crumpling grout -- what is the cause?

After an cheap contractor did my bathroom, I found the grout crumples into powder with moderate scraping -- as if scraping a piece of chalk.
What might be the cause of this so I don't make the same mistake, if I decide to regrout?
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incorrect amount of water used to mix the grout, or water was added after it started setting rather than throwing it out.
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peter wrote:

Old material and/ or improperly mixed.
http://www.super-tek.com/doit7.htm
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