## 6 Conclusions  
1. The mixing ratio of cement is critical to the strength development of cemented soil; the mixing ratio of FA exerts an obvious influence on strength development in the middle and late stages; and the mixing ratio of WG only slightly affects the strength. The ratio of cement, FA, and WG of 9%:12%:3% is the optimal engineering mix proportion of the composite-cemented soil. Compared with the ordinary-cemented soil and FA- and WG-cemented soil, the composite-cemented soil has significantly improved compressive load-bearing capacity. 2. The composite-cemented soil always has a permeability coefficient obviously lower than the ordinary one after any curing period. Despite the mass loss, the composite-cemented soil still has superior endurance to the ordinary one after wetting-drying and freeze-thaw cycles. 3. Through SEM and XRD analysis, the content of hydration products in the composite-cemented soil is always significantly higher than that in the ordinary one after any curing period. In addition, the bonding action of hydrates on soil particles is more obvious in the composite-cemented soil. The contents of C-S-H gel and Aft crystals in the composite-cemented soil are also apparently higher than those in the ordinary one. Under the alkali activation, FA produces free \(\rm SiO_{3}\)\({}^{2-}\) and \(\rm AlO^{2-}\), which then have polymerization with \(\rm Ca^{2+}\) to generate C-S-H and C-A-H gels, further promoting the hydration reaction of cement. 4. The FA-WG-composite-cemented soil overcomes the large difficulty in backfilling of ultra-deep and ultra-narrow foundation trenches and compensates for the shortcom \begin{table}
\begin{tabular}{c c c c c c} \hline
**Curing Stages** & **Cement** & **WG** & **FA** & **Soil** & **Main Hydrates** \\ \hline \multirow{3}{*}{Initial stage} & \multirow{3}{*}{Hydration reaction begins} & Strong alkali and & & & \\  & & silica gel are & & & C-A-H, C-S-H, CH, \\  & & produced by & & & Aft \\  & & hydrolysis of WG. & & & \\ \hline \multirow{3}{*}{Middle stage} & \multirow{3}{*}{Hydration reaction continues} & Silica gel aqueous solution produced by hydrolysis is & & WG begins to & \\  & & & undergo the & Reactions of CH & \\  & & solution produced & pozzolanic reaction & with Si and Al & C-S-H, CH, Aft, \\  & & by hydrolysis is & with CH under & activating oxides. & C-A-H \\  & & reacted with CH. & alkali activation. & & \\ \hline \multirow{3}{*}{Middle and late stage} & \multirow{3}{*}{CH is consumed to promote hydration} & Water-containing silica gel is consumed to & Under further & & \\  & & silicon gel is & alkali activation, & Stable hydrates are & \\  & & consumed to & hydrates of more & generated after & \\  & & promote & stable structures & sufficient reactions. & \\  & & hydrolysis of WG. & are generated. & & \\ \hline \multirow{3}{*}{Late stage} & \multirow{3}{*}{Hydration reaction is basically finished} & \multirow{3}{*}{WG is basically transformed into} & The residual FA & Cementation with & \\  & & & further undergoes & hydration & \\ \cline{1-1}  & & transformed into & the hydration & products to form & C-S-H \\ \cline{1-1}  & & hydration products & reaction under the & high-strength & \\ \cline{1-1}  & & effect of OH\(-\). & structures. & \\ \hline \end{tabular}
\end{table} 
Table 8: Curing process of the composite-cemented soil. ings of ordinary-cemented soil, including the poor strength, impermeability, and frost resistance. Therefore, it provides a reference for similar engineering. Conceptualization, X.D. and N.H.; methodology, X.D. and M.S. ; software, M.W. ; validation, X.D., N.H. and M.S. ; formal analysis, X.D. and M.W. ; investigation, M.S. ; resources, M.W. and M.S. ; data curation, N.H.; writing--original draft preparation, X.D. and M.S. ; writing--review and editing, N.H.; visualization, M.S. ; supervision, M.W. ; project administration, X.D. and M.S; funding acquisition, X.D. All authors have read and agreed to the published version of the manuscript.