## 1 Introduction  
The manufacturing process for ordinary Portland cement (OPC) accounts for 12-15% of the consumption of all industrial energy usage and emits a large amount of greenhouse gas, producing 800-1000 kg of carbon dioxide per ton of OPC. In general, the amount of carbon dioxide emitted during OPC production is thought to be responsible for approximately 6-8% of worldwide carbon dioxide emissions [1; 2]. To reduce the current consumption of OPC, alkali-activated inorganic binders, which utilize industrial waste by-products such as coal fly ash and ground granulated blast-furnace slag (GGBFS), have been developed as an alternative to OPC [3; 4]. Particularly, alkali-activated GGBFS is a strong candidate which is suitable for meeting established OPC standards (e.g., American Concrete Institute [ACI] or CEB-FIP codes) because it has characteristic similarities to OPC; it generates comparable binding properties with an analogous major reaction product, known as calcium silicate hydrate 1 [C-S-H(I)] [5; 6], to those of OPC. Despite the aforementioned potential, alkali-activated GGBFS binder is still not widely used commercially in the construction industry, mainly because of the low cost-effectiveness of alkaline activators and the safety issues connected with their high pHtoxicity [7]. Considering these issues, the choice of an appropriate alkali-activator is significant because it determines strength development and safety, and it represents a production cost of alkali-activated GGBFS. Thus, calcium-based activators [e.g., Ca(OH)\({}_{2}\) or CaO] have gained attention [8; 9; 10; 11; 12] as alternatives to alkaline activators, due to their significantly lower pH and lower material costs [10] than those of alkaline hydroxide or silicates. [PERSON] et al. reported the superior strength development of CaO activation over that of Ca(OH)\({}_{2}\)[10]. CaSO\({}_{4}\cdot\)2H\({}_{2}\)O (gypsum) and CaSO\({}_{4}\) (anhydrite) have been applied to activate fly ash, known as geopolymer [13; 14]. These studies showed that both sulfate sources have different effects on compressive strength of geopolymers even for an equivalent SO\({}_{3}\) content. Geopolymer activated by anhydrite had an improvement in early-age strength, but not in long-term strength compared to one activated by gypsum [13]. However, no detailed study has been conducted in influences of both sulfate sources on alkali-activated GGBFS. The use of calcium sulfates as an additive in GGBFS systems significantly impacts not only reaction products in terms of type, quantity, or rate of formation, but it also influences strength development in CaO-, Ca(OH)\({}_{2}\)-, or alkali-activated GGBFS binders [15; 16; 17; 12] because it may influence the degree of GGBFS dissolution and it modifies sulfate-bearing reaction products, such as Al\({}_{2}\)O\({}_{3}\)-Fe\({}_{2}\)O\({}_{3}\)-tri (AFt) (e.g., ettringite) or Al\({}_{2}\)O\({}_{3}\)-Fe\({}_{2}\)O\({}_{3}\)-mono (AFm) (e.g., monosulfate, monocarbonaluminate). In particular, the formation of ettringite may boost the early strength of cementitious binders. There are three types of calcium sulfates: CaSO\({}_{4}\) (anhydrite), CaSO\({}_{4}\cdot\)0.5H\({}_{2}\)O (hemihydrate), and CaSO\({}_{4}\cdot\)2H\({}_{2}\)O (gypsum) [18]. These calcium sulfates have different crystalline forms and thermodynamic stabilities. In particular, they show a significantly different solubility below 150 \({}^{\circ}\)C. At room temperature, the high solubility is in the order of hemihydrate, anhydrite, and gypsum [18]. As the concentration of sulfate ions often greatly affects the dissolution degree of raw GGBFS [15], the solubility of the sulfate source may play an important role in enhancing the strength of a cementitious binder [16]. Therefore, selection of the calcium sulfate type may be a key issue in the achievement of the desired strengths. However, we know little about the influence of CaSO\({}_{4}\) type on GGBFS activations, or its possible dependence on different types of activation (e.g., CaO- vs. NaOH-activation).