Cake Alum or aluminium sulfate, alternatively spelt aluminum sulfate, aluminium sulphate, or aluminum sulphate; is a chemical compound with the formula Al2(SO4)3. It is mainly used as a flocculating agent in the purification of drinking water and waste water treatment plants, and also in paper manufacturing.

Cake Alum is used in water purification and as a mordant in dyeing and printing textiles. In water purification, it causes impurities to coagulate which are removed as the particulate settles to the bottom of the container or more easily filtered. This process is called coagulation or flocculation.

When dissolved in a large amount of neutral or slightly-alkaline water, Cake Alum produces a gelatinous precipitate of aluminium hydroxide. In dyeing and printing cloth, the gelatinous precipitate helps the dye adhere to the clothing fibers by rendering the pigment insoluble.

Cake Alum is sometimes used to reduce the pH of garden soil, as it hydrolyzes to form the aluminium hydroxide precipitate and a dilute sulfuric acid solution. An example of what changing the pH level of soil can do to plants is visible when looking at the Hydrangea macrophylla. The gardener can add Cake Alum to the soil to reduce the pH level which in turn will result in the flowers of the Hydrangea turning a different color.

Cake Alum is the active ingredient of some antiperspirants. It is usually found in baking powder, where there is controversy over its use due to concern regarding the safety of adding aluminium to the diet.

In construction industry it is used as waterproofing agent and accelerator in concrete. Another use is a foaming agent in fire fighting foam.

It is also used in styptic pencils, and pain relief from stings and bites.

It can also be very effective as a molluscicide, killing spanish slugs.