Making cement is one of the dirtiest jobs in heavy industry accounting for roughly 8 percent of the world’s carbon dioxide emissions, much of it from burning limestone at very high temperatures.
That’s what makes an odd little experiment worth a look.
The claim is simple. Take waste eggshells, clean them, dry them, grind them into a fine powder, and use that powder in place of about 10 percent of the cement. Test the result at 28 days, the point at which concrete has done most of its early hardening, and the eggshell batch bears more weight before it cracks than the plain batch beside it.
Why eggshells and cement are closer cousins than you’d think
The surprise fades once you know what an eggshell is made of. Eggshells are primarily composed of calcium carbonate (CaCO3), a material similar to the limestone used in cement production, as civil engineering researcher Blasius Henry Ngayakamo puts it in a 2025 study. Limestone is the raw material cement is made from. So the powder you’re adding isn’t a foreign body. It’s a close chemical relative of what the cement started as.
Eggshells are an abundant poultry-industry waste. Using them to replace some cement means less limestone quarried, less fired in a kiln, and less waste sent to landfill. The chemistry gives the notion a real footing before a single cube is ever poured.
Where the stronger claim gets ahead of itself
In Ngayakamo’s tests on mortar cubes, the results indicate that CESP enhances compressive strength up to an optimal replacement level of 10%, achieving maximum values of 41.80 N/mm² at 7 days and 54.70 N/mm² at 28 days. That sounds impressive on its own. Set the 28-day figure against the plain control, though, and the picture softens. The author calls the peak at 10 percent a modest increase over the control sample’s 52.4 N/mm². From 52.4 to 54.7 is a gain of about 4 percent. Real, measurable, but small.
A second study, this one on high-strength concrete rather than mortar, points the same way. A team at Universiti Malaysia Pahang found the mix containing 10% eggshell specimen achieved the highest compressive strength of 68.4 MPa at 28 days. Again the highest, but only just, edging past the control. These are different mixes with different baselines, so the two numbers can’t be compared directly.
What they share is the shape of the result: ten percent wins, and it wins by a little.
So the direction is right. The eggshell batch does tend to edge out the plain one at 28 days. Any hint of a large margin, though, is more than the evidence will carry.
Past 10 percent, the trend reverses
More is not better. Ten percent looks like a sweet spot, not a floor you can keep raising. In Ngayakamo’s study, beyond 10%, strength declined, likely because the extra powder left more tiny holes in the mix and weakened the bonding. At 12.5 percent replacement the strength dropped to 45.30 N/mm², below the plain control rather than above it.
A separate group publishing in the journal Materials saw the same slide when they pushed harder, testing 0, 7.5, and 15 percent. Their 28-day strengths came in at 54.8, 43.4, and 35.5 MPa. Taken together, the studies sketch a hump: helpful up to around 10 percent, unhelpful after it.
What it would take for this to matter
A 4 percent strength bump on a lab cube is not, by itself, a reason to change how the world builds.
But the real pull here isn’t strength at all. It’s the carbon and the waste. If a tenth of the cement in a mix can be replaced by a material that would otherwise be thrown away, and the concrete performs at least as well, that’s a small dent in one of industry’s biggest emissions problems.
Each of these is a single study on a specific mix, cured and tested in a lab, not a settled property of eggshell-cement that holds everywhere. The best result sits near 10 percent in the mixes tested so far, the gain is modest, and the strength falls off if you overdo it. None of that adds up to a finished technology. It adds up to a promising result that has held up enough times, in enough hands, to be worth watching.
