Few reports in the literature adequately address the early phases of the hydration reaction, which leads to crystallization of the hydration product of tetracalcium aluminoferrite (brownmillerite). In the present report, the surface of C(4)AF was vitrified by spray granulation. We investigated the early hydration reaction of C(4)AF for surface vitrified brownmillerite (SVB) under rapid freeze-drying in a dilute solution with w/s equal to 5. Based on the results of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), the crystallization mechanism of hydration product C(4)AF was proposed. After C(4)AF was completely dissolved, an 'etch pit' appeared on the surface of the particles. The hydrated nanogel particles accumulated and expanded at the 'etch pit'. Subsequently, a bulge was found on the surface of the grains. This was formed by the iron-rich hydrated gel. With increasing hydration time, flaky sheets and poor crystals were discovered, which were then converted into C-4(A, F)H-19 and C-3(A, F)H-6. (C) 2020 Elsevier B.V. All rights reserved.