Ye'elimite (Ca4Al6SO16) is a main mineral in calcium sulfoaluminate cements. Aluminum hydroxide is one of the products formed by hydration of ye'elimite. To characterize this phase, various aluminum hydroxides were synthesized from their chemical constituents using sol-gel processing and compared with the aluminum hydroxide formed during the hydration of ye'elimite. The nanostructure of aluminum hydroxide formed during the hydration of ye'elimite was investigated in detail using X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, inductively coupled plasma optical emission spectroscopy, and Al-27 magic angle spinning nuclear magnetic resonance spectroscopy. No evidence was observed that indicated the existence of amorphous aluminum hydroxide in the hydration of ye'elimite. The pH values, thermogravimetric analysis, particle morphology, and Al-O coordination indicated that the aluminum hydroxide from the hydration of ye'elimite had a crystal-like structure. The X-ray diffraction analysis, particle sizes, and ion activity product showed that the aluminum hydroxide from the hydration of ye'elimite had a microcrystalline structure.