Technical reportNatural radioactivity and human exposure by raw materials and end product from cement industry used as building materials
Introduction
Cement is a widely used building material. Hence, it is of a great benefit for the entire society to examine the radioactivity of the raw materials used in its manufacture. Evaluation of the specific activity (Bq kg−1) of these raw materials is an important issue, for they could be a source of considerable indoor dose rate. They consist mainly of the natural occurring uranium (238U) and thorium (232Th) series, and potassium (40K). In the 238U series, the contribution of the radionuclides in the first half of the series between 238U and 230Th is negligible relative to the second half comprising 226Ra to 210Pb. Consequently the measurement of the activities of 226Ra, 232Th and 40K in all the component materials is relevant to a study of cement.
Naturally radioactive materials, manufactured products and industrial residues are widely used in the cement industry as raw materials. Residues from industrial processes such as fly ash from coal-fired power plants produced in large quantities can be recycled and used as a supplement in cement production. This process could yield technological, economical and environmental benefits, but if not subject to regular control, can yield elevated indoor radioactivity exposure rates.
The safety requirements for building materials refer to the excess exposure rate caused by these materials in addition to terrestrial and cosmic radiation. In this study, safety requirements proposed by the European Commission (EC, 1999) have been examined. The basic concept in determination of the excess exposure rate consists of (1) determination of the total exposure resulting from the building material including the background and (2) background subtraction (Markkanen, 1995).
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Sampling and sample preparation
The samples included the following: cement collected since 2005 on a monthly basis from the Cement Factory in Skopje; 218 samples (of which 49 were fly ash samples) from the deposits of two power plants, REK Bitola Fly ash I and REK Oslomej Fly ash II; 45 samples from Pozzolana (originating from Strmoš-Pozzolana I and Češinovo-Pozzolana II), 16 natural gypsum samples, 42 clinker samples and 66 cement samples. The specifications of the cements under investigation are given in Table 1. The cement
Results and discussion
The specific activities (Bq kg−1) of 226Ra, 232Th and 40K (minimum, maximum and mean values) of the raw materials and cements are given in Table 2. From the results of all the analyzed materials it is evident that lowest activity mean values of 226Ra, 232Th and 40K were found for gypsum (5.9 ± 1.1 Bq kg−1, 1.44 ± 0.44 Bq kg−1 and 11 ± 5.2 Bq kg−1, respectively). On the other hand, the highest activity mean value for 226Ra (140 ± 89 Bq kg−1) was found in Fly ash I samples. Likewise, the highest values
Conclusions
In this study we have found that the highest values for the mean activity concentrations are in fly ash (226Ra, 107 ± 45 Bq kg−1, 232Th, 109 ± 30 Bq kg−1; 40K, 685 ± 171 Bq kg−1), the specific activities of which are above the world mean values for building materials (50, 50 and 500 Bq kg−1 for 226Ra, 232Th and 40K, respectively; UNSCEAR, 1993). However, the mean specific activities of the cement products (226Ra, 42 ± 10 Bq kg−1; 232Th, 28 ± 6 Bq kg−1; 40K, 264 ± 50 Bq kg−1) are similar to those reported by other
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