# Pore Solution Chemistry of Calcium Sulfoaluminate Cement and Its Effects on Steel Passivation
## 1 Introduction  
Calcium sulfoaluminate cement (CSA) is an environmentally friendly material providing rapid hardening and excellent mechanical performance. In recent years, it has been increasingly applied as an alternative binder for concrete production. Compared with Portland cement (PC), CSA generates different hydration products, which may give rise to the issues related to the durability of reinforced concrete structures. Steel corrosion is a crucial factor in concrete durability. Generally, the high alkalinity of PC-based concrete can protect steel from corrosion by forming a stable passive film. The application of new cement, however, changes the internal environment of concrete, which may influence the properties of the passive film [1, 2]. Currently, only a few experiments on steel passivation have been conducted in relation to CSA; therefore, the condition of steel embedded in CSA-based materials is not yet fully understood [3, 4, 5, 6]. As an important part of the concrete internal environment, pore solution plays a significant role in preventing or accelerating steel corrosion. According to experiments by [PERSON] [5] and [PERSON] [7], the time it takes to form a stable passive film decreases in solutions with higher pH values. Furthermore, if the solution alkalinity drops under 11.5 [8], the rebar is not passivate. The influence of ion composition has also been observed in previous works. [PERSON] [9] reported that, with the addition of Ca(OH)2 in a solution of 0.1 M NaOH and 0.175 M KOH, the constitution of passive film changed, and the charge transfer resistance also increased noticeably. Additionally, SO4\({}^{2-}\) may also affect the condition of the steel bar, as indicated by [PERSON] and [PERSON] [10, 11], who claimed that SO4\({}^{2-}\) can cause more severe corrosion than Cl\({}^{-}\). However, this conclusion was notsupported by some other researchers [12; 13; 14]. Some other ions such as cation types, carbonate and sulfur species have also been investigated, and the results indicated the importance of ion composition in steel passivation [2; 13; 14; 15]. However, current studies on the effects of pore solution chemistry on passivation are concentrated on PC. Considering the different hydration mechanism of CSA [16; 17; 18; 19], there is a lack of knowledge about the passivation behavior in this special environment. The pore solution of PC-based materials, with a pH value fluctuating around 13.5, comprises Na\({}^{+}\), K\({}^{+}\) as well as a small amount of SO\({}_{4}\)\({}^{2-}\) and Ca\({}^{2+}\)[20; 21; 22]. Nevertheless, the pH value of the CSA pore solution varies from 12.5-12.9 [17; 22; 23], and in some cases, values under 11.3 have also been recorded, i.e., much lower than that of PC. [PERSON] [23] has confirmed that Na, K, Al, S were the four principle elements in the liquid phase of CSA-based material. It should be noted that Al and S are nearly undetectable in OPC concrete. [PERSON] [17] speculated that S was SO\({}_{4}\)\({}^{2-}\) from gypsum contained in the cement clinker, but currently no experimental evidence supports this point. As for element Al, few studies have been conducted to discover the ions it represents and its function on passivation. From the review of past research work, it may be concluded that the pore solution of CSA, compared with that of OPC, features a lower pH and a completely different chemical composition. It is necessary to discover the mechanism of how the chemistry of the CSA pore solution affects passivation, the results of which can offer a better understanding to the steel passivation in CSA-based materials and support the service life prediction of structure constructed with CSA. This work firstly aims to investigate the alkalinity and ion composition of CSA pore solution, establishing the basis of preparing simulated pore solution of CSA.