many tiles are either powdering or flaking

Hi,

The original owner of my home (I'm the third) installed slate tiles outside on a base of concrete, I believe. I'm not sure what kind of slate it is, but it's dark grey with some brownish hues.  Anyway, the slate doesn't seem to be weathering well in some areas.  On the covered front porch, many tiles are either powdering or flaking (with thin, brittle layers washing away with just a simple hose spraying). In the back where it's sunnier, some of the tiles are doing the same thing, though to a lesser extent.  In a few places, the tiles seem to be doing fine.  We're in a Southern California coastal valley where it gets both very hot (95ish for a month or so) and very cool and damp (when the ocean fog rolls in).  A few years ago, we had someone spray the slate with a water-based sealant, but that didn't seem to do much, and I'm sure it's long since worn away.  I see lots of conflicting info about sealing or treating slate.  Is there anything we can do to both protect our slate and, potentially, make it look a little more lusterous ("wet") without spending a bunch of money?  I appreciate any expert advice you can offer.  Thanks.

S. Nelson

Aliso Viejo