For this current work, we took cement samples from companies whose products are marketed in Côte d’Ivoire. These samples are:

- Sample of CEM II/B 32.5 R (CG1);

- Sample of CEM II/A-L 42.5 N (CB1);

- Sample of CPJ 35 R (CG2);

- Sample of CPJ 32.5 R (CG3);

- Sample of PCJ 32.5 R (CG4).

Samples taken were weighed and sealed during 30 days to reach the secular equilibrium between ²²⁶Ra, ²³²Th and their daughters.

A scintillation counter with a Thalium-doped NaI scintillation crystal (Diameter 38.1 mm and thickness 50.8 mm) [1] has been used. This detector designed by the German manufacturer LD Didactic has a resolution of 662 keV of about 7.5%. In order to constitute a measurement chain, the NaI crystal (Tl) was polarized by a high voltage generator (at 8.5 kV). At the output of the crystal, the analog signal is converted into a digital signal by the multichannel analyzer (AMC) to be transmitted to the Sensor-Cassy acquisition system. The collected information is carried out to the computer and appears as a spectrum. This equipment is located in the practical room of nuclear physics at the Felix Houphouet Boigny University of Cocody.

Analysis of the cement samples was performed using gamma spectrometry. However, its implementation requires two essential operations which are: the energy calibration and the determination of the detection efficiency.

Calibration is a tuning process that determines the energy range while eliminating measurement errors. It is, therefore, a prerequisite for any measure. This operation is intended to establish a relationship between the position of the peak in the spectrum and the corresponding γ line energy. For this work, a mixed source of americium (²⁴¹Am) and cesium (¹³⁷Cs) emitting respectively -peaks of energy at 59.54 keV and 661.66 keV have been used. Efficiency reflects the fact that all gamma photons emitted by sources are not detected. Then the detection efficiency noted ε is the efficiency of the detector used in the chain measurement. In this study, the efficiency curve was obtained using reference materials as sand containing a multi-photon radionuclide which is the ²²⁶Ra [2] [3] of 5 kBq activity. After carrying out these operations, we determined the specific activity of the radionuclides like radium (²²⁶Ra), thorium (²³²Th) and potassium (⁴⁰K). ²²⁶Ra was measured using the ²¹⁴Pb peak (351.93 keV) and the ²¹⁴Bi line (609.31 keV) [2]. For ²³²Th quantification, we used the ²¹²Pb (238.63 keV) line and the ²²⁸Ac (911.20 keV) line. For the ⁴⁰K, it was determined using its line emitted at 1460.83 keV. From these specific activities measured, we have been able to achieve the quantities that are the Equivalent Radium Activity and the Annual Effective Dose using the mathematical relationship below:

- The Equivalent Radium Activity noted Ra_eq

R a e q = A R a + 1.43 A T h + 0.077 A K ( Bq ⋅ kg − 1 ) [4] (1)

A_Ra, A_Th and A_K are the specific activities of ²²⁶Ra, ²³²Th and ⁴⁰K respectively.

- The Annual Effective Dose noted H_R (mSv∙year^-1):

H R = D ( nGy ⋅ h − 1 ) × 8760 ( h ) × 0.8 × 0.7 ( Sv ⋅ G y − 1 ) × 10 − 3 (2)

with D ( nGy ⋅ h − 1 ) = 0.92 A R a + 1.1 A T h + 0.08 A K ;

D: is the absorbed dose rate given by UNSCEAR and EC;

8760 h: is the number of hours in a year;

0.8: is the weighting factor that implies that 80% of the time is spent inside a build in;

0.7: represents the conversion factor from an absorbed dose to an effective dose.

In the present work the specific activity of ²²⁶Ra, ²³²Th and ⁴⁰K radionuclides have been determined. This choice is mainly motivated by the fact that these three radionuclides are naturally present in the Earth’s crust. Therefore, these radionuclides are also found in the cement whose basic constituents are clay and limestone. The concentrations of these radionuclides are shown in Figure 1.

The values of the specific activities presented in Table 1 are below the limit values recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) [4]. These limit values for building materials are 50 Bq∙kg⁻¹ for ²²⁶Ra and ²³²Th, and 500 Bq∙kg⁻¹ for ⁴⁰K. The specific activity values of ²²⁶Ra range from 18.85 ± 1.68 Bq∙kg⁻¹ to 50.60 ± 2.26 Bq∙kg⁻¹ with an average of 29.66 Bq∙kg⁻¹. The specific activities of other radioelements vary respectively from 22.94 ± 1.96 Bq∙kg⁻¹ to 41.82 ± 3.54 Bq∙kg⁻¹ for ²³²Th with an average of 34.88 Bq∙kg⁻¹, and from 111.10 ± 7.03 Bq∙kg⁻¹ to 309.74 ± 19.59 Bq∙kg⁻¹ for the ⁴⁰K with an average of 178.424 Bq∙kg⁻¹. The specific activities of radionuclides are generally lower in CB1 white cement than grey cement samples (except for ⁴⁰K potassium where the minimum value corresponds to CG4 cement). This finding could be explained by the fact that white cement consists essentially of pure chalk and white clay (sedimentary rocks). While the grey cement contains pozzolana (volcanic rock). In fact, the natural radioactivity of soils and rocks is due to the presence of three long-lived natural radioelements: ²³⁸U, ²³²Th, ⁴⁰K and their daughters such as radium (²²⁶Ra), which have a higher content in volcanic rocks (igneous rocks) than in sedimentary rocks.

Equivalent radium activity was introduced to evaluate the radiation risks from ²²⁶Ra, ²³²Th and ⁴⁰K radionuclides in building materials. It is the most widely used index for radiological risk assessment and can be calculated using Equation 1 proposed by Beretka and Mathew [4]. It is a weighted sum of specific activities based on the assumption that 370 Bq∙kg⁻¹ of ²²⁶Ra, 259 Bq∙kg⁻¹ of ²³²Th and 4810 Bq∙kg⁻¹ of ⁴⁰K produce the same gamma radiation dose rate [4]. Any building material whose equivalent radium activity exceeds 370 Bq∙kg⁻¹ is not recommended and is considered as an unhealthy material [4]. The values of this factor are indicated and compared to the UNSCEAR [5] limit value of 370 Bq∙kg⁻¹ in Figure 2 and Table 2.

The equivalent radium activity ranges from 60.21 ± 5.02 Bq∙kg⁻¹ to 118.57 ± 5.70 Bq∙kg⁻¹ with 93.28 Bq∙kg⁻¹ average. These values are well below the value recommended by UNSCEAR (370 Bq∙kg⁻¹). The values of equivalent radium activity were also compared to those obtained in other studies conducted in some countries as shown in Figure 3 and Table 3.

The average equivalent radium activity for grey cement samples (101.55, in this work) is lower than that obtained in Algeria, Australia and India. But this activity is higher than those obtained in Lebanon, Tunisia, Ghana and Nigeria. Regarding white cement, the equivalent radium activity is higher than that measured in Tunisia and lower than that of Cuba and Lebanon.

The effective dose is a protection quantity expressed in millisievert (mSv). It allows taking into account for an individual the effect of ionizing radiation on the whole organism from equivalent doses weighted in all tissues and organs of the body. The annual effective dose noted H_R (mSv∙year⁻¹) is defined from this quantity. It is evaluated by equation 2 given by the European Commission (EC) and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSEAR) [11]. For this quantity, the limit value recommended by the ICRP is 1 mSv∙year⁻¹ for the public.

Measurements on cement samples yielded annual effective dose values ranging from 0.252 ± 0.021 mSv∙year⁻¹ to 0.513 ± 0.021 mSv∙year⁻¹ with 0.427 mSv/year average. We also note that grey cement values remain the lowest. Table 4 and Figure 4 illustrate these results.

These values are below the limit value set at 1 mSv/year by the ICRP (Table 5 and Figure 5).

It is noted that the annual effective dose for grey cement samples (0.427 in this work) is higher than that obtained in Ghana, Tunisia and Egypt. But this dose is lower than that obtained in Turkey. For white cement, the annual effective dose is roughly equal to that measured in Tunisia and Ghana.

*Grey cement; **White cement; ( ) Average.

*Grey cement; **White ciment; ( ) Average.