Hi all-
Lurker here:) I'm curious about the lime mix too. I've done a few tufa planters and have varing degrees of thickness and would love to try a thinner more delicate mix, but I dont want a piece that is too fragile.

btw! This forum is awesome! I have learned so much from you all!

Thank you so much for the tips. I think I'll start experimenting with different mixes, like the ones you described. Is there a 'best' mix (or a mix anyone considers 'best') that doesn't use peat? I'd like to try a much smoother one and if a recipe already exists less mess for me!

So far I've been happy with my rock-like appearances, as I am a real rock hound. Lately I've been looking to try and get smoother finishes, but I still love the rough texture that peat gives.

I went to a horticultural suppliers, and bought a very large bag of fine vermiculite (much finer than I've been able to find at garden centres). I haven't got all my moulds lined up yet, so haven't started on the nitty-gritty of casting, but initially I intend using a 1-2-3 mix (one cement, two sand, 3 vermiculite) in any pieces that need a fine surface.

Obviously, this results in a mix that's half the weight of 'proper' concrete. That's still quite heavy when you're making large objects. Has anyone upped the proportion of vermiculite, or dropped the sand from the mix, and if so what was the finished strength like?

The 1:2:3 mix is quite strong and can be very smooth especially if you puddle the surface. It is possible to make it without sand but to me it seems really weak, only half the strength of the sand mix according to the charts but also only half the weight. I've seen mixes specified up to eight parts vermiculte, but they are considered to have no structural strength (barely more than expanded polystyrene) and are just used as insulating covers or layers in other structures.

The small planter at the back in this picture is made from the 1:2:3 vermiculite mix.

{{gwi:75089}}

A stronger recipe can be made using fine Perlite instead of vermiculite.
I do a very nice
2 Portland
1 sand
5 Perlite recipe that is quite strong.
I use a 10% Acrylic admix as well.
The same recipe using vermiculite is much weaker.

Regarding Lime..
Mason's use lime in mortar for two reasons, for added workability...and (as odd as it sounds) to actually reduce the strength of the mix. Concrete and sand alone would yield a mix that could easily be stronger then the bricks being set, which could lead to an uncontrolled failure in a wall or structure. But if the mortar gives way before the bricks, the integrity of the structure is less likely to be compromised and can easily be repaired.

As for workability, lime makes for a stickier and creamier mix that bonds well to other materials and resists being squeezed out between the joints. I use lime only for the workability benefit myself. It helps considerably when I am building up layer upon layer and sculpting by hand.

But use extreme caution. Hydrated Lime is a very powerful caustic and can cause severe eye damage and blindness. You also do not want to get it into any kind of open cut or wound. Trust me...you really, really don't.

I avoid adding lime and just use S type Portland cement when extra stickiness and working time is required.

David, are you actually using a Type S Portland cement, which is a slag-based product for making very strong concrete, or are you using Type S blended lime/cement? The blend would be basically what I am asking about, an extra workable mix which doesn't set as rigidly. The actual portland cement Type S (confusing, huh?) is at least 70% blast furnace slag. The slag-based cements are fascinating in the way they react to form very strong concrete, but I doubt that's what you are using. There is another Type S, which is a 2:1:9 cement:lime:sand mortar mix. All very confusing!

Shrubs, By slag based do you mean fly ash? Fly ash is a pozzolan which is reactive and actually reduces the amount of portland needed/used in the mix.

Yes this can get confusing with all the different types we can use. Most of the readers here do not however go to a masonry supplier(s) to get those specialized types. We go to the local hardware or renovation center to get our products and have to work with what they can get. Then this being an international forum the types are rated/named differently from country to country. It's hard to get everyone the same page so to speak.

Tango, You are so correct about lime used in mortars. Having done restoration work for limestone buildings, I've seen up close and personal the damage that can be done by using the incorrect mortar. A harder mortar actually will break the limestone away in big chunks at the mortar joint. This happens because all things move with the changes in weather conditions, even stone, brick and mortar, although much slower than wood.

I personally like the look of vermiculite over perlite in the things I make. So the strength/hardness of one over the other doesn't matter too much. It all depends on what we are making, the uses they're put too and the makers preference. I pulverise my vermiculite to a much smaller size and find it acts as a smaller aggregate without much loss in the strength of the product.

Hi shrubs
I'm using Type S blended lime/cement. It's cheap and you don't have to mess with pure lime. It's still more caustic than regular Portland so carefull handling is needed.

Rock I use cryogenic perlite which is like a fine sand, carves beautifully without tear out and finishes up very nice. Like sandstone, and it can be colored using cement pigments.

To understand what fly ash and slag do in cements, first you need to know what non-hydraulic and hydraulic cements are, and what pozzolans are.

Hydraulic cement is a cement which sets by a chemical reaction with water, often called hydration. Ordinary Portland Cement (OPC) is a hydraulic cement, where Calcium Silicon Hydroxides react with water to form Calcium Silicates (the tough part of concrete) and Calcium Hydroxide (the lime that comes from concrete). It is worth remembering that ther Calcium Hydroxide produced in this reaction does not contribute to the strength of the concrete. Hydraulic cements react until all the calcium silicon hydroxide is gone and then it is finished. The stories of roman concretes still getting stronger today are something of an exaggeration, although the hydration does continue exponentially slowly for months and years.

Non-hydraulic cements set by reaction with carbon dioxide in the air to form Calcium Carbonate (same as limestone). This occurs when the Calcium Hydroxide (lime) cement is dissolved in water, but isn't actually a reaction with the water. Non-hydraulic cements never achieve a final set because the calcium carbonate can always be dissolved back into calcium hydroxide and then react again to form calcium carbonate in a slightly different place. This is part of the process that allows lime mortars to move under shifting loads such as expanding stonework.

Pozzolans are substances which react with non-hydraulic cements to form calcium carbonate without needing carbon dioxide from the air. This enables them to set much faster. Non-hydraulic cement plus a pozzolan behaves a lot like a hydraulic cement. Very pure lime cements set very poorly since the interior cannot react with the air. Historically, lime mortars and concretes include natural pozzolans such as clays or other impurities.

Fly ash is what is left after burning coal. Fly ash is quite a strong pozzolan and this is its main purpose, although it does also improve workability. Fly ash includes varying proportions of lime and is used as a substitute for some or all of the OPC in concrete or mortar mixes.

Slag is what is left after smelting iron ore. Slag is not a strong pozzolan. Its main purpose in cements is to react with the calcium hydroxide produced by the hydrating cement to form more calcium silicate which is strong. The slag fines also makes concretes mixes more workable and typically will also include about 50% lime. The slag/lime combination is called slag cement and is used as a substitue for some or all of the OPC in concrete or mortars.

There are now many combination cements which include OPC, lime, fly ash, and slag in various quantities to produce concretes with specific properties. Such combinations often have both hydraulic and non-hydraulic properties, which can allow for a fast initial set from the hydraulic part followed by the slow hardening and ability to shift under load from the...

Very interesting shrubs, I just wish there was more info about additives for making LIGHT WEIGHT concrete strong.

This is a very infomative thread!!!

So, do you think you could make a light-weight, non-hydraulic hypertufa using lime, fly ash, peat, and perlite?

There is a vast amount of research on lightweight concretes. Unfortunately most of it is focussed on materials and techniques that are not readily available to amateurs.

Structural lightweight concretes are widely made, typically by replacing some or all of the aggregates and cement with various types of fly ash. These are roughly 2/3 the weight of regular concrete and are about the same strength as regular concrete. This is probably stronger than most of use need for our projects and heavier than we would like.

Really lightweight concretes in construction are mostly aerated concretes where some or all of the aggregate is replaced by air bubbles. This provides for densities from about half of regular concrete down to a little heavier than expanded polystyrene. The lighter grades have no structural use, they are only really strong enough to support themselves. The technique is probably beyond anything any of use can do.

Simple lightweight aggregates such as perlite, polystyrene, and vermiculite are used in industry mostly for non-structural projects such as poured insulation. Little research has been done on strengthening these products because alternatives are available. It is probably fruitless to try since the aggregates themselves are so weak.

There are some simple approaches to creating strong lightweight concretes. Leaving out the sand from a regular concrete mix makes a big difference and you can gain back a lot of the strength using admixes and fibres. Using strong lightweight aggregates such as pumice, slags, or fly ash is another approach that is fairly simple. The problems come with finding and using the aggregates. Fly ash in particular is nasty stuff and slag cement also includes lime which is bad, while pumice is hard to find and expensive.

The fundamental hypertufa idea of using peat is bad for strength because the voids it creates are too large and you need to use relatively high-cement mixes to be able to join the coarse aggregates. I find that hypertufa recipes need to include sand to get good strength. Again, there are lighter alternatives such as fibres or a reinforcing interior framework.

One thing I would suggest to anyone who is struggling to combine strength and light weight is to learn how to mix and cure your concrete. Too much water, improper dry mixing, and insufficient or incorrect curing will dramatically reduce the strength of your work. Also correct laying of the concrete to exclude air bubbles is important, although not really appropriate to hypertufa.

For anyone thinking of experimenting with non-hydraulic cements, here is what slaked lime can do when it eats the skin off your fingers.

{{gwi:75506}}

Oh, the things I do just to provide informative photographs!

That's why lime is still a favorite for disposing of bodies.:)
I get good protection from Bag Balm when I have to 'man handle' lime sans gloves. In fact I've found a glove with a hole in it is worse than no glove at all when handling lime.

I use cryogenic perlite in my light weight concrete.
I'm constantly experimenting to get it as strong as possible.
You are right there is next to no info on strengthening this product.
I find admix, latex paint, and a bit of sand to be just about as good as it gets. Fibers, silica fume, fly ash don't seen to make much difference. They just mess up the color and the texture.
My light weight perlite cement is much stronger than Hypertufa made with peat at the same weight. Peat also decomposes over time, perlite does not.
If I want the peat look I first make a shell from light weight concrete then apply a layer of hypertufa to that.
This works very well and you can have a beautiful smooth colored interiors that are waterproof with a nice crusty peat on the outside. Great look for water features or birdbaths. All my tufa now incorporates a light weight concrete interior shell made with cryogenic perlite.

David, thats a great idea to put a hypertufa crust over a concrete base. Now if only you'd told me a few weeks ago before I started playing! Hypertufa is a new material to me and I'm concerned about its longterm strength and durability outdoors.

Peat is certainly the weak link, because over time it is going to completely decompose leaving the other elements in your recipe to keep the piece hanging together.
I have a very nice trough that is only peat and Portland and it appears to be very strong but as the peat rots away it is certainly going to fall apart.

I love making stuff in layers. If you make a shell and let it cure a couple days you can then apply your finish material without worrying about removing molds, etc. You can handle the piece right away and that gives you much more control over finishing.

And people seem to love the two toned look and it's really not much more work.

Couple of questions :)

What exactly is perlite? I've bought a large bag of vermiculite, but haven't come across perlite as yet.

David - how do you prevent the different layers from laminating apart? Do you apply some kind of sealer/adhesive to the base layer?

Okily dokily - googled perlite, see that I should be able to get it at any horticultural suppliers.

Perlite from horticultural suppliers is usually expensive because it comes in little 5l or 10l bags for maybe 5 euros. At builders suppliers, or occasionally horticultural suppliers, you can get big 50l or 100l bags for 10-15 euros. Same with vermiculite.

The cryogenic perlite that David is using is much smaller balls than horticultural perlite. It is intended as an insulation product at extremely low temperatures, either loose or with a binder.

Perlite is a very weak aggregate and is mostly just maintaining air pockets in a concrete with the strength provided by the cement and any other aggregates. On this basis cryogenic perlite, which I haven't used, should give a stronger result than regular perlite. Adding sufficient fine aggregate, like sand, to just fill the gaps between the perlite balls makes perlite concretes dramatically stronger.

Hi Dog
Most of the time I do all the layers at once or within a couple days but waiting even 3-4 days is not a problem if you keep your work damp.
I increase the amount of admix in subsequent layers as time goes by (but rarely over 25%) and if a piece does sit more than a few days I soak it over night in water and coat the older layer with a concrete bonder.

I have tested these bonds and they are just as strong as the entire pot. Even breaking them with a hammer will not seperate them at the joins.

That said I'm still a newbie and made my very first pot this way in January and it is still like new.

Ferro cement is applied in layers and it lasts decades.:)

Talk about clever ways to create "lightweight" concrete...the Romans used to make small, hollow ceramic spheres that they would mix in with their concrete. An inch or two in diameter, they would displace an equal volume of concrete resulting in a lower density. (Imagine really BIG perlite balls!) The best known use was to lighten the upper portions of the magnificent 142 foot high dome that tops the Pantheon that was engineered by Agrippa, the son-in-law of Emperor Augustus. Built in 27 BC, it still stands as a testiment of Roman ingenuity and the enduring properties of the concrete they invented to build their empire.

I see this is an old post .. but I was curious if anyone has ever used wood ashes from your fireplace for an additive as it is composed of lime (I think).. Any comments on this would be great .. thanks.. Also if anyone has added unusual stuff to their mixes that has worked .. would love to hear about it.. Thanks