Introduction

Geopolymers are inorganic polymers formed by the reaction of concentrated aqueous alkali hydroxide or silicate solution with a solid aluminosilicate precursor in the presence of alkaline activators at room or high temperature. A geopolymer is a semi-crystalline or amorphous compound with a network of three-dimensional cementitious structured material1. They display a low carbon footprint and excellent mechanical properties, low thermal conductivity, high thermal stability, and good fire and acid resistance properties that account for their wide applications2,3,4,5,6,7. They have been deployed as building materials, thermal insulators, radiation and heavy metals adsorbents8,9,10. They offer equivalent strength performance with conventional binders11. Geopolymer is known as an effective immobilization system for a variety of hazardous wastes due to its low permeability, long-term durability and resistance to acid attack12. The compressive strength of the synthetic geopolymer is similar to that of regular Portland cement with physical characteristics stable enough to encapsulate hazardous metals and suitable for use in future construction projects9,18.

Geopolymers have displayed good prospects in radiation shielding22. When gamma radiation passes through a geopolymer, it undergoes three main interaction processes: photoelectric absorption, Compton scattering and pair production. These interactions are a direct function of the density, the energy of the incident photon and the effective atomic number of the geopolymer17,22. The photoelectric effect is dominant at lower energies (below 0.1 MeV), where the photon energy is completely absorbed by an atom, ejecting an electron. Compton scattering is dominant at higher energies (above 0.1 MeV), where the photon scatters off an electron, losing some of its energy. Pair production is significant at higher energies, typically above 1.022 MeV where a high-energy photon interacts with the nucleus of an atom, converting its energy into an electron-positron pair. This process usually occurs in materials with high atomic numbers, as the strong electric field of the nucleus is required for this interaction.

In recent years, researchers have explored the incorporation of various additives to enhance the properties of geopolymers. One such additive is lead (II) oxide (PbO), which has shown high prospects in the modification of mechanical, physical and radiation shielding properties of geopolymer matrices by introducing a high bonding background within the geopolymer network13,14. Lead (II) oxide (PbO) has shown a high prospect of modifying the gamma shielding characteristics of geopolymers due to the high bonding matrix. Before then, most studies concentrate on the possible use of geopolymer to immobilize heavy metals11,12,14, radioactive waste15,16and buildings17,18 with less concentration on X-rays and gamma radiation shielding applications. Considering many, advantages of PbO, the low permeability of geopolymer, the introduction of PbO in geopolymers substantially improves the radiation shielding properties of geopolymers. Therefore, the objective of this study is to comprehensively evaluate the impact of 10% and 20% PbO addition on the gamma radiation shielding characteristics of geopolymers. This research will enhance the application of PbO-doped geopolymers in environmental protection construction. The findings will provide valuable insights for designing environmentally friendly and high-performance geopolymer-based composites for radiation shielding in medical imaging facilities, nuclear power plants and industrial radiography.

Materials and methods

Investigated samples were code-named: GEO, GEO-10Pb, and GEO-20Pb, to represent, undoped geopolymer, 10% PbO doped geopolymer and 20% PbO doped geopolymer respectively as shown in Table 1. The PbO provided by Merck Germany with an estimated purity of 99.99% was used in this study. Throughout the experimental procedure, ethyl alcohol was used to clean instruments and equipment to prevent cross-contamination.

Table 1 Sample code, mole (mol%) and weight fraction (wt%) of the element present in the prepared GEO-xPb glasses including their measured density.
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The undoped GEO and doped GEO with different amounts of PbO content as 10% and 20% wt samples were fabricated and sintered above 1100oC for 1 h to obtain a denser material. The introduction of different percentages of PbO (lead (II) oxide) in the presence of NaOH (sodium hydroxide) and water gave distinct Pb-doped geopolymers with special mechanical, and physical features as final products. The choice of PbO decreases the sintering temperature and reduces energy loss. The step-by-step fabrication and doping process are as follows:

(a) The geopolymer powders were first synthesized using kaolin mineral, sodium silicate and sodium hydroxide as shown in Alouani, et al.19. (b) The undoped and PbO doped powders were separately allowed to pass through the ball milling process to obtain homogeneous particle distribution in each of the samples, (c) To obtain bulk materials, the doped and undoped GEO powders were prepressed in a bidirectional steel mold under 250 Bars. (d) The cold isostatic pressing (CIP) was carried out under 250 Bars after the pre-pressing process to increase the green density of the samples before the sintering process. (CIP). (e) The doped and undoped GEO samples were sintered in the alumina crucible above 1100ºC for 1 h and the final products were obtained at this condition. The fabrication process of GEO, GEO-10Pb and GEO-20Pb are shown in Fig. 1.

Fig. 1
figure 1

The production process of the samples. Reproduced from22, where we discussed the preparation, physical and structural properties of the samples.

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Result and discussion

Mass attenuation coefficients (MAC)

MAC gives the effectiveness of a material to shield gamma radiation per unit mass. Higher values of MAC designate higher radiation shielding characteristics20,21. Table 2gave a comparison of MAC (µ) for different GEO-xPb polymer compositions. These coefficients signify the likelihood of gamma interaction (absorption or scattering) per unit mass thickness of a material. Higher values imply greater shielding ability. As the energy of photons increases, the MAC generally decreases for all materials22. This is because higher energy photons have less probability of interacting with matter. Increasing the lead content of geopolymer (GEO-10Pb to GEO-20Pb) usually increases the MAC, especially at lower photon energies. This is a result of the high atomic number of Lead, which boosts the probability of photon interaction.

To corroborate the precision of computer programs XCOM and FLUKA deployed in this study, the results obtained were compared. The relative deviation in MAC values between results from XCOM and FLUKA were computed for all samples at selected gamma energies are shown in Table 2using Eq. 122.

$$\:RD\%\left(\text{X}\text{C}\text{O}\text{M}-\:\text{F}\text{L}\text{U}\text{K}\text{A}\right)=\:\frac{\text{X}\text{C}\text{O}\text{M}\:\:-\:\text{F}\text{L}\text{U}\text{K}\text{A}}{\text{X}\text{C}\text{O}\text{M}} \times \:100$$
(1)

From the obtained results, there is largely good agreement between XCOM and FLUKA values, with most deviations below 5% percent. However, slight discrepancies were observed at higher energies for lower Lead content. The XCOM and FLUKA are valuable tools for radiation transport simulations, but their limitations should be considered when choosing the appropriate tool for a specific application. For simple photon transport problems, XCOM may be sufficient, while for more complex scenarios involving multiple particle types and detailed geometry, FLUKA may be a better choice. Geopolymer samples with higher lead content at lower gamma energy exhibit improved radiation shielding properties due to higher MAC. This comparison between XCOM and FLUKA offers veritable insights into the precision of simulation codes for radiation transport calculations23.

Table 2 Mass attenuation coefficients (µ/ρ; cm2 g−1) of the prepared GEO-xPb geopolymer samples evaluated by FLUKA code and XCOM database.
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Figure 2 displayed a comparison chart of MAC for different materials at 0.2 MeV, 0.662 MeV and 1.25 MeV. These geopolymer materials, mostly those with higher lead content, demonstrated more effective radiation shielding properties at lower energy radiation. Higher energy radiation requires thicker geopolymer or combinations with other shielding materials.

Fig. 2
figure 2

Comparison of MAC of the prepared GEO-xPb geopolymer with those in commercial glasses at selected energies (0.2, 0.662, and 1.25 MeV).

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Linear attenuation coefficient variation

LAC denotes the segment of a beam of photons that is absorbed or scattered per unit thickness of the material. Figure 3illustrates the connection between LAC and photon energy for three different geopolymer compositions: GEO, GEO-10Pb, and GEO-20Pb. LAC decreases as photon energy increases for all geopolymer compositions22,30. Therefore, higher energy photons are less expected to interact with the material. The addition of Pb to the geopolymer significantly increases the LAC, mostly at lower photon energies. This is a result of the higher atomic number of Lead, which improves the possibility of photoelectric absorption which is the predominant photon interaction. The figure displayed the K-edge of lead, which signifies the photon energy where the likelihood of photoelectric absorption by lead atoms rises sharply30. This displays an abrupt drop in the LAC values for the GEO-10Pb and GEO-20Pb geopolymer at around 0.08 MeV.

Fig. 3
figure 3

LAC variation as a function of photon energy in the prepared GEO-xPb geopolymer.

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Half-value layer variation

HVL is the thickness of material needed to reduce the strength of an X-ray or gamma radiation to half of its original value30. Figure 4shows the connection between the HVL and density (ρ) for geopolymer at three different photon energies 0.2 MeV, 0.662. As the density of the geopolymer samples increases, the HVL decreases across all photon energies. This is a pointer that denser geopolymer materials offer better radiation shielding thereby requiring a thinner layer to shield radiation by half of its original intensity. Higher energy photons give larger HVL values compared to lower energy photons for the same density. This implies that higher energy radiation infiltrates deeper into the geopolymer material before significant attenuation31. Figure 4 also validates the importance of density and photon energy in deciding the shielding efficiency of a geopolymer material. Denser geopolymer materials with lower photon energy led to lower values of HVL hence improved shielding performance. Applications that involve low photon energy require higher density geopolymer to minimize the sample thickness. The linear relationship between density and HVL could be deployed in the estimation of the required thickness of geopolymer to attain a desired level of radiation shielding. Figure 5 depicts a 3D illustration of the HVL of the investigated geopolymer samples as a function of both lead concentration (x) and photon energy (E). As the percentage of lead (Pb) was increased from GEO to GEO-20Pb, the HVL decreased for a particular photon energy. This indicates that geopolymer samples with higher lead content are more efficient in shielding gamma radiations.

Fig. 4
figure 4

HVL with respect to a function of mass density in the prepared GEO-xPb geopolymer at 0.2 0.662 and 1.25 MeV.

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Fig. 5
figure 5

HVL with respect to the concentration of Pb and a function of photon energy in the prepared GEO-xPb geopolymer.

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Mean free path variation

MFP is the average displacement of gamma radiation in a material before interacting with the material32. Higher MFP values imply less ability of a material to stop gamma radiation. Figure 6 shows a comparison of MFP in different selected materials as a function of their photon energy. Investigated geopolymers gave lower MFP values than most other materials, particularly at lower energies. This signifies that the investigated geopolymers are more effective in gamma radiation attenuation, mostly at lower photon energies.

Fig. 6
figure 6

Comparison of MFP of the prepared GEO-xPb geopolymer with those in recently developed materials (SPCNCF423, S-TV/Ti2024, NBFL525, STV-YLi226, BSY227, LBZV428, and CNBF-C29) at energies between 0.015–15 MeV.

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Effective atomic number (Zeff) variation

Figure 7 displayed the Zeff of investigated geopolymers as a function of photon energy. As the concentration of Pb increases, the effective atomic number (Zeff) also increases for a given photon energy. This indicates that geopolymers with higher Pb content have a greater capacity to interact with photons thereby improving their shielding capabilities. For geopolymer samples without Pb (GEO), Zeff decreases as the photon energy increases. The implication is that the interaction probability between gamma radiations and geopolymer diminishes at higher energies. At higher photon energies greater than 1 MeV, the Zeff values for all GEO-10Pb and GEO-10Pb geopolymers converge towards a similar value, meaning that the impact of Pb concentration on photon interaction becomes less obvious at higher energy levels. Higher Zeff values of geopolymers with more Pb content show higher radiation shielding capabilities. The observed sharp decrease in Zeff with increasing photon energy implies that different radiation interaction mechanisms predominate different energy ranges. For example, photoelectric absorption was more prevalent at lower photon energies, while Compton scattering dominated higher photon energies.

Fig. 7
figure 7

Effective atomic number (Zeff) with respect to the concentration of Pb and a function of photon energy in the prepared GEO-xPb geopolymer.

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Exposure build-up factor (EBF)

EBF gives the degree of increase in radiation exposure at a given material depth compared to the material exposure32. Figure 8 shows that EBF is a function of photon energy for all investigated geopolymers. EBF increases as the concentration of Pb increases across all photon energy. This indicates that geopolymers with higher Pb content display a higher build-up of radiation exposure within the geopolymer. EBF of all glass compositions reaches a maximum value at 0.1–1 MeV (intermediate photon energies). This specifies that the build-up of radiation is maximum at this energy range. After the peak, the EBF reduced as gamma energy increased. EBF becomes less obvious at higher photon energies. The higher EBF values for geopolymers with higher Pb content suggest that these geopolymer materials have improved radiation shielding at intermediate energies. However, their improvement diminishes at higher photon energies. The peak in EBF at intermediate energies suggests that Compton scattering, which is dominant in this energy range, contributes significantly to the build-up phenomenon.

Fig. 8
figure 8

Exposure buildup factor (EBF) with respect to a function of photon energy in the selected GEO-xPb geopolymer.

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Energy Absorption Build-Up Factor (EABF)

EABF is the ratio of the absorbed energy at a specific depth within a material to the absorbed energy at the same point if there were no build-up. Figure 9 displayed the variation of EABF with photon energy for all investigated geopolymer samples. As the concentration of Pb increases across all photon energy, the EABF increases. This suggests that geopolymers with greater Pb content display an obvious build-up of absorbed energy within the geopolymer. EABF reaches a maximum value between 0.1 and 1 MeV which is the intermediate photon energy for all geopolymer samples. This shows that EABF is most significant in this energy range. The peak in EABF at intermediate energies suggests that Compton scattering, which is dominant in this energy range, contributes significantly to the build-up phenomenon.

Fig. 9
figure 9

Energy absorption buildup factor (EABF) with respect to a function of photon energy in the selected GEO-xPb geopolymer.

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Conclusion

Geopolymer-based composites with different amounts of lead (II) oxide (PbO) were prepared to evaluate the effect of PbO addition on the gamma radiation shielding characteristics of the geopolymer. The results showed that adding 10% and 20% lead oxide significantly improved the investigated geopolymer’s shielding properties, mostly at lower photon energies. The higher values of MAC of 59.66728 cm2/g at 0.015 MeV, and 0.03916 cm2/g at 15 MeV for the investigated geopolymers suggest that these materials have improved radiation shielding at intermediate energies due to higher photoelectric absorption and Compton scattering. The findings of this study provided valuable insights for designing high-performance radiation protection in medical imaging facilities, nuclear power plants, industrial radiography and other special radiation installations. Further research studies could be explored by incorporating elements and compounds with higher effective atomic numbers like Bi₂O₃ or WO₃ into the investigated material matrix to enhance its shielding features. Subsequent research may explore other alternative simulation tools like WinXCOM, PHIT and Py-MLBUF which are fast and free online interactive Python programs for the computation of about 36 gamma-ray shielding parameters33. These computer programs have been suggested as excellent programs for the theoretical evaluation of most radiation interaction parameters based on the accuracy of their results with experimental results33,34.