The sorption experiments of carbon-14 on the mortar grain (grain size : 0.50 – 1.0 mm) focused on the chemical form of the carbon-14 were carried out by the batch method. Three kinds of carbon-14 chemical form were used for the experiments : sodium carbonate (Na2
14CO3) as the inorganic radiocarbon, and sodium acetate (CH3
14COONa) and acetaldehyde (14CH3
14CHO) as the organic radiocarbons. 0.30 gram samples of mortar were soaked in the solution with carbon-14 at 15°C for periods of up to 160 days. At the end of each run, carbon-14 concentrations in the supernatants were determined before and after centrifugation (3,500 rpm., 1 hr.).
In the mortar-sodium carbonate system, the retention process of carbon-14 related to reaction on the surface of the mortar was speculated as follows. First, 3CaO-SiO2. and 2CaO-SiO2, of the mortar components contact with water and produce Ca(OH)2. Ca(OH)2 produces Ca2+ and OH− in the solution. Then, calcite forms from Ca2+ and CO3
2− in the solution. Thus, the sorption ratio of carbon-14 onto mortar will be high until mortar nas been completely carbonated because Ca2+ is rich in the mortar and the solubility of calcite is low.
In the mortar-organic carbon system, the soluble organic carbon-14 is hardly sorbed on the surface of the mortar. Therefore, the cementitious materials may not inhibit the release of organic radiocarbons from the low-level radioactive wastes, contrary to the case of inorganic radiocarbon.