The toxic sludge that plunged from an alumina plant and collapsed a dam wall in western Hungary on Oct. 4 has flooded three towns, killed four people and injured at least 120 as it spread to the Danube River. What exactly is this mysterious red sludge made of?

The sludge is aptly called "red mud" by those who work with it industrially, and is a solid waste product made during the production of alumina, or aluminum oxide. The stuff is made mainly of iron oxide particles, which give the slurry gets its hue, according to Chandra.

However, the chemical composition of red mud varies with the quality of the processing procedures it has undergone.

The process used to make alumina was developed by Karl Bayer in 1888, and consists of treating bauxite with caustic soda (sodium hydroxide) at high temperatures and pressures, according to "Waste Materials Used in Concrete Manufacturing" (Standard Publishers Distributors, 2002).

"The products of this reaction are a solution of sodium aluminate and an insoluble 'red mud residue' which is removed by decantation and filtration," Satish Chandra states in "Waste Materials."

Statements made by Hungary's National Disaster Unit (NDU) describe the sludge as caustic, slightly radioactive and as containing heavy metals that can burn through clothing and skin. It is toxic if ingested, and inhaling its dust could lead to lung cancer, according to the NDU.

It is unclear what caused the reservoir to break and release an estimated 1 million cubic meters (35 million cubic feet) of red mud liquid waste, which rose to a height of over 6 feet (1.83 meters) in some areas, into Hungarian towns and rivers, according to the NDU.

The NDU is currently working to neutralize the caustic sludge. Red mud can be neutralized with seawater, which will lower its pH and reduce its alkalinity, according to "Mine Wastes: Characterization, Treatment, and Environmental Impacts" (Springer, 2003).

Although it's a byproduct, red mud can serve several purposes. Ceramic materials can be made from red mud, and it's also used as a filler in mortars and road construction, according to Chandra. It is also used in the making of rubber and plastic products, and can be added to raw cement mix to change the consistency and color of concrete and bricks.

This article was provided by Life's Little Mysteries, a sister site to LiveScience.