The mechanical and physical behaviour on the cemented waste forms after one year disposal of real conditions

To prove the durability of cemented waste forms, an experimental program started in 1997 to establish the alteration in time of mechanical and physic-chemical properties of different simulated and real matrices in laboratory and Baita-Bihor disposal conditions. The selected matrices for characterization are normal cement and concrete compositions with and without mineral additives (bentonite and volcanic tuff) as reference samples, the conditioned matrices prepared with radioactive sludge non-active components (iron hydroxides and phosphate, calcium phosphate and cooper ferrocyanide) and non-active matrices prepared with organic acids and salts (oxalic, citric, tartaric, EDTA). All samples will be positioned in five locations of Baita-Bihor repository and kept 1, 3, 5 and 10 years before mechanical and structural characterization. These locations have different humidity conditions decreasing from location 1 to 5. These results will be compared with results obtained for the same type of samples kept in laboratory conditions. The research program requires detailed identification of the complex phenomena involved, long investigation times and extending basic data determined from laboratory tests in simplified conditions. The basic information on the waste matrix are obtained by mechanical strength tests. In this program, up to now, samples were prepared and disposed as follows: 1)cement-water with cement/water ratio 2:1 and 2)cement-sand-water with a cement/sand/water ratio 1:2:0.5. For the mechanical tests the sizes of the samples were: 40 x 40 x 160 mm (tested at bending and compression) and 20 x 20 x 20 mm (tested at compression). After the preparation, the samples were kept 24 hours in pattern, then in tight plastic bag for 7, 14 and 28 days for reinforcement. The bending and compression tests were done in a construction materials laboratory, using an electric press machine. After 28 days the samples of reference were introduced in the National Repository. Some of these samples are kept in simulated conditions of laboratory. The mechanical characterization of the samples with mineral additives are in progress. The following results were obtained. Physical-mechanical tests were done on cement and mortar samples with different sizes: prisms (40 x 40 x 160 mm) and cubes (20 x 20 x 20 mm). The samples were submitted to mechanical tests after 28 days and after one year disposal in real and simulated conditions (in laboratory). The resistance values were calculated depending on applied forces. Comparing the obtained results for the samples kept in real and simulated conditions, we can see that the mechanical resistance of the samples kept in real conditions is similar to that of those kept in laboratory conditions. Slight increases of the resistance values were found for samples kept in bentonite with distilled water as compared with those kept in distilled water. Bentonite is used as back-filling material for the volumes between radioactive waste packages from Baita-Bihor repository. In conclusion, the mechanical resistance obtained for all the samples, after one year disposal, is within the accepted limits for the matrix durability. Thus, the disposal conditions have not influenced the mechanical properties of the cemented matrix after one year disposal. The present results, show that bentonite seems to have a positive influence on the quality of examined matrices. (authors)