# Gypsum-Dependent Effect of NaCl on Strength Enhancement of CaO-Activated Slag Binders
## 1 Introduction  
Ground-granulated blast furnace slag (GGBFS) is an industrial waste byproduct generated in the steel manufacturing industry. Extensive investigations have examined the use of GGBFS as a construction material to replace Portland cement for concrete production; in particular, numerous studies have focused on utilizing GGBFS as a precursor material in producing cementless binders [1; 2; 3; 4; 5; 6; 7; 8].  
Sodium-based alkaline activators (e.g., NaOH, Na-silicate) have been widely used to activate the binding properties of GGBFS to produce strong cementless binders for construction purposes (e.g., alkali-activated slag) [5; 9; 10]; however, because these activators are significantly more expensive than Portland cement, alkali-activated GGBFS binders are not commonly used in construction sites. Furthermore, alkaline activators are not safe for workers due to their high pH levels (>14) resulting in high toxicity [11].  
Recently, calcium-based activators (e.g., Ca(OH)\({}_{2}\), CaO) have been reported for their use of activating GGBFS because these activators are less expensive and have less alkaline than sodium-based activators [6; 11; 12; 13]. In the manufacturing process of commercial GGBFS, sulfate sources (e.g., gypsum (CaSO\({}_{4}\) 2H\({}_{2}\)O), anhydrite (CaSO\({}_{4}\))) are often incorporated to improve grinding efficiency and satisfy the required national standards of chemical composition for industrial uses [14]. When gypsum is present, CaO activation produces significantly higher strength when activating GGBFS, mainly due to the formation of ettringite (Ca\({}_{6}\)Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)(OH)\({}_{12}\) 26H\({}_{2}\)O) [12]; Thus, the solubility of gypsum is an important factor in increasing the strength of the CaO-activated GGBFS system.  
The use of sodium chloride (NaCl) in cementless binder systems may also affect strength development; For instance, [PERSON] et al. [1], reported that the addition of NaCl notably increased the compressive strength of activated GGBFS with calcium hydroxide (Ca(OH)\({}_{2}\)) and calcium carbonate (CaCO\({}_{3}\)). Further, in alkali-activated GGBFS, the addition of NaCl changed strength development and setting time, which is defined as the measured amount of time it takes for a freshly mixed paste to solidify [10].  
Most previous studies have looked at chlorides and sulfates as aggressive environmental agents, focusing primarily on the durability issues of cementitious materials (e.g., chloride binding capacity, embedded steel corrosion, or sulfate attack) [15; 16; 17; 18; 19; 20; 21; 22; 23] rather than their effects as additives.  
Thus, this study investigated the influences of adding NaCl to the CaO-activated GGBFS system on strength development and the dependence of strength enhancement effect on the presence of gypsum in the system. However, the significance of chloride-induced corrosion is not specifically considered because this study was a preliminary study to develop cementless binders for manufacturing blocks or bricks, which do not contain any steel.  
To this end, compressive strength testing, powder X-ray diffraction (XRD), thermogravimetry (TG), and mercury intrusion porosimetry (MIP) were conducted.