Lightweight concrete block that has cracked and fallen loose. I have scraped and knocked off everything loose around it. It is about ~5cm deep at the deepest point. What should I use to fill this in? What's the easiest way to go about it? I have only cast with regular C-mortar before when I cast new around the smoke pipe.
If it's a dry and nice space, gypsum plaster works well.
If there's a risk of moisture, I would use thin joint mortar, e.g., http://www.bauhaus.se/tunnfogsbruk-20kg-sack.html. It can also be applied in very thin layers (looks like you have rather thin plaster). If you use just a little more water than stated on the bag, it is very easy to work with.
Lightweight concrete prefers low-alkaline mortars, so common cement mortars are usually avoided.
Are 2 different rooms, one is a boiler room that is dry and warm. The other is a laundry room, but with a dehumidifier running constantly. How thick dare you build?
Are you completely sure that it is lightweight concrete blocks?
The structure suggests that they are cement blocks, and normally one does not build lightweight concrete blocks with cement joints and in that thickness?
Is it an exterior wall?
Is it a basement?
If it is a basement house, it is not uncommon for the basement to be built with concrete hollow blocks (even an advantage) and the rest with aerated concrete.
Aerated concrete works but concrete blocks are more suitable in basements.
Yes, not sure what you mean by "rebar". It's not clear in the pictures either. Normally, neither concrete hollow blocks nor aerated concrete blocks are reinforced. The reinforcement is usually placed in the joint about every 4th course.
It seems like you have moisture problems from somewhere. This must be addressed first, otherwise the problem will return and a newly plastered wall will not last long.
