Mitigating the ecological footprint of alkali-activated calcined clays by waste marble addition

Low-CO2, Portland-free cement binders can be produced by reaction of calcined clays in alkaline solutions, although the environmental footprint associated with alkaline activators may represent a point of concern. The amount of alkaline activator, and its embodied CO2, can be reduced by partial replacement of calcined clay with waste marble. Fresh and hardened state properties, as well as compositional and microstructural evolution of alkali-activated blends of calcined clay and waste marble, were investigated by combining experimental methods with thermodynamic modelling. It was shown that substitution of calcined clay with 30% waste marble was beneficial to the workability and mechanical properties of alkali-activated binders and can mitigate drying shrinkage. The physical and chemical role of calcium carbonate and the composition of the reaction products were discussed, along with the role of carbonation reactions occurring by uptake of atmospheric CO2. This approach can contribute to both reducing the CO2 footprint of cement and upcycling potentially hazardous waste from the dimension stone industry.