GOLDSCHMIDTCONFERENCE TOULOUSE 1998 Hydrothermal formation of hydrated calcium silicates: an in situ synchrotron study S. Shaw C. M. B. Henderson S. M. Clark Y. Wang Department of Earth Sciences, University of Manchester, Manchester, M13 9PL, UK & Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK GSE-CARS, Advanced Photon Source, Argonne National Laboratory, Itlinonois, USA Hydrated calcium silicate minerals, e.g. tobermorite (CasSi6016(OH)2.4HeO) and xonotlite (Ca6Si60(OH)2), are rare phases formed in hyper- alkaline, hydrothermal environments. They usually occur where fluids saturated with respect to calcium hydroxide react with basic igneous rocks e.g. Okayama, Japan. These phases are also known to form in cements and in the hyper-alkaline environ- ments surrounding cementitious nuclear waste sites making them both mineralogically and environmen- tally important. There has been a lot of work in this field aimed at understanding the structural relations and the formation mechanism and kinetics. Few conclusive results have been published due to the slow reaction kinetics and the large number of stable phases, more than 30 in the calcium silicate hydrate (CSH) system. This study was initiated to investigate, in situ, the formation of tobermorite and the higher temperature phase, xonotlite, the main aims being to investigate the influence of temperature and alumi- nium content on phase stability, kinetics and reaction mechanism. Experimental methods We used station 16.4 of the Synchrotron Radiation Source (SRS) at the Daresbury Laboratory, and line 13 BM of the Advanced Photon Source (APS) at the Argonne National Laboratory for this work. White beam energy dispersive powder diffraction (EDPD) was used to follow the hydrothermal formation of tobermorite and xonotlite in situ. The synthesis equipment is shown in Fig. 1 (Evans et aL 1994) and consists of a stainless steel pressure vessel surrounded by a heater unit which has 2 holes to allow the incident and diffracted X-ray beam to pass through. This system relies on the ability of the high energy X-rays produced by a synchrotron to pass through the steel walls of a reaction cell. The maximum usable energy of the synchrotron radiation utilised determines the maximum wall thickness of the cell since the higher the X-ray energy the more penetrating the beam. Ultimately, the energy of the X-ray beam determines the cell wall thickness that can be used, and because the wall thickness controls the safe working temperature, the maximum temperature permitted in the cell is a function of X- ray energy. At the SRS, cells of wall thickness up to 0.4 mm can be used; above this the diffraction data become unresolvable, limiting experiments to below 240~ Beam line 13 BM at the APS has a X-ray beam with higher energy enabling the use of cells with wall thicknesses up to 3mm. This allowed experiments to be performed at over 300~ The synthesis experiments were performed by mixing an anhydrous amorphous gel of tobermorite composition, and a saturated calcium hydroxide Pressure Transducer Injection R e s e r vo i r ""--...,,~ Air Gate Thermocouple PTFE liner Parr Cell - X-Rays Out 4- Sample \ \ 6- Pressure Release Valve Heating Block .__~<-~ X-Rays In "Magnetic Stirrer FIG. 1. Schematic diagram showing on-line hydrother- real synthesis equipment. 1377