## 2 Characteristics of Colloidal Silica (CS)  
### Physical Properties of CS  
Colloidal silica (CS) is an aqueous suspension of microscopic nanosilica, extracted from saturated solutions of silicic acid. In general, the particle size of nanosilica is uniform, ranging from 7-22 nm. CS is nontoxic, chemically and biologically inert, and environmentally friendly [38; 39]. When diluted to a 5% concentration by weight, the viscosity of CS solution is only 1.5-2 cP, and is quite similar to  
Figure 1: Concept for passive site stabilization [7; 32].  
pure water. With the addition of an electrolyte, a CS solution can be destabilized in order to start the gelation process.  
CS nanoparticles are formed as a result of the interaction between H4 SiO4 and other molecules. During the manufacturing process, alkaline solutions are added into the CS to keep the silica particles ionized and repelled from each other, so that gelation can be inhibited, and the CS solution can stay steady [7]. The surfaces of the CS particles are initially distributed by uncombined silanol (Si-O-H) groups. When exposed to an alkaline environment, the alkali materials react with the surfaces of nanoparticles, leading to negative charges (O\({}^{-}\)) on the particles and resulting in repulsive forces (Figure 2a). As the amount of charge on particle surfaces is proportional to the amount of hydroxyl ions, interparticle repulsive force weakens and particles start to interact with each other when the pH drops to a relatively low value. Under this circumstance, some of the silica particles still have negative charges (O\({}^{-}\)), while the others become uncharged (-OH). Here, siloxane bonds (Si-O-Si) start to develop (Figure 2b) and the gelation of the CS suspension is motivated (Figure 3). As the pH drops continuously below 5, positive charges attach to the silica particles, and repulsive forces dominate once again (Figure 2c). Therefore, the gelation time increases with a decreasing formation rate of siloxane bonds. It has been investigated and determined that the minimum gelation time can be obtained when the pH ranges from 5 to 7 [40; 41; 42].  
In addition to pH, the critical factors that could influence gelation process also include the ionic strength, silica solids concentration, and temperature [7; 36; 45; 46]. Since silica concentration and temperature are relatively constant, gelation time can be controlled by adjusting the ionic strength  
Figure 3: Formation of siloxane bonds and a schematic of the colloidal silica (CS) particle [40; 43; 44].  
Figure 2: Surface charge characteristics of silica particles on the condition of different pH values: (a) pH \(>\) 8, O\({}^{-}\) on the particle surface and repulsive forces demonstrated; (b) 5 \(\leq\) pH \(\leq\) 8, some particles still with O\({}^{-}\) and others become uncharged; (c) pH \(>\) 8, H\({}_{2}\)O\({}^{+}\) on the particle surface and repulsive forces indicated [7].  
and pH value. The evolvement characteristics of viscosity in the gelation process for solutions with 5 wt % colloidal silica have been measured and depicted quantitatively [7; 40; 45], as shown in Figure 4. It is obvious that the shape of gelation curve is similar regardless of gel time. Moreover, silica particles have an induction period in the gelation process, during which viscosity stays fairly low and silica suspension has a flow rate close to pure water. With an appropriate amount of time elapsed, viscosity starts to increase dramatically, and a firm gel is finally obtained [47; 48]. In detail, gel time increases with pH value on the condition of pH > 7.5, while it decreases as normality (i.e., ionic strength) rises from 0.225 N to 0.3 N. As a consequence, the time from mixing to the point where viscosity increases has a significantly wide scope, ranging from a few minutes to several months. In other words, CS has a widely ranged controllable gel time, which is favorable for the transportation of grouting materials to the desired location.