Analysis of chemical degradation of fly ash blended concrete in a vault-type radioactive waste repository using reactive transport modeling

Abstract

This study presents a chemical degradation behavior of fly ash blended concrete in a vault-type radioactive waste repository by “pure” thermodynamic modeling as well as a reactive transport model that coupled a pure thermodynamic model with water and solute transport simulation. Since rainwater plays a predominant role as leachate for the concrete vault in the repository, the effect of chemical properties of seasonal rainwater on the chemical degradation of concrete was analyzed. In addition, the impact of blending fly ash on the chemical degradation was assessed through a gradual replacement of ordinary Portland cement (OPC) by fly ash. The pure thermodynamic modeling results revealed that autumn rainwater shows the greatest influence on the concrete degradation owing to the presence of inorganic carbon species, which induce the carbonation. The resistance of concrete to chemical degradation by rainwater was persistently reduced with increasing replacement level. In a qualitative manner, the reactive transport simulations showed comparable results with respect to pure thermodynamic modeling. For example, the types of cementitious minerals formed and the correlation between them have rarely changed. Quantitatively, however, the two models showed different trends. The pure thermodynamic modeling indicated that winter rainwater is more aggressive towards concrete degradation compared to spring rainwater, but the opposite tendency was observed with reactive transport modeling. Compared to winter rainwater, the concrete was very vulnerable to spring rainwater. A decrease in the porosity of concrete due to the increased formation of calcite during degradation by winter rainwater appeared to be the main reason for the observed phenomenon. In addition, reactive transport modeling indicated that the chemical condition of concrete pore water, due to the dissolution of concrete, maintains a high pH and lasted for tens of thousands of years.