The mineralogy of the CaO–Al 2 O 3 –SiO 2 –H 2 O (CASH) hydroceramic system from 200 to 350 °C Nicola Meller, Konstantinos Kyritsis, Christopher Hall ⁎ Centre for Science at Extreme Conditions and School of Engineering & Electronics, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3JL, UK abstract article info Article history: Received 23 April 2008 Accepted 7 October 2008 Available online xxxx Keywords: Hydration products X-ray diffraction Oil well cement Geothermal well cement We describe a quantitative mineralogical study of the hydrothermal reactions of an oil well cement with added silica and alumina, hydrated at temperatures from 200 to 350 °C. We compare the products with pure end member systems and find phase stability can be altered radically, even by small amounts of additive. The upper temperature limits of α-C 2 SH (b 250 °C), and 1.1 nm tobermorite C 5 S 6 H 5 (b 300 °C) are increased. C 8 S 5 , reported in a cement-based system for the first time, is stable to 300 °C and is believed to prevent foshagite C 4 S 3 H formation below 350 °C. Hydrogarnet C 3 AS 3-y H 2y is the only aluminum bearing phase at b 300 °C but it coexists with C 4 A 3 H 3 and bicchulite C 8 A 4 Si 4 H 4 at higher temperatures. The presence of alumina increases the stability of 1.1 nm tobermorite greatly and also to a lesser degree of gyrolite. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction Various potential sources of geothermal energy exist in many regions, for example in the UK in the form of radiothermal granites [1– 5]. In geothermal systems, cool fluids are pumped down an injection well, heated at depth by the surrounding rock formation and returned to surface via a production well. Such methods, formerly known as hot dry rock (HDR) technology and now as enhanced geothermal systems (EGS), require the annulus between the well casing and rock formation to be sealed with cementitious materials to prevent the escape of the working fluid in much the same way as an oil or gas well is sealed [6]. This paper reports a detailed examination of the mineralogy of sealants based on an oil well cement with additions of silica and alumina for use in geothermal wells over the temperature range 200 to 350 °C. The engineering properties of these sealants are described elsewhere [7]. In oilfield engineering, it is standard practice to use special cement formulations when the well temperature exceeds 110 °C. Above this temperature the predominant phase formed in the sim- ple hydration of an oilwell cement is α-dicalcium silicate hydrate α-C 2 SH 1 [Ca 2 SiO 3 (OH) 2 ], which forms bulk materials that are too weak and permeable to seal the well [6,8]. On initial hydration cement first forms a C–S–H gel which on heating converts to crystalline α-C 2 SH. This crystallization causes a reduction in solid volume and is accom- panied by an increase in permeability and the reduction in compressive strength known as strength retrogression. Therefore silica is commonly added to cement (typically approx 35% by wt of cement BWOC) to reduce the C/(C+S) mol ratio to about 0.5. This prevents the formation of α-C 2 SH and instead numerous other crystalline calcium silicate hydrates are produced. Fig. 1 redraws the well known summary diagram of Taylor [9] based on information available in 1964 which shows the hydrates commonly found under various conditions. We emphasize, as did Taylor, that these are not necessarily the equilibrium phases. Such cement +silica formulations often yield bulk materials with greater strength and lower perme- ability than cement alone. While such formulations are normally acceptable for oil wells they are not always durable in the hostile chemical environments encountered in geothermal wells, and forma- tion damage or sealant deterioration or both can occur. With high temperature well cementing in mind, Barlet-Gouédard et al. [10,11] and Meller et al. [12–14] have recently designed hydroceramics based on the CaO-Al 2 O 3 -SiO 2 -H 2 O (CASH) system at 200 and 300 °C. A hydroceramic is defined here as any ceramic composition containing chemically combined water as H 2 O or OH or both. The studies carried out by Barlet-Gouédard et al. used the principle, first described by Roy et al. [15–17], that the sealant should have the same overall chemical composition as the surrounding rock formation: hence if the rock formation is stable in that environment the sealant should be too. For this reason a limited number of compositions was assessed. Our research extends the CASH system to cover a wider range of compositions and temperatures. A detailed knowledge of the mineralogy of a complete system delivers more options should some compositions be unsuitable in certain chemical environments. We describe here the detailed mineralogy of a suite Cement and Concrete Research xxx (2008) xxx–xxx ⁎ Corresponding author. E-mail address: christopher.hall@ed.ac.uk (C. Hall). 1 Note that cement nomenclature will be used generally throughout: C = CaO, S = SiO 2 , A = Al 2 O 3 , F = Fe 2 O 3 ,H=H 2 O, _ S = SO 3 , _ C = CO 2 . The complete chemical formula of each individual mineral is given in Table 4. CEMCON-03834; No of Pages 9 0008-8846/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2008.10.002 Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp ARTICLE IN PRESS Please cite this article as: N. Meller, et al., The mineralogy of the CaO–Al 2 O 3 –SiO 2 –H 2 O (CASH) hydroceramic system from 200 to 350 °C, Cement and Concrete Research (2008), doi:10.1016/j.cemconres.2008.10.002