Effect of pre-introduced sodium chloride on cement hydration process

Chloride-based accelerators are widely used to accelerate cement hydration; however, there is no focus on the effect of the interaction between Cl⁻ and SO₄²⁻ on cement hydration. Hence, the interaction effect between initial gypsum and sodium chloride (NaCl) in the cement hydration process was investigated. Isothermal calorimetry, scanning electron microscopy, thermogravimetric analysis, X-ray diffraction and inductively coupled plasma spectroscopy were used to evaluate the hydration process of cement paste. Moreover, a thermodynamic model and boundary nucleation and a growth model were applied to calculate the hydration products and describe the hydration process, respectively. It is clear that sodium chloride can accelerate cement hydration, but as the incorporated sodium chloride increases, the acceleration effect is weakened. The incorporation of chloride salts will promote the dissolution of minerals and increase the growth rate. In the early stage of hydration, the formation of ettringite precedes formation of Friedel's salt, owing to the fact that tricalcium aluminate (C₃A) favours binding with SO₄²⁻, which can be demonstrated by the change in ion concentrations and the evolution of hydration products. The formation of Kuzel's salt depends not only on the chloride concentration but also on the hydration time.