Problems of pop-outs and associated iron staining on unplastered clinker ash/cement brick walls in Windhoek after a particularly wet rainy season were investigated. The problems were found to be due to sulphurous nodules present in the bricks. The nodules were the result of the burning of pyritic fragments present in the coal used as fuel for the local power station, which produced the clinker ash used in making the bricks. The nodules before expansion were found to be distinctly zoned, with centres consisting of free sulphur, amorphous to poorly crystalline iron sulphides (pyrite, marcasite and pyrrhotite) and unburned carbonaceous material, enveloped by a red-coloured zone of iron oxides which was in many cases surrounded by a silicate zone. In the presence of water the iron sulphides and sulphur became oxidized to iron sulphates and sulphuric acid. A resultant large increase in volume caused the pop-outs. The sulphuric acid dissolved iron oxides in the adjacent zone, enveloping the centre zone which, when free water was available, resulted in severe iron staining in the area around the pop-out.