Admix doesn't make your piece waterproof,it makes it tougher!
For waterproofing,you can't beat a good sealer.And,remember,if you're building a birdbath,use a sealer that's kind to the birds.There was a discussion a while ago about this.I found a non-toxic 2-part epoxy sealer,but the cost was WAY up there!
Others on the forum will possibly suggest that their pieces are waterproof and they didn't need any admix.Good for them.Good ratios and good mixing can do that.But admix reduces your water need,as prev. stated,so pieces are less impacted by that old freeze/thaw cycle.I use about 25%(depends on the conditions of the day,doesn't it?)admix in my masks(coulda done without,but want my clients to get bang for buck)and 100% in anything that has to hold water.
Now,I'm branching out to make thinner,harder concrete pieces for your wall(about 3/8-1/2 inch down to an edge)so I'm trying other additives like Silica fume and superplasticizer to see what I can make concrete do.As they used to say,so very long ago,"Verrrrrrry interesting!"

Paul, I've been using acrylic polymers for quite a while in my work, laying stone floors etc, and I find they are excellent at bonding, and imparting strength and also a little flexibility to concrete/adhesives, however they don't make the product entirely 'water-proof'. You need an admix that is designed specifically for that, and add it into the mix water before adding the water to the sand/cement. These products are expensive, but you may be able to purchase them in small quantities at your local hardware store.
The polymer acrylics will make the product a lot more waterproof than it would have been without it, though.
A product I have used and am very impressed by, is Laticrete, from the good ol' USA. It's the best I've used.
I used Laticrete 4237 to add to the adhesive to lay clay pavers around my pool, onto concrete, and 11yrs later it is the only part of the whole backyard hardscape that hasn't 'let go'.
The product for Mixing with concrete/sand-cement mixes is called Laticrete 3701.
In my experience, Laticrete is hard to beat.
Just my experience, though.
I'm going to try it in Tufa when I get a chance, and see if it gives the same results. I'm only a newbie at 'Tufa-ing', so I've got to learn what works with that. As for dosage, I found that half the recommended dosage, mixed into the water you add to the dry mix, gives excellent results.
Ric, Silica Fume and superplasticisers are products I haven't used, what kind of results have you had with them so far? I did a half inch thick ramp, tapered down to
a feather-edge, coming off a stone floor, and drove a fork-truck over it for 10yrs, and it's the same now as the day I laid it. That's good enough for me. I just mixed in a locally available PVA additive, called Bond-crete. PVA additives aren't as good as the new generation acrylic polymers though. But a good enough substitute in a pinch.
Slate

Paul,

I haven't used any admix or sealer with any of my birdbaths and as far as I can tell they don't leak.

Amy

What was your recipe Amy.
Some cure much less porious than others.

Thanks for the input everyone!

Btw, Amy, what 'recipe' did you use if I may inquire?

So far I've put a Quart of acrylic add mix into a gallon jug and fill with water.

The label says how much to use per bag or yard and too use water after-wards for the desired consistency.

A little confusing (*-*?)