Closing empty conduit slots in shell construction - which mortar?

Good evening forum members,

we are allowed to seal the slots of the empty conduits at the interior walls ourselves, and it is not as simple as it seems. If you ask the "www," there is a range of answers on this. However, whether they are technically correct is mostly questionable when you think about it more closely.

For the exterior masonry, we closed the slots with quick-mix LM5/21, since we have perlite-filled hollow bricks, so the insulating properties were the main focus. We noticed that this mortar shrinks (significantly) when drying and repeatedly left cracks. Unfortunately, there was no alternative mortar for us because of the mentioned insulating properties of this lightweight masonry mortar. Now we have looked at all these slot mortars that are offered. The prices range between 20 and 30€ per bag. Of course, one wonders whether they are necessary or if there is a technically equivalent alternative at a lower price.

Unfortunately, our plasterer is already on summer vacation and will only be available again at the end of August; until then, we unfortunately have to be finished. Which mortar did you use? Was a mortar perhaps even recommended to you by your plasterer, or did you receive important tips on this topic? It would be great to get some helpful information here.

Thanks in advance for reading/responding.

There are good reasons for slot mortar. It also matches the thermal insulation of the brick easily. It is a bit unusual to mix, but you get used to it.