Old brick wall - lime or cement mortar?

As I understand it, modern mortar sets rigid, and, if your old wall is not equally as solid as the modern cement mortar then the brickwork may part at the join between old and new as the wall expands and contracts. Also, if the wall lacks modern solid foundations, the lime mortar allows a certain amount of movement rather than cracking the bricks.

Interestingly, I dug up an old wall while investigating dead patches on our lawn. The lime mortar was still holding the bricks together even though it had a decidedly cheesy texture, and it could be scraped off so the bricks could be put to good use elsewhere in the garden. Old bricks and modern mortar on the other hand: the brick breaks when trying to hack off the mortar.

Not a brickie so I stand to be corrected.

Hmm. I find that unless you tie a wall together hard with steel ties, at some point it will crack unless its keyed into an existing structure.

So if the wll can be keyed in to the existing with ties or with proper binding, use modern.

Otherwise use a lime.