Utilization of tunnel waste slurry and steel slag for preparation of backfill grouting materials: Properties and hydration behavior

Abstract

The utilization of steel slag (SS) and tunnel shield muck (including tunnel sand (TS) and mud cake (TM)) for the preparation of synchronous grouting materials is a key technological pathway to enhance the recycling of industrial solid waste and achieve decarbonization goals in tunnel engineering. This study proposes a novel ternary cementitious system composed of SS, fly ash (FA), and cement, utilizing TS and TM generated from the shield tunneling construction on the Tianxiang Avenue section of Nanchang as fine aggregates for the preparation of synchronous grouting materials. The mechanical, rheological, hydration characteristics, and hardening mechanisms of the developed grouting materials were systematically evaluated. The results show that when the SS content is 20 %–30 % and the TS:TM ratio is 7:3, the 28-day unconfined compressive strength (UCS) reaches 5.85 MPa, which is 24.5 % higher than that of the traditional cement-FA system. Additionally, the flow diameter of the slurry increases to 280 mm, and the bleeding rate is reduced to less than 1 %. At 20 % SS content, the slurry’s yield stress and viscosity significantly increase, enhancing structural stability. However, excessive SS content weakens the network integrity. As the TS:TM ratio increases, the yield stress and thixotropy of the slurry also increase. Mechanistic analysis indicates that the highly alkaline microenvironment generated by the early hydration of cement clinker promotes the depolymerization of the amorphous phase in SS and the aluminosilicate glass phase in FA. The hydroxyl ions released by the hydration of SS further accelerate the dissolution and reaction of the active components in FA, significantly promoting the formation of C-S-H gel and densifying the skeleton structure. The Ca²⁺ provided by the FA participate in the formation of C-S-H, resulting in a pronounced synergistic effect. This study elucidates the synergistic activation mechanism of shield muck and SS and their coupled impact on the macro- and micro-performance of the grouting material. It provides a theoretical basis and engineering strategy for the high-value utilization of shield muck in backfill grouting, which aligns with the principles of cleaner production and sustainable material engineering by turning waste into valuable resources, reducing carbon footprint, and minimizing environmental impact.