Abstract
A theoretical understanding of chloride binding is urgently needed if we are to use the new low-CO2 composite cements in reinforced concrete structures exposed to chloride-loaded environments. The empirical models and “deemed-to-satisfy” rules currently in use will not help in the face of the wide variety of new SCMs now being proposed. Engineers need generic chloride ingress models that can handle the variations in binder chemistry and exposure conditions. This paper reviews a selection of recent investigations by a team of NTNU researchers and international collaborators on various Portland composite cements using a combination of equilibrium chloride-binding experiments and thermodynamic modelling. One of our main findings is that “leaching” simulated by varying the pH and the calcium concentration has a dominant effect on chloride binding. However, challenges regarding the quantification and characterization of the chloride binding phases have yet to be overcome. To advance in this field we need additional crucial thermodynamic data for chloride-binding hydrates formed by low-CO2 cements containing SCMs, and we need to overcome some experimental challenges. A major break-through would come from understanding the interactions between chlorides and C(-A)-S-H. Part of the answer might be found at the atomic level. Finally, mechanistic numerical models are needed to upscale the findings from chloride binding to chloride ingress models. We conclude by showing the significance of such theoretical work on chloride binding for engineers tasked with the construction and maintenance of the reinforced concrete structures that are so vital a part of modern infrastructure.
Highlights
Chloride-induced corrosion of steel reinforcement is one of the main degradation mechanisms for reinforced concrete structures worldwide and is a major threat to our global infrastructure [1]
This paper focuses on one crucial piece of the puzzle, chloride binding, which reflects the ability of hydrated cement paste to accumulate chlorides in the hydrates formed
An alternative explanation for the differences between the measured and the modelled pH and chloride binding for higher chloride concentrations could be the formation of some other phase which takes up chloride, hydroxyl and calcium ions and which is not included in the thermodynamic model, e.g., a calcium hydroxychloride compound
Summary
Chloride-induced corrosion of steel reinforcement is one of the main degradation mechanisms for reinforced concrete structures worldwide and is a major threat to our global infrastructure [1]. We are designing structures for much longer service lives and want to extend those of existing structures All these sustainability factors combine to make it urgent to develop generic mechanistic models for the accurate prediction of the durability of the concrete we make. It should be noted that generic mechanistic chloride ingress models should account for the impact of new sustainable binders on the porosity and moisture state of the concrete. This very challenging work is at least as important as chloride binding when it comes to understanding chloride ingress in concrete [6, 7], but lies beyond the scope of this paper. The third section indicates the main knowledge gaps in this field with a view to focusing the efforts of the research community and encouraging support from funding institutions and industry, and the conclusion summarizes why such research is so important for the engineers responsible for the construction and maintenance of reinforced concrete structures exposed to chlorides
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