I shall stick my head above the parapet and call myself an expert ...
There is already mortar between the bricks, so the pointing is not
there to stick the bricks together. It is there as a sacrificial
Rain (water, salt, acid ...) is driven into the brick surface and then
when the weather improves, leaves by the easiest escape route. If the
pointing is weaker than the brick, then the easier route is through
the pointing. Over the (many) years this causes the pointing to wear
away and eventually the bricks will need repointing. This is a much
better situation to be in than if the pointing is stronger than the
bricks because then the surface of the brick wears away leaving the
soft core exposed which wears away quickly. OUCH!
Consider three situations:
Old soft bricks, pointed with lime mortar. Lime mortar is weak, even
with a pozzolan added, so the bricks will be OK.
Modern, hard bricks pointed with cement. So long as the cement mix is
fairly weak this should be OK too. I'm not a cement expert so I assume
that others know better than me and that a that a
1cement:1lime:6aggregate mix is OK. (The lime is just there as a
plasticiser and a red herring)
Old soft bricks pointed with cement. The cement is stronger than the
bricks so producing the effect which I can see outside the window as I
sit here. An old red brick wall has a grid of cement pointing but the
bricks in between the pointing have worn back by about 10mm since it
was done. This is a problem which has shown up particularly in the
last 50 years, partly because late 20thC builders have no experience
of lime mortar, but also because Portland cement today is MUCH
stronger than the cement of 100 years ago.
The thing to understand is that a weak mortar is not a poor mortar.
The mortar must be weaker than the brick.
I shall create another message about pozzolans which seem to be
causing a lot of confusion.
~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plasterwork, plaster conservation
/ ^^ \ // Freehand modelling and pargeting
|____| www.kettlenet.co.uk 07976 649862