FORMATION PROCESSES OF BETA-C2S 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 w as hydrothermally synthesized at 80-degrees to 200-degrees-C, and the thermal decomposition behavior of the products (C-S-H with Ca(OH)2) wa s analyzed using XRD, Si-29 MAS NMR, and the trimethylsililation metho d (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 Si5O1 6) and that polymerization advanced with an increase of the synthesizi ng temperature. On heating, the products decomposed to form beta-C2S. It was found that the decomposition was gradual and that the higher th e temperature of hydrothermal synthesis, the lower was the temperature of the decomposition into beta-C2S. Although the decomposition procee ded to form a monomer (beta-C2S) from the polymer and dimer, this dime r was resistant to heat and did not decompose unless heated to above 4 00-degrees-C.