Effect of silica fume on shrinkage of cement-based materials mixed with alkali accelerator and alkali-free accelerator

Abstract

The current price of silica fume has a more significant advantage than cement. Application in shotcrete can improve early strength and the durability of shotcrete, and has a good synergistic effect with accelerators to promote setting and hardening. This paper mainly studies the impact of silica fume (FS) on the shrinkage of cement-based materials mixed with the alkali accelerator and alkali-free accelerator. Test autogenous shrinkage, drying shrinkage, mass loss, and internal humidity change, and analyze with XRD, MIP, SEM and other test methods. At the age of 180 days, the autogenous shrinkage of 9FSAR-4 increased by 29.1% and the drying shrinkage increased by 33.5%, and the autogenous shrinkage of 9FSAS-7 increased by 23.1%, and the drying shrinkage increased by 33.8% compared with the blank group. The results show that: FS can significantly increase the autogenous shrinkage and drying shrinkage of cement mortar mixed with alkali accelerator and alkali-free accelerator. MIP tests found that the incorporation of FS could refine the pore structure of the cement paste, increasing the number of 5–50 nm pore sizes, which would increase the capillary tensile stress in the cement paste. Nevertheless, the addition of silica fume reduces the total porosity and makes the pore distribution shift towards the decreasing direction. It reduces the mass loss and internal humidity change of cement mortar under dry conditions. XRD test found that FS can significantly reduce the content of CH in cement paste, weakening the confinement effect of CH on C–S–H and C-A-H.