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

摘要

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_2SH) and Ca_6[Si_2O_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℃-420℃. 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_2O_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_2O as shown by thermogravimetry (TG) coupled with IR spectros-copy. 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_2S materials. The influence of the type of grinding on the thermal behavior of α-C_2SH upon its transformation into water-free Ca_2SiO_4 modifications is discussed.