I've just started removing the mortar from quite a large area of brick wall due to the fact it was quite powdery. The Mortar rake on the grinder went through it like butter. However, when I went to remove mortar from the verticals it was so bad that, without effort it went clean through to the void at the rear of the bricks. What’s the best way of dealing with this? I'm considering using hydrated lime in the mix rather that 4:1 sand, cement. Don't know if that's wise.