# Gypsum-Dependent Effect of NaCl on Strength Enhancement of CaO-Activated Slag Binders
## 4 Conclusions  
This study investigated the influence of adding NaCl to a CaO-activated GGBFS system on strength development, and its dependence on strength enhancement effects on the presence of gypsum in the system. When gypsum was not present, adding NaCl exhibited either no effect or a negative effect on increasing compressive strength, while introducing the inclusion of gypsum used with NaCl allowed NaCl to observably increase the strength, particularly in early days of curing (i.e., before 7 days). The most significant contributor to this strength improvement was likely the increased solubility of gypsum and Ca(OH)\({}_{2}\) due to the presence of NaCl in a fresh mixture paste, leading to the more efficient formation of ettringite accompanied by strength improvement in early days of curing. Our conclusions are as follows:  
Figure 5: Mercury intrusion porosimetry (MIP) results of hardened paste samples at (a) 7 days and (b) 28 days.  
1. Considering that most strength gains of samples with gypsum were obtained by 7 days and the 7-day strength of Ca-N1-G5 was approximately 22% higher than that of Ca-N0-G5, the combined effect of NaCl with gypsum on strength seems to have mostly occurred before three days.
2. In XRD and TG testing, the use of NaCl alone notably formed hydrocalumite, while the incorporation of only gypsum clearly produced ettringite. When NaCl and gypsum were used together, hydrocalumite and ettringite were simultaneously formed; however, no other type of new reaction product was identified.
3. In TG/DTG testing, the use of NaCl likely boosted the formation of ettringite by notably increasing the concentration of sulfate and calcium ions in early days (i.e., before 7 days), resulting in the improvement of strength early in the process.
4. In MIP testing, the use of NaCl did not yield a beneficial influence on pore size distribution for strength improvement at either 7 or 28 days when no gypsum was present. Rather, the use of NaCl hindered pore size refinement from 7 to 28 days. These observations are fairly consistent with their corresponding strength results. However, when gypsum was present, the use of NaCl yielded a significantly advantageous influence on pore size distribution; in particular, at 7 days, the addition of NaCl had removed relatively large pores, possibly due to the increased formation of ettringite.  
[PERSON] and [PERSON] designed the experiments for this study. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] obtained the materials and prepared the samples. All authors participated in conducting the experiments, as well as contributing to the analysis and discussion. [PERSON] wrote the first draft of manuscript. [PERSON], [PERSON] and [PERSON] reviewed the final manuscript.  
This work was funded by the grant (144132) from the Technology Business Innovation Program (TBIP) funded by the Ministry of Land, Infrastructure and Transport of Korea. It was also funded by the Basic Science Research Programs (NRF-2016R1D1A1B03932908) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Republic of Korea.  
A contributing author, [PERSON], was financially aided by the NRF-2017-Global Ph.D. Fellowship Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Republic of Korea. The authors declare no conflicts of interest.