'bastard' mortar (portland cement + lime) for BBR outer skin OK?

Hi all. Tried to ask this questions a few times on my build thread but got no real response.
I'm nearing the height where the inside skin temperatures of the bell will be the highest. I recall a post from Fieda, stating the temperatures dropped very quickly from the top of the bell down.

I'm now at 1m20 and the final height will be around 1m90. I've been using mortar labeled 'masonry mortar with lime' so far.

I just want to make sure this will also be OK for the hottest (highest) part of the bell.

I've seen masonry stove builders refer to type N mortar for the outer skin, but I do not have any idea on the mixing ratio lime/portland cement that is in my own mortar.

I can get my hands on NHL 3.5 lime. If I use this, should I make a pure lime mortar, or type N mortar?

My outer skin will be load-bearing so it's quite important the strength of the mortar is maintained.

You need structural  refractory cement for the upper level of brickwork.
Your profile does not say we’re you live but in Europe you would need to have used refractory cement for all the brickwork in order to pass building regulations.
Where I live, the standard high temperature cement is called Fondue, it is readily available and mixed at 4-1 with silica sand but the product is very fast setting and you can only mix small batches at a time.
In America, I don’t know what the trade name for high temperature cement is but, I think you can buy ready mixed bags, just add water.

Julian Adam wrote:My outer skin will be load-bearing so it's quite important the strength of the mortar is maintained.

I understand you are building a double-skin heater with a batchrocket core inside. The outer skin will be fine when done with lime/sand/portland mortar. The inner skin is an entirely different story, it should be separated from the outer skin. Most people use 1/2 inch superwool between the skins, or cardboard that is pulled up every 5 layers or so. This separation is there to provide an expansion joint as well as a slip joint, the inner skin will get hotter and will expand as a consequence, mainly in vertical direction.

Now, the inner skin can be done in the same mortar and bricks as the outer one, *up to the height of the riser*. Above that level, you should switch to firebricks (on edge) and real refractory mortar. Care must be taken that the firebricks don't make any connection with the outer skin, otherwise the outer skin will crack, badly.

The top of the bell could be done with upside down steel T-profiles and firebricks between the steel. All faces of the steel should be separated from any bricks by means of 1/2" superwool or ceramic paper. When done properly, this construction will hold indefinitely.

James, thank you for your input. I am based in Belgium. I have looked into regulations but could not find anything. Anyway, what is built, is built. I was assuming the temperatures in the bottom 2/3rd of the outer skin would not be high enough to justify the high-temp (aluminium silicate) mortar. The fast-setting would be my biggest problem, because I can now only lay one row before my mortar starts going off.

I will use lime mortar for the rest of the outer skin of the bell, as research has shown this has also quite high 'refractoriness'.

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.sciencedirect.com/science/article/abs/pii/S0950061818322049&ved=2ahUKEwi29PvnirGJAxVaR6QEHTxZMOgQFnoECBMQAQ&usg=AOvVaw2tb10EVAGRrR3yG9kmtGi4  

Peter, you understood correctly. Thank you again for your input!
I picked up a bag of NHL 3.5 lime this morning for making pure lime mortar just to be safe, as the lime should produce a more flexible and heat resistant mortar but yield similar strength.

Thank you for pointing out again the importance of separating outer and inner skin. My idea was to line the outer skin completely with (double or single corrugated?) cardboard, and just leave it in place, and lay the firebrick against it. I was planning to use sand + fireclay. Would it be prefered that I use real refractory mortar for the top of the bell?
As I understood from masonry stoves, the cardboard just disintegrates from the high heat. I'm not a fan of superwool myself, so I'm trying to limit the use of it wherever I can.

I will be building the inner skin entirely out of reclaimed firebricks placed on edge (6 cm side facing down). I was wondering if it could be an added value to put the upper 40 cm on their flat (11 cm) side to form a bit of a buffer from the heat? This would however increase my bell height slightly, which is less desirable.