production starts with mining bauxite, a
rock rich in aluminum oxide that also con-
tains a wealth of other elements, includ-
ing silicon, iron, and titanium. Workers
extract the aluminum with a combination
of treatments, including caustic chemicals,
heat, and electricity. What remains is usu-
ally red, because of the iron, but its exact
makeup can vary from region to region, de-
pending on the ore, making it still harder
to contend with. “The composition of [red
mud] varies so much it means one [type
of solution] will not work,” says Brajendra
Mishra, a materials scientist at the Worces-
ter Polytechnic Institute.
One approach that does seem to be work-
ing is tapping red mud as a source of scan-
dium, a rare earth metal used to strengthen
metal alloys. Researchers have recently
shown that scandium-aluminum alloys are
as much as 40% stronger than pure alumi-
num. That has manufacturers eagerly eye-
ing the alloy; aircraft manufacturers, for
instance, could use it to build planes that
have lighter aluminum framing and burn
less fuel. But scandium currently costs
$3500 per kilogram, so there’s plenty of in-
centive to find new, cheaper sources.
Scientists have come up with several
ways to purify scandium from red mud.
Balomenos’s group, for example, has shown
it can use both sulfuric acid and compounds
called ionic liquids to extract the rare earth.
Ultimately, red mud could meet 10% of Eu-
rope’s demand for scandium, Balomenos
says. Rusal, one of the largest aluminum
producers in the world, is already building
a pilot plant that uses related methods to
extract scandium from red mud at one of
its facilities in the Ural Mountains of Rus-
sia. But scandium makes up only about 140
parts per million of red mud, Pontikes notes,
so “99.99% of the residue” still remains.OTHER APPROACHES aim to use more of
the waste. One idea is to harness red mud,
which is typically 40% to 70% iron oxide,
to make iron-rich cements. The world uses
more than 4 billion tons of cement per year,
mostly as the binder in concrete. The most
common version is Portland cement, made
from calcium silicates that react with water
to make create a tough, hard matrix.
But in 2015, researchers in New Zealand
reported that by adding a common cement
additive called silica fume to red mud,
together with a modest amount of iron,
they could create a cement with roughly
the same hardness as Portland cement.
Pontikes and his colleagues are working to
extend these findings, by developing recipes
that would enable manufacturers to make
cement from a wide range of red muds
with varying iron concentrations. The team
hopes red mud could become a source of
both the extra iron added to their cements
and the alkaline compounds needed to cata-
lyze the hardening reactions.
In the meantime, Pontikes’s lab is already
producing about 1000 kilograms of iron-
rich cements per day. They’ve even used
their product for demonstration projects,
such as a 2-ton staircase made with ultra–
high-strength concrete. “This is no longer a
lab-scale endeavor,” Pontikes says. He’s be-
gun to talk with companies about making
the cement on an industrial scale.
Red mud could form the basis for other
construction materials. Pontikes and his
team have found that if they add about
10% clay and silicate minerals to red mudand bake the mixture in a furnace, they can
make bricks able to withstand 80 megapas-
cals of compressive force, 40 times more
than conventional bricks. They’re now look-
ing to scale up the technique, which could
be used to make everything from roofing
tiles to sidewalk pavers.
Because of its chemistry, red mud can also
capture and lock away carbon dioxide (CO 2 ),
the major climate warming gas. In Australia,
aluminum producer Alcoa bubbles CO 2 into
red mud, creating a mild acid that reacts
with the alkaline waste, forming carbon-
ate minerals that turn the red mud into red
sand that can be used to level road beds. The
company estimates that the red mud from a
single aluminum refinery can lock up 70,000
tons of CO 2 per year, equivalent to taking
more than 15,000 cars off the road.YET THESE GLIMMERS of progress could fade,
Balomenos says, just as earlier hopes have.
Since 1964, he notes, researchers have pat-
ented some 700 uses for red mud, includ-
ing tapping it to make decorative ceramics,
dyes, and even fertilizer. Yet just 3% of red
mud is currently recycled.
One major reason is that many schemes
envision using red mud to make commodi-
ties that are already cheap and produced
with methods that have been optimized
over a century or more. In addition, red
mud isn’t easy to handle. The iron industry
has shied away from extracting the metal
from it, for example, because the caustic
waste destroys key components in their
smelters. “The industry has iron ore avail-
able with much better quality,” Mishra says.
Balomenos argues that countries could
push progress by establishing zero waste
mandates for aluminum makers, or other in-
centives that force companies to recycle red
mud instead of letting it pile up. The Euro-
pean Union has considered instituting a tax
on waste deposited in landfills, for example.
But it hasn’t done so, and there appears to
be little appetite elsewhere for similar ideas.
Another obstacle, Balomenos says, is in-
ternational opposition to allowing hazard-
ous materials to cross borders. As a result,
it can be cumbersome and costly to move
red mud that contains even trace amounts
of heavy metals or radioactivity. For now, he
says, simply putting the waste in a landfill is
both cheaper and far simpler.
Finally, there is the question of con-
sumer acceptance. Even if scientists and
engineers manage to come up with a suite
of practical uses for red mud, consumers
still have the final say in whether they will
buy products with such a noxious start-
ing point. “Will you use roofing tiles made
with red mud?” Pontikes asks. “It’s up to
the market to say ‘yes.’” jSCIENCE sciencemag.org
A worker inspects ponds
holding 30 million tons of red
mud at an aluminum plant
in Hungary. A 2010 spill from
the ponds killed 10 people.NEWS | FEATURES | MUD21 AUGUST 2020 • VOL 369 ISSUE 6506 911
Published by AAAS