My misunderstanding of lime mortar

[malcolm]

I'm still building my wall very slowly using NHL3.5 because that's what the local builders merchants have. I've just knocked off a reference brick used to align the rest of the wall because I put it in the wrong place. I placed the brick 2 or 3 weeks ago but the mortar was still soft like damp sand. I noticed 1mm on the outside face of the mortar was quite hard, and indeed for the rest of the wall the mortar seems hard on the outside.

After some reading the hardness at the outside face was down to carbonisation. The wall carbonates at 1mm or 2mm per month. That's what's going to hold the wall up in the long term not the faster initial set offered by increasingly high NHL numbers. And I don't want a hard mortar. When I have to rebuild the wall again in 200 years time I would rather the mortar was soft.

Anyway I'm confused. I understand that higher NHL numbers result in less breathability. Presumably due to something added to make the initial set harder. Does that make any difference in the long term hardness of the wall if carbonation is the thing that provides the strength? Of is it just so more proficient bricklayers can lay 9 courses a day while I manage 0.4 courses. They use NHL 5 for chimney cappings. Why?

 

[a twig]

In my experience all NHL "set/cures" in a similar time frame, 30-90 days depending on weather etc.

That "crust" you saw will end up being the case all the way through eventually. Hydraulic lime mortars do get harder as they cure, which is why some of the conservation lot get nervous about it potentially pulling or cracking stone work, and have all got enthusiastic about hot lime etc again, the last advice I was was to always use the lowest NHL possible for that reason. Of particular note is the lack of consistency in rating - there were arguments a while back that the St Astier were overly hard and that their 3.5 was actually more like other brands NHL 5 for example.

NHL 5 for chimney cappings is because your primary concern is keeping water out and resisting erosion rather than breathability - I never really saw the benefit of using NHL 5 in that situation and usually ended up using a cement based flaunching.

 

[88v8]

I think St Astier has some sort of chemical set, it is very fast and can catch you out if you want to use the churn brush at the right moment.

The mortar will go off quicker if you damp the bricks as you lay them.

Ivor