You are quite right Stella, apparently, archiologists have beev scratching their heads for centuries, wondering why Roman seawalls had lasted 2000 years.
It seems the answer was discovered in 2017 - sea water and volcanic ash/sand together with lime creates a certain (very rare in nature) chemical reaction which builds a crystalline structure, reinforcing the bonds between the materials.
Richard, being the smartarse he aspires to being, thought ‘wouldn’t that be cool for my bathroom floor’.
The pouzzolane should arrive this morning.
If you hang about for a couple of thousand years, I’ll let you know if I stumbled on the right recipe