Ceramics-Silikáty 63, (2) 239 - 247 (2019)

Silica flour is used to mitigate the strength retrogression and microstructure degradation problems of oil well cement paste in high temperature wells. This study investigates the effects of the particle size distribution of silica flour on the thermal stability of cement paste. After 7 days of hydrothermal curing at 80 °C, the cement paste was exposed to further aging for 7 or 28 days under high temperature and high pressure (HTHP) conditions in a sealed chamber at 260 °C (500 °F) and 21 MPa (2900 psi). The results show that 43% silica flour by weight of cement (BWOC) mitigates the strength degradation of silica-stabilized Portland cement (SSPC), and course silica flour (63.13 and 48.25 μm) maintains the strength stability of SSPC better than fine silica flour (3.25 μm). The transformation of C-S-H gel to xonotlite in SSPC was influenced by the silica particle size. The relatively fine silica flour (d d₅₀=3.25 μm) accelerates the consumption of calcium hydroxide during the pozzolanic reaction, resulting in a poorer strength stability of SSPC than the courser silica flour (d₅₀=63.13 and 48.25 μm). The silica flour with an average particle size of 48.25 μm is more conducive to crystallization of xonotlite on the crystal plane corresponding to a diffraction angle of 25.5 ° than the other size silica flour.