I’ve been trying to understand the differences in mortars and can only find bits and pieces on about a dozen websites, so I figured I’d put it together here.
Mortar, in its simplest form, is made out of “glue” (e.g. Portland cement, lime) and “filler” (e.g. sand, fire clay). I’ll limit myself to type N, S and High Temp and ignore all the additives and various types of sand, at least for this post.
Volume Ratios (Sand, Portland Cement, Lime, Fire Clay)
Type N: (6, 1, 1, 0)
Type S: (9, 2, 1, 0)
High Temp: (3, 1, 1, 1)
To me this is confusing so I looked at percent ratios:
Type N: (75%, 13%, 13%, 0%)
Type S: (75%, 17%, 8%, 0%)
High Temp: (50%, 17%, 17%, 17%)
Wow! Type N and S have the same ratio of filler to glue and it’s just the ratio of glue materials that differ. To explain why type S is strong we have to look at the material properties of the two “glues”. Portland Cement will bond stronger but is less flexible and does not breathe. Lime is more flexible, less strong and will breathe.
So what’s up with high temp mortar? If has a higher ratio of glue to filler, as much Portland cement as type S and fire clay. Fire Clay is just mud BUT with an Alumina content of 24-34% and a Silica content of 50-60%. Another light bulb moment: looks exactly like fire brick. Of course, this is no accident. We have a strong, flexible mortar that matches the material properties of the fire brick which means as they expand and contract due to heating and cooling they should do so together. Note, this is why it’s so important to use silica sand. Other sands may have other materials (rocks) with different properties than the firebrick we are trying to match.
Bonus: You should be able to use your high temp mortar left overs (unmixed) to mix your own Type N & S mortars.
Disclaimer: I pieced this together from various sources and make no claims of being infallible. Feel free to point out where I may have got something wrong.