Effect of CO2 surface treatment on penetrability and microstructure of cement-fly ash–slag ternary concrete

Abstract Permeability is an important indicator of the durability of cement-based materials. Surface treatments are effective in reducing permeability, and thus improving durability. In this study, CO2 surface treatment, as a new method, was investigated explore the efficiency on cement-fly ash-slag ternary concrete. The results show that the CO2 surface treatment densified surface microstructure and reduced both air and water permeability of concrete. With increase in fly ash and slag contents, the efficiency of CO2 surface treatment in reducing concrete permeability was enhanced. The results of thermogravimetric analysis (TGA), X-ray diffraction (XRD), environmental scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) indicated that CO2 surface treatment consumed calcium hydroxide (CH) and formed calcium carbonate (CaCO3). Moreover, the calcium-to-silicon ratio (Ca/Si) of calcium silicate hydrate (C–S–H) decreased after surface treatment and CaCO3 was covered by C–S–H during further hydration.