Understanding the Carbonation Phenomenon of C–S–H through Layer Structure Changes and Water Exchange

The carbonation process of C–S–H in cement-based materials has the potential to capture carbon dioxide from the atmosphere. However, understanding this process has been hindered by the amorphous nature of C–S–H and the various pore structures that result from carbonation. To better understand this phenomenon, we have carefully synthesized and carbonated C–S–H with 100% CO2 for 24 h and then investigated it using solid-state 29Si NMR and 1H relaxometry under relative humidity (RH) conditions. We found that the degree of collapse and reconstruction of the silicate chain structure during carbonation strongly depends on the Ca/Si ratio of the C–S–H and the RH conditions. The polymorphism of carbonates resulting from carbonation also varies in their parameters. The collapse of the chain structure produces bridging silicate structures like amorphous glass, resulting in the development of large-sized pores filling with water. Interlayer, gel pore, and capillary pore water in C–S–H can be determined by 1H NMR relaxometry using spin–spin relaxation time (T 2). Careful T 2 analyses of the decay data determined the T 2 and quantity of the complex pore created by carbonation. The size of the gel pore predicted from T 2 increased with higher RH. C–S–H with relatively small gel pores produced more unstable vaterite than thermodynamically stable calcite, which was linked to pore size. Carbonation experiments in an atmosphere using D2O-saturated salts directly revealed the exchange of water filled with various pores as detected by 1H NMR. The changes in layer structure and water movement associated with carbonation discovered by a series of experiments shed light on the phenomenon of C–S–H carbonation.