Answer:
This boardis similar in composition to Type X drywall,
except that it has more glass fibers,
and, it has another ingredient in the
gypsum core. This ingredient is a
shrinkage-compensating additive, a
form of vermiculite. When exposed to
high heat, the gypsum core shrinks
but the shrinkage-compensating element
expands at about the same rate
as shrinkage occurs in the waterdepleted
gypsum.
The gypsum core then is more
stable in a fire and remains in place
even after the combined water in the
gypsum has been driven off.