rafter size and spacing , lean to.

[Flyfisher]

i have heard tales of adding portland cement to lime mortar to help it go off quicker , ever tried this?

Err, you might want to remove that post Rob before the SPAB police get you. I believe it is a complete no no.

A 1:3:9 mortar mix was required as part of the LBC for recent work on our G2* house. It was only to be used for new work, not as a repair mortar.

I've read that the high purity of modern lime and well-washed sand can give rise to problems in lime mortars due to greatly lengthened chemical setting times. Traditionally, the natural impurities acted as a pozzolan that ensured a relatively fast chemical setting time. I also recall reading somewhere (here perhaps?) about adding soil to lime mortar mixes, presumably to overcome this modern purity.

It's ironic that our modern obsession with purity, analysis and reproducibility might actually work against the very characteristics we now prize in lime mortars compared to the way they were historically mixed with slaked lime from a shallow pit and unwashed sand straight from the ground or river bed. Perhaps this accounts for lime's reputation as being a bit of an art rather than a science?

Some very interesting reading here:
http://www.ihbc.org.uk/context_archive/53/Limemortar.htm

 

[MdB]

Ok. It looks like I am wrong but didn't someone in the past quote a report which showed that mixing cement into lime made the result worse than lime on its own or cement on its own?

Also, what is the difference between white cement and bog standard blue circle grey cement? Is there an apart from the colour?

 

[Flyfisher]

I don't know what makes white cement white, but my 1:3:9 mix was specified as being similar to the BS 1:2:9 mix but with extra lime to compensate for the extra hardness of white cement compared with OPC and to make a 'fatter' mix that wouldn't need any plasticiser additives.

What I do know is that I've had 3:1 lime mortar fail during winter frosts even though the work was done during the summer. Based on what I've read I now suspect this was because the purity of the mortar was such that it dried out before the chemical setting took place. Then the rain and frost made it crumble. Perhaps I should have thrown a spadeful of soil into the mixer or used sand from our river!