Monitoring Cement Quality By Microscopy
Authors: Caveny W.J. , Ripley H.E. 
Published: March 2010

Abstract
In the late 19th century Henry Le Chatelier was perhaps the first to appreciate that chemical analysis revealed little concerning the nature of the compounds formed during the fusion and crystallization of cement. He was the first to apply the microscope consistently to this problem and recorded numerous microscopic observations made from thin sections of cement and the various crystal forms. A.E. Thornebohm, at about the same time, was also using the microscope to study crystals in cement. He assigned the names alite, belite, celite and felite to these crystal forms. The term alite is still used today as the name for the angular tricalcium silicate crystal (C3S) and belite is still commonly used as the name for the spherical dicalcium silicate crystals (C2S).
The chemical compounds found in Portland cement are identified within the cement industry using the unique symbols shown in this article.
Other pioneers in early cement microscopy were Stern and Wetzel who worked with polished section techniques, also Tevasci who developed several chemical etchants. Improvements in qualitative methods were given by Insley, Flint, Newman and Swenson.

Methodology
Microscopic examination of dry cement samples has led to better oil well cement job performance through more accurate job design and rejection of poor quality and contaminated cement. It may also indicate whether selected additives have actually been added to the mix.
Two methods, each of which require microscopes with 100x to 400x power, are used in the preparation and examination of samples.Examination of neat cement (pure cement without additives and before mixing with water) is accomplished by following the techniques described by Bogue in the late 1940s. A sample of neat cement is mounted in an epoxy binder on a glass slide, then polished by a series of pastes containing progressively finer abrasives. The sample is then chemically etched to highlight the colours. A variety of filters may be added to the microscope to cause cement crystals and contaminants to show up in distinguishing colours. This is the principal method used for examination of cement quality because colours that indicate quality show up vividly in the opaque sample when using a reflected light for illumination (Figs. I, 3, 5 and 6). Transmitted, polarized light may also be used if the sample is thin enough to be translucent (Figs. 3 and 4).Identification of additives and contaminants is achieved by mounting a sample of dry cement on a clear slide and illuminating it with polarized light generated from below. No particular preparation of the sample is necessary (Figs. 7 through 12). This method does not always show vivid colours, thus the illustrations are in black and white. Particle shapes and sizes key the identification of additives and contaminants seen by transmitted, polarized light shining through the samples.

Quality of Cement
Cement microscopy visually reveals some deficiencies that may have existed in the manufacturing process. Microscopic examination may reveal the following qualities referenced to API Spec. 10.