Officially, CosIng (the official EU ingredient database) lists Aluminum Hydroxide 's functions as opacifying (making the product white and non-transparent), as well as emollient and skin protectant.
However, with a little bit of digging, it turns out Aluminum Hyroxide often moonlights as a protective coating for UV filter superstar Titanium Dioxide. Specifically, it protects our skin from the harmful effects of nasty Reactive Oxygen Species (free radicals derived from oxygen such as Superoxide and Hydrogen Peroxide) generated when Titanium Dioxide is exposed to UV light. Btw, chlorine in swimming pool water depletes this protective coating, so one more reason to reapply your sunscreen after a dip in the pool on holiday.
Other than that, Aluminum Hydroxide also often shows up in composite pigment technologies where it is used the other way around (as the base material and not as the coating material) and helps to achieve higher color coverage with less pigment.
Show me some proof
- Shimizu, Katsumi, and Shoji Takekawa. "Aluminum hydroxide pigments having improved coloring capability." U.S. Patent No. 9,011,893. 21 Apr. 2015.
- Virkutyte, Jurate, Souhail R. Al-Abed, and Dionysios D. Dionysiou. "Depletion of the protective aluminum hydroxide coating in TiO2-based sunscreens by swimming pool water ingredients." Chemical engineering journal 191 (2012): 95-103.