US scientists turn 95% calcium-rich eggshell waste into stronger magnesium alloys

U.S. researchers have demonstrated a way to turn powdered eggshells into stronger, harder magnesium alloys, using waste material to replace conventional calcium ingredients produced from mined ore.

The proof-of-concept study uses friction stir extrusion to incorporate eggshell powder into magnesium. The process generates calcium materials during manufacturing, bypassing the separate ore-processing step.

Magnesium alloys have applications across automotive, aerospace and biomedical industries. The researchers’ approach offers a cheaper, less energy-intensive route to improving their mechanical properties.

Eggshells replace mined calcium materials

Eggshells are 95 percent calcium carbonate, making them a source of calcium for alloy production. During processing, that calcium carbonate converts into calcium oxide and nascent calcium.

“For example, calcium carbonate and calcium oxide are important materials for manufacturing metal alloys,” said Bharat Gwalani, corresponding author of the paper and an assistant professor of materials science and engineering at North Carolina State University.

“But producing those calcium materials relies on a complex process making use of mined materials. We’ve demonstrated a technique that allows us to skip a step,” he added.

Instead of processing ore into calcium products before adding those products to metals, the researchers introduced finely ground eggshells directly into the manufacturing process.

“There are many benefits to this. There are fewer steps. You have a reliable, sustainable supply chain. Eggshells are inexpensive. And you use far less energy because you do not have to go through the process of creating calcium products from ore,” Gwalani said.

For the demonstration, the team focused on magnesium, whose combination of strength and low weight makes its alloys useful for electronics and aerospace equipment.

A spinning tool transforms the powder

The process begins with a cylindrical magnesium block. Researchers drill evenly spaced holes into it, fill them with finely ground eggshells, and place the block inside a steel cylinder.

A steel mandrel with a central hole is then lowered into the cylinder. Acting like a pestle in a mortar, it presses down on the magnesium while rotating at 300 revolutions per minute.

The pressure and rotation mix the eggshell powder into the surrounding metal. Friction between the eggshell particles and magnesium drives the conversion of calcium carbonate into calcium oxide and calcium, leading to the formation of Mg2Ca.

At the same time, the mandrel’s downward force pushes the processed material through its central hole. The result is an extruded rod of the finished magnesium alloy.

“Calcium is added to magnesium to improve its mechanical properties,” said Gwalani.

“We wanted to see if we could use biogenic waste—eggshells—to produce the necessary calcium materials during the manufacturing process,” he added.

Applications extend beyond eggshells

The researchers describe the approach as scalable and energy-efficient, with applications beyond this particular combination of waste material and metal.

Earlier this year, the team demonstrated another application of friction stir extrusion, using magnetic samarium-cobalt powder with scrap aluminum to produce magnetic composites.

The eggshell study is published in the Journal of Magnesium and Alloys. The work demonstrates how discarded eggshells can supply useful alloy ingredients directly during magnesium processing.