It sounds to me that what you have is plain portland cement which is just the cement powder from your description Jonna.
Quickrete here has too much sand and our concrete has rocks. My first attempt at hypertufa substituting sand mix (Quikrete) was a crumbly disaster.
I never have added additives other then vermiculite, perlite, or seived or hand uncluped peat moss. For this cloth technique it is important to have more cement. 2/3 cement, 1/3 perlite or other choice.
Think really thick cake batter. Pourable and easy to squish your pre-dampened cloth in, and not loose and runny.
Good Luck! I'm glad you're gonna' try it and these will look great in your tropical world. Looking forward to seeing them soon!
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Thank you Jean, Chelle, and Jonna!