We bought a house with this problem (see photo), can anyone advise what to do about it? There is a bathroom inside this area, I'm assuming the blocks got damp because they didn't fix the leak in time. Everyone who has seen it advises not to sweat it, and just fill/plaster with concrete. But I'm confused by the fact that these blocks are very porous, and crumble easily. Maybe who has experienced something like this, it would be very interesting to hear opinions.
If there really was a leak and it was repaired, then make a repair compound of aerated concrete sawdust and aerated concrete glue. Take an aerated concrete block, put a wooden feather in the drill and start drilling holes in the block, collecting the resulting sawdust. After that, mix these sawdust with glue in a 1:2 ratio, plug with water and with the resulting mixture plaster the damage.