You can use crushed shell, and you will have the control of any weeds below
the mat, they will not be able to get through the geotextile, but please be
aware that the shell is weak. Under foot traffic it will gradually crush and
compact into the Natural Paving grid in the trafficked area. This will
result in the surface at the top of the grid becoming flat and compact, and
the benefits of the Natural Paving will be compromised.

In contrast, when you use 8 to 14mm pebbles, the stones that overlay the
grid are stabilised because they sit in what are effectively ‘nests’ formed
by the gaps between the pebbles below. Those pebbles are held in place by
the Natural Paving grid. The grid is covered in pebbles and is hidden by the
next two layers of pebbles which is the recommended coverage. The resulting
surface is made up of pebbles all stable because they are nested like eggs
in a crate. The surface looks completely natural but is so stable that even
stiletto heels or the skinny tyres of a racing bicycle traverse it without
displacing the pebbles, and the surface stable.

With shells, you lose all the stabilising influence of shells on top sitting
in ‘nests’ if the shells are crushed underfoot, and compacted into the grid.
If you sprinkle fresh shells on top it would be like sprinkling shells on a
hard flat surface. The time it takes for the shells to crush and compact
depends on the amount of traffic, but, for a while, Natural Paving will
stabilise shells.

If it is the colour you want, then you could consider using white quartz
pebbles which are smooth and nice to walk on, but expensive. You could also
consider using lime chips that are the colour of shells and economical, but
not so nice to walk on..

If you really want the shell finish you could use Natural Paving to hold the
shells in place, but accept that in the longer term maintenance will be
required if the shells become crushed and compacted. This would entail
lifting each mat, tipping out the crushed shells and filling the mats with
fresh shells.