence (XRF, PANalytical Axios mA×4 KW, Malvern Panalytical, Almelo, The Netherlands), and the results are illustrated in

Table 3. Major components in seven solid wastes.

Component	Concentration (%)
	PS	RM	FA	PG	EMR	EMR-Na	EMR-Ca
CaO	44.77	14.35	2.83	34.07	7.32	9.1	15.43
Fe2O3	0.45	21.53	2.83	0.20	-	-	-
Al2O3	5.55	20.89	14.75	0.13	8.85	11.27	7.99
SiO2	37.81	16.75	45.72	5.29	22.85	30.29	20.35
MgO	2.61	1.55	1.18	0.01	1.86	2.42	1.80
P2O5	2.86	0.31	-	0.75	-	-	-
TiO2	-	4.59	1.74	-	0.25	0.33	0.21
K2O	1.43	0.98	1.24	-	1.75	2.22	1.66
Na2O	0.33	4.93	0.55	-	0.14	0.42	0.12
SO3	-	1.19	-	40.24	-	-	-
Mn	-	-	-	-	4.92	5.57	6.25
S	-	-	-	-	8.19	3.2	7.08
Others	0.04	12.04	17.03	19.31	2.88	3.75	2.18

. As shown in Table 3, PS, RM, FA are mainly composed of CaO, SiO₂, Al₂O₃, and Fe₂O₃ [35], PG is composed of CaO, SiO₂, Al₂O₃, and SO₃ [36], while CaO, SiO₂, Al₂O₃, Mn, and S are the main components in EMR, EMR-Na, and EMR-Ca.

2.2. Methods

2.2.1. 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 ²²⁶Ra 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 ²³⁸U, ²²⁶Ra, ²³²Th, ⁴⁰K,²⁴¹Am, ¹³⁷Cs, and ⁶⁰Co 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 ²³²Th, ²²⁶Ra, and ⁴⁰K. The ⁴⁰K and ²¹⁰Pb were determined from their single photo peaks of 1460 keV (1.2%) and 46.5 keV (4.26%), respectively. The ²³⁸U and ²³²Th are not gamma ray emitters. However, it is possible to measure the gamma rays of their decay products. The decay product taken for ²³⁸U was ²³⁴Th (63.3 keV (3.81%)). The intensity of gamma rays emitted by ²³²Th is very weak, and its decay product ²²⁸Ra has a long half-life without gamma rays. The half-life of ²²⁸Ac, the daughter of ²²⁸Ra, 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 ²³²Th and ²²⁸Ra in solid wastes such as red mud, phosphogypsum, and fly ash are approximately balanced [38,39,40]. Therefore, the equilibrium between ²³²Th and ²²⁸Ra was assumed, and the ²³²Th was determined using ²⁰⁸Tl (583.2 keV (30.78%) and 2614.5 keV (35.7%)) and ²²⁸Ac (911.2 keV (26.6%)). In addition, the ²¹⁴Pb (351.9 keV (35.8%)] and ²¹⁴Bi [609.3 keV (45%)) were used to determine the ²²⁶Ra, and the decay product taken for ²³⁵U was ²³⁵U (185.7 keV (57.5%)). As there is an interference between 185.7 keV from ²³⁵U and 186.2 keV from ²²⁶Ra, the ²³⁵U activity can be deduced from the 186 keV multiplet after removal of the ²²⁶Ra contribution. Thus, the activity concentration of ²³⁵U radionuclide is given by Equation (1) [41]:

$$A_{\mathrm{U}-235}=1.75\times \frac{C{R}_{\mathrm{total}\left(186\mathrm{keV}\right)}}{{\epsilon }_{186\mathrm{keV}}}-0.06{\mathrm{A}}_{\mathrm{Ra}-226}$$

(1)

where CR and CR_{total(186 keV)} are calculated by using Equations (2) and (3):

$$CR=A\times BR\times \epsilon$$

(2)

$$C{R}_{\mathrm{total}\left(186\mathrm{keV}\right)}=C{R}_{\mathrm{Ra}-226}+C{R}_{\mathrm{U}-235}$$

(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 ²³⁵U contribute about 58% and 42% of the total count rate of the 186 keV peak with the existence of equilibrium, respectively [42].

2.2.2. Internal and External Illumination Index

The internal exposure index (I_Ra) refers to the specific activity ratio of ²²⁶Ra in the building materials to the ²²⁶Ra 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 ²²⁶Ra, ²³²Th, and ⁴⁰K in building materials to their respective standard limits. The I_Ra and I_γ were calculated by using Equations (4) and (5):

$$I_{{\mathrm{R}}_{\mathrm{a}}}=\frac{C_{{\mathrm{R}}_{\mathrm{a}}}}{200}$$

(4)

$$I_{\mathsf{\gamma }}=\frac{C_{{\mathrm{R}}_{\mathrm{a}}}}{370}+\frac{C_{\mathrm{Th}}}{260}+\frac{C_{\mathrm{K}}}{4200}$$

(5)

where C_Ra, C_Th, and C_K are the mean radioactivity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K (Bq/kg), respectively. The specific activity limit of ²²⁶Ra 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 ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively, in building materials when they exist separately under the external irradiation condition.

2.2.3. Radium Equivalent Activity

The distribution of ²²⁶Ra, ²³²Th, and ⁴⁰K in building materials is not uniform [44]. The radium equivalent activity (Ra_eq) [45] was used to compare the relative gamma radioactivity of ²²⁶Ra, ²³²Th, and ⁴⁰K in building materials. It has been estimated that 1 Bq/kg of ²²⁶Ra, 0.7 Bq/kg of ²³²Th, and 13 Bq/kg of ⁴⁰K produce the same gamma ray dose [46,47]. Thus, the radium equivalent activities (Ra_eq) can be estimated using Equation (6) [48,49,50]:

$$R{a}_{\mathrm{eq}}=C_{\mathrm{Ra}}+1.43C_{\mathrm{Th}}+0.077C_{\mathrm{K}}$$

(6)

2.2.4. Indoor External Dose (D_in) and Outdoor External Dose (D_out)

An irradiation scenario is required to evaluate the ²²⁶Ra, ²³²Th, and ⁴⁰K 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):

$$D_{\mathrm{in}}\left(\mathrm{nGy}/\mathrm{h}\right)=0.92C_{\mathrm{Ra}}+1.1C_{\mathrm{Th}}+0.081C_{\mathrm{K}}.$$

(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 ²²⁶Ra, ²³²Th, and ⁴⁰K was supposed to be equally distributed at 1 m above the ground surface. Therefore, the D_out was calculated using Equation (8) [53]:

$$D_{\mathrm{out}}\left(\mathrm{nGy}/\mathrm{h}\right)=0.462C_{\mathrm{Ra}}+0.604C_{\mathrm{Th}}+0.0417C_{\mathrm{K}}.$$

(8)

2.2.5. 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]:

$$E_{\mathrm{in}}=D_{\mathrm{in}}\times 365\times 24\mathrm{h}\times 0.8\times 0.7\left(\mathrm{Sv}/\mathrm{Gy}\right)=D_{\mathrm{in}}\times 4905\times {10}^{-6}\left(\mathrm{mSv}/\mathrm{y}\right)$$

(9)

$$E_{\mathrm{out}}=D_{\mathrm{out}}\times 365\times 24\mathrm{h}\times 0.2\times 0.7\left(\mathrm{Sv}/\mathrm{Gy}\right)=D_{\mathrm{out}}\times 1226\times {10}^{-6}\left(\mathrm{mSv}/\mathrm{y}\right)$$

(10)

2.2.6. 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):

$$\frac{{\mathrm{f}}_{\mathrm{s}}·C_{\mathrm{Ra}}+\left(1-{\mathrm{f}}_{\mathrm{s}}\right)·C_{\mathrm{Ra}}^{\prime }}{200}\le 1.0$$

(11)

$$\frac{{\mathrm{f}}_{\mathrm{s}}·C_{\mathrm{Ra}}+\left(1-{\mathrm{f}}_{\mathrm{s}}\right)·C_{\mathrm{Ra}}^{\prime }}{370}+\frac{{\mathrm{f}}_{\mathrm{s}}·C_{\mathrm{Th}}+\left(1-{\mathrm{f}}_{\mathrm{s}}\right)·C_{\mathrm{Th}}^{\prime }}{260}+\frac{{\mathrm{f}}_{\mathrm{s}}·C_{\mathrm{K}}+\left(1-{\mathrm{f}}_{\mathrm{s}}\right)·C_{\mathrm{K}}^{\prime }}{4200}\le 1.0$$

(12)

where $C_{\mathrm{Ra}}^{\prime }$, $C_{\mathrm{Th}}^{\prime }$, and $C_{\mathrm{K}}^{\prime }$ are the mean radioactivity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K (in Bq/kg) for other components in building materials, respectively. When the ratio of $C_{\mathrm{Ra}}^{\prime }$, $C_{\mathrm{Th}}^{\prime }$, and $C_{\mathrm{K}}^{\prime }$ 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.

3. Results and Discussion

3.1. Activity Concentration

The average values of the activity concentration of six nuclides for seven samples were calculated and are illustrated in

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

Name of Solid Wastes	Activity Concentration (Bq/kg)
	40K	210Pb	226Ra	232Th	235U	238U
PS	461.0 ± 23.6	15.1 ± 9.9	187.4 ± 5.8	233.7 ± 9.6	9.1 ± 0.3	199.8 ± 17.9
RM	259.5 ± 18.9	324. 8 ± 18.1	462.7 ± 14.5	457.7 ± 13.4	22.4 ± 0.7	513.0 ± 43.1
FA	529.4 ± 31.1	210.5 ± 11.8	208.2 ± 7.0	165.6 ± 5.6	10.1 ± 0.3	234.9 ± 27.2
PG	3.3 ± 6.0	86.0 ± 4.8	61.0 ± 2.1	2.3 ± 0.5	2.7 ± 0.1	28.0 ± 13.0
EMR	443.8 ± 26.7	34.3 ± 2.3	26.6 ± 1.6	24.8 ± 1.7	1.6 ± 0.1	48.3 ± 15.9
EMR-Na	423.9 ± 25.9	45.9 ± 2.9	24.5 ± 1.6	22.2 ± 1.5	1.5 ± 0.1	37.9 ± 15.7
EMR-Ca	321.4 ± 20.9	37.7 ± 2.5	21.1 ± 1.5	20.9 ± 1.6	1.3 ± 0.1	39.8 ± 15.8

. It can be concluded that the seven industrial solid wastes all contained ²³⁸U, ²³⁵U, ²³²Th, ²²⁶Ra, ²¹⁰Pb, and ⁴⁰K. The activity concentrations of ⁴⁰K 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 ²²⁶Ra were relatively higher than those of the other samples, at 208.2, 462.7, and 187.4 Bq/kg, respectively. The activity concentrations of ²³²Th in FA and RM were as high as 165.6 and 457.7 Bq/kg, respectively. In addition, activity concentrations of ²³⁸U 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 ²¹⁰Pb and ²³⁵U 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.

3.2. The Source Analysis of Uranium

The analysis of the relationship between ²³⁸U and ²³⁵U and ²²⁶Ra can be used to trace the source of the radioactive contamination by uranium in the environment. The activity concentrations of ²³⁸U were plotted against the activity concentrations of ²³⁵U and ²²⁶Ra, as shown in Figure 1. The ratios of ²³⁸U/²³⁵U and ²³⁸U/²²⁶Ra were calculated and are shown in Table 4. As shown in Figure 1a, the linear fitting of the graph shows a good correlation between ²³⁸U and ²³⁵U (R² = 0.990). From the natural isotope abundance of uranium isotopes (²³⁸U is 99.2%, ²³⁵U is 0.72%) and its half-life, it is well known that naturally occurring uranium has a constant ²³⁸U/²³⁵U activity ratio of 21.7 [58]. The results given in Table 4 indicate that the ratios of ²³⁸U/²³⁵U for the seven samples vary from 10.37 to 30.62, and the ²³⁸U/²³⁵U of PG is the lowest at 10.37. The ratio of PS, RM, and FA is close to the constant value, while the ²³⁸U/²³⁵U 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, SiO₂, Al₂O₃ 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; ²³⁸U 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 activity ratio for different samples under investigation.

Name of Solid Wastes	238U/235U	238U/226Ra
PS	21.96	1.07
RM	22.90	1.11
FA	23.26	1.13
PG	10.37	0.46
EMR	30.19	1.82
EMR-Na	25.27	1.54
EMR-Ca	30.62	1.88

, the ratios of ²³⁸U/²²⁶Ra for the seven solid wastes varied from 0.46 to 1.88, and the concentrations of ²³⁸U were commonly lower than ²²⁶Ra. The reason for this may be that ²³⁸U/²²⁶Ra was disturbed in favor of ²²⁶Ra [59]. As shown in Figure 1b, the linear fitting of the graph shows a good correlation (R² = 0.985), and the slope of the line has a value (1.09) close to the average value of 1.29 for ²³⁸U/²²⁶Ra activity ratios. This indicates that there may exist a radioactive balance between ²³⁸U and ²²⁶Ra. According to previous literature reports, there is depleted uranium pollution in addition to natural uranium in the sample when the ²³⁸U/²²⁶Ra 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.

3.3. Radiological Impact Assessment

3.3.1. Analysis of Radiation Hazard Indexes

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, I_γ, Ra_eq, D_in, D_out, E_in, and E_out for industrial solid wastes.

Solid Wastes	IRa (Bq/kg)	Iγ (Bq/kg)	Raeq (Bq/kg)	Din (nGy/h)	Dout (nGy/h)	Ein (mSv/y)	Eout (mSv/y)
PS	0.94	0.71	557.03	404.67	246.93	1.98	0.30
RM	2.31	3.07	1137.18	888.07	501.03	4.36	0.61
FA	1.04	1.33	485.78	354.49	218.29	1.74	0.27
PG	0.30	0.17	64.54	−3.19 *	29.70	−0.02 *	0.04
EMR	0.13	0.27	96.23	25.60	45.77	0.13	0.06
EMR-Na	0.12	0.25	88.99	19.28	42.45	0.09	0.05
EMR-Ca	0.11	0.21	75.78	6.38	35.80	0.03	0.04

* The minus sign represents that the D_in of PG is lower than the natural radiation background value.

, the I_Ra and I_γ of the seven industrial solid wastes were 0.11–2.31 and 0.17–3.07 Bq/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 the Radionuclide of Building Materials (GB/6566-2010) [43].

Standard Name	Building Materials		Decoration Materials
	Hollow Rate ≤ 25%	Hollow Rate > 25%	Class A	Class B	Class C
Radionuclide limit	$I_{{\mathrm{R}}_{\mathrm{a}}}$ ≤ 1.0	$I_{{\mathrm{R}}_{\mathrm{a}}}$ ≤ 1.0	$I_{{\mathrm{R}}_{\mathrm{a}}}$ ≤ 1.0	$I_{{\mathrm{R}}_{\mathrm{a}}}$ ≤ 1.3	$I_{\mathrm{r}}$ ≤ 2.8
	$I_{\mathrm{r}}$ ≤ 1.0	$I_{\mathrm{r}}$ ≤ 1.3	$I_{\mathrm{r}}$ ≤ 1.3	$I_{\mathrm{r}}$ ≤ 1.9

, “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 1137.18 Bq/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 29.70–501.03 nGy/h, respectively. Specifically, the D_in of PS, RM, and FA were as high as 404.67, 888.07, and 354.49 nGy/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 0.03–4.36 mSv/y for E_in and 0.04–0.61 mSv/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.

3.3.2. Contribution Analysis of Nuclides to Radiation

According to Equations (1)–(7), the contribution of radionuclides ²⁶Ra, ²³²Th, and ⁴⁰K to radiation hazard indexes varies in different solid wastes. The contributions of ²⁶Ra, ²³²Th, and ⁴⁰K 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, ²²⁶Ra 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 ²²⁶Ra 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, ²³²Th 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 ²³²Th 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 ⁴⁰K 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 ²²⁶Ra and ²³²Th were the main contributors to radiation hazard indexes in PS, RM, FA, and PG, while the contributions of ²²⁶Ra, ²³²Th, and ⁴⁰K 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.

3.3.3. 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.

4. Conclusions

(1)

⁴⁰K, ²²⁶Ra, ²³²Th, and ²³⁸U are the main nuclides in FA, RM, and PS. ²¹⁰Pb and ²²⁶Ra are the main nuclides in PG, while ⁴⁰K is the main nuclide in EMR, EMR-Na, and EMR-Ca. The uranium contents of all samples are all from natural uranium.

(2)

The values of I_Ra and I_γ were all less than 1 except for FA (I_Ra > 1 and I_γ > 1.3) and RM (I_Ra > 2 and I_γ > 2), and the values of Ra_eq, D_in, D_out, E_in, and E_out were higher than the world’s recommended values (i.e., 370 Bq/kg, 84 nGy/h, 59 nGy/h, 0.4 mSv/y, and 0.07 mSv/y, respectively) for PS, RM, and FA due to higher concentrations of ²²⁶Ra and ²³²Th.

(3)

PG, EMR, EMR-Na, and EMR-Ca could be used for building materials unlimitedly. However, RM, FA, and PS could be used as additive or auxiliary materials for building materials by means of doping and mixing, with maximum portions of 75.44%, 29.72%, and 66.01%, respectively. These findings provide a basis for the restriction of the aforementioned solid wastes in Guizhou in the field of building materials. Further research on the phase analysis and treatment of known nuclides in solid wastes are necessary in the future.