dissociation of aluminum sulfate in water
Aluminum sulfate has special properites and uses. It is true that aluminum sulfate is soluble in water, and it has specific solubility values for certain temperatures:


$31.2\ \mathrm g$ per $100\ \mathrm{mL}$ at $0\ \mathrm{^\circ C}$
$36.4\ \mathrm g$ per $100\ \mathrm{mL}$ at $20\ \mathrm{^\circ C}$
$89.0\ \mathrm g$ per $100\ \mathrm{mL}$ at $100\ \mathrm{^\circ C}$


You stated you used $12\ \mathrm g$ in $100\ \mathrm{mL}$, so it would seem to completely dissolve. However, it this situation, since it isn't being dissolved in very large amounts of water, it doesn't dissociate and form aluminum hydroxide. Aluminum sulfate can be used in water purification in small amounts, because it causes impurities to group up into larger particles and then settle to the bottom of the container (or be filtered out) more easily. The solid you see sometimes are the impurities of the water that have been grouped up and look like precipitate. If you use more pure water, you will see less of these impure solids, or possibly nothing if the water is pure enough (such as if it's distilled water).