You just reduced the required amount of shells to maybe just the whole of the middle east and even that is questionable. Hamas has access to enough beach to fix some cracks and maybe even piece together one wall out of very large pieces of a building that fit together very well over 20 years but not much else. The amount of seashells you need for a decent amount of cement is colossal, for anything serious you need a limestone mine.

Not to mention the facilities to produce the cement that would give off massive amounts of heat and smoke.