Effect of Alkali Activation Modulus on the Performance and Mechanism of Geopolymer-stabilized Bentonite-containing Shield-Separated Fine Sand

A large amount of sand separated from slurry shield tunneling spoil often contains clay fines exceeding specification limits, necessitating the development of low-cost valorization approaches for such high-clay aggregates. This study investigates geopolymer-solidified materials prepared using high-clay separated fine sand derived from shield tunneling projects in Shenzhen. Under fixed mix proportions with a binder-to-sand ratio of 1∶3 and a water-to-binder ratio of 1∶2, the effects of bentonite content (0%~10%) and alkali activation modulus (1.2, 1.6, and 2.0) on the workability of the geopolymer-solidified materials were systematically examined. The results show that bentonite significantly reduces the fluidity of the geopolymer mixtures, and the fluidity is almost completely lost when the bentonite content exceeds 5%. However, within the modulus range of 1.2~1.6, an appropriate amount of bentonite (2.5%~5%) enhances both compressive strength and flexural strength, whereas at a higher modulus of 2.0, the addition of bentonite leads to a substantial reduction in strength. Analyses by nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) reveal that a moderate amount of bentonite partially dissolves under alkaline conditions, releasing additional Si and Al components, promoting the formation of C-(A)-S-H gel, and making the gel structure denser, thereby improving strength. Excessive bentonite adsorbs OH^-, inhibits slag dissolution, reduces gel formation, and induces the formation of a loose and porous N-(A)-S-H gel, leading to increased porosity and decreased strength. This study confirms that high-clay shield fine sand can be effectively valorized by low-modulus alkali activation, making it suitable for applications with low fluidity requirements but certain strength demands, such as roadbed filling, precast blocks, and dry-hard concrete products, providing a feasible technical approach for the reduction and low-carbon utilization of shield tunnel slurry waste.