For decades, scientists have been looking for more environmentally friendly and robust alternatives to Portland cement – the main product used in building construction. Current methods of producing cement contribute to greenhouse gases in two ways. First, through the production of carbon dioxide when calcium carbonate is thermally decomposed, producing lime and carbon dioxide. Second, through the use of energy, particularly from the combustion of fossil fuels.
These researchers have now unearthed three new types of concrete. One uses bacteria, another has a structure similar to a human’s femur, and the third is based on a mistake previously thought to have occurred in ancient Roman building techniques.
Basilisk Self-Healing Concrete
While not as easy to say as ‘Portland’ cement, this newly developed concrete gets its self-healing properties from bacteria mixed into it. These bacteria produce new limestone when the concrete cracks, allowing it to completely mend the cracks without needing concrete reapplied.
Here’s how it works. Let’s assume a floor in a building built with BSHC cracks due to traffic vibrations around the block on which the building sits. Repair crews can simply wet down the cracked areas and the bacteria in the concrete will convert nutrients to calcium carbonate (limestone), which fill in the crack and heal it.
Closer to home, imagine your driveway as poured BSHC. Stress cracks from vibrations, weight, or tree roots will heal themselves when it rains, or you spray it with water. No more having to hire contractors to come in and patch and seal it.
Hollow Concrete
Engineers at Princeton have developed a “hollow” concrete that is 5.6 times stronger than regular concrete. The team was inspired by human cortical bone, the dense outer shell of human femurs that provides strength and resists fracture.
Cortical bone is made up of elliptical tubular components known as osteons. The osteon’s hollowed-out shape (think of a honeycomb with elliptical shapes rather than octagonal holes) keeps cracks from progressing. Cracks only go until they reach the first osteon and then stop, allowing the concrete block to have only small cracks that are much more resilient to pressure than longer ones would be.
This seems contrary to logic, as hollowing out the inside of concrete seems counterproductive, but lab tests have proven otherwise. Further testing of the orientation, size, and even shape of the tubes is being done to see if its strength can be increased even further.
One of the other significant advantages of this concrete is that it eliminates the need for additives like fibers or plastic for added strength. The geometric design outperforms any other concrete that includes these environmentally toxic elements.
Do As The Romans Did
Have you ever wondered how the Roman viaducts, that still function today, have not crumbled into dust millennia after they were constructed? What about the Parthenon or even the Roman Colosseum? Scientists have spent decades trying to figure out what makes Roman concrete so stable, and believe they have finally discovered what gives it so much resiliency.
The scientific community refers to Roman concrete as pozzolanic concrete. Researchers used to believe the secret to its durability was a mix of volcanic ash and lime, but a team from MIT has discovered the secret is something called ‘lime clasts.’
Lime clasts are white inclusions visible in the concrete that were once thought to be a sign of sloppy mixing or inferior materials. Slaked lime, a paste-like substance formed by combining lime with water, was thought to be the source of these clasts, which were, in turn, not well-blended into the mix. But how could a society known for its meticulous construction allow for such low-quality mistakes?
They wouldn’t, according to Admir Masic, professor of civil and environmental engineering at MIT. Using advanced imaging and chemical mapping techniques, the team at MIT discovered that the lime clasts weren’t accidental at all. They were a deliberate addition that gave the concrete a self-healing ability.
Instead of lime slack, the Romans used lime in its more reactive form, quicklime. The lime clasts were formed when quicklime was added to concrete heated to extremely high temperatures (a process called “hot mixing”). Exothermic reactions occurred that significantly enhanced the properties of the concrete and formed compounds that contributed to its durability.
Using higher temperatures also reduced curing and setting times by accelerating these reactions. The result – faster construction.
Like the Basilisk concrete, the Roman concrete is self-healing. As tiny cracks form over time, water that passes through comes into contact with the lime clasts and forms a calcium-saturated solution that recrystallizes and fills the cracks, preventing them from spreading.
A more durable concrete means the service life of buildings and other structures made with it will be longer. Masic’s lab is also working to develop a concrete that can absorb carbon dioxide. It hopes to couple this with the stronger Roman concrete production method and have an even bigger impact on the environment.
Better for The Environment and Your Pocketbook
Given that cement manufacturing accounts for 8% of global greenhouse gas emissions, innovations like these are critical to developing a more resilient infrastructure while contributing to carbon reduction efforts.
Self-healing concrete will have an extended lifespan and significantly cut down on the need for expensive repairs. It will also have lower maintenance costs and minimal repair downtimes. The result is lower CO2 emissions as less concrete will need to be produced, a reduction in the amount of steel reinforcement required, and ultimately the elimination of toxic waterproof coating production.
Homeowners will also benefit from their self-healing properties, which will reduce construction maintenance costs and the hassles that come with frequent repair needs.
You can read more about the research and development of more environmentally friendly concretes (one researcher is looking at how coffee grounds may make a difference), by visiting any of the sources listed below.
Sources: earth.com, BGR.com, Princeton.edu
Fascinating story about the Roman builders