You might try the hypertufa forum, they may have some answers for ya.

Marquisa, As soon as you fill your molds, slide them into a plastic bag and seal the bag with a twist tie or knot. You should be able to unmold in 24 hours. Clean up your piece, spray with water and place back in the plastic bag for a few days. The signs should be hard enough to use by the end of the week. Even then, they'll still be a little fragile so be careful. Planters or birdbaths are placed in a water bath to leach out the lime from the portland cement. The lime can hurt some plants and you don't want to give birds a drink of water with a lime twist. It usually only takes 3 days submerged in water and change the water every day to remove the lime. Good luck. Billie

Curing concrete takes 28 days no matter what you do. Even after that, the concrete will continue to harden throughout its life.

For regular concrete: as soon as you can't dent it with your fingernail, you can immerse it in water. Two weeks isn't too long. Forget about it & let it go a month? No problem.

Since you live in such a cold area, there are two things in particular that you can do to make your concrete more frost resistant.

The first is to make your mix with as little water as you can. It has to be completely moistened, but not wet. Mix the concrete VERY well -- using gloved hands is best. If you can form a ball of it in your hand and it doesn't slump too much, that's about right. This will make the air pockets in the mix very small; it is the water that works into these air pockets and freezes that causes the concrete to crack, & eventually break.

The second way is to add a latex polymer mix to the water. If you have a commercial brick/mortar place nearby, they may help. Some people use Elmer's Carpenters Glue. Mix 1 part glue to 4 parts water. Always mix well before adding to your dry components, & don't let the glue or the mix freeze. Be careful about adding this to the dry mix, as it tends to change from not-wet-enough to too wet VERY EASILY. I pour about half of what I think I'll need into the dry mix and mix it well with my gloved hands until all is uniformly damp. Then I start adding more liquid mix slowly, maybe only 2 tablespoons at a time. When I'm close, I drop down to just one tablespoon at a time. Latex polymer additives seal the air spaces in the concrete so water can't move from one area to another, thus helping elminate water traveling through, and freezing inside, your work. Be prepared to wash your hands & mixing utensils frequently when using this material. NOTE: keeping your pieces moist for 2 days with this method is good. DO NOT keep wet or immerse for longer than 2 days or the moisture will WEAKEN your concrete.

One thing I am wondering about is the thickness of your pieces. Would increasing the thickness to 1/2" for added strength make them too clunky looking?

'Pup

Hi Pup
The bars are 1 AND 1/4 inch thick. That is the depth of the molds.
I've been using the same molds for my tufa and making bars of each recipe.
Even freezing and thawing them to see what happens.
My strongest tufa bar so far is not so good with the kit in that it has very course sand that doesn't work well with the letters. But it seems impervious to ice.
The recipe contained a small amount of a product called anti hydro. All I know about it is that it's used in making gravestones and costs 20 bucks agallon.:)

Does hypertufa crack in the winter with frost and freezing temperatures?