A new phase diagram is reported for the CaO-Al2O3-SiO2-H2O (CASH) system at 200 degrees C. This system is rare in nature but has applications in cementing geothermal and deep oil wells. The phase diagram was constructed by synthesising a range of hydroceramics with CASH assemblages from oilwell cement, silica flour (quartz) and alumina (corundum). A hydroceramic is defined as any ceramic material incorporating water as H2O or OH. At 200 degrees C, gyrolite, hillebrandite, jaffeite, portlandite, quartz, 11 angstrom tobermorite, xonotlite, hibschite and katoite were observed as product phases. The mineral assemblages produced the following three-phase triangles in the CaO-Al2O3-SiO2 diagram: Gyr + Qtz + Xon; Cm + Tob + Xon; Crn + Hib + Xon; Crn + Hib + Jaf; Crn + Jaf + Kat; Hib + Jaf + Por; Hib + Jaf + Xon; and two reactions are found to be in progress at 200 degrees C. When alumina is present in the reaction mixture, the thermal stability of tobermorite is extended to higher temperature, and the crystallinity of tobermorite and xonotlite enhanced. (c) 2005 Elsevier Ltd. All rights reserved.