Preparation of M15 cement mortar and its adsorption property as well as mechanism for Co2+

BackgroundCement mortar is an essential building and barrier material for geological disposal repositories of radioactive waste. Therefore, it is of great significance to study the preparation of the cement mortar and its adsorption property and mechanism for radionuclides.PurposeThis study aims to study the preparation of M15 cement mortar and its adsorption property as well as mechanism for Co2+.MethodsA cavern-type low and/or intermediate-level radioactive waste repository under construction in the South China was selected as a real reference for the design and manufacture of M15 cement mortar. Adsorption properties of the M15 cement mortar for Co2+ were determined through the static batch test method combined with the inductively coupled plasma atomic emission spectrometry analysis. Characterization analyses of the M15 cement mortar were performed by X-ray fluorescence spectrometer, X-ray diffractometer, infrared spectrometer and scanning electron microscope.ResultsCompressive strength tests show that the M15 cement mortar with cement:sand:water mass ratio of 1.0∶4.84∶0.84 meets compressive requirements. The characterization analyses reveal that the M15 cement mortar had good crystal structure, large specific surface area, amorphous distribution of particles and surface distributed with a large number of groups, such as OH-, HCO3-, CO32-, SiO42-. Adsorption results from the static batch test show that: (1) the adsorption equilibrium time, adsorption distribution coefficient (Kd) value, adsorption rate and adsorption capacity of the M15 cement mortar are 6 h, 1 453.93 mL∙g-1, 74.14% and 73.37 mg∙g-1, respectively, (2) the Kd value, adsorption rate and adsorption capacity increase with the decrease of particle size, (3) the Kd value is 1 626.02 mL∙g-1 when the solid-liquid ratio is 2 g∙L-1, indicating that the M15 cement mortar has good absorption capability for Co2+, (4) the Kd values rise with the increase of pH values of water phase, and (5) the order of inhibition effects of ions on the Co2+ adsorption is Mg2+>Ca2+>SO42->NO3->Cl->K+. Adsorption mechanism of the M15 cement mortar for Co2+ can be described with the pseudo-second-order kinetic model and Langmuir adsorption isotherm model.ConclusionsThis study provides a useful reference for the preparation of superior performance cement mortar to build disposal repositories of radioactive waste.