finding lime

[Tscarborough]

Re: finding lime

That is agricultural, i.e. hydrated lime. Hydrated lime is difficult to fully slake naturally. For laying a rock wall in the garden, no problem.

Type S lime is double hydrated, usually done in an autoclave to get full hydration of the magnesium component. It slakes easily, i.e. it is usable as is.

Type N can be used, but it should be slaked prior to use.

[Seth Hensel]

Re: finding lime

This from the National Lime Association website FAQ:

Q: Is aglime the same as lime?

A: The term agricultural lime, or "aglime," usually refers to crushed limestone. Limestone (calcium carbonate) is not the same as hydrated lime (calcium hydroxide).

And this:

Q: What are the differences between Type N, NA, S, & SA hydrated lime used for mortar and other building applications?

A:
Hydrated limes used in building applications are divided into four types, as described in ASTM Standard Specification C 207 (Hydrated Lime for Masonry Purposes):
•Type N – normal hydrated lime
•Type NA – normal air-entraining hydrated lime
•Type S – special hydrated lime
•Type SA – special air-entraining hydrated limeTypes S and SA are differentiated from Types N and NA principally by the unhydrated oxide content and their water retention value. Type S must meet a water retention value of 85%, while Type N hydrate lime must have a water retention value of 75%. No distinction is made based on the nature and source of limestone.

That 10% difference in water retention value might make it worth slaking if you are using Type N. I didn't and it worked fine. Maybe I just got lucky. If you have any doubts, go with Tscarborough's recommendation. I've been reading his posts for years and he clearly knows what he is doing.

[Tscarborough]

Re: finding lime

Agricultural lime is usually not just crushed limestone, it is calcined (burnt) then milled to a high degree (and sometimes pressed into pellets). This does give it some degree of desirable properties for mortar as a plasticizer, giving a better bond strength and workability, but it does not provide any hydraulic or cementious properties when used in mortar.

Type S (or N) is limestone that is calcined, autoclaved, then milled and it will provide various degrees of hydraulic and cementious properties when used in a mortar, a well as working as a plasticizer.

This matters for a refractory mortar more than for a normal mortar because when the portland cement paste in the matrix fails at high temperatures, the cementious properties of the lime and fireclay will maintain the integrity of the mortar, more or less.

Refractory cements like Heatstop are based on an entirely different type of chemistry.