Aluminium + Fluoride = ?

One point I think needs a better understanding is the reaction between the aluminium and fluoride.

When looking into the causes of dementia aluminium comes high on the list of potential causes. However aluminium does not cross the blood/brain barrier, I am told. Aluminium with fluoride does, again needs confirmation.

Therefore when aluminium sulphate is added to the water (as it is, to ‘clean it’) and then silicon hexafluoridealuminium sulphate is added to the water (as it is, to ‘clean it’) and then silicon hexafluoride (or tetrafluoride, which I think is a solid, or liquid hydrofluorosilicic acid, H2SiF6)?) is also added, one may expect a reaction between them, especially if there are free ions of each.

Aluminium tetrafluoride may be produced or aluminum fluosilicate solution. Either seem potential culprits of the brain problems associated with both aluminium and fluoride (i.e. the combination may be much more dangerous than each separately).

Aluminium and Silicon are right next to each other on the Periodic Table of Elements. An expert chemist’s view of this would be useful. Does anyone know a good one?