Far-Field assessment of radioactive Co transport with polycarboxylic ester (PCE) cement admixture from the LILW disposal facility

Abstract

Modern construction relies heavily on cement and concrete, and often uses comb-like polymers, particularly polycarboxylic esters (PCEs), to increase hydration and workability. PCE is an organic material with a very high molecular weight that induces specific sorption patterns on cementitious particles. It is characterized by the interaction of carboxylic main chains and ester side chains with adjacent Ca particles, thereby initiating a dispersion effect in the cement paste. The organic materials involved in these interactions increase the mobility of specific cationic nuclides. However, there is limited research describing the mobility of nuclides in cement structures containing these organic components. Therefore, this study explores the complex behavior of Co ions in the cement matrix under different conditions, while evaluating the direct influence of cement admixtures in far-field conditions. The main findings reveal that external weathering has little effect on the structural integrity of the cement. Even with a water- to-cement ratio of 0.3 and PCE of 5.0 wt%, the maximum desorption was only 2.3%. It is noteworthy that increased calcite formation slightly decreases Co desorption. And also the solubility of Co is highly pH dependent, with limited influence of carboxylic groups at neutral pH but a pronounced interaction at pH above 10. The solubility of the leachate in the cement paste was tested using a 2.0 wt% mixture and was observed to be almost unchanged compared to the 0.0 wt% mixture, attributable to the alkaline pH. Importantly, the Co ion adsorption efficiency of the cement remained satisfactory even after weathering, which emphasizes the pivotal role of the cement mixture in protecting waste stability, especially in low- and intermediate-level repositories.