The other day someone asked me about what ratio they should
use for a lime mortar to repair some fragile masonry which has suffered in past
from erosion due to hard cement repairs. We rapidly got onto the subject of
using a traditional 1(lime):3(sand) mix. This led to a discussion on ratios and
limes. A suggestion had been to use a hot lime, and in recent years I have
found that this now common phrase can mean different things to different
people, depending upon their understanding and knowledge of it.
For those who are new to my blogs, or new to lime, you may
want to take a look at my other blog ‘the facts about lime’. This should give
you the basic facts on what lime mortar is and the different types:
Personally when I look at what the mortar should be for a
specific project or area then many different things should be taken into
consideration. The most important rule here is that the mortar should never be
harder than the masonry, brick or stone, as this will result in damage to the
masonry and create various other issues.
If you look in the traditional text books it talks about a
mortar mix of 1 (lime):3 (sand) and this was often recommended as being the
standard mix before any adjustments in materials or ratios were required to
suit the individual project. This mix has remained in use since by a surprising
amount of people, and to some people this may sound a reasonable mix. However,
the problem with using this ratio with modern hydrated NHL’s (Natural Hydraulic
Lime) is that this does not create the same mixture of material that the
original 1:3 mix was intended to produce.
When lime was delivered to a building site in the 18th,
19th or early 20th century, the lime would have been
quicklime and would be mixed on site. These days finished lime such as bags of
NHL are brought to, and mixed, on site. You may say what’s the big issue with
this? Well the main issue is that when mixed, the quicklime will expand during
slaking, this will increase lime ratio in the mix, especially if it is sand
slaked, which was one of the most common methods. This resulted in a high lime
content compared to the original mix ratio, compared with the material made
from a finished lime such as an NHL, this will not change in volume during the
mixing process. Therefore this makes the mix higher in sand content than would
have been planned.
The diagram below hopefully helps to describe the different
types of lime during mixing a 1:3 mix:
As can be seen in the sketch
above the difference between the two mortars is quite a lot. I can hear some
people saying “but I need a hydraulic mix- this 1:3 business is all very well
and good for scenarios where you can just use a non-hydraulic mortar, but what
about stronger more practical mortar?”.
During the 19th
Century if the quicklime was not a good hydraulic lime such as Arden Lime, it
was common to gauge the non-hydraulic lime with a pozzolan such as smithy ashes
or hydrated hydraulic lime. This gave the mixture the benefit of the free limes
of the quicklime and also the benefits of the strength of the NHL or pozzolan.
This is where modern lime mixtures look like they are heading, where the
hydrated lime is used in the way that you would use a pozzolan. I believe
Historic England will be issuing guidance on the use of pozzolans shortly to
encourage the use of hot limes.
See my other blog about
strong lime mixes used in the industrial chimneys of the 19th
Century for some of these mixes:
An example of a hot mix gauged
with an NHL which seems to work for some people is 1 (quicklime): 1 (NHL): 6
(sand). This is such an interesting subject, as some people develop what works
well for them, while others will go another way, often neither way is wrong as
long as the main principles are respected. I suspect another blog will come the
subject of mixes.
Thank you for reading.
James
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