Water: Dissolves the wood pulp; activates and hardens the cement.
Wood pulp: Improves flexibility and resilience.
Fly ash: Acts as a filler. (Some makers use silica sand instead.)
Portland cement: Binds the ingredients. Made with limestone, clay, and iron.
The external cladding products require very little maintenance once installed and painted. The thicker/denser fiber cement products have excellent impact resistance but the thinner less dense products need to be protected from impact. Compared to wooden siding, fiber cement is not susceptible to termites or rot.