Friday, 21 June 2013

It's either lime mortar or it's not.

I live in Oxfordshire and like many other residents am acutely aware of our national hertitage because I'm surrounded by drystone walls and stone built properties. However, as someone who has been working exclusively with lime mortar for well over ten years, I probably see the built environment a little differently to most.

The issue I have is with newly built houses and extensions made of Cotswold stone. What I see is creamy mortar and lovely stone which both provide the impression of traditional craftsmanship. However, inherent in the build is one big problem and that's white cement. The issue is the fact that cement and natural stone should never be combined - ever. The reason is that buildings put together with natural stone need a way in which moisture can be wicked away from the surface of the masonry. Unfortunately, once something's been built with cement mortar, this is made impossible. This is because moisture - instead of being pulled away by porous lime mortar joints - evaporates away from the face of the stone leaving behind harmful chemical deposits. In addition to this, moisture unable to evaporate during winter will freeze causing further damage.  The result is eroded stone which must be chisled out and replaced. Furthermore, cement mortar binds like glue to stone  and this makes any repointing impossible stone replacement very difficult.
Although some tradespeople are aware that only lime mortar should be used when building with stone, they often lack the skills to work with pure natural hydraulic lime (NHL). Therefore a common practice of combining cement with natural hydraulic lime has emerged - this process is known as gauging and it's bad form. Unfortunately, the many builders to whom I talk have no issue with this because in their world both NHL and cement both do the same thing, i.e. bind bricks and stone together. The rationale behind gauging is that it keeps mortar porous and flexible whilst enabling a quick set. Although true, it is a fact that both are mutually exclusive products and should never be combined.

My argument is this: champagne and Newcastle Brown are both alcoholic and can do the same thing, i.e. get you drunk, but you wouldn't want them in the same glass.

The bottom line is that cement mortar and lime mortar are intrinsically different from one another and combining them creates a mortar which is neither something nor nothing. Cement contains no free lime because its all burnt away and it will not absorb carbon dioxide from the atmosphere. In direct contrast, all limes (whether they're NHLs, putties or hot-mixed mortars) will carbonate. 

No comments:

Post a Comment