Introduction
It is widely acknowledged that chloride-induced corrosion is one of the main degradation mechanisms in civil structures based on reinforced concrete. Corrosion starts as soon as the chloride comes in contact with reinforcement steel bars. The source of chloride can be natural, e.g., sea water or from de-icing salts. In general the chloride will enter a concrete by advection with moisture or diffusion within the moisture present in concrete. Gaining insight into these transport phenomena can not only improve the assessment of durability aspects of existing structures, but might lead to improved design for new reinforced concrete structures. At the moment there is lack of experimental data on this topic that can be used to validate or discard the wide range of models available in the literature. One problem is to obtain experimental data.  There are various methods available to measure ionic chloride content. Most common method is to drill a specimen out of a concrete structure and analyse it chemically in the laboratory. The most obvious drawback of this method is its destructiveness, but there are more shortcomings, such as its poor spatial resolution and irreproducibility.  Despite the low sensitivity of nuclear magnetic resonance (NMR) for 35Cl this method might still be preferred, as it allows to measure different nuclei, 1H, 23Na and 35Cl, simultaneously with a high spatial resolution and give a full insight into dynamic interactions between the ions taking place.


    Short overview of the NMR setup
A picture of the NMR setup is shown in Fig. 2. The main field of the NMR is 4.7 T and is generated by a vertical superconducting magnet with a 200 mm wide bore. Assisted with an anti-Helmholtz gradient coil set, a maximum gradient of 0.6 T/m is provided inside the NMR, resulting in a millimeter resolution. Using 3 step-motors, the RF coils, i.e., 1H, 23Na and 35Cl (optional 7Li ), can be moved through the magnet, which allows the 1H, 23Na and 35Cl of sample to be measured.


Fig 1: An overview of the NMR setup for measuring 1H, 23Na and 35Cl in porous materials


A short presentation giving a more detailed explanation of the NMR setup.
    Movie. Short video presentation of the NMR setup

 

    Conclusions
Using a specially designed NMR setup, the 1H, 23Na and 35Cl content in cementitious materials can be measured quasi-simultaneously. The sensitivity of the current setup is sufficient to study the intrusion of seawater in cementitious materials. Moreover, the setup has shown that by multi nuclei measurements more insight can be gained on interactions with the cement matrix. It is observed that the Na/Cl ratio changes during the hydration, indication a chemical/physical interaction with the cement matrix.


     

 Yanliang Ji, Leo Pel, Xiaoxiao Zhang, Zhenping Sun, Cl- and Na+ ions binding in slag and fly ash cement paste during early hydration as studied by 1H, 23Na and 35Cl NMR, Const. Build. Mat. 266B,121606 (2021). 

Yanliang Ji, Leo Pel, Zhenping Sun, NMR study on the early-age hydration and ion binding of the cement paste prepared with NaCl solutions, Cement and Concrete Composites 129, 104489 (2022).