Here's some tips and links:
Lifewater A page about concrete from a Well-making charity.
Art Concrete Tips on how to use concrete for art.
Technical Concrete Information
Here's some artists:
David French Some cast sculptures.
Carved Concrete With ash from Mount St. Helens!
Hannes Kuehtreiber Small scale concrete block art.
Joaquin Aviña Concrete furniture.
Carol Boram-Hays Abstract sculpture.
Deborah Brackenbury Concrete Bowls
Curt Pieper Concrete Furniture