Preparation of a Novel Cementitious Material from Hydrothermally Synthesized C–S–H Phases

New cementitious materials based on calcium hydrosilicate hydrates were recently developed as potential substitutes for ordinary portland cement, but with a reduced CO 2 footprint. The materials are produced by hydrothermal processing of SiO 2 and Ca ( OH ) 2 , giving rise to calcium silicate hydrates, followed by mechanical activation of the latter via cogrinding with various siliceous materials. Thus, the chemical composition in terms of C/S ratio could be adjusted over a broad range (1–3). In this study the synthesis of a previously unknown cementitious material produced via the combination of mechanical activation in a laboratory mill and thermal treatment of a mixture of quartz and hydrothermally synthesized calcium silicate hydrates: α‐ Ca 2 [ HSiO 4 ]( OH ) (α‐ C 2 SH ) and Ca 6 [ Si 2 O 7 ]( OH ) 6 (jaffeite) are reported. It forms independently of the type of mill used (eccentric vibrating mill, vibration grinding mill) after thermal treatment of the ground materials at 360°C–420°C. The new material is X‐ray amorphous and possesses a CaO / SiO 2 ratio of 2. A characteristic feature in regards to the silicate anionic structure is the increased silicate polymerization (up to 27% Si 2 O 7 dimers) as revealed by the trimethylsilylation method. Infrared ( IR ) spectra show a very broad absorption band centered at about 935 cm −1 . Another characteristic feature is the presence of ~2.5 wt% H 2 O as shown by thermogravimetry ( TG ) coupled with IR spectroscopy. As this water is bound mostly as hydroxyl to Ca, we refer to this new cementitious material as calcium‐oxide–hydroxide–silicate ( C – CH – S ). Calorimetric measurements point to a very high hydraulic reactivity which is beyond that for typical C 2 S materials. The influence of the type of grinding on the thermal behavior of α‐ C 2 SH upon its transformation into water‐free Ca 2 SiO 4 modifications is discussed.