High-Temperature Cement Compositions - Pectolite, Scawtite, Truscottite, or Xonotlite: Which Do You Want?

Abstract

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^

Effect of Autoclave Curing on the Microstructure of Blended Cement Mixture Incorporating Ground Dune Sand and Ground Granulated Blast Furnace Slag

“…The formation of tobermorite filled the pores and enhanced the compactness of the hydrated mixture. It has been reported that tobermorite is associated with high strength and lower permeability features (Eilers et al 1983, Jupe et al 2008. The inclusion of slag as a further PC replacement (M3, M4 and M5) maintained the compressive strength between 66 (M1) and 72 MPa (M2).…”

“…This analysis indicated that the crystalline structure phases are newly formed CSH (i.e., tobermorite). The formation of CSH phases with a Ca/Si ratio close to unity is a favored result for the concrete strength and physical properties (Eilers et al 1983;Yazıcı et al 2008). …”

“…The primary formed C-S-H gel converts to crystalline alpha dicalcium silicate hydrate (a-C 2 SH) or C 3 SH 1.5 (Jupe et al 2008;Mindess et al 1981;Taylor 1997). These phases are porous and weak, which leads to a deterioration in the compressive strength and concrete durability (Eilers et al 1983;Grabowski and Gillott 1989). To avoid the formation of undesired phases (a-C 2 SH and C 3 SH 1.5 ), fine siliceous material should be added at 30-40 % by weight of the cement.…”

“…To avoid the formation of undesired phases (a-C 2 SH and C 3 SH 1.5 ), fine siliceous material should be added at 30-40 % by weight of the cement. The added siliceous materials react with the CH generated from the PC hydration to produce new C-S-H phases, such as tobermorite and xonotlite (Berardi et al 1975;Eilers et al 1983;Mindess et al 1981;Taylor 1997). The hydration product features of autoclaved mixtures depend on many parameters, including the curing conditions, calcium to silica ratio (Ca/Si), and type of added silica (Bresson et al 2002;Eilers et al 1983;Hope 1981;Kołakowski et al 1994).…”

“…The added siliceous materials react with the CH generated from the PC hydration to produce new C-S-H phases, such as tobermorite and xonotlite (Berardi et al 1975;Eilers et al 1983;Mindess et al 1981;Taylor 1997). The hydration product features of autoclaved mixtures depend on many parameters, including the curing conditions, calcium to silica ratio (Ca/Si), and type of added silica (Bresson et al 2002;Eilers et al 1983;Hope 1981;Kołakowski et al 1994). It has been stated that, under autoclave conditions, the use of crystalline silica produces high strength compared to the use of amorphous silica (Assarsson and Rydberg1956;Grabowski and Gillott 1989;Jupe et al 2008;Luke 2004;Sanders and Smothers 1957;Yazici 2007).…”