Shells are strong alternative to crushed rocks and its abundant near the seashores,,This are widely used at least 15-30 kms away from the ocean...But learn how to differentiate from petrified shells to disturbed shells(human interference)...similarity almost makes it difficult to identify as in some cases,,ACID may not work at all! Burned clay mixed with shells are even used and strong enough to hold load similar to a pottery making process!! Remember that during the war,,scarcity of cement products were all over the country...But they always have good alternatives...