Sorption of chlorides on hydrated cements and C[sub(3)]S pastes

Abstract A concentration dependent adsorption of chloride ions for hydrated cements and C3S pastes was detected. The capacity of adsorption is mainly determined by the specific surface, the pH value and the associated cation. Chloride adsorption is obviously preceded by a sorption of calcium ions. The adsorption of Ca2+ ions increases the positive charge on the surface of the used materials and induces further adsorption of chloride. Keywords: deicing agents, chloride adsorption, calcium-induced adsorption

1 Introduction

It has been known for a long time that chloride ions lead to corrosion in reinforced concrete under the presence of oxygen and water. On the other hand chlorides have been used as hydration accelerators. Chlorides penetrate hardened concrete since they are used as deicing agents. Therefore, research work has been done very early on binding of chloride ions. According to well-known works of Richartz [1] chloride ions during hydration are principally bound by the aluminates of cement paste. The mechanism of chloride binding on hardened cement paste is not clarified. According to Binder the aluminate phases are the chief part of the chloride sorption [9], However, results of Arya et al. show that C3A content has little effect on the binding of chlorides introduced after 2 days of hydration [2]. The experimental results of Tang und Nilsson [3] show that the chloride binding capacity of concrete strongly depends on the content of CSH gel in the concrete. These results are confirmed by Beaudoin et al. [6]. The authors examined the chloride content of CSH-phases after the action of a 3,8% CaCl2 solution.