Summary The crystalline nature of hydrated Portland cement is dependent primarily on temperature. The calcium silicate hydrate (CSH) gel is produced at low temperatures and, upon curing at higher temperatures, will convert to one or more crystalline phases. The better cementing compositions contain a low lime-to-silica (C/S) ratio. Xonotlite is a phase commonly produced above 150 deg. C (302 deg. F) when approximately 35% fine silica is added to Portland cement. Generally, it has good strength but moderate permeability, Truscottite, produced when an even larger quantity of silica is added to the cement, has lower permeability than xonotlite but is slightly more difficult to produce and to stabilize. Pectolite can be produced by introducing sodium into a truscottite-type formulation. Once formed, pectolite is very stable but typically has high permeability. The addition of carbonate to any of these formulations may produce scawtite. Scawtite appears to be an inferior phase by itself, but in small quantities it can be helpful in strength development. Introduction The failure of wells in several geothermal fields has been directly attributed to degradation of cement. This implies that the cementing materials used to complete geothermal wells had not been sufficiently evaluated. For the past 3 years, under the auspices of the U.S. DOE, we have studied geothermal cementing materials in an attempt to identify suitable systems. A major portion of this study was devoted to research on the behavior of calcium silicate hydrates at the high temperatures found in geothermal zones. The literature contains many references pertaining to calcium silicate hydrates in wells at temperatures up to 150 deg. C (302 deg. F), but little has been published concerning higher temperatures. Portland cement is the material normally used to seal steel pipe in a borehole. Originally designed for hydration at or near atmospheric temperature, Portland cement can be adapted for use in petroleum or geothermal wells with bottomhole temperatures approaching 370 deg. C (700 deg. F). The hydration chemistry and phase equilibria of Portland and similar calcium silicate cements change with increasing temperature. At atmospheric temperatures, tricalcium silicate (C3S)* and dicalcium silicate (C2S), which comprise about 75% of the dry Portland cement composition, react with water to form a CSH gel with variable composition and calcium hydroxide (CH). A cement slurry becomes rigid when less than one-half, and sometimes less than one-fourth, of the cement has hydrated. At this point, pores begin to close and free movement of water is no longer possible within the cement. Consequently, a true gel is formed that is strong and impermeable. Calcium ions migrate from C3S and C2S particles into the water trapped in pores. Silica migrates from quartz (sand) grains into the water at various locations. The resulting calcium silicate reaction products are high in calcium at one point and high in silica at another. Aluminum ions released by another important compound in Portland cement, tricalcium aluminate (C3A), are also of concern. Considering the number of calcium silicate compounds and aluminum substitutions possible, it is surprising that reasonably pure cement phases are commonly obtained. As temperature increases to about 120 deg. C (247 deg. F), CSH gel converts to other crystalline forms. If excess calcium hydroxide is present, alpha dicalcium silicate hydrate (alpha-C2SH), a very weak and porous material, is produced. Fine silica is normally added to Portland cement to prevent this. If at least 35% silica is added to Portland cement, to bermorite (C5S6H5 approximately), also a strong and impermeable binder, usually is formed. JPT P. 1373^