What to use to fill gaps between bricks in fireplace (and other uses)?

[DIYNotIan]

Hi all

I have an old stone and brick fireplace and will soon be having a wood stove installed. There are a handful of gaps between bricks, some of which are quite deep. Would this be suitable to use to fill the gaps:

https://www.screwfix.com/p/cementone-general-purpose-mortar-grey-5kg/36857

Or should I use something more specifically heat resistant? There is of course a lot of existing mortar which I'm guessing is probably normal stuff but who knows?!

Many thanks

 

[DIYNotIan]

Hi again

I've read through the Q&As on that page and the chap says this is not suitable around fireplaces.

So just wondering what would be best... I've read lime based mortars are good for around fires, but they take a long time to cure and the install is quite soon.

Would a basic mortar be ok do you think and if so...:

- What's the best ratio?
- What type of sand?
- Do I need to use a plasticiser or any additives?

I can't work out if the ready made mortars in tubs are literally just cement and sand mixed, or if there's more to it?!

Also, would it be best to ram bits of stone or similar into the gaps so that they're not as deep?


Many thanks

 

[DIYNotIan]

Here's some photos of the holes in question: