Cl- and Na+ ions binding in slag and fly ash cement paste during early hydration as studied by 1H, 35Cl and 23Na NMR

Lacking effective and in particular direct methods, the study on Cl- and Na+ ion binding during hydration has rarely been reported. In this study, a specially developed Nuclear Magnetic Resonance setup was employed to study the Cl- and Na+ ion binding of cement pastes during hydration. Influence factors, such as slag and fly ash replacement, on the H-1, Cl-35 and Na-23 relaxation of the cement paste made with 1 mol/L NaCl solutions were investigated. It was found that the increase of slag in the cement gives rise to bigger pores during hydration, and the fly ash resulted in less water being consumed. The Cl- signals would first be stable then gradually decrease as a function of time. The slag in cement has minor impacts on the Cl- ions binding, and the fly ash replacement in the cement would result in less Cl- ions binding during the hydration. Results also suggest that the slag could result in later Na+ binding, and the fly ash in the cement will significantly reduce the Na+ binding. Besides, the Cl- ions will bind earlier compared to the Na+ ions, which is probably related to the characteristics of the hydrates. The normalized Cl-35 and Na-23 signal intensity, in general, decrease with water consumption reflecting the hydration. Furthermore, the early formed hydrates would consume relatively less water but bind more Cl- and Na- ions. (C) 2020 The Authors. Published by Elsevier Ltd.