Scientists who tested eggshells as cement substitute found an unexpected result: Adding 10% processed eggshell powder made mortar stronger, but increasing it further had opposite effect

Waste eggshells could have a useful second life in construction, according to a study that tested processed eggshell powder as a partial replacement for cement in mortar. Researchers found that mortar containing 10% calcined eggshell powder recorded a compressive strength of 54.7 N/mm² after 28 days, compared with 52.4 N/mm² for conventional mortar without eggshell powder.

Read more - Mushrooms sent to space return to Earth & end up in pasta: Australian researchers send lion’s mane, turkey’s tail and cordyceps to the ISS, then watch what happens when they return

Published in Discover Concrete and Cement under the title Transforming cement mortar performance: impact of eggshell powder as an eco-friendly cement substitute, the study examined different levels of eggshell powder replacement and assessed properties including compressive strength, water absorption, bulk density and microstructure.

10% eggshell powder produced the highest strength

The researchers tested mortar mixtures in which eggshell powder replaced cement at levels of 2.5%, 5%, 7.5%, 10% and 12.5%.

The 10% mixture recorded the strongest performance among the tested formulations. Its compressive strength was 41.80 N/mm² after seven days of curing and increased to 54.70 N/mm² after 28 days.

Read more - Driver lost both hands in construction site accident, but refused to give up: From spending weeks in a hospital bed to getting back behind the wheel & supporting his family

By comparison, the conventional mortar reached 52.40 N/mm² after 28 days.

However, increasing the eggshell powder beyond 10% did not produce further gains. At a 12.5% replacement level, the 28-day compressive strength fell to 45.30 N/mm².

How researchers converted eggshell waste into powder

For the experiment, researchers collected discarded eggshells from a canteen and restaurants in Dar es Salaam, Tanzania.

The shells were first washed and dried at 90°C for five hours. They were then heated to 800°C for 1.5 hours before being ground into a fine powder and passed through a 100-micrometre sieve.

The resulting calcined eggshell powder was used to replace a portion of cement in the different mortar mixtures. Samples were subsequently cured in water and tested after seven and 28 days.

Why eggshells can be used in cement-based materials

Eggshells contain a high proportion of calcium-based compounds. According to the study, the calcined eggshell powder contained about 80.22% calcium oxide.

The researchers used X-ray diffraction, infrared spectroscopy and scanning electron microscopy to investigate the composition and internal structure of the material and the hardened mortar.

The analysis identified hydration products such as calcium silicate hydrate, portlandite, ettringite, monosulfoaluminate and calcite. These materials are associated with the development and structure of cement-based products.

More eggshell powder was not necessarily better

The study found that the physical properties of the mortar changed as the proportion of eggshell powder increased.

After 28 days, water absorption rose from 6.7% in the conventional mortar to 8.6% in the mixture containing 12.5% eggshell powder. Bulk density also declined, falling from 2.1 g/cm³ to 1.38 g/cm³ at the highest replacement level.

The researchers associated these changes with increased porosity. This also helps explain why compressive strength declined when the eggshell powder content exceeded the 10% level.

Could eggshell powder reduce cement use?

The findings suggest that a controlled quantity of calcined eggshell powder could partially replace cement in the mortar tested in the study.

The 10% replacement level offered the strongest balance between cement reduction and compressive strength among the formulations examined. However, the findings do not establish that eggshell powder can replace cement at higher proportions while maintaining the same performance.

The researchers also highlighted the need for further research into the material's long-term durability and performance under real-world construction conditions.

Turning food waste into a construction material

The study adds to research exploring ways to incorporate waste materials into construction products while reducing reliance on conventional cement.

For eggshells, the potential lies in processing a commonly discarded material into a calcium-rich powder that can be incorporated into cement-based mixtures. More testing will be needed before such applications can be assessed for large-scale or structural construction.


Inputs from TOI