"Ferrocement" isn't a type of cement or concrete. Ferrocement refers to a method of construction used in boat building. Sometimes called a concrete boat, a ferrocement boat hull is made by forming a framework of steel reinforcing bars and wire mesh, over which cement--concrete with no aggregate added--is applied. Because of the nature of ferrocement hulls, surface cracks in the cement that forms the skin of the hull are the primary form of damage these vessels suffer, and the damage is easily repaired.
Grind both sides of the damaged area into a shallow V-shaped trench. This removes loose material from the area and allows the concrete repair patch to adhere properly to the surrounding concrete.
Add the quick-setting liquid to the cement. Use a drill-driven paint mixer to mix the cement for the repair. Form golf ball size clumps of cement and apply them by hand to the crack. Application of the cement by hand ensures that no voids will exist in the concrete.
Keep pressure on the cement after you apply it to the crack. The ferrocement fishing boat construction project sponsored by the Food and Agricultural Organization of the United Nations discovered that the concrete mixture "will adhere to the inside surface of the ferrocement if hand pressure is applied to the mixture for approximately three minutes."
Allow the cement to cure thoroughly. After the cement has cured, the surface may be ground flat with the grinder and painted or sealed, if desired.
Temporary repairs under way can quickly be effected by applying an epoxy filler like A+B Marine Putty to fill the crack. Temporary repairs should be monitored for watertightness and be properly done at the earliest possible time.