The one thing I am a bit unclear on is the curing process – there seems to be different methods from doing nothing to water baths. I need these markers to be as winterproof / crack proof as possible (I am in S. Ontario Canada with lots of freeze/thaw cycles). After reading different forums on this site, it seems that a good plan would be to let the
pieces air dry for 24 hours, then place in a wet towel in a plastic bag for 3 days and into a water bath for a further week. Does this sound OK or is there a better way to ensure these markers will last in my garden?

Thanks for your advice!
Marquisa

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