AbstractUltrafine ettringite (UE) can accelerate the hydration of sulfoaluminate cement-based grouting materials (SCGMs). However, the mechanism of UE on the hydration of SCGM is not clear. Many researchers believe that ultrafine materials can serve as crystal nuclei for hydration products. How to prove that UE can supply crystal nuclei for hydrated products remains an open question. It is still unclear for which SCGM hydration products UE can supply a crystal nuclei. In this article, UE was synthesized in different solvent compositions by a separate nucleation and growth step method. Synthetic UE was enclosed in a dialysis bag with a molecular weight cutoff of 7 kDa, which allowed the diffusion of water and ions, and the mechanism of UE’s effect on the hydration of SCGM was investigated. The results indicated that with the increase of the amount of ethanol in the water–alcohol mixed solvent, the particle size of the prepared UE gradually became smaller. UE only provided a nucleation site for ettringite, which was one of hydration products, and thus promoted a cement hydration reaction. The smaller the size of the UE, the higher the cement hydration rate and the greater the amount of hydration products produced.