The utilization of alkali-free accelerators, primarily based on aluminum sulfate, in shotcrete often leads to significant shrinkage and cracking, jeopardizing long-term durability. This study aims to mitigate this issue by investigating the efficacy of silica fume (SF) and fly ash (FA), individually and combined, in controlling the shrinkage deformation of shotcrete.

Shotcrete mixtures were prepared with cement partially replaced by SF, FA, or their combination (SF-FA). Aluminum sulfate was used as an accelerator. The shrinkage behavior under sealed and dry conditions was monitored. The underlying mechanisms were elucidated through hardened airvoid analysis, mercury intrusion porosimetry (MIP), and internal humidity tracking.

Contrary to some previous studies, both individual and combined incorporation of mineral admixtures reduced the 28-day shrinkage. The SF-FA composite exhibited the most substantial reduction (23.72% sealed, 17.76% dry), followed by SF alone (18.11% sealed) and FA alone (11.35% sealed). Mechanism analysis revealed that the admixtures refined the pore structure, reduced the volume of harmful pores, and, crucially, optimized the air-void system by increasing the population of fine bubbles. This created an internal buffering effect that alleviates capillary stress. The synergistic effect in the SF-FA group is attributed to complementary pore-filling at dual scales.

This work demonstrates that, within an aluminum sulfate-accelerated shotcrete system, silica fume can effectively reduce shrinkage when its pore-refining effect is counterbalanced by a well-optimized micro-bubble network. It provides the first comprehensive evidence of the synergistic shrinkage-reducing mechanism of SF and FA in such systems, offering a practical strategy for mix design optimization to enhance the volume stability of shotcrete.