Concrete is made by mixing cement with sand and water. Construction companies in Turkey often get their sand from beaches; it may be illegal to do so, but it also costs probably much less than it would if they purchased good, clean sand from somewhere else.
One can often tell if uncleaned beach sand was used in a construction by looking at the walls surrounding new buildings, especially exclusive residential compounds.
The sidewalk along a busy street not too far from where my mother lives in Istanbul is bordered by long and tall walls that were built not too long ago from large rock fragments held together by concrete. During several walks along those walls during my recent stay in Istanbul, I had opportunities to photograph some of the members of the mollusk fauna of the beach the sand had come from.
This one seems to be one of the common cardiid shells (Cardiidae).
Here is a small scallop shell, a member of the Pectinidae, probably.
This one is complete with both valves still attached. I can't put a name on it, though.
I got the impression that, for whatever reason, bivalve shells were more common than snail shells. More careful looking, however, did reveal occasional gastropod shells.
And here is another snail, perhaps a trochid (Trochidae).
I hope the future paleontologists will be smart enough to tell that what they are looking at is artificial concrete not some unusual conglomerate fossil formation.