The rapid conversion of calcium aluminate cement hydrates, as revealed by synchrotron en.ergy -dispersive diffraction S. Rashid,* P. Barnes* and X. Turrillas* The exceptional speeds of data capture available with synchrotron dzffraction systems have been adapted and used to examine the structural/dynamic order of events in rapid hydration and conversion of calcium aluminate cements at high temperatures (50-90°C). The time resolution of the results obtained is suficient to reveal effects that were not detectable or even suspected from conventional laboratory analysis. In particular, C, AH, emerges as a ubiquitous transitionary phase, initiating the formation of C,AHh in both the hydration and conversion processes. The dynamic diffraction data are presented and interpreted in terms of model descriptions of the order of events in high-temperature hydration and conversion. Introduction It has recently been shown' that the technique of synchrotron radiation energy-dispersive diffraction (SR-EDD) is ideally suited to studying rapid solid- state crystalline transformations and reactions, par- ticularly those involved in hydrothermal conditions. The intense and energetic X-radiation produced by a synchrotron source is easily capable of yielding dif- fraction patterns in seconds or under, and with bulk specimens of thicknesses up to 12mm. Thus it has been to study rapid hydrating cement systems such as are available with calcium aluminate cements. However, the technique is also ideally suited to the study of other cementitious reactions such as the con- version of unstable calcium aluminate hydrates at higher temperatures. It is well known4 that the hydrated product of CA, *Industrial Materials Group, Department of Crystallography, Birkbeck College, Malet Street, London WClE 7HX, UK. S. Rashid is also at Lafarge Special Cements, 730 London Road, Grays, Essex, RM 16 IUJ, UK. according to CA + 10H -, CAH,, (1) is the metastable hydrate CAH,,, which at tem- peratures above 28°C converts to the more stable cubic hydrate C,AH, according to 3CAHl, + C,AH, + 2AH3 + 18H (2) The role of the other metastable hydrate C,AH, is interesting in view of its presence during hydration and/or conversion, according to the following equations, which cover a range of possible reaction processes 2CA + 11H + C,AH, + AH, (3) 2CAHl, + C2AH, + AH, + 9H (4) 3C2AH, --+ 2C,AH, + AH, + 9H (5) which might suggest a possible role for C,AH, as a transitionary intermediate in the conversion reaction of expression (2). Currently, the only detectable crys- talline hydrates above 50°C are considered to be C,AH, and AH, .5 However, it has been suggested by Rettel et al.,' from NMR results, that C2AH, could form above 50°C simultaneously with C,AH, and crystalline AH,. The speed of data capture possible with synchrotron techniques allows us to demonstrate that current theories are incomplete. The technique When electrons are accelerated around circular paths at relativistic speeds in a synchrotron, they give off electromagnetic radiation in the forward direction. Until the 1980s such radiation was utilized only para- sitically, from machines that were primarily designed for the needs of particle physics experiments. Since the commencement in 1980 of the Daresbury Laboratory SERC synchrotron source, the 'SRS', many more Advances in Cement Reseurch, 1991192, 4, No. 14, Apr., 61-67