can i use fondant icing sugar to make royal icing?!

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It is,I believe also called confectioner's sugar or powdered sugar and before adding liquid would be the same.
Fondant and Icing Sugar
A very fine form of sugar that is used to make a Fondant, a dessert icing for cakes and sweets, or a garnish to be sprinkled lightly on plates or over fresh berries. Cane sugar is the type of sugar commonly processed for use in making the Fondant and Icing Sugar. It is a key ingredient for producing a rolled Fondant, a sugar paste that is popular in many countries as a candy or an icing. Often referred to as icing sugar, confectioner's sugar, or Fondant and icing sugar, this type of sugar has been ground and pulverized over 100 times more than powdered sugar, so it becomes exceptionally fine in texture. The finely powdered form results in icings and Fondants that are much smoother in consistency and glossier in appearance.

http://www.recipetips.com/glossary-term/…

I don't think it would set as hard as Royal icing because fondant icing is meant to be soft. Maybe if you add as little water or fat as possible it would be close but Royal icing should set hard and be crisp to bite through.