Formation Processes of β‐C₂S by the Decomposition of Hydrothermally Prepared C‐S‐H with Ca(OH)₂

Abstract

A mixture of CaO and silicic acid prepared with a CdSi 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),) was analyzed using XRD, 29Si 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,O,,) and that polymerization advanced with an increase of the synthesizing temperature. On heating, the products decomposed to form p-C,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 p-C,S. Although the decomposition proceeded to form a monomer (p-C,S) from the polymer and dimer, this dimer was resistant to heat and did not decompose unless heated to above 400°C.

Silicate Anion Structural Change in Calcium Silicate Hydrate Gel on Dissolution of Hydrated Cement

“…These operations were conducted in an atmosphere-controlled glove box where air was replaced with nitrogen gas. After a predetermined period (1,4,8,13, 53 weeks), the sample was placed in a centrifuge separator to separate the aqueous phase from the solid phase at 3,500 rpm for 10 min. The procedure of the dissolution test is shown in Fig.…”

“…Recently from the analysis of the structure of C-S-H gel using 29 Si-NMR, a model representing the structure of the silicate anion of tobermorite as a double chain structure and the change in the structure of the silicate anion of C-S-H as dependent on the Ca/Si molar ratio and other coexisting elements have been reported. 12,13) The evolution of the silicate anion associated with the dissolution of C-S-H gel was investigated using 29 Si-NMR. The SiO 2 of the unhydrated cement is present as a monomer (Q 0 ) in alite and belite.…”

Low Temperature Synthesis of Belite Cement Based on Silica Fume and Lime

“…Hillebrandite was synthesized by hydrothermal treatment of quartz and lime with Ca/Si = 2.0 at 200°C for 10 hours or at 250°C for 5 hours [15]. The main silicate anion structure of hillebrandite studied by 29 Si MAS NMR analysis is 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 [16]. Different polymorphs of calcium silicate hydrates with Ca/Si = 2.0, namely, α -C 2 SH [Ca 2 (SiO 4 H)OH], dellaite [Ca 6 Si 3 O 11 (OH) 2 ], and hillebrandite, were synthesized and their stability and phase transformation were investigated under hydrothermal conditions up to 400°C.…”

“…Hillebrandite starts to decompose at about 500°C producing low-crystalline β -C 2 S [15]. Moreover, 29 Si MAS NMR analysis illustrates that the temperature at which β -C 2 S begins to form decreases as the Ca/Si ratio of hillebrandite becomes higher [19] or as the temperature of hydrothermal synthesis of hillebrandite becomes higher [16]. On the other hand, XRD analysis illustrates that the crystallographic properties and density of dissociation products of α -C 2 SH depend on the calcination temperature [20].…”

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“…However, it should be noted that the weight loss is overlapped by calcium silicate hydrates, which are known to dehydrate gradually and differently over a wide temperature range ≤ 800°C. 62 Differences in their dehydration behaviour are due to compositional and structural variations. 63 In general, the results of the thermogravimetric analysis indicates that the addition of CaO to kaolinite and metakaolinite produced several water containing phases in the temperature range 25-350°C.…”