Rhode Island mortar
[...]
-39-
The lack of lime in many regions was a serious obstacle. Limestone was uncommon in Massachusetts and almost wholly lacking in Connecticut. Rhode Island, however, had good quarries and exported lime to neighboring colonies after the mid-century. Seacoast towns made an inferior sort of lime by burning oyster and mussel shells; sometimes these were incompletely calcined and bits of shell are visible in the mortar. [...]
Source: Hugh Morrison, "Early American Architecture from the First Settlements to the National Period" (New York: Courier Dover Publications, 1952), 39.