A new approach to coupled two-phase reactive transport simulation for long-term degradation of concrete

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Publisher: Elsevier B.V.
Document Type: Report
Length: 14,740 words
Lexile Measure: 1440L

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ABSTRACT

We present a new model for fast and efficient simulation of long-term concrete degradation due to alkali-silica reaction (ASR) and carbonation. The novel model provides an alternative coupling solution of reactive transport and multiphase multi-component flow by approximating the complex chemical reactions into a quickly calculating look-up table, which can further be integrated into a two-phase multi-component transport model via source/sink terms. The complex dynamic interplay between chemistry and multi-phase transport are well addressed in this approach. A 1-D reactive transport benchmark is proposed by taking into account the two main chemical reactions which drive the concrete degradation: ASR and carbonation caused by transport of C[O.sub.2] in a gas phase. We contrast three different sets of simulations to explore the pattern of competition between ASR and carbonation in the long-term degradation of concrete. The numerical model derived from the look-up table approach is compared to a full reactive transport code to validate its accuracy and efficiency. It is shown that the look-up table approach and the full reactive transport code produce very similar results for degradation of concrete even for the case of competition between ASR and carbonation. However, in terms of performance, it is observed that the look-up table approach leads to a considerable reduction in calculation time. Future work will be focused on incorporating the proposed model with a geo-mechanical model for multi-chemo-physics analysis of the concrete evolution.

Keywords:

Concrete degradation

Carbonation

ASR

Multiphase reactive transport modelling

Look-up table

1. Introduction

In the field of nuclear waste disposal research, the long-term stability and properties of cement-based materials is of great importance with the aim of ensuring safe disposal over very long time periods [1]. Cement-based materials have been used for several decades for the conditioning of specific waste types (e.g. [2]). The materials are also important components of the engineered barrier system of the planned deep geological repositories for low- and intermediate-level radioactive waste in Switzerland [3]. Therefore, characterizing the long-term evolution of such materials is a topic of great interest, and thus receives continuous attention by the scientific community (e.g. [1,4,5]). The spatial and temporal evolution of cementitious materials in a deep geological repository is influenced by several external and internal processes involving chemical reactions and water/humidity transport, which are usually tightly coupled with each other [6,7].

In the Swiss disposal concept, the use of cement-based materials shall provide a stable mechanical and a high pH chemical environment throughout the repository for a very long period. Highly alkaline conditions are favorable as the sorption of radionuclides is enhanced on cement phases in these conditions and further corrosion of metals and microbial degradation of organic wastes is decelerated. The two latter processes limit gas generation. The long-term chemical stability of cement materials is further of great importance because in the course of cement/concrete degradation, cement phases are successively dissolved which reduces the pH buffering capacity of cement paste and results in a continuous drop in pH with time (e.g. [7,8]). Once the chemical environment evolves to near-neutral conditions,...

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Source Citation   

Gale Document Number: GALE|A569159234