Effect of Limestone Powder Content on the Early-Age Properties of CO2-Cured Concrete

Abstract

The effect of limestone powder content on the CO2 curing degree (the carbonation rate after the CO2 curing process), compressive strength, reaction products, and microstructure of concrete cured in a CO2 environment were investigated in this study. Various proportions of limestone powder (, 1, 2, 3, 4, and 5%) were blended in concrete as partial mass replacement for cement. Concrete specimens with dimensions of φ5 mm×1 mm were manufactured and initially preconditioned at 2±2°C with a relative humidity (RH) of 65±5% and circulated air for durations up to 5 h. CO2 curing was then carried out for 1–3 h in a chamber at a pressure of .2 MPa. Results showed that concrete incorporating limestone powder demonstrated a higher CO2 curing degree as compared with those without limestone under the same conditions. The remaining water-to-binder ratio was also critical for the CO2 curing of concrete regardless of the incorporation of limestone powder. There may exist an optimal remaining water content or remaining water-to-binder ratio for which fast diffusion and dissolution of CO2 results in the highest curing degree of concrete. Concrete incorporating limestone powder demonstrated higher optimal remaining water-to-binder ratio as compared to those without limestone. Quadratic response surface analysis indicated that a combined effect of the limestone powder content and the remaining water-to-binder ratio, rather than the individual factors, play a critical role in improving the CO2 curing degree of the concrete specimens incorporating limestone powder. The microstructure and reaction products of the CO2-cured concrete were characterized by thermogravimetric analysis (TG), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and scanning electronic microscopy/energy-dispersive spectroscopy (SEM/EDS) examination. It was found that limestone powder with a maximum particle size of 2 μm could serve as a nucleus for precipitation of reaction products, especially calcium carbonates, and induce the consumption of calcium silicates (C3S and C2S). Incorporating limestone powder can also generate more carbonation products and make the microstructure less porous.