Red mud spill - neutralising red mud alkalinity with calcium sulphate

Source of the photo: 
http://index.hu/belfold/2010/10/04/iszapomles_veszprem_megyeben/?p=0
Author of the description: 
GK
Red mud

Calcium sulphate transported by truck to neutralise the alkaline red mud flow in the local creek  Addition of calcium sulphate reduced the pH of the devastating slurry to 10.