## 2 Characteristics of Colloidal Silica (CS)
### Gelling and Curing Process for CS  
[PERSON] (1999) divided the gelation process into 11 gel states to describe the course during which a CS solution evolves into a rigid gel [49], as shown in Figure 5. Gel state 1 is such a process, where an original CS solution has a viscosity quite close to pure water and no detectable formed gel. During this stage, CS acts like a Newtonian fluid. When the pH or salt concentration is adjusted, the gelation process is triggered, and CS enters into gel state 2. Here, viscosity increasing gradually, and the gel becomes a non-Newtonian fluid. Since then, CS transforms from a flowing gel into a nonflowing gel, and finally into a rigid gel. When arriving at gel state 10, CS particles gel into chainlike structures and then into uniform three dimensional networks. As a result, a CS resonating, rigid gel can be obtained. Therefore, gel time can be defined as the time needed for a CS solution to evolve from gel state 1 to gel state 10 [40; 45]. Figure 4: Viscosity increasing law of CS suspension in the gelation process at different (a) pH values and (b) ionic normalities [40; 45]. Figure 5: Gel state classification and description [49]. Since the relationship of gel time with pH value is not as clear as that with ionic strength, the salt concentration has always been adjusted to control the gel time in previous experimental studies in the literature [14; 17; 20; 32; 50]. Sodium chloride (NaCl) or calcium chloride (CaCl\({}_{2}\)) has mostly been adopted as the electrolyte solution of choice to adjust salt concentration in order to start the gelation process. The addition of salt will shrink the double layer of CS particles, which will increase the probability of interparticle collisions [45]. These collisions will undoubtedly the motivate gel formation process and facilitate the CS suspension to transform into a rigid, firm gel. With NaCl addition to adjust the salt concentration, the CS gelling process was monitored by [PERSON] et al. (2018), as shown in Figure 6.  
[PERSON] and [PERSON] (2010) performed a series of gel experiments with different salt concentrations and salt solution quantities, and the gel time curves for three magnitudes of salt concentrations were plotted against the addition quantities of the salt solution. It was concluded that gel time decreases with an increasing salt concentration and an increasing ratio of salt solution volume to CS volume [14]. [PERSON] proposed that a 1.7 mol/L NaCl solution should be hand mixed with CS in a 1:5 ratio by volume to obtain a 1 h gel formation time [50]. The required salt concentrations to obtain the targeted gel time can be determined with the analytical model proposed by [PERSON] et al. (2017) [51]. Upon rigid gel formation, the gelation process is accomplished, and the curing period begins. The curing time is the time from where gel formation has finished to strength testing starts. It has been verified that curing time has a significant effect on the strength properties of a CS-soil mixture. [PERSON] et al. (2018) investigated the influence of curing time and curing conditions on the strength properties of CS specimens. Four CS samples were placed under de-aired water to cure for 1, 2, 4, and 8 weeks after gelation was accomplished. At the same time, another CS sample was cured for 1 week at 90% relative humidity (RH). Oedometer tests were performed on the five samples, and the corresponding compression curves are shown in Figure 7. It can be seen in Figure 7a that the samples cured for 1 and 2 weeks had a single compression curve, while the samples cured for 4 and 8 weeks have another single (but different) compression curve. The samples cured for 4 and 8 weeks tended to have a stiffer behavior at low vertical stress. Figure 7b depicts that the sample cured at 90% relative humidity suffered some water loss during the curing period, leading to a lower initial void ratio compared with the specimens immersed in de-aired water. Moreover, the 90% RH sample showed a stiffer response than the immersed samples. [PERSON] et al.