Innovative Processing for Cement Materials
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Innovative Processing for Cement Materials E. Hideki Ishida and Norifumi Isu Introduction It is not an overstatement to say that there would be no purpose in developing new materials unless due consideration were given to the load they exert on the global environment. The consumption of energy and other resources when a product is made, used, and returned should be minimized wherever possible. Belite cement is receiving attention as a low-heat cement that does not use additives. Since belite cement has more belite ( -C2S)* and less alite (C3S) than ordinary portland cement, less limestone (CaCO3) as a raw material is required for its production, resulting in a savings of natural resources and a reduction in overall CO2 emissions. However, since the reaction rate of belite cement is low, a longer time is required for the development of initial strength. For this reason, increasing the reactivity of belite is an important target in the development of low-heat cements. Belite is generally synthesized by adding a stabilizing agent to the mixture of lime and silica and heating to 1400C to prevent transformation to the stable phase at room temperature.1–6 However, Pritts and Daugherty1 and Shibata et al.7 indicate that -C2S with no stabilizing agent has a relatively high rate of hydration. Activation of -C2S by another method has been reported: firing precursors from Ca(NO3) · 24H2O solution with colloidal silica8–10 and CaC2O4 · H2O with amorphous silica.11 However, these methods are difficult to put into practice. The need for lowtemperature synthesis and simplification of the manufacturing process leads to the conclusion that the most advantageous method would be decomposition of calcium silicate hydrates with a Ca/Si ratio of 2. This article presents the status of investigations on the low-temperature (i.e., energy-saving) synthesis of highly reactive
* Cement notation is used throughout: C is CaO, S is SiO2 , H is H2O, and C-S-H is calcium silicate hydrate gel.
MRS BULLETIN/NOVEMBER 2001
-C2S and its hydration, without being limited by conventional methods of cement synthesis.
Calcium Silicate Hydrates Having a Ca/Si Ratio of 2 There are three calcium silicate hydrates having a Ca/Si ratio of 2: dellaite [Ca6(Si2O7) (SiO4) (OH)2], -C2S hydrate [Ca2(SiO4H) (OH) or -C2SH], and hillebrandite [Ca2(SiO3) (OH)2]. The synthesis of dellaite is difficult because of the very high temperature and pressure conditions (800C and 100 MPa) required.12 -C2SH can be produced from C2S13 or lime and silicic acid14 by hydrothermal treatment in saturated steam pressure at 140–200C. On heating, -C2SH dehydrates at 390–490C to an unidentified phase similar to Li-C2S15 and transforms to -C2S at 900–1000C.14 Since the unidentified phase has high hydration activity, there is a strong possibility of it being a new cement phase,14 but further investigation is required. Although hillebrandite can be synthesized at relatively low temperatures, there are large discrepancies in reported preparation conditions and start
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