## 5 Mechanism Analysis of Composite Slag Solidifying Agent  
Based on the results of the microstructure investigation, it can be inferred that the analysis of the mechanism of a composite slag solidifying agent is essentially the clarification of the process of formation of CSH, AFt, and calcium aluminate chloride sulphate hydrate. 1. CSH In the solidified soil, the source of CSH is the hydration of composite slag solidifying agent and the reaction of SiO\({}_{2}\). Based on the XRD results of unsolidified soil, it can be stated that SiO\({}_{2}\) was a main component in the dredger fill. In addition, the slag was essentially composed of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), CaO, and other oxides. NaOH and Ca(OH)\({}_{2}\) from sodium silicate and quicklime hydrolysis increased the pH value of the sample. Next, SiO\({}_{2}\) and other oxides were activated. Finally, CSH was obtained. The reactions are expressed as:  
\[\text{CaO}+\text{H}_{2}\text{O}\rightarrow\text{Ca(OH)}_{2} \tag{2a}\] \[\text{Na}_{2}\text{O}\text{-SiO}_{2}+\text{H}_{2}\text{O} \rightarrow\text{2 NaOH}+\text{SiO}_{2}\] (2b) \[\text{SiO}_{2}+\text{Ca(OH)}_{2}+\text{H}_{2}\text{O} \rightarrow\text{CaO}\text{-SiO}_{2}\text{-H}_{2}\text{O(CSH)} \tag{2c}\]
2. AFt In the solidified soil, AFt (3 CaO-Al\({}_{2}\)O\({}_{3}\).3 CaSO\({}_{4}\).32H\({}_{2}\)O) was formed by the transformation of CAH. As introduced in Section 4.2.4, CAH was obtained in the solidified soil with slag and quicklime. The formation of the CAH was similar to that of CSH: Al\({}_{2}\)O\({}_{3}\) was activated in an alkaline environment and combined with Ca(OH)\({}_{2}\) to form CAH colloids. The reactions are expressed as follows:  
\[\text{CaO}+\text{H}_{2}\text{O}\rightarrow\text{Ca(OH)}_{2} \tag{3a}\] \[\text{Al}_{2}\text{O}_{3}+\text{Ca(OH)}_{2}+\text{H}_{2}\text{O} \rightarrow\text{CaO}\text{-Al}_{2}\text{O}_{3}\text{-H}_{2}\text{O} \tag{3b}\]  
Next, AFt is formed by the combination of CAH and sulphate ion, which is obtained from gypsum powder:  
\[\text{CaO}\text{-Al}_{2}\text{O}_{3}\text{-H}_{2}\text{O}+\text{CaSO}_{4}+ \text{H}_{2}\text{O}\rightarrow\text{3 CaO}\text{-Al}_{2}\text{O}_{3}\text{ 3 CaSO}_{4}\text{-32H}_{2}\text{O} \tag{3c}\]
3. Calcium aluminate chloride sulphate hydrate Calcium aluminate chloride sulphate hydrate formed from the interaction between the composite slag solidifying agent and the soluble salt in the saline dredger fill. As presented above, a certain amount of CAH combined with sulphate ions in the gypsum powder to form AFt. Part of the remaining CAH reacted with Cl\({}^{-}\) in the saline soil to form [PERSON]'s salt (Fs: 3 CaO-Al\({}_{2}\)O\({}_{3}\).CaCl\({}_{2}\).10H\({}_{2}\)O). Next, the sulphate ions from the gypsum powder continuously replaced part of the chloride ions in [PERSON]'s salt in the sulphate ion-enriched environment to eventually yield calcium aluminate chloride sulphate hydrate. The reactions are expressed as follows:  
\[\text{CaO}\text{-Al}_{2}\text{O}_{3}\text{-H}_{2}\text{O}+\text{NaCl} \rightarrow\text{3 CaO}\text{-Al}_{2}\text{O}_{3}\text{-CaCl}_{2}\text{-}1 0\text{H}_{2}\text{O}+\text{NaOH} \tag{4a}\] \[\text{3 CaO}\text{-Al}_{2}\text{O}_{3}\text{-CaCl}_{2}\text{-}10\text{H}_{2} \text{O}+\text{CaSO}_{4}\text{-2H}_{2}\text{O}\rightarrow\text{CaCl}_{2} \text{+3 CaO}\text{-Al}_{2}\text{O}_{3}\text{-(0.5 CaCl}_{2}\text{-0.5 CaSO}_{4}) \text{-}12\text{H}_{2}\text{O} \tag{4b}\]  
The NaOH generated during the reaction, which provided an alkaline environment, further promoted the hydration of the slag powder and maintained the reactions. Therefore, soluble salts (such as Cl\({}^{-}\)) in saline dredger fill can be reduced effectively and, in turn, the solidified soil can exhibit good road performance. To summarize, the mechanism of composite slag solidifying agent for saline dredger fill is essentially the formation of CSH, AFt, and calcium aluminate chloride sulphate hydrate. CSH is a colloid, which has a good gelling property.