## 4 Conclusions  
This study investigated the influences of the CaSO\({}_{4}\) type (i.e., anhydrite vs. gypsum) on strength development and hydration products in the activation of GGBFSs when different activators (i.e., CaO vs. NaOH) and GGBFS sources (i.e., different steel makers) were used. This study found that the combination of CaO and calcium sulfate sources is a more effective means of activating GGBFS than any other combinations in order to gain greater strength and better quality control, while the use of gypsum with NaOH-activation was found to be the worst mixture for strength. Detailed conclusions of the study are as follows:  
1. The CaO-activated GGBFS achieved a great strength enhancement from the addition of CaSO\({}_{4}\) sources, and the type of calcium sulfates and source of GGBFS did not significantly affect the strength development in this study, while the NaOH-activated GGFBS showed its complicated dependency on the choice of CaSO\({}_{4}\) type and GGBFS source for strength development; thus, the use of CaO was found to be better than that of NaOH for quality control (meaning less change in strength by input parameters) when calcium sulfates were present in raw GGBFS.
2. In the ANOVA study, the influential degree of each variable for strength was found to be greater in the following order: activator type > CaSO\({}_{4}\) type > GGBFS source; in particular, the type of calcium sulfate was highly influential in NaOH-activation, while it was relatively insignificant in CaO-activation.
3. When calcium sulfate sources were added, the NaOH-activated samples showed significantly complex phase composition of reaction products, while the CaO-activated ones exhibited a much simpler composition.
4. In the NaOH-activation, due to the substantial formation of U-phase, the ettringite formation seemed to be suppressed. As the formation of ettringite generally contributes to strength evolution to some extent, more formation of ettringite in CaO-activation may be the cause for the higher strength of CaO-activated samples than those of NaOH-activation.
5. The TGA results indicated that the use of calcium sulfate sources in the CaO-activation clearly increased the quantity of reaction products; however, in the NaOH-activation, this barely increased, or even reduced, the quantity of reaction products.
6. In the MIP results, in the CaO-activation, the use of calcium sulfates significantly reduced the pore size and the total porosity from 7 to 28 days, while, in the NaOH-activation, it showed almost no reducing effect on total porosity and pore size; even the use of gypsum increased the porosity.  
All the authors have contributed equally to the idea of the paper, performing the experiments, analyzing and discussing the experimental results, and preparing this manuscript.  
This research was supported by a grant (18 TBIP-C144132-01) from Technology Business Innovation Program funded by Ministry of Land, Infrastructure and Transport of Korean government, and Dr. [PERSON]'s stipend was supported by Korea Research Fellowship Program through the NRF, funded by the Ministry of Science, ICT & Future Planning (2015H1D3A1061913).  
The authors declare no conflicts of interest.