The Initial Carbonation Behavior of Synthetic C-S-H by Relative Humidity

Nowadays, CO2 emissions caused by cement production have increased. Since Calcium silicate hydrate (C-S-H), which contains the largest composition in waste concrete, can reabsorb CO2, it is necessary to improve the carbonation efficiency of C-S-H. Water is an important factor affecting the carbonation degree of C-S-H; too little water leads to a lack of dissolved calcium ions in C-S-H layers, whereas too much water hinders CO2 dispersion inside the C-S-H layers. In this study, to clarify the carbonation mechanism of waste concrete in wet-dry cycle environment, the effect of relative humidity on the carbonation of C-S-H was investigated. The C-S-H synthesized in molar ratio Ca/Si = 1.7 and put in constant relative humidity, which is 40% RH, 70% RH, 100% RH, and wet-dry cycle environment, which is 40–60% RH, 50–90% RH, 60–80% RH and 80–100% RH. To quantify the calcium carbonate in samples, thermogravimetric analysis (TGA) was conducted. Also, we analyzed the spectra from infrared spectroscopy (IR) to check the structure of carbonated C-S-H. The result of TGA showed that the carbonation degree was the highest at 80–100% RH, 50–90% RH, and 60–80% RH in 7 days of carbonation; this implies that the carbonation degree in wet-dry cycle is higher than that in constant RH. From the result of IR, vaterite, and aragonite were generated at 40% RH, 70% RH, 40–60% RH in 3 and 7 days. However, most of the calcium carbonate polymorphs generated during the carbonation process of C-S-H were calcite, which is the most stable polymorph of calcium carbonate.