In a more recent study, Gupta’s team checked to see what happens when carbon dioxide is sequestered in excavated soil to develop cement-lime-soil materials, which are then used to replace up to 25% and 50% of fine aggregates by mass in mortar. “When you sequester carbon dioxide in cement-soil materials, tiny crystals of calcium carbonates are formed, which reduce the fraction of medium capillary pores, densify the interfacial zones and thus improve the compressive strength,” explains Ashutosh Dwivedi, first author and PhD student at CST. Exposing the cement-lime-soil block to carbon dioxide increased their early-age strength by about 30%, the team found. Gupta explains that carbon dioxide exposure after preparation also cuts down their curing time – the time they require to harden before being used in construction.
The team has also developed 3D-printable materials made of excavated soil stabilised with a combination of binders like Portland cement, blast furnace slag (a granular calcium-silicate byproduct) and fly ash. They found that the non-expansive clay in the excavated soil acts as an excellent thickening agent (rheological modifier) and causes the material to display superior extrusion and buildability when compared to conventional cement-sand mortars. Using these materials could also reduce the amount of cement and natural sand needed in mortar by 30% and 50% respectively, they found.
Moving forward, the team plans to examine the effect of industrial and simulated flue gas – which contains a combination of gases like carbon dioxide, sulphur dioxide, nitrogen oxide and others – on the micro and macro properties of these newly formulated materials. This would help them understand whether gases other than carbon dioxide released by industries affect the materials’ carbon-capturing potential and engineering properties.
The team is in talks with a couple of major construction companies to apply these findings to their manufacturing plants. Gupta is also part of a national committee currently working on revising the standards for natural and recycled aggregates in cement-based construction materials.
“The climate change impacts of construction materials, exacerbated by increasing CO2 emissions and sand scarcity, are increasing every year,” he says. “Finding alternatives to natural sand is the need of the hour.”
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