Study of carbonation rate of synthetic C-S-H by XRD, NMR and FTIR

Bei Wu ¹, Guang Ye ^{1, 2}

¹ Delft University of Technology, the Netherlands
² Magnel Laboratory for Concrete Research, Ghent University, Belgium

Supplementary cementitious materials (SCMs) like fly ash (FA) and blast furnace slag (BFS) are broadly used in the concrete to replace parts of Ordinary Portland Cement (OPC) due to both economic and environmental issues. In the concrete blended with SCMs, C-S-H with different C/S ratios, formed from the hydration and pozzolanic reactions of blended cement paste, are is the major calcium-bearing phases which reacts with CO2 during the carbonation with the a CO2 concentration of 3%±0.2. Therefore, it i’s important to study the carbonation rate of different C-S-H phases. In this paper, the C-S-H phases (C/S ratio: 0.66 to 2.0) were synthesized and used for the accelerated carbonation testing. Synthesized C-S-H phases with different C/S ratios were identified by X-ray diffraction and 29Si nuclear magnetic resonance (NMR). Carbonation rate and products of different C-S-H phases are also determined. The results show that C-S-H phases with different target C/S ratio (lower than 1.40) were synthesized in the lab, the structure of which is similar to the C-S-H(I) reported by Taylor. The portlandite appears in the products when the designed C/S ratio is higher than 1.40 . C-S-H with lower C/S ratio is decomposed faster than that with a higher C/S ratio. After three days’ accelerated carbonation, the C-S-H phases with different C/S ratio were all fully decomposed to CaCO3 and silica gel.

Key words: C-S-H, C/S ratio, structure, carbonation rate