Correct mix for repointing a slate slabbed internal floor

1.25K viewsLime Mortars
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Good evening, I have two questions please – Firstly, what is the best lime mortar mix to repoint a 200 year old slate slab floor? The slabs are laid on an earth base which is still very damp. I have removed all the pre-existing mortar from round the slabs as the floor had been repaired with all sorts of materials over the years, including lime mortar, various sorts of soft and very hard cement mixes and even what appears to be pink ‘bonding’ plaster. Several slabs have decayed on the edges and corners and so the gaps between the slabs vary dramatically, and some need big sections replacing and renewing. Secondly, is it wise to add a plasticiser where possible so I can get the mortar into the very narrow gaps with a hand held mortar pump? Thank you.

Justine Changed status to publish August 24, 2020
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Hi Bob,

I assume you’re the gentleman I spoke with on the phone this morning?
Due to the damage to the slate an NHL3.5 for repointing is going to be more appropriate than NHL5 in this scenario. A good quality coarse sharp sand is highly advised mixed at 2:5. It will need to be cured for longer than an NHL5 but should be more than strong enough by the times it’s cured.
Plasticisers should not be required for a lime mortar like this and would likely not help with a skeleton type pointing gun. Mortars don’t often run through pointing guns without being designed to, we can offer a pre-blended product if you would prefer to use something like that.
Kind regards, Edd.

Justine Changed status to publish August 24, 2020
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