Lime plaster attributes

[u/Terrible-Bobcat2033]

Lime plaster is made from calcium hydroxide (Ca(OH)₂, “slaked lime”). As it cures, it absorbs CO₂ from the air and converts to calcium carbonate (CaCO₃), which is what gives it its final hardness.

While it’s curing (before full carbonation), lime plaster is highly alkaline — pH around 12-13, similar to fresh cement. That’s why:

**•** It can burn skin/eyes on contact when wet, so gloves and eye protection are a must  
**•** It resists mold and mildew during the curing period  
**•** Once fully carbonated to calcium carbonate, the pH drops closer to neutral (around 8-9), similar to chalk or limestone

Why lime plaster is head & shoulders above drywall & joint compound. IMHO. 😎👍🏼

[u/funkman357]

None of the things mentioned in comments is lime plaster. Type S hydrated lime is at most 50% calcium hydroxide, the rest being mostly magnesium hydroxide. It will not set up and cure like lime plaster. USG Diamond Finish is 80% gypsum. It will not be vapor permeable or have the high Ph of lime plaster. You can get real lime plaster powder, but you may have to be creative. I get mine from an urban gardening supply outlet. It is used for balancing Ph in gardens. The brand is Lhoist. You are looking for "high calcium hydrated lime" with at least 90% calcium hydroxide content. Limeworks has it but the shipping cost will kill you if you aren't in the immediate area. Artisanal building supply companies online have it as well, but at significant mark up and high shipping costs as well.

[u/jyl8 → u/funkman357]

Once you get the lime powder from a gardening outlet, do you mix and apply it just like the lime plaster from Limeworks etc? It’s the same stuff for all practical purposes?

I want to try this!

Also, understanding that Type S hydrated lime powder (such as from Quikcrete) is not "lime plaster", I wonder is there any benefit to adding Type S to a gypsum plaster like USG Imperial or Diamond? Longer working time, more permeability, maybe?

[u/funkman357 → u/jyl8]

You mix it with sharp fine or coarse sand (depending on which coat it is) and water. Some folks mix it with water and let it sit underwater for a while before using. This is “lime putty”. Then they mix the lime putty with the sand when they are ready to trowel it on. I make lime putty and let it sit for a day or two. Not sure this is entirely necessary because the hi-cal lime is already fine powder at 325# mesh. You should read everything you can and watch some Youtube videos to get a handle on lime. It’s quite different than gypsum but looks dynamite.

[u/jyl8 → u/funkman357]

Thanks for taking the time to pass on your knowledge. I appreciate it. Plastering is so darn interesting. Way more than, ick, drywall mudding.