I would go with mortar. Figure that the unglazed clay roofing
tiles and standard brick are basically the same material, terra
cotta, just in a different shape. Whatever works for brick will
work to join the terra cotta roofing tiles, too.
Trying mortar vs silicone is a good idea, of course, to find out
for yourself which one works better with your materials. Mortar
is more forgiving of rock/clay dust, etc on the surfaces to be
joined than adhesive silicone caulk. If you can get good
adhesion on those surfaces, though, 50-year silicone caulk is
a very strong glue.
Question: How are you going to drill the overflow holes for
the reservoir? Those high speed steel masonry bits take
forever to drill a 1" deep hole in concrete, for example.
Your tiles are thinner than that, but a carbide-tipped
or diamond bit is still going to be a considerable timesaver.
Using a hammer drill speeds up the drilling but risks cracking
the tiles. (I guess at 30 cents each you can risk one for a test.)
Since the sides are probably more than one roofing tile long,
you could perhaps put the overflow holes in the vertical seam
between them (just put a piece of wood stick in the seam at
the desired height above the bottom when joining the bottom
row of side pieces together, then drill it out after the mortar
or glue sets up).
To understand mortars, one should first understand
what portland cement is and its relation to concrete
(people tend to use the terms interchangeably, but
cement is only a component of concrete, mortars,
grouts, etc):
http://www.cement.org/basics/concretebasics_faqs.asp
Mortar types:
http://www.mc2-ice.com/support/estre...nry/mortar.htm
It sounds like a fine self-watering container to me, that will
not crack after a few years of UV exposure, etc.