Is cement a rock

From the Portland Cement Association:

Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove. By this crude method he laid the foundation for an industry which annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water. Cement is so fine that one pound of cement contains 150 billion grains.

So, I think you could say that cement is a rock (limestone) derivative.