Carbonation mechanism of different kind of C-S-H : rate and products

During the carbonation of cement paste blended with supplementary cementitious materials, the C-S-H with different Ca/Si ratios, formed from the hydration and pozzolanic reactions, are the major calcium-bearing phases which react with CO2. Therefore, it’s important to study the carbonation mechanism of different C-S-H phases. In this paper, the pure C-S-H phases (Ca/Si ratio: 0.66 to 2.0) were synthesized and used for the accelerated carbonation test. Synthesized C-S-H phases with different Ca/Si ratio were identified by X-ray diffraction and 29Si nuclear magnetic resonance. The carbonation rate and products of different C-S-H phases are also determined. The results show that pure C-S-H phases with different Ca/Si ratio (lower than 1.40) can be synthesized in the lab. The structure of synthesized C-S-H is similar to the C-S-H(I) reported by Taylor. The mean chain length of the C-S-H decreases dramatically when the Ca/Si ratio increases from 0.66 to 1.40, then it keeps no change. The portlandite appears in the products when the designed Ca/Si ratio is over 1.40. The C-S-H with lower Ca/Si ratio is decomposed faster than that with a higher Ca/Si ratio. All the C-S-H phases are fully decomposed to CaCO3 and silica gel after 3 days’ accelerated carbonation.