Unexpected Chemical Activity of a Mineral Surface: The Role of Crystal Water in Tobermorite

The most prominent phase of hydrated cement paste, as a model for calcium silicate hydrate (CSH) gel, is tobermorite with varying Ca/Si ratios. In the present work, carbon monoxide (CO) gas is introduced onto the surface of tobermorite at 62 K. The experimental results from infrared spectroscopy reveal a chemical reaction of CO with tobermorite, leading to the formation of carbonate (CO32-), which is adsorbed on the surface. This chemical reaction is driven by the dissociation of crystal water incorporated in the tobermorite structure. In infrared spectroscopy, the saturation of this carbonate growth process is revealed by the adsorption of unreacted CO molecules sticking to the surface of the fresh CaCO3. Interestingly, the decomposition process performed by heating the system leads to CO2 emission rather than CO. Our results are underlined by density functional theory calculations that bring out the high significance of interlayer water.