Controlling chemistry in sustainable cements aids durability
A new study reveals tactics to increase the longevity of common cement alternatives by manipulating their internal structure to control their properties.
Production of cement, the powder substance mixed with water and aggregates to make concrete, currently generates 5—8% of the world’s carbon dioxide emissions, but there are alternatives to commonly used ordinary Portland cement that could reduce this environmental burden. Alkali-activated materials (AAMs) are more sustainable than Portland cement because they are manufactured from industrial by-products such as blast furnace slag or coal-derived fly ash instead of limestone, but they can have comparable mechanical properties. AAMs were first used industrially in the 1970s and could reduce the carbon dioxide emissions associated with concrete production by 70% or more if they were more widely exploited.
AAMs and cements more generally react with atmospheric carbon dioxide during their life time in a process called carbonation. During carbonation, the carbon dioxide forms stable carbonate compounds with calcium, magnesium, sodium, or potassium within the material. However, carbonation can degrade concrete made with AAMs or Portland cement by changing their internal structure and pH and making them weaker.
In new research led by Claire White, professor of civil and environmental engineering and the Andlinger Center for Energy and the Environment, state-of-the-art analysis techniques were used to study the structural details of alkali-activated slag (AAS), a promising type of AAM. The team subjected several samples to intense carbonation conditions such as would be found in carbon dioxide rich environments such as geological carbon dioxide sequestration wells.
Eric McCaslin, who was a chemical and biological engineering graduate student and postdoc at Savannah River National Laboratory, and Michael James, an Andlinger Center associate research scholar, worked with facilities and scientists at Argonne National Laboratory to show how the nanometer-scale structure of alkali-activated slag samples with different magnesium contents evolves during the carbonation process.
The alkali-activated slag is made up of zones where the layouts of the atoms are different at the nanometer scale, and these differences impact the material’s properties. The zones include amorphous slag and an amorphous calcium-rich gel phase from which the material derives its strength, and which weakens when calcium is removed. It is very difficult to isolate and assess the unique atomic layouts of these different amorphous phases, but White and her team have shown it is possible using a synchrotron technique that employs X-rays to probe local atomic structures in heterogeneous materials.
During carbonation, the acidic carbon dioxide molecules dissolve in the cement altering the internal conditions. Acidification encourages the removal of calcium from the calcium-rich gel which weakens the cement, and calcium carbonate regions emerge.
The team’s analysis showed that magnesium is likely incorporated into amorphous calcium carbonate regions as they form, stabilizing them by hindering their crystallization. This stabilization reduces further removal of calcium from the calcium-rich gel, meaning carbon dioxide can no longer weaken the material. Greater magnesium concentrations therefore limit the amount of carbonation that can occur and increase the durability of the cement as less calcium can be removed from the strength-giving gel phase.
“Our findings are relevant for increasing the resistance of cements to carbon dioxide-induced degradation in places such as geological carbon dioxide sequestration and oil and gas wells,” said White. “However, they also provide clues as to which chemistries are best avoided if the aim is to maximize carbon dioxide uptake during carbonation curing of concrete.”
Likewise White explains that the presence of silica, which would be common under highly alkaline conditions, could suppress carbon dioxide uptake, so pH should be carefully considered. “Nano X-ray fluorescence revealed silicon atoms within the amorphous calcium carbonate regions, so together with magnesium the soluble silica is having a stabilizing effect on this phase,” said White. She considers the results of the study to be at least partially applicable to ordinary Portland cements.
More studies are needed to fully understand the intricacies of the phases of alkali-activated slag – and ordinary Portland cement – and the impact of external factors. “For instance, humidity and temperature are known to have a big effect on the carbonation of cement-based materials, so looking at these behaviors across a range of humidity and temperature levels will be important for alkali-activated slag,” says White. “Using large-scale facilities like those at Argonne to investigate industrial materials can help us improve their environmental impact and mechanical performance simultaneously.”
The article, Spatially resolved nanostructural analysis of disordered phases in carbonated alkali-activated slag, was published Dec. 11 in the journal Nature Communications. Besides White, authors include: Eric McCaslin and Michael James of Princeton University; Jonathan Almer, Zhonghou Cai, Peter Kenesei and Jun-Sang Park of Argonne National Laboratory. The work was in part supported by the National Science Foundation, Grant No. 1553607.