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 CO2 footprint. The materials are produced by hydrothermal processing of SiO2 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-Ca2[HSiO4](OH) (α-C2SH) and Ca6[Si2O7](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/SiO2 ratio of 2. A characteristic feature in regards to the silicate anionic structure is the increased silicate polymerization (up to 27% Si2O7 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% H2O 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 C2S materials. The influence of the type of grinding on the thermal behavior of α-C2SH upon its transformation into water-free Ca2SiO4 modifications is discussed.