Thermal Decomposition of Tricalcium Silicate Hydrate

C 3 SH 1.5 can be prepared in stirred suspensions using lime and silicic acid with Ca/Si = 3.0 for 14 days at 270°C under saturated steam pressure. The C 3 SH 1.5 which forms consists of long, fibrous or prismatic aggregates having a specific surface area of 14.0 m 2 /g. The specimen gives an average Ca/Si ratio of 2.98 (29 crystals) and lattice parameters of a = 1.00291(8) nm and c = 0.74842(11) nm by ATEM and XRD examinations, respectively. NMR examination confirms the existence of dimer silicate anions in C 3 SH 1.5 , suggesting the possible composition Ca 6 (Si 2 O 7 )(OH) 4 . Upon being heated, C 3 SH 1.5 dehydrates at 520–550°C with a sharp endothermy and decomposes to CaO plus a calciochondrodite‐like phase, which on further heating dissociates, with a gradual weight loss, to α′ L ‐C 2 S with CaO at 800°C. The intermediate calciochondrodite‐like phase has low crystallinity, but 29 Si MAS NMR and infrared spectra identical for highly crystalline calciochondrodite. CPMAS measurements indicate the possibility that the calciochondrodite‐Iike phase contains both protonated and unprotonated monosilicate anions. The amount of CaO which forms through the two‐step process agrees with the calculated values from the stoichiometry of decomposition: [Ca 6 (Si 2 O 7 )(OH) 6 ] → [Ca 5 (SiO 4 ) 2 ] + CaO + 2H 2 O →α L ‐(Ca 2 (SiO 4 ) + 2CaO + H 2 O.