What can I treat this with

Rusty190

to stop it crumbling?
It's a sheared off brick face which developed after new back door fitted in an old house.
I just want to stop the crumbly bits dropping off. 

WIAWSNB

A soaking coat of SBR? Or Stabilising Liquid?
PVA, slightly diluted, should do it too.
What's going to go over it?

ButterCheese

Quick and easy method:  Some form of spray waterproof primer.  For best results, a mortar mix and try to build it up to the rest of the wall.  You could then paint the whole thing to get a better match

Rusty190

Nothing is going over it - it's the outside bit of the doorway (? jamb).
I wondered if there was some product that I can paint on which will dry matt and just stop the crumbly bits dropping off.

stuart45

Can you replace the brick?

WIAWSNB

Rusty190 said:

Nothing is going over it - it's the outside bit of the doorway (? jamb).
I wondered if there was some product that I can paint on which will dry matt and just stop the crumbly bits dropping off.

Well, the usual treatment for porous crumbly masonry is Stabilising Solution, such as Everbuild 406. 
Once dry, overpaint.

stuart45

Ethyl silicate is used for stone and old porous brick consolidation.
However, intros case a replacement brick might be easier.

Rusty190

stuart45 said:

Can you replace the brick?

Sadly - not me. I don't have those skills though, I fully appreciate this is what really needs to happen. Will look at some of those other suggestions. Thank you all. 

Rusty190

stuart45 said:

Ethyl silicate is used for stone and old porous brick consolidation.
However, intros case a replacement brick might be easier.

I looked at Ethyl Silicate after your post @stuart45 and it looks to be exactly what I need but, blimey, it's expensive stuff.
Any option b recommendations for someone on a budget? 

WIAWSNB

Stabilising Solution - from less than a £enner for 500ml (eg Rustin's). Or around £15 for 5 litres (other makes) if you think you'll have future uses for it.
Make sure brick is bone dry, so it penetrates as far as possible - this hot spell is ideal!
Apply liberally and repeatedly so it soaks in. Clean the ground in front of the brick, and collect and reapply run-off. Observe what's happening to the surface - if it 'absorbs', keep applying until it doesn't!
Wipe away any runoff using a damp cloth before it dries. 
Jobbie should be jobbed.
Are you planning to overpaint? That would be ideal.

stuart45

An old school brick sealer was a 50/50 linseed oil and turps mix.