Preparation and performance of alkali-activated synchronous grouting material incorporating shield muck

With the rapid development of urban underground space, the large volume of muck generated from shield tunneling has posed significant environmental and engineering challenges in terms of disposal and stockpiling. To promote the resource utilization of shield tunneling muck and reduce the consumption of natural sand, this study employs tunneling muck as a fine aggregate to partially or fully replace natural river sand, while fly ash, ground granulated blast-furnace slag, steel slag, and silica fume are used as cementitious components to prepare an alkali-activated synchronous grouting material. The effects of muck replacement ratio (0%–100%) on the workability, mechanical properties, durability, and microstructural characteristics of the grouting slurry were systematically investigated.The results indicate that as the muck replacement ratio increases from 0% to 100%, the flowability of the slurry decreases from 385 mm to 144 mm, while the setting time is shortened from 118 min to 30 min; notably, the bleeding rate remains zero throughout the entire replacement range. In terms of mechanical performance, both 3-day and 28-day compressive strengths exhibit a trend of initial increase followed by a decline with increasing replacement ratio, reaching peak values of 6.98 MPa and 11.54 MPa, respectively, at a replacement ratio of 40%. Durability tests reveal that resistance to salt attack and freeze–thaw cycles gradually deteriorates with increasing muck content. The mixture with a 20% replacement ratio (M1) demonstrates the best durability performance, exhibiting corrosion resistance coefficients of 0.916 and 0.905 after immersion in 5% NaCl and 5% MgSO₄ solutions for 56 days, respectively, and a compressive strength loss of only 8.76% after 20 freeze–thaw cycles.However, when the replacement ratio exceeds 40%, the slurry flowability deteriorates sharply, accompanied by pronounced reductions in mechanical properties and water-dispersion resistance. SEM and XRD analyses reveal that at muck contents of 40% or lower, muck particles are effectively encapsulated by C–A–S–H gels, forming a dense microstructure; at higher replacement levels, insufficient cementitious phases lead to weakened interfacial bonding and increased porosity. Considering the overall performance and resource utilization efficiency, it is recommended that the muck replacement ratio be controlled within 40% under the investigated mix design to achieve an optimal balance between engineering performance and sustainable resource utilization.