Cement Quality Evaluation by Polished-Section Microscopy

Abstract

Summary With the increased use of polarized and reflected light microscopy in the oilwell cementing industry has come an intensified application of the microscope in cement quality evaluation and control. By the very nature of cement manufacture, variances occur within and among cement plants because of raw material differences, kiln condition variations, and plant design characteristics. When cement crystalline properties are monitored, these variations can be readily evaluated, and properties are monitored, these variations can be readily evaluated, and with physical data correlations, changes in slurry design can be made when Introduction During cement production, chemical changes and recombinations in the raw materials yield four basic chemical constituents: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). The physical and chemical properties of the internal cement microstructures are properties of the internal cement microstructures are dependent on the raw materials and kiln conditions. By the very nature of this manufacturing process, variances occur within and among cement plants as a result of raw material differences, kiln condition variations, and plant design characteristics. All these accepted variations affect such aspects of cement performance as thickening time, retarder sensitivity. and strength development. When cement crystalline properties are monitored, these variations in microstructure can be readily evaluated, and with physical data correlations, changes in slurry design can be physical data correlations, changes in slurry design can be made when warranted. As this paper will illustrate, correlations can be made between the observed crystalline characteristics and the data generated by physical testing. With these correlations, predictions can be made by polished-section evaluation predictions can be made by polished-section evaluation for cement reactivity on subsequent production runs of cement without the time and expense involved in the actual physical testing, such as thickening-time and compressive-strength tests. Sample Preparation The steps for preparing the sample are as follows. 1. Fifteen to twenty grams (accurately weighed) are sieved through a 200-mesh screen. The percentage of cement passing the screen is noted. 2. A thin layer of the + 200-mesh material is affixed to a properly labeled petrographic slide (45 x 25 mm) and allowed to dry for 5 minutes. 3. Excess cement is removed, and the slide is placed in the petrographic attachment of the polishing machine. 4. The sample is polished on a 30-mu m diamond disk (optional), a 15-mu m diamond disk, and then with a 6-mu m diamond paste and a 1-mu m diamond paste. Polishing with pastes is done on a nylon polishing cloth. Anhydrous ethyl pastes is done on a nylon polishing cloth. Anhydrous ethyl alcohol or lapping oil is used as lubricant. After each polishing step, the sample is cleaned with alcohol. polishing step, the sample is cleaned with alcohol. 5. The sample is washed with alcohol to ensure that all remnants of diamond paste and lapping oil are removed. The sample is allowed to air dry. 6. The sample is completely covered with etchant. Etching times are dependent on the etchants used. Etchants provide phase differentiation for more complete crystal provide phase differentiation for more complete crystal analysis. 7. The sample is rinsed immediately with anhydrous ethyl alcohol for 20 seconds. 8. The sample is allowed to dry without the etched surface being touched. The slide is now ready for viewing. 9. Evaluations are made with reflected white light with magnifications of 200X and 400X. All photomicrographs were taken at a microscopic magnification of 400X.