first aerated concrete blocks...

[anra]

Here's what it says on the manufacturer's website:

Uneven surface?
Answer: If the surface is not completely flat or level, the first layer of the multiplate should be laid on soil-damp mortar. It may be possible to use a base beam of 2400 x 200 x 75 mm, or 2400 x 200 x 100 mm to make the surface flat. This should also be laid on soil-damp mortar.

Can anyone explain what exactly soil-damp mortar is?

 

[anaitis]

A consistency designation. The mixture is given just enough water so that it resembles soil.

 

[anra]

Ahh, thanks!