Sodium silicate hydrogels for reducing aquifer permeability: Physicochemical, mechanical, and aging characterisation

Sodium silicate grouts are widely used to reduce permeability in unconsolidated sands, but their in-situ performance remains difficult to predict because injection behaviour, gelation, aging, degradation, and groundwater interactions are rarely evaluated within a single framework. This study compares four sodium silicate grouts prepared with different hardeners: Condat Stab Soft (C-gel), Triacetin (T-gel), sodium aluminate (A-gel), and Stabisil® SE (S-gel). Using experiment and simulation, this study assesses: Injection properties, gelation behaviour, mechanical performance, structure, syneresis, degradation, leaching, and leachate–groundwater interactions. The grouts showed distinct performance profiles, indicating that there is no universally optimal grout; rather, the ideal grout is application-specific. C-gel had the lowest viscosity (1.22 mPa·s) and developed moderate early shear strength (7.6 kPa after 4 h), but showed high syneresis (16.5% after 7 days) and lost structural integrity within 37 days under saturated aging conditions. T-gel showed the highest early shear strength, combined with low viscosity (1.37 mPa·s), low surface tension (46 mN·m⁻¹), low syneresis (<4%), and a low degradation rate (0.04 mg·cm⁻²·day⁻¹), indicating the most balanced long-term performance. A-gel developed the highest stiffness after 7 days (88 kPa), but produced highly alkaline leachate (pH ≈ 12) and high mineral precipitation potential. S-gel showed the lowest degradation rate (0.03 mg·cm⁻²·day⁻¹) and near-neutral leachate (pH ≈ 7.8), but limited mechanical strength. XRD and FTIR analyses showed predominantly amorphous silicate networks, with Na₂CO₃ in C-gel, Na₂SO₄ in S-gel, and Si–O–Al bonding in A-gel. PHREEQC simulations confirmed secondary mineral precipitation and indicated higher clogging potential for C-gel and A-gel leachates, whereas T-gel and S-gel showed lower mineral precipitation risk. Overall, the results provide quantitative input for grout selection, injection modelling, durability prediction, and coupled reactive transport assessment.