For beginners, there are two very common mistakes:
1) Buying a concrete mix instead of portland cement. The concrete mix already contains the correct proportions of portland cement and sand so if you try to add peat moss or another ingredient your 'tufa probably won't cure properly.
2) Not being patient! Many 'tufa projects are ruined because they're disturbed before they've sufficiently cured.
It's a good rule of thumb to wait 24 hours before moving or otherwise disturbing your hypertufa project. Even better is to use the fingernail test: if you can make an impression in the 'tufa with your fingernail, wait another day. If it's hard enough to resist your fingernail, proceed!
Entered by peakpoet
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