NMR study on the early-age hydration and ion binding of the cement paste prepared with NaCl solutions

Potentially one could use seawater to make concrete, i.e., in non-reinforced applications and in reinforced concrete with non-corrosive fibres. By doing so one could reduce the environmental footprint of concrete as fresh water is getting scarce, but also could reduce the CO₂ impact by using fewer resources for making concrete. There have been various studies on the final properties of concrete made with NaCl solutions, mimicking seawater. In this study a specially designed NMR was used to look at the hydration of cement paste with a NaCl solution looking at the pore-structure formation and the dynamic consumption of Na and Cl, i.e., the binding of the ions and incorporation in the hydration products in time. The results confirm that the NMR setup is able to simultaneously and quantitatively measure the H, Na and Cl content during hydration, giving information on both the time dependent pore-structure change as well as the decrease of free Cl⁻ and Na⁺. Influence factors such as the water-cement ratio and concentration of NaCl on the microstructure development and on the decrease of both Cl⁻ and Na⁺ were determined, as well as how the Freundlich binding isotherm develops in time. Moreover, it is found that the Na⁺ and Cl⁻ ion binding can be directly related to microstructure development during the stages of hydration, which can be identified using the T₁ relaxation of hydrogen.