Sulphate attack is characterised by an initial horizontal cracking of the mortar joint. As it progresses it causes expansion and destruction of the mortar leading ultimately to structural failure.
It is caused by an excess of sulphates in the brick or other sources reacting with the cement in the mortar and typically affects S1 and certain F1 bricks in situations where saturation of the brickwork is common. Unfortunately there is no known cure but the Code of Practice for Masonry recommends the use of sulphate resisting cement in a (i) or (ii) mix to give additional strength to the joint when using bricks in vulnerable locations.