I can't find a picture but the best concrete has enough aggs to fill all the spaces with just a thin coating of Portland covering each one.
Therefore the ratio of course to fine in good concrete will have an aggregate for every space.:)
They interlock, this is why you want sharp aggregates not round. Round aggregate does not work well at all.

Put down a 3 inch thick layer of sharp concrete gravel and then step on it. Then try stepping on 3 inches of marbles.
A good maximum size for your aggregates is no more than 1/3 the thickness of your finished concrete.

IMO a good (sweet spot) ratio of Portland to aggregates for tufa and decorative objects is a 1 to 4.
That said I generally use a 1 to 3 ratio for white Portland.

I'm a newbie so please forgive this if it's a seriously dumb question - but the recipes I have for hypertufa don't call for any sand at all. Just portland cement, perlite and/or vermiculite, and peat. Is sand a common addition? doesn't that make the object heavy - which is what I am trying to avoid?
So many of the postings I see here look more like straight concrete art instead of hypertufa. I'm not saying that's wrong, I'm just trying to understand the differences.

Tufaenough,
thanks! That makes total sense, very useful imagery!

My river stone pea gravel, however, runs more toward round than sharp.
It's what I've got, so I have to use it, and the exposed pebbles look good.
The size mix is excellent, I'm hoping that will mitigate to some degree.
Also, there won't be great demands on my edging as far as strength,
all it has to do is lay still in its trench and carry a mower wheel now and then.

For other applications though, I'll bear this in mind.

Getting a feel for the dry mix, it's a different beast, I think my ratios
are ok, but time will tell. I never really thought about it and don't recall
anyone here addressing aggregate ratios, though I'm sure someone has.

Thanks again!

Hi Solstice
You are correct many recipes here are for concrete. Leaves are generally cast from sand only cement or mortar mix. Stepping stones are generally a concrete recipe.

If a recipe contains peat it's Tufa.
If it contains peat and perlite and or vermiculite it's light weight tufa...I guess.:)
If it contains no peat but sand perlite and or vermiculite it would probably be called light weight concrete.

Unless it contains additional lime. Then it's called mortar not concrete.:)
But if it contains additional lime and peat then it's tufa once again. ;-)
Peat = Hypertufa
No Peat = concrete, cement, or mortar.

If you are making any large object that you wish to be able to move then avoiding sand is probably a good choice. It will be weaker without sand but not really that much weaker.
I make mostly concretes but my favorite lightweight Tufa recipe is
1 peat
1 fine vermiculite
1 fine or course perlite, or another vermiculite (you can opt for a second part peat but considering the peat will eventually rot out your tufa might not last as long as you like).
1 Portland
And water that is 10-20% admix.

All I know for sure is if you are making 'cement overshoes' avoid peat and light weight concrete and generally you can dispense with the curing.:)

Thanks! You've cleared up a lot of the confusion I felt reading so many of the postings.

Hi Solstice
To expand on my comment about sand not adding much additional strengh.
If you add enough sand the strength improves dramatically and you can make thinner wall projects.
But my personal findings are that adding one part sand to a 4-6 part mix of peat perlite or vermiculite doesn't add enough additional strength to overcome the additional weight. So if your choice is to add just a bit of sand my feeling is...don't bother. Especially if all you have is playground sand. I think one part of playground sand does more harm than good in tufa. If you are going to use a part of sand make sure it is 'sharp' masonary sand.
Tufa without sand seems to handle drops and hard knocks at least as well as tufa with sand.
In my personal opinion it handles them better.
If you increase the sand to 2 parts in a 4-6 part recipe it is harder to 'break' the tufa but it's also much heavier.

My personal findings subject to change as I experiment further:

Tufa made with vermiculite is stronger than tufa made with perlite but it will absorb more water that perlite so a watered vermiculite trough will be heavier.

Tufa made with a good mix of fine and course vermiculite or perlite is considerably stronger than one made with the chunky stuff from the garden centers. This comes down to there being an aggregate to fill every hole in the mix.
The fewer holes filled with portland the better.

Adding old frozen latex paint as an admix makes for stronger tufa that sets up fast and cures to a beautiful soft looking finish. Even more like aged natural tufa.
So if you have old latex paint products laying around that have frozen and are useless as paint throw them into your water as admix. You will not be sorry.:)

Tufaenough, you are a fount of information!
Never heard about the old latex paint before! Does this work in
regular concrete as well? I have a lot of it and have been wondering
how to get rid of it.
Does the pigment figure as well?

Solstice, our posts crossed, didn't mean to ignore you.
You lucked out Tufaenough got to you first, I don't know anything
about hypertufa and apparently not that much about concrete either,
it's, happily, a rather forgiving material.

Many cities and states are 'disposing' of latex paints by using it in their concrete and pavements.
It strengthens, adds water resistance, and gives the concrete a nicer finish.
I doubt you will see much color as it's diluted in water.
But if you have dark colors and use white Portland you might see color.
All my latex has been of the off white eggshell variety.
I did pick up a gallon of yellow for 25 cents at a garage sale the other day.:)

I hope my opinion is not redundant but when talking about cement mixes and aggregate it is really quite simple:
Cement + water + aggregate.

Aloha, tufaenough and Coastal Concepts, for your information and advice. The link on concrete composition and mixtures was very helpful. I have tried to learn more, especially about using pozzolans, particularly silica fume and metakaolin, with their particular advantages for strength, resistance to water absorption, as well as superplasticizers and water reducers. For very small batch mixes, it looks like it would be hard to use an appropriate dosage (to borrow a champagne-making term) of each of these admixtures.The added strength, durability, and resistance to water absorption could be of value in making very strong and lightweight planters. Perhaps you might give us a few more analogies to understand these? Mahalo nui loa for your replies!