Compressive Strength and Hydration Products of Oil Well Cement Mixed with Fly Ash at Ultra-high Temperature

The compressive strengths and hydration products was investigated of oil well cement mixed with fly ash at ultra-high temperatures. Cured at 150 degrees C, the strength for test samples with 30% and 50% fly ash was relatively high in the early days but declined in subsequent days. However, the samples with 70% fly ash showed a continuous trend. The compressive strengths were relatively high at 28 days, and all the samples met the cementing requirements at this temperature. At 220 degrees C, the mix with 30% fly ash had the least compressive strength, while that of 70% fly ash content was comparatively high. X-ray diffraction analysis shows that the main crystalline hydration products at 150 degrees C were tobermorite and hibschite. At 220 degrees C, tobermorite decomposes into xonotlite because it has limited high temperature stability. The pozzolanic reaction of fly ash and portlandite Ca(OH)(2) produced a comparatively stable phase known as hibschite. The scanning electron microscopy analysis shows that the surface morphology of tobermorite is fibrous while hibschite is vitreous. Both compounds have high strength characteristics due to their structural interconnectedness. Xonotlite crystal is coarse and plate-shaped with poor bonding, which negatively impacts its strength stability. Due to the presence of hibschite, cement mix with high amounts of fly ash is suitable for application at 220 degrees C.