Study of fracture filling colloidal silica grouts under groundwater conditions in the engineered barrier system
Keywords: Engineered Barrier System
grouting
erosion
Eka EXP36
groundwater
MEYCO
colloidal silica
Cembinder
dissolution

The present work has established a series of grouting performance criteria for low-pH non cementitious colloidal silica grouts, a relatively new fracture filling material which has been suggested for the construction of an underground disposal repository for high-level nuclear waste, known as the Engineered Barrier System (EBS). The research investigation reported here includes experimental analysis of the gelling during the grouting period; a study of the physical and chemical stability of the grout during the post-gelling period; and preliminary first-order modelling of the grout removal process by physical and chemical erosion.
Performance comparisons were conducted for three commercially available colloidal silica products; Cembinder, Eka EXP36, and MEYCO MP320, all obtained from Akzonobel Company (Bindzil®). The performance criteria of these three products were subdivided. For the gelling process, the sub-categories of performance studied were the gel induction time, the gel time and the zeta potential. For the physical and chemical stability studies, the sub categories of performance studied were sensitivity analysis with respect to grout age, channel length and salt release, and groundwater salinity, cation content and flowrate. For a low saline groundwater condition, an overall comparison of these sub-categories was expressed in property Radar charts.
It was found that the gelling time increased in the order of Eka EXP36 < Cembinder < MEYCO, similar trend was found in grout removal resistance for both initial erosion and4 Public chemical dissolution at high salinity groundwater condition. This observation suggests that the grout size distribution is the major contributing factor in the overall grout removal analysis, grout with larger particle size tends to have stronger grout removal resistance in high saline groundwater intrusion. This comparison provides a simple guidance for future grout selection.
An alternative method was introduced for gel induction time measurement during the gelling process. It was defined as the point of intersection between loss and storage moduli. Furthermore, a grout mixing recipe was introduced for each of the three grouting products, in order to maintain a target operational time between 10-33 minutes.
Grout stability post-gelling was studied. The physical stability (resistance towards physical erosion) was measured by the turbidity of outflow solutions, whereas the chemical stability was measured by dissolved ion concentration due to silica dissolution at the outflow. The initial physical erosion was characterised using the power-law relation 𝜀 = 𝑀(𝜏 − 𝜏𝑐)𝑛 . At geodisposal groundwater condition (0.5M NaCl), the critical shear stress ( 𝜏𝑐 ) values were obtained 1×10-10 ~ 2.14×10-5 Pa, with n values fluctuating in the range 0.70 ~ 1.03, for a variety of groundwater salinity conditions. The dissolution concentration could be described by the expression 𝐶𝑆𝑖 = 𝐶𝑆𝑖 𝑒 (1 − 𝑒𝑥𝑝 − (2𝜋𝑟/𝑄𝐶𝑆𝑖 𝑒 𝑘1𝑙)) . The dissolution reaction rate constants (k1) for Cembinder, Eka EXP36 and MEYCO were 2.90, 3.04, 2.89 ×10-7 mol m-2 s -1 , respectively. The preliminary first-order life span estimation of the grout barrier in the Engineered Barrier System was found to be relatively short.