Formation Processes of β‐C ₂ S by the Decomposition of Hydrothermally Prepared C‐S‐H with Ca(OH) ₂

A mixture of CaO and silicic acid prepared with a Ca/Si ratio of 2.0 was hydrothermally synthesized at 80° to 200°C, and the thermal decomposition behavior of the products (C‐S‐H with Ca(OH) 2 ) was analyzed using XRD, 29 Si MAS NMR, and the trimethylsililation method (TMS). It was found that the main silicate anion structure of C‐S‐H was a mixture of a dimer and a single‐chain polymer (larger than Si 5 O 16 ) and that polymerization advanced with an increase of the synthesizing temperature. On heating, the products decomposed to form β‐C 2 S. It was found that the decomposition was gradual and that the‐higher the temperature of hydrothermal synthesis, the lower was the temperature of the decomposition into β‐C 2 S. Although the decomposition proceeded to form a monomer (β‐C 2 S) from the polymer and dimer, this dimer was resistant to heat and did not decompose unless heated to above 400°C.