Methods are provided that include a method comprising providing magnesium oxide; placing the magnesium oxide within a portion of a subterranean formation; contacting the magnesium oxide with a brine that comprises water and a divalent salt; and allowing the magnesium oxide and brine to react to form a hardened mass in the portion of the subterranean formation. In some embodiments, the methods comprise providing a subterranean drilling fluid that comprises magnesium oxide; and adding a divalent salt to the subterranean drilling fluid to form a settable composition. In other embodiments, the methods comprise providing a subterranean drilling fluid that comprises magnesium oxide; and adding a divalent salt to the subterranean drilling fluid to form a settable composition. Additional methods are also provided.