Wednesday, 12 May 2010
We took advice about the cracks we noticed in the lime mortar bed yesterday. We were told that the shallower cracks can be smoothed out with a trowel. The deeper cracks can be very lightly damped down and then smoothed out. We carried this out before we started building up the blocks. The mortar could have cracked for a number of reasons: it could have been because the mix was too wet when it was laid, this will lead to excessive shrinkage (visible in the sign of cracks); or it could have dried out too quickly due to the warm weather and would have benefitted from being wet down to keep it moist for longer.
Almost the entire outer leaf was built up to two blocks high today. The mortar bed is now much harder, we have been protecting it from frosts completely covering it every night.
Originally we were proposing to use NHL 3.5 lime mortar, with a mix of 1 (lime) : 2 (sharp sand). However, we were advised to use NHL 5 for all the below ground work, particularly as we were using it to bed concrete blocks (which are very hard). The idea is that your mortar should always be softer than the material it is bedding. The other advantage of using NHL 5 rather than 3.5 is that it sets a lot faster. This means it should be sufficiently dry before we lay the limecrete slab in the first week in June. The mix we are using with the NHL5 is 1 (lime) : 2.5 (well graded sharp sand). With naturally hydraulic lime the sharp sand should be 3mm down, if we were using a lime putty we could use a courser sand.
Apparently last night it was the coldest night in May for about 14 years! I have now had to de-ice my windscreen two mornings in a row. We never thought we would be having problems with frost when starting out lime work in almost the middle of May.