Re Drew's inquiry:

GFRC is indeed cementitious (glass fiber reinforced concrete).  This implies
a formed cement paste typically with small aggregate and reinforced
with mat and or loose fiberglass, allowing much thinner sections than
steel reinforced precast.  Sometimes it comes to the job with a case
of the "fuzzies", or can develop same over time.  (The little fibers
randomly prickle out across the surface.
Usually these are precast sections or panels.  Can also be used to
replicate terracotta since it takes the same shape without the
shrinking of terracotta, often with similar or lesser thickness.
Polimer may also be employed for similar applications, called GFRP,
FRP panels, etc, usually thinner so that when you tap on it it often
betrays its non-masonry nature and stands up to ultraviolet like most
polimers.  I've used GFRC as a repair to form the arch to which
Guastavino replica tile are adhered, and to replicate lost terracotta
modillions.  In either case, look to how the units are weatherproofed
and anchored.  If by ferrous anchors...look out if water gets behind.

What you have could be any of many mixes.  Like any good patch, cut
it in and bond the new without feathering, but realize you don't have
the depth of mass concrete to play with.

Rev. Jim