Crustaceans make cement stronger

Concrete is second only to water when it comes to humans’ material consumption. And cement—a combination of powdered calcium oxide, silica, alumina, and other ingredients that makes up about 10–15% of concrete—binds the mixture together, making it critical to modern infrastructure. But processing the limestone (calcium carbonate) that goes into cement produces roughly 5% of total greenhouse gas emissions. Finding ways to minimize cement use is therefore of great practical importance.

A promising strategy for reducing consumption is making the material stronger so it doesn’t have to be repaired or replaced as often. Mostofa Haider at Washington State University and coworkers have now shown that they can increase cement’s flexural strength by up to 40%, and its compressional strength by up to 12%, by adding nanostructures made from the biopolymer chitin to the mix.

Nature has already demonstrated that incorporating chitin makes for strong composite materials: A typical crustacean shell is around 20–30% chitin. It’s an appealing additive because its electric charge can be controlled by adding different chemical groups to its surface; cellulose, another additive that has been recently studied, lacks that flexibility. When put in a cement solution, the charged chitin stabilized the cement particles through electrostatic repulsion. That effect slowed the drying process, which is desirable in construction settings because it gives workers more time to transport and pour the material.

The researchers tested cement samples with up to 0.1% added chitin. The material was in one of two forms: small nanocrystals (NCs), shown on the left in the figure, or larger nanofibers (NFs), shown on the right. The NCs extended the usual set time of about one to two days by up to 106 minutes; the NFs, by up to 78 minutes. That result points to the NCs as better stabilizers. The nanostructures have similar surface charges, though, so the researchers suspect that the difference is caused by the NCs having a smaller size and a more elongated geometry, which allows them to more effectively adsorb onto particle surfaces and prevents the formation of tangled fiber networks.

Adding chitin had little effect on the cement suspension’s flow properties, so it wouldn’t change the energy required for mixing. But it did affect the dried material’s strength. After 28 days, the flexural strength of concrete with NCs or NFs was about 40% higher than that of regular cement. In terms of compressional strength, the NFs came out ahead; they caused an improvement of up to 12% over regular cement, compared with 5% for NCs.

The seafood industry generates about a million tons of chitin waste annually. Repurposing that material could benefit the environment if it prevents the untreated waste from being dumped back into the ocean, as it sometimes is . In addition to polluting coastlines, the material can harm aquatic species by altering the chemical environment as it breaks down. (M. M. Haider et al., Cem. Concr. Compos. 132, 104623, 2022. )