Polyaluminium chloride (PAC) is the go to chemical currently for use in rainfall activated systems on earthworks sites. Polyaluminium chloride (PAC) is a strong coagulant which uses aluminum ions as its active ingredient. Clays are generally negatively charged with clay particles which causes other particles to repel other particles i.e two negative charges will repel each other like two magnets; preventing settling. Aluminum ions are positively charged which means aluminum reacts with the clay and the charges neutralise each other, leading to a electrically neutral particle. The particles no longer repel each other and they will therefore settle. Mixing is useful to get efficient settling as the particles will collide and bind to each other, causing flocculation to occur with the generation of "flocs" which are larger and heavier than the individual particles.
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Polyaluminium chloride is used in the water treatment industry for the coagulation of
organic and mineral colloids prior to sedimentation and/or filtration. The aluminium
destabilises fine colloidal suspensions and promotes the forming together of large
conglomerations of this material bound in a chemical precipitate (called floc) which is able
to be removed from the water by sedimentation, flotation and/or filtration. In general,
polyaluminium chloride is preferred over aluminium sulphate if a larger, faster forming floc
is desired, as this may remove the need for a flocculant to facilitate sedimentation.
Alternative chemicals such as PolyDADMAC have also proven useful on difficult to treat clays and soils. PolyDADMAC is an organic polymer which acts as both a coagulant (it carries a positive charge) and a flocculant with its long polymeric chains able to bind multiple particles together.