More lime if you're interested

People who have rediscovered the benefit of lime-based technology developed over thousands of years tend to be annoyingly evangelistic about it. 

I was taught the basics of lime and its uses by people who were taught by the very man who looked into the science of it during the conservation of the West Front of the beautiful Wells cathedral - Professor Baker. I never met him because I never worked at Wells.

Ordinary Cement mortars have been doing damage to limestones for only about 150 years, but the masons who continue to use it see stone conservators as untrained, mystical hippies who get in the way of serious builders and masons that just want the job done as quickly and as inexpensively as the contractor demands. 

In the old days, there were not many conservators who had a grounding in masonry, and they tended to give the others who did a bad name. Ordinary builders would groan when they were mentioned, and the general stereotype was of a woman with a degree in Art History, wearing dungarees and spending two months wielding a toothbrush or scalpel against a square yard of stone. They were seen more as archeologists than anything associated with serious building.

These days you cannot work on an historic building without knowing about lime and lime-based procedures, but only a few years ago, masons who were forced to use lime mortars on important buildings would wait until the conservation officer or architect's back was turned, then put handfuls of white cement into the laboriously mixed real thing, killing it off completely. The biggest obstacle was getting them to understand that mortars are not supposed to be used as glue, and the mortar should never be harder than the stone itself. The science behind it all could wait for them to muster any interest - or stifle any hostility - toward the whole process, just so long as they followed the instructions to the letter.

Of course you have to use ordinary cement in modern construction, but remember that concrete made from lime is nothing new. The very word 'concrete' comes from a reference to Crete, which had large quantities of volcanic ash used by the ancients to make their concrete set, even under sea water. There are concrete supportive pillars beneath the Great Bath here which are as solid as they were 2000 years ago, thanks to the addition of pumice.

There are only a few things you need to understand to get your head around lime. The first is the chemical make-up. A good mortar should have a mineral and chemical composition as close to the natural stone as possible. Over the years it bonds to the stone by replenishing the minerals leeched out of it by weathering, so it protects and strengthens. Lime is highly alkali, so it also neutralises the acids which accumulate on surfaces from a number of sources.

The key is crystallisation.  Sharp sand - or even sharper pumice - sets off a crystallisation process which takes weeks, months and years to develop, so the maturation of lime mortar is not measured in the couple of hours that ordinary cement  takes to chemically set.

The Victorians were entranced by the potential convenience of Ordinary Portland Cement and over a generation all of this ancient knowledge was forgotten. Now when you talk about it, it sounds like magic. 

In a way, it is.