Engineers at North Carolina State University have incorporated ground eggshells into magnesium, producing small metal samples that resisted crushing more strongly after they began deforming. The laboratory work found chemical bonding between shell particles and metal, but no measurable improvement in the finished material's hardness.
The research, led by Bharat Gwalani, an assistant professor of materials science and engineering, tested whether household waste could supply calcium-based ingredients for lightweight metals, according to Earth.com. The study appeared in the Journal of Magnesium and Alloys. Originally reported by Earth.com.
Kitchen waste goes through a spinning press
Eggshells consist almost entirely of calcium carbonate, a compound manufacturers also obtain from mined rock. Calcium can improve magnesium's mechanical properties, giving researchers a reason to investigate shells as an alternative raw material.
The team washed household eggshells, dried them for four hours at about 300°F (150°C), then ground them using a mortar and pestle. Researchers packed the powder into grooves cut into a magnesium cylinder about an inch (25.4 mm) across, with shells accounting for roughly 8.4 percent of its volume.
A die rotating at 300 revolutions a minute pressed into the cylinder, forcing the material into a rod. Friction and pressure raised its temperature to roughly 930°F (500°C), softening the magnesium without melting it.
The resulting rod contained evenly distributed shell particles, most measuring 2 to 4 micrometers across. Its metal grains averaged 8 micrometers, compared with 16 micrometers in plain magnesium processed the same way.
Strength gains appeared under microscopic crushing
Smaller grains generally strengthen metals, but tests with a diamond tip found no measurable hardness advantage for the eggshell-containing rod. A difference emerged when researchers crushed microscopic pillars carved from grains with matching orientations in the two materials.
Both sets of pillars started deforming at similar stress levels. After that point, the eggshell-containing pillars increasingly resisted compression, reaching peak strength of about 32,600 pounds per square inch (225 megapascals), against about 29,000 pounds per square inch (200 megapascals) for plain magnesium.
The researchers think shell particles encourage defects within the metal's crystals to accumulate during deformation. That buildup could make further compression harder.
Shell chemistry changed inside the metal
Some smaller shell fragments partly decomposed despite the overall processing temperature being well below the usual breakdown range for calcium carbonate. The team suspects hotter conditions at particle boundaries and abrupt pressure changes helped drive the reaction.
Measurements also indicated magnesium oxide around some particle edges. Researchers concluded that carbon dioxide released by decomposing shells most likely reacted with nearby magnesium, helping chemically bond the particles to the metal.
The reaction was uneven: larger particles remained mostly intact, leaving a mixture of unchanged shell, partially decomposed material and calcium-oxide-rich regions. The researchers tested only one rotation speed and feed rate, and did not measure energy savings against conventional production.
The study's purpose was to examine the internal changes, rather than maximize strength. Calcium remained largely bound in calcium oxide; the team did not detect Mg2Ca, a magnesium-calcium compound that can strengthen magnesium alloys.
Originally reported by Earth.com.