Abstract
An alkali-activated blend of aluminum cement and class F fly ash is an attractive solution for geothermal wells where cement is exposed to significant thermal shocks and aggressive environments. Set-control additives enable the safe cement placement in a well but may compromise its mechanical properties. This work evaluates the effect of a tartaric-acid set retarder on phase composition, microstructure, and strength development of a sodium-metasilicate-activated calcium aluminate/fly ash class F blend after curing at 85 °C, 200 °C or 300 °C. The hardened materials were characterized with X-ray diffraction, thermogravimetric analysis, X-ray computed tomography, and combined scanning electron microscopy/energy-dispersive X-ray spectroscopy and tested for mechanical strength. With increasing temperature, a higher number of phase transitions in non-retarded specimens was found as a result of fast cement hydration. The differences in the phase compositions were also attributed to tartaric acid interactions with metal ions released by the blend in retarded samples. The retarded samples showed higher total porosity but reduced percentage of large pores (above 500 µm) and greater compressive strength after 300 °C curing. Mechanical properties of the set cements were not compromised by the retarder.
Highlights
Due to their resistance to high temperatures and aggressive acidic environments, high-performance ceramic materials are of great interest for use in constructing high-temperature geothermal wells
In addition to the chemically aggressive environment, the cement may suffer a significant thermal shock when wells constructed under relatively mild temperatures of about 80 to 110 ̋ C are moved into production, and the temperature may rise by several hundred degrees
The present paper focuses on the effect of a known calcium aluminate cement (CAC) set retarder, tartaric acid, on mechanical properties, the mineralogy, and the morphology of a CAC-fly ash blend activated with sodium metasilicate after curing at elevated temperatures of 85, 200, and 300 ̋ C and under the respective pressures of 0.1, 6.89 and 8.27 MPa
Summary
Due to their resistance to high temperatures and aggressive acidic environments, high-performance ceramic materials are of great interest for use in constructing high-temperature geothermal wells. The use of fly ash in cementitious materials contributes to abating the environmental impact of Portland cement, such as the release of CO2 and NOx during its manufacture. The low reactivity of fly ash under ambient conditions is not a limiting factor for its downhole applications in high-temperature geothermal wells. In addition to the chemically aggressive environment, the cement may suffer a significant thermal shock when wells constructed under relatively mild temperatures of about 80 to 110 ̋ C are moved into production, and the temperature may rise by several hundred degrees. Differences in temperature between the injection- and production-heat-carrier fluids may further impose a constant thermal-shock on the cement
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