## 6 Conclusions  
The solidification of Caofeidian dredger fill is difficult because of salinization, as well as environmental and economic factors. In this study, the composite slag solidifying agent proposed was experimentally detected by an unconfined compression test and microanalysis. Based on the results, the following conclusions can be drawn:  
1. The composite slag solidifying agent can substantially improve the unconfined compressive strength of Caofeidian saline dredger fill. The additive quicklime contributes the most to this improvement. The optimal proportion is obtained as 10% slag + 1.0% quicklime + 0.8% sodium silicate + 1.5% gypsum powder.
2. The microanalysis illustrates that a compact overall structure was obtained and few voids were observed in the solidified soil with the composite slag solidifying agent. This is an apparent improvement in the microstructure compared with that of unsolidified soil or solidified soil with slag and a single additive (quicklime, sodium silicate, or gypsum powder).
3. The mechanism of composite slag solidifying agent for saline dredger fill is essentially the formation of CSH, AFt, and calcium aluminate chloride sulphate hydrate. First, the CSH colloid formed by slag hydrolysis improves the compactness of the soil. Second, AFt (formed by the combination of CAH and SO42-) can further strengthen the microstructure of the solidified soil. Finally, CAH can combine with SO42- and Cl- in the soil to form calcium aluminate chloride sulphate hydrate, which can effectively reduce the content of soluble salts in saline dredger fill and substantially improve the engineering characteristics of solidified soil.  
These observations are helpful for understanding the solidification mechanism of saline dredger fill and can provide a theoretical basis for future solidification projects in coastal saline dredger fill areas. In a future study, these observations could be verified with different dredger soils from other locations and field tests.  
Conceptualization, Y.S.; methodology, Y.S.; validation, P.L.; formal analysis, P.L. and P.J.; investigation, P.L.; resources, R.F.; data curation, X.L.; writing--original draft preparation, P.J.; writing--review and editing, P.J. and Y.L.; supervision, Y.S.; funding acquisition, Y.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Fundamental Research Funds for the Central Universities, grant number 2020 JBZD013, the State Key Laboratory of Frozen Soils Engineering, grant number SKLFSE201808, and the National Natural Science Foundation of China(NSFC), grant number 42172291.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.  
Figure A3: EDS results of solidified soil with slag and sodium silicate. (**a**) Representative region; (**b**) EDS diffractogram.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Element** & **C** & **O** & **Na** & **Mg** & **Al** & **Si** & **Cl** & **K** & **Ca** & **Fe** & **Zr** \\ \hline Mass percent (\%) & 0.60 & 55.56 & 1.24 & 1.71 & 5.99 & 21.54 & 1.50 & 1.81 & 5.19 & 3.85 & 1.68 \\ Atomic percent (\%) & 0.26 & 71.61 & 1.11 & 1.45 & 4.57 & 15.82 & 0.87 & 0.95 & 2.67 & 1.42 & 0.38 \\ \hline \hline \end{tabular}
\end{table}
Table 4: EDS element determination results of solidified soil with slag and quicklime.