If I'm correct cement contains lime---and shouldn't be worked by
hand--it's an industrial building material that's best handled by guys
in cement trucks, or rubber boots, rubber aprons, and long, long rakes!
Plaster, or related materials such as hydrocal, hydrostone ($$$) are
more appropriate for art projects. In California, an excellent,
inexpensive material is known as Casting Plaster (also known as Plaster
of Paris--but then the price will triple)--get it from a building
materials company, about $12 per 100lb.
Students should wear aprons, dust masks and latex (or better) waterproof
gloves when working. However, it's a messy material and you may never
get the artroom clean again!