New method stores CO2 in cement, enhances strength, durability

An engineering team led by Northwestern University has found a novel method for storing carbon dioxide (CO2) in concrete, a common building material.

This is done by utilizing a carbonated water-based solution throughout the manufacturing process instead of a still one.

The innovative method produces concrete with unmatched strength and durability and aids in sequestering CO2 from the ever-warming environment.

The technique achieved a CO2 sequestration efficiency of up to 45 percent in laboratory studies. It effectively trapped and stored nearly half of the CO2 introduced during the manufacture of concrete.

The cement and concrete industries contribute 8 percent of the world’s greenhouse gas emissions, and the team expects that their innovative approach will help offset these emissions.

The details of the team’s research were published in the journal Communications Materials .

Eco-friendly concrete advances

Concrete is one of the most utilized materials in the world, second only to water, and is an essential component of infrastructure.

In its most basic form, concrete is made by mixing water, coarse aggregates (like gravel), fine aggregates (like sand), and cement (which holds everything together). Researchers have been investigating different approaches to storing CO2 inside concrete since the 1970s.

“The idea is that cement already reacts with CO ₂ . That’s why concrete structures naturally absorb CO ₂ . But, of course, the absorbed CO2 is a small fraction of the CO2 emitted from producing the cement needed to create concrete,” said Rotta Loria, an assistant professor of Civil and Environmental Engineering at Northwestern, in a statement .

The carbonation of fresh concrete and hardened concrete are the two types of carbonation processes used to store CO2. The toughened method involves inserting solid concrete blocks under high-pressure injections of CO2 gas into chambers.

According to the team, workers inject CO2 gas into the mixture of water, cement, and aggregates as concrete is being made in the new version. A portion of the CO2 that is put into the cement in both methods reacts to form solid calcium carbonate crystals. Nonetheless, both methods have extremely restrictive features.

Researchers say their high energy consumption and low CO2 capture efficiency are impediments. Even worse: The resulting concrete frequently lacks strength, which limits its usefulness.

Enhanced CO2-storing concrete

The Northwestern team used a fresh concrete carbonation process to solve such challenges in their novel strategy.

However, initially, CO2 gas was injected into water that had been mixed with a tiny bit of cement powder, as opposed to injecting CO2 while mixing all the materials together. This carbonated suspension was combined with the remaining cement and particles to create concrete that, when manufactured, actually absorbed CO2.

The carbonated cement suspension has a much lower viscosity compared to the traditional mixture of water , cement, and aggregates used to carbonate fresh concrete. This allows for quick mixing and leverages the rapid kinetics of the chemical reactions, resulting in the formation of calcium carbonate minerals.

“The result is a concrete product with a significant concentration of calcium carbonate minerals compared to when CO ₂ is injected into the fresh concrete mix,” said Loria.

After testing, the team discovered that their carbonated concrete’s strength rivaled that of ordinary concrete’s endurance.

A typical limitation of carbonation approaches is that strength is often compromised by the chemical reactions. However, their experiments have shown that the strength might actually be even higher.

“We still need to test this further, but, at the very least, we can say that it’s uncompromised. Because the strength is unchanged, the applications also don’t change. It could be used in beams, slabs, columns, foundations—everything we currently use concrete for,” said Loria.

Researchers say that the findings highlight that, despite the well-known carbonation of cement-based materials, there is still potential to optimize CO2 uptake by better understanding the mechanisms involved in materials processing.