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question about lime putty

 
pollinator
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I have a bucket of lime putty that I mixed using bagged hydrated lime several years ago. It has been sealed ever since, and still has about an inch of water standing over the putty.

I can't remember if I knew enough at the time to avoid hydraulically setting lime for this. The mass of lime putty is still soft; should I assume this means that I used the right kind of lime and it is still usable? Or would a weakly hydraulic lime still be soft under such conditions?

Also, does anyone have any experience with lime concrete vs lime mortar or lime cement? I want to use some lime mortar to make a pebble mosaic stepping stone; it will be about three inches thick and would be more like a concrete situation.
 
pollinator
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Sounds like you have been slaking it.
Slaking is good for hydrated lime, makes it better from what I hear from my expert.
Low quality hydrated lime is slaked for years in water,
to improve the calcium to magnesium ratio, I think.
I have a bucket of slaking lime in my basement.

If you mix lime with clay and bake it in a kiln again it makes cement/concrete.
It is then cured by water and sets much quicker and is then cured by water.
Whereas lime is cured by air/CO2 and can therefore take a long time to set and get increasingly harder for a hundred years.
Also can self heal cracks using the minerals in rain.
If they build a brick wall using lime mortar I don't think they build much more than 3ft a day and let it cure.

So even though lime is better in many ways; takes less energy to make and improves over time
it has mostly been replaced by concrete because it sets faster.
There are different types like type S but I can't remember the difference.
Some are for mortar and some are for surface plaster.
 
pollinator
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Yes, lime putty improves with time. I've recently gotten into some buckets that I mixed several months ago and it's good stuff. If what you have is pliable then it's fine...hydraulic lime would have gotten crunchy by now and even plain lime putty can get a carbonated skin on it if you leave it long enough.

For making a stepping stone, consider pozzolanic additives like volcanic ash, clay brick dust, diatomaceous earth, or even ash from high-silica plant material like rice hulls, straw, or horsetail. These materials speed up the setting, as well as increase the tensile strength and durability of the lime cement, and in a paving situation you're going to want that.
 
Gilbert Fritz
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Thanks for your responses!

A somewhat related question; would "dead" portland cement work as a pozzolanic additive? I've got some old bags of portland cement sitting around; it is probably not very reactive anymore. But Portland cement contains both fine silica materials and fired clay minerals.

I can't find any info online about this.
 
Phil Stevens
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I've used small amounts of stale portland cement in lime mortar, and it certainly hardened up quickly. So I'd say yes, that's a decent pozzolan. If you trace the development of concrete, it seems to have come about in experimentation with additives to lime to increase durability and hydraulic attributes:

Pozzolan article
 
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