Abstract: To investigate the influence of different sizes of ettringite on the early strength performance of foundry sand, various sizes of ettringite (AFt-0, AFt-1, and AFt-2) were first prepared using different dispersants. The prepared series of AFt were characterized by infrared spectroscopy (IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Comparative analyses were conducted on the mechanical properties, microscopic morphology, hydration heat of cement, and setting time of foundry sand using different analytical methods to assess the effects of various sizes of ettringite on the foundry sand. The results show that adding dispersants could control the size of ettringite and improve its suspension stability in solution. The addition of ettringite of different sizes could enhance the early compressive strength of foundry sand, with a greater increase observed as the size of ettringite decreases. Specifically, incorporating AFt-2 into the foundry sand increases the compressive strength by 225% compared to the blank sample at the same age, approaching the compressive strength of the blank sample at 1 day. Microscopic structural analysis of the foundry sand also reveals that with decreasing ettringite size, the early formation of ettringite in the foundry sand increases, which facilitates the formation of an early skeleton system in the foundry sand. Hydration heat analysis further indicates that as the size of ettringite decreases, the rate of cement hydration accelerates, leading to a shorter cement setting time.