I used Rapidset Mortar Mix. It is scary-fast! It kicked off in about 10 minutes! I says it hardens in 15, full strength in one hour and that is about right. I had to mix very small amounts--like a 1/2 gallon worth, slap it on, wash the bucket and trowel and then mix another. The mortar is made to harden underwater, stirring or trying to wet the mix won't slow it. I had two gallons of it kick off on me when I first started before I figured out to do small batches. Now I have a nice 5-gallon bucket doorstop.