Simulating concrete degradation processes by reactive transport models

Abstract
Cement-based materials are commonly used in the multibarrier systems of radioactive waste repositories. Under the sub-surface environmental conditions they are exposed to during service-life, the chemical composition of the initially highly alkaline cement pore fluid may be altered by the influence of external ions and the leaching of dissolved species present in the cement interstitial solution, both of which processes are mainly ruled by ionic diffusion. Furthermore, the perturbation induced in the local thermodynamic equilibrium of the system yields to a series of dissolution/precipitation reactions which may result in a significant reorganization of the microstructure of concrete, in terms of both the distribution of mineral phases and the physical morphology of the capillary pore network, thus causing the concrete properties to undergo a gradual decline. Therefore, the long-term performance of concrete structures is a relevant issue in relation to the safety assessment of radioactive waste disposals. The analysis of the evolution of concrete degradation is a challenging task. It is also one that stresses the relevance of the development of reliable modeling techniques aimed at the prediction of long-term concrete behavior. The present work deals with the conceptualization of concrete both as a mineral aggregate, thus susceptible to deterioration, and as a porous material, where transport processes are expected to take place. Coupled reactive transport models are required to cope with the highly complex cyclic interactions arising between the chemical reactions which take place in the water-concrete interface and diffusive and advective transport in the aqueous phase. The approach taken herein aims at formulating and testing reactive transport numerical models by reproducing recent experiments reported in the scientific literature. Such procedure is intended to provide insight into the very nature of the phenomena involved, particularly those related to the appropriate methods available to describe ionic diffusion and the accuracy of the constitutive laws (e.g., porosity/permeability, porosity/diffusivity, etc.) developed for cement-based materials.