Alkali-activated calcined smectite clay blended with waste calcium carbonate as a low-carbon binder

Abstract Reconciling the need of producing reliable building materials, with that of drastically cutting greenhouse gas emissions represents one of the current fundamental technological and societal challenges. To this end, we assessed the performance of a binder alternative to ordinary Portland cement (OPC) based on the alkali activation of a blend of locally sampled, impure calcined clay of smectitic composition, with waste calcium carbonate from the marble industry. The microstructure of the final product was investigated by means of scanning electron microscopy and X-ray tomography, and the phase composition by X-ray diffraction, implementing the PONKCS (partial or not known crystal structure) approach. Results of compressive strength tests show an adequate mechanical performance of these materials, with values of the compressive strength as high as 60 MPa after 20 days under room temperature curing. Replacement of a small quantity of the alkaline activator with sodium citrate induces enhanced workability and mechanical performance, reducing the amount of entrained air voids. The use of alkali activated calcined smectite clay – waste calcium carbonate blends represents a valuable complementary approach with respect to alkali activation of metakaolin and calcined kaolinite clay – OPC blends, towards the development of carbon-free binders.