Ferrous Sulfate — Frequently Asked Questions

To reduce hexavalent chromium. Portland cement contains traces of chromium in the hexavalent state from the raw materials and the kiln, and hexavalent chromium is a potent skin sensitiser responsible for chromate dermatitis in construction workers. Ferrous iron reduces it to the trivalent form, which is not a sensitiser. EU regulation has required the treatment since 2005.

Because the ferrous iron slowly oxidises to the ferric state in the bag and loses its reducing power. Once the additive is spent, the hexavalent chromium is no longer being reduced and the sensitisation risk returns. That is why bagged cement carries a shelf life for the chromate reduction specifically, independent of its setting performance.

Oxidation. The divalent iron that gives the pale green heptahydrate its colour oxidises in air to ferric compounds, which are brown. Discolouration is therefore a visible indication that reducing capacity has been lost, and material that has browned significantly should be assessed rather than used on the assumption that it is still active.

The iron is directly toxic to moss at concentrations that grass tolerates, and it also acidifies the immediate surface slightly, which moss dislikes. The blackening seen within hours is the moss dying. Iron simultaneously greens the grass, which is why lawn moss killers are typically ferrous sulfate based and give both effects from one application.