Do not use lime mortars below 5C
If that means you have to put off work or cancel a winter job then that’s better than having your work be damaged by frost and you having to come back and re-do it at your own expense. Having a failure like that also can damage yours and your businesses’ reputation.
What happens when it’s cold
There is a significant risk of frost damage and strength development is significantly slowed. When water freezes it expands by 9%, this expansion will push a fresh mortar apart and if this happens it cannot be re-tempered, the work will have to be redone. During the summer your NHL3.5 mortar will normally take 3 weeks to get to a frost resistant strength, with the cold weather we’ve been having this winter it will take roughly double that time.
Accelerators for Lime Mortars
There are no ‘accelerators’ for Lime Mortars that we would advise using, they are commonly based on chloride salts which are still soluble after the mortar has set; this can lead to spalling of the mortar/render through salt crystallising on or under the surface. “Chloride Free” accelerators are not designed for use with lime and will normally cause more problems than they solve, this can include; stopping the mortar setting, severe loss of workability and creating a patchy finish.
The only thing we can advise is the use of Prompt in the mortar, this can help to increase the speed of set, however, if you want frost resistance within 24 hours you will need to use a mortar which is 4:1:15 (Prompt : Lime : Sand), this kind of mix ratio means you’re no longer using a Lime mortar, you’re using a Prompt mortar. Prompt is good for many things but it’s not a lime mortar; it’s less breathable, it’s less flexible and it’s harder.
Can I use a Pozzolan?
Pozzolans take 3-4 days to start reacting, they don’t accelerate the set but they do accelerate development of strength over time. We would advise not to use a Pozzolan in NHL mortars; the strength increase will be too high and lead to damage of the mortar or the stone around it. We would not advise a pozzolan and putty mortar in this weather either; the strength development is not fast enough and adding ‘more’ pozzolan will just damage the mortar through shrinkage and very high final strength.
For further information, guidance and advice please call us on 01208 79779 or ask a question via our Forum