Strength retrogression of set Portland cement at elevated temperatures, for example at temperatures greater than about 230°F, has been known for many years, and is believed to be due to the formation of a variety of crystalline phases at the expense of amorphous calcium silicate hydrate phase. Ground quartz silica is typically added to prevent the strength retrogression. Calcium hydroxide that is generated from cement hydration reacts with silica in pozzolanic reactions to generate amorphous calcium silicate hydrate. It appears from the literature search that there is no consensus as to how much silica is needed to effectively prevent strength retrogression. The amount of silica proposed as sufficient varies from the generally accepted value of 35% by weight of cement to more than 60%. In offshore situations, the problem is more complicated due to the high content of salts present in sea water. The chloride content of seawater has the potential to form crystalline phases, for example Friedel’s salt. The effects of seawater on the required amounts of silica flour for prevention of strength retrogression have not been explored. The objective of this study is to identify the optimized amounts of silica required for a given application temperature in both fresh water and sea water, and search for correlations between macroscopic strength related-properties and molecular level structural features. It is hoped that the information and knowledge will be useful in designing cement slurries for any offshore hot zones.
As part of this study, cement formulations containing different amounts of silica were tested in both fresh and sea water in the 250-400°F range using Ultrasonic Cement Analyzer (UCA). The strength development/retrogression patterns were observed. The cured samples were analyzed by X-ray diffraction studies. In these studies, cement to water ratio is maintained constant to ensure similar hydration rates, and the densities were allowed to vary based on the amount of silica added.
The results clearly indicate that the amount of silica needed to prevent strength retrogression depends on temperature. Highly intriguing strength development patterns, dependent on temperature and the amounts of silica, were observed prior to stabilization at ultimate strengths. Implications of such patterns on well construction strategies are discussed, and molecular level clues from X-ray studies for such patterns are pursued.