Comparing the effects of in-situ nano-calcite development and ex-situ nano-calcite addition on cement hydration

A comparison between the effects of in-situ formation and ex-situ addition of calcium carbonate nanoparticles (NPs) on cement hydration is herein investigated. The in-situ formation was achieved by CO2 injection during mixing (doses of 0.1%, 0.3%, 0.5%, and 0.7% by weight of cement), while NPs of precipitated CaCO3 (0.1%, 0.5%, 1%, and 5% by weight of cement) were incorporated for ex-situ addition. The hydration kinetics were investigated through isothermal calorimetry, the hydrates were quantified using thermogravimetric analysis (TGA), and the microstructure was examined with scanning electron microscopy (SEM). Both the systems (in-situ and ex-situ) accelerated hydration and increased heat of hydration compared to a reference system without calcium carbonate additions. Increasing the dosage of ex-situ NPs lead to greater acceleration and energy release, while the effects of increasing CO2 dosage were negligible. However, increasing CO2 dosage resulted in increasing amounts of carbonate formation through CO2 mineralization. Unlike those of the control system, micrographs of the CO2-activated system showed evidence of C-S-H precipitation as early as within 40 s of the start of hydration. The experimental evidence is used to develop mechanistic insights into the two modes of NP incorporation in cementitious systems.