Mechanical properties and attenuation ability of biowaste-derived CaCO 3 for radiation shielding applications

Abstract

Lead, the common radiation shield, poses significant risks to medical personnel's organs despite its shielding properties. Concrete, increasingly utilised as another radiation barrier than lead, shifts towards sustainability due to depleting resources like cement. This research evaluated fabricated concrete samples’ mechanical stability and radiation shielding using EpiXS software for gamma radiation and a medical linear accelerator for x-rays. Local green mussel shells underwent cleaning and calcination at 550°C for 2 h and then pulverised and sieved for uniformity. Concrete cylinders and slabs were cast with 2, 4, 6, 8, and 10% cement replacement for mechanical and attenuation tests. Debris from mechanical tests was pelletised for XRF analysis. Results indicated 2% green mussel shell powder (GMSP) concentration optimised concrete's mechanical properties. EpiXS analysis showed almost the same N_eff trend for control and all concentrations, while GMSP4 consistently produced the highest Z_eff for all energies. MAC values peaked at GMSP4, and TVL was lowest at GMSP8. In linear accelerator tests, GMSP4 had the lowest TVL yield. Future studies should explore MRI and XRD analysis for powder characterisation and consider larger slab surfaces.

1. INTRODUCTION

Technological improvements pave the way for expanding the utilisation of ionising radiation especially in the medical field. Thus, radiation shields are employed to make sure no unwanted radiation exposures are made to workers and the public during operations. Concrete, usually made up of 10–15% cement, 15–20% water, and 65–75% aggregates, is the most common material for wall shielding in radiation facilities. However, with the price increase of concrete materials (Jalil et al., 2019), there is a need to look for alternatives which can lower the cost of the usual concrete mixture. Furthermore, Ismail et al. (2019) stated that production of biowastes such as green mussel shells (GMS) has become a local issue as they add to air pollution due to smell and slow decaying rate. Lertwattanaruk et al. (2012) highlighted that 95–96% of the GMS's composition of CaCO₃ with regard to its weight matched the calcium content in the ordinary Portland cement. These GMS have the potential to absorb and scatter ionising radiation, making them a promising material for shielding. This study aimed to use GMS as alternative cement in the concrete mixture to assess its mechanical properties and photon attenuation ability. Not only does this provide a sustainable solution to the problem of biowaste, but it also offers a cost-effective alternative material to traditional ones for composing the concrete walls as radiation shielding.

2. METHODS

2.1. Sample preparation

Seven kilos of GMS were obtained from public markets in Quezon City, Metro Manila, Philippines. The preparation was adopted from Adewuyi et al. (2015), Islam et al. (2011), and Razali et al. (2017) where excess mussel meats were removed using tap water and then GMS were boiled for 2 h to remove the smell. GMS were cooled and washed with distilled water and then air-dried at room temperature for 12 h prior to the calcination process.

GMS were reduced in size using a mortar and pestle and then refined using a mechanical grinder. The calcination process was conducted at UP Drugs of Abuse Research Laboratory in the Department of Pharmaceutical Chemistry, College of Pharmacy, University of the Philippines Manila. GMS were placed in evaporating dishes and crucible and then put inside the furnace at 550°C for 2 h in accordance with the methods of Chiou et al. (2014). Dried samples were cooled for an hour before being powdered using the same grinder and had undergone 200-mesh 75-μm sieving.

Six samples of shields were prepared by mixing water, ¾ grade gravel, sand, and GMS with Portland cement with a replacement ratio starting from 0% for the control up to 10% with 2% increments. A 5:3 cement-water ratio was utilised. Concrete samples were then moulded in cylinders for mechanical tests and slabs for x-ray shielding test.

2.2. Instrumentation and testing

2.2.1. Mechanical test

Mechanical properties of the concrete samples, which include compressive strength and split tensile strength, were tested at the Construction Materials and Structures Laboratory, Institute of Civil Engineering, University of the Philippines Diliman, Quezon City. Circular base dimension cylinders measuring 20 cm (height) × 10 cm (diameter) were moulded for both tests.

2.2.3. X-ray shielding using a medical LINAC

A 6 MV of x-ray energy aligned to the study of Demir et al. (2010) was used to get the electric charge reduction of concretes via Varian medical linear accelerator in the Philippine General Hospital. A field size (FS) of 10 cm × 20 cm was set to align the area of the sample slabs with the source-to-surface distance (SSD) of 100 cm. SSDs after the samples were piled up were noted for every reading. The measurements were repeated five times to vary the thickness of the concrete slabs by placing them on top of each other.

2.2.4. Gamma ray shielding using EpiXS software

Debris from mechanical tests were obtained and ground using a pulveriser and ball mill for five minutes before sieved through a 53-μm sieve. Powders were further homogenised using an agate mortar and pestle. A 225-μL polyvinyl alcohol (PVA) binder was then dropped onto the sample before it was transferred to the mould and pressure press. The press was set to 20 T with 4 min pressing and 1 min release. Lastly, the sample pellets were dried in an oven at 105°C for 2 h. The Bruker S1 Titan XRF device was used for the characterisation of these pellets. The percentage yields of all the elements and oxides were input to EpiXS software that interpolates between 1 keV and 100 GeV to obtain the photon shielding parameters (Hila et al., 2021). These processes were all completed in the Nuclear Material Research Section at the Philippine Nuclear Research Institute.

3. RESULTS AND DISCUSSION

3.1. Mechanical test results

Table 1 shows the results of compressive strength and split tensile strength tests of cylindrical concretes. GMSP2 increased an 11.69% to the mechanical property as compared to the control. This was followed by the GMSP10 and GMSP8. This means that 2, 10, and 8% were replacement ratios which possessed greater property over the control. Meanwhile, GMSP6 decreased a 9.91% to its property. These findings were comparable to those of Ismail et al. (2019) who incorporated 1–4% of shell ashes as an admixture. They found that 2% was the strongest and 4% was the weakest concrete. Meanwhile, Razali et al. (2017) who used calcined green mussel shell powder (GMSP) as cement replacement with 10–40% with 10% increments found out that 10% yielded the highest compressive strength after a 58-day curing.

Table 1.

Results of compressive strength and split tensile strength tests of cylindrical test concretes.

% replacement	Compressive strength (MPa)	% difference*	Split tensile strength (MPa)	% difference*	Average % difference*
Control	12.89	-	8.72	-	-
GMSP2	13.88	7.66	10.09	15.72	11.69
GMSP4	11.30	12.34	8.94	2.52	4.91
GMSP6	10.09	21.70	8.89	1.89	9.91
GMSP8	13.16	2.13	9.32	6.92	4.52
GMSP10	15.63	21.28	8.61	1.26	10.01

*

The green arrows represent the positive percent differences of the sample's yield with respect to the control, while the red arrows represent the negative percent differences.

3.2. X-ray shielding test results

 Fig. 1 shows the bar graph that demonstrates TVL in centimetres for the different GMSP concentrations. GMSP4 had the lowest TVL with 28.71 cm followed by GMSP10, GMSP2, GMSP6, GMSP8, and lastly control with 28.83, 28.91, 28.95, 29.19, and 29.85 cm, respectively. This showed that the GMSP helped decrease the TVL and improved the shielding capacity of the concrete as compared to the normal ones.

Fig. 1.

Tenth-value layer of the different GMSP concentrations for 20-cm-thick slabs showing GMSP4 having the least yield of TVL.

3.3. Gamma ray shielding test results

3.3.1. Electron density

 Fig. 2 shows the N_eff as a function of energy graph of pelletised concretes. It was seen that N_eff was highest in the control for lower energies (1–10 keV), while GMSP10 had the highest trend in the latter half of this range. For middle to higher energies (10–1,000,000 keV), all trends moved in unison. This suggested that N_eff trends of concretes with GMSP were almost the same overall against increasing energy.

Fig. 2.

The electron density N_eff vs energy graph of pelletised concrete samples where control was dominant for lower gamma energies.

3.3.2. Effective atomic number

Fig. 3 shows the effective atomic number Z_eff vs energy graph of pelletised concretes. It was presented that Z_eff was highest in GMSP4 which was followed by GMSP6, GMSP2, and the overlapping GMSP8 and GMSP10. Control had the lowest Z_eff suggesting that the addition of GMSP to samples had increased the Z_eff and the attenuation ability since the higher the Z_eff, the more photon–electron interactions will occur, thus more shielding will happen.

Fig. 3.

The effective atomic number Z_eff as a function of energy graph of pelletised concrete samples where 4% GMSP was dominant for all gamma energies.

3.3.3. Mass attenuation coefficient

 Fig. 4 presents the behaviour of MAC of control and concretes with GMSP against increasing energy. It was consistent across all graphs that MAC decreased rapidly in the low-energy range (1–100 keV) where the photoelectric process was dominant. For the mid-energy region (100 keV–5 MeV) where incoherent scattering dominated, a slight decrease in MAC values occurred. Above 5 MeV where nuclear pair production was dominant, MAC values tend to increase.

Fig. 4.

The variation in MAC of the control, GMSP2, GMSP4, GMSP6, GMSP8, and GMSP10 against increasing gamma energy.

For comparison, the mean MAC was obtained from all samples against 1 keV to 1 GeV. Results showed that GMSP4 had the highest mean MAC with 59.29 cm² g⁻¹ followed by GMSP6, GMSP2, GMSP8, and GMSP10 with 59.02, 58.93, 58.78, and 58.54 cm² g⁻¹, respectively. Control yielded the lowest MAC mean with 58.24 cm² g⁻¹ which indicated the addition of GMSP as partial cement replacement had increased the total MAC values of concrete shields against increasing gamma energies.

3.3.4. Tenth-value layer

Fig. 5 shows the graphs of TVL for all the concrete samples. It was presented that trends were increased from 1 to 10,000 keV range and peaked from 10,000 to 100,000 keV before it decreased beyond this range. GMSP8 yielded the lowest TVL trend followed by GMSP6, GMSP4, GMSP2, and GMSP10. Control possessed the highest TVL trend indicating that GMSP helped the shields since the lower the TVL, the better the shields will be. These results might be due to the density of the pellets where GMSP8 had 1.77 g cm⁻³, followed by GMSP6 with 1.73 g cm⁻³, GMSP4 with 1.72 g cm⁻³, GMSP2 and GMSP10 with both had 1.70 g cm⁻³, and lastly, the control with 1.69 g cm⁻³. This density–TVL relationship was also found in the results of the study of Akkaş (2016) where the TVL and half-value layer (HVL) of concretes were observed with different sample densities.

Fig. 5.

Variation in the tenth-value layer of the control and all the concentrations with increasing energy showing 8% GMSP having the least yield.

4. CONCLUSION

This study investigated the mechanical properties and photon attenuation parameters of concretes with calcined GMSP as partial cement replacement. Compressive and split tensile strength tests showed that a 2% GMSP concentration was most effective in improving mechanical properties. Medical linear accelerator and EpiXS software were used for x-ray transmission and gamma ray irradiation simulation, revealing that different GMSP concentrations were optimal for each parameter. In fact, 4% GMSP yielded the highest effective atomic number and mass attenuation coefficient, while 8% GMSP resulted in the lowest tenth-value layer via LINAC.

Additional tests, such as the homogeneity test, bubble formation check, SEM-EDX or XRD analysis for further verification of the chemical reaction brought by calcination, and exploration of slab dimensions to attain a wider surface size for reduced backscattering, are recommended. The study provides insight into the potential benefits of using GMSP in concrete applications.