Chloride binding in concrete: recent investigations and recognised knowledge gaps: RILEM Robert L’Hermite Medal Paper 2021

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.

Phase changes in cementitious materials exposed to saline solutions

“…2b). In addition, chlorides can physically be bound in the C(-A)-S-H [5,[53][54][55][56][57][58][59][60][61], although the thermodynamic model does not account for this yet. The physical binding can stand for a large part of the chloride uptake in hydrated cement paste [60,62].…”

“…2. The C(-A)-S-H will also take up some chlorides by physical binding [5,[53][54][55][56][57][58][59][60][61], which is not considered in the thermodynamic model.…”

Physicochemical Analysis of Chloride Diffusion and Adsorption in Water-saturated Concrete: Theory and Measurement

“…Therefore, once the spatial distribution of Cl − ions in concrete is measured at specific time t s , the penetration profile of Cl − ions expressed with x/t s 1/2 as the abscissa is the same even the distribution map is re-measured after ten or twenty years. The nature of concrete is changed by the chemical reactions of concrete ingredients with diffusing ions (Samson et al 2000;Weerdt 2021), which may lead to the change of the diffusion characteristics of concrete. However, as long as the reactions are much faster than Cl − diffusion, the x/t 1/2 law is still valid no matter how the nature of concrete is altered.…”

Chloride profiles with a peak – why and what are the consequences for predictions?

“…Several models describing ion transport in cementitious materials include the chemical interactions between the ions in the pore solution, sea water and the binder reactions products. A few very sophisticated models so far [7] include many of the consequences of leaching on chloride binding and chloride transport properties. The codes, however, are not open for public use which means that it is not quite clear exactly what and how various parts are included.…”