As one of our most relied upon construction materials, concrete makes a significant contribution to our overall carbon emissions. Calcium-based substances are heated at high temperatures to form the cement, a process that produces carbon dioxide. But by slightly altering the quantities of materials used, scientists from MIT have uncovered a new method of cement mixing that could reduce these emissions by more than half.
Concrete is generally made by mixing gravel, water, sand and cement. To produce the cement, calcium-rich materials such as limestone are cooked up, typically with clay, at temperatures as high as 1,500° C (2,732° F). The energy required to heat up the mix combined with a resultant chemical reaction generates carbon dioxide, a process the researchers say is responsible for between five and ten percent of total industrial greenhouse gas emissions.
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By examining the makeup of cement, the MIT team, led by senior research scientist Roland Pellenq, found that reducing the ratio of calcium to the silicate-rich clay substantially reduced the output of carbon dioxide. Typically, calcium to silica ratios can vary between 1.2 to 2.2, though 1.7 is seen as the standard for producing cement. By compiling a database that compared the chemical makeup of the different ratios, the researchers determined that 1.5 ratio was in fact the optimal mix. According to the researchers, this slight change in calcium levels could reduce carbon emissions by as much as 60 percent.
Furthermore, with the altered ratio the mix proved to have a higher resistance to fractures, with the researchers claiming this is due to the molecular structure transforming from a tightly ordered crystalline to a disordered glassy structure. Pellenq describes the mix of 1.5 to be "magical ratio," with twice the mechanical resistance to fractures of normal cement.
The analysis of the cement mix was carried out at a molecular level, meaning the team will need to conduct further research to make sure it can apply to engineering scale applications. Pellenq says the formula could find a home in oil and gas industries, where more resistant cement could be particularly useful in preventing leaks and blowouts.
The research was published in the journal Nature Communications.