Looking at clinker through a microscope is a powerful technique that can improve clinker production and cement quality. Using microscopy, one can gather remarkable information about clinker history and predict cement performance. A look in the microscope can determine the temperature profile in the kiln and provide clues to improve clinker grindability, optimize raw feed fineness, or increase 28-day strength. This process allows one to troubleshoot and identify the causes of poor clinker grindability or low cement mortar strength with just a few minutes of lab work. Not surprisingly, clinker microscopy has become an essential tool at cement plants worldwide.

Three purposes Clinker microscopy serves three basic purposes at the cement plant: perform quality control, troubleshoot problems, and monitor process changes.

Quality control-When clinker microscopy is performed routinely, operators get to know the microstructure of the plant's "typical" clinker. Then, if there is a change in the microstructure (for example, if crystals become larger, irregular, or poorly distributed), kiln operators can react to modify pyroprocessing parameters to improve clinker quality. Physical tests (such as strength and fineness) and chemical tests (such as free lime) provide valuable quality control information but don't tell the whole story. The relatively quick preparation time of clinker microscopy and the wealth of pyroprocessing information it can reveal make it a valuable supplementary diagnostic tool.

Troubleshoot problems-When there is a problem with clinker production or cement performance, the clinker microstructure can provide a clue as to the cause. Again, routine physical and chemical tests often provide insight, but microscopy can provide a complete solution.

Monitor process changes-If a change in any aspect of clinker production is anticipated (new raw materials, different fuel, change in burner pipe position), analysis of the microstructure is important before, during, and after the change. The operator can pinpoint the effects of the change-positive or negative-and predict changes in cement performance.

Preparation and evaluation Although records of clinker microscopy date back to 1887, the technique has gained a great deal of popularity in the past 20 years. Along with the increasing use of microscopy have come the introduction and evolution of new preparation techniques, etches, and equipment. Currently, the two most common methods of observation are the polished cross section and the powder mount.

Polished Cross Section, or "polished section," is the most common means of evaluating clinker. With this method, the clinker-either uncrushed or partially crushed-is impregnated in epoxy. The section is cut to reveal a cross section, then ground and polished. To reveal the various phases, the section is usually chemically etched. Two of the easiest and safest etches are water and nital (1% nitric acid in 99% isopropyl alcohol). Exposure to warm water (40degrees C) for 5 to 10 seconds reveals brightly colored free lime. A section immersed in nital colors the silicate phases.

The prepared polished section is observed with a polarized-light microscope using reflected light. Magnifications between 100x and 600x are commonly used. The phases that can be observed include: alite, belite, ferrite, tricalcium aluminate, periclase, free lime, alkali aluminate, alkali sulfate, and calcium sulfates. The important properties of these phases to evaluate include size, morphology, distribution, and reactivity to etchants. Porosity of the clinker also should be noted.

When observing the cement or clinker in the powder state, Powder Mount, or "Ono Method," is usually used. Dr. Yoshio Ono developed the method during his more than 40 years of experience at the Chichibu Onoda Cement Corp. (now Taiheiyo Cement Co.). Ono studied burning conditions and related clinker properties. He also investigated the relationship between clinker properties and compressive strength, producing a multilinear regression equation to predict 28-day mortar-cube strength. The Ono method involves observing a sieved fraction of clinker or cement powder mounted on a glass microscope slide using a refractive index oil. A polarized-light microscope is used with magnifications of about 400x.

The microscopic observations used in Ono's method and the related kiln conditions are:

* Alite size/Heating rate-Length of the "most commonly occurring" alite crystals is measured;

* Alite birefringence/Maximum temperature-Birefringence, or the difference in indices of refraction of relatively slow and fast light rays as they pass through an anisotropic crystal, is measured;

* Belite size/Burning time-Diameter of the "most commonly occurring" belite crystals is measured; and

* Belite color/Cooling rate-Belite color is estimated from belite crystals free from interstitial; the reported value is the most commonly occurring color. The color scale used includes amber, yellow, pale yellow, and clear.

Once these four microscopic measurements are taken, they are evaluated on a scale ranging from "poor" to "excellent" and assigned a numerical value. These values are then used in an equation to predict 28-day, mortar-cube strength.

Ono's method is a great complement to the polished section method. It involves little preparation time and provides information not obtainable with polished section (alite birefringence and belite color). A disadvantage of using the powder method in place of the polished section method is that only the silicates are observed (ignoring aluminates, ferrite, free lime, and more). Also, phase distribution and porosity cannot be evaluated, and color observation is subjective.

Clinker microscopy and production How does microscopy reveal so much information about the production of clinker? The crystal microstructure of clinker is formed by everything that goes into it and what happens to it along the way. In other words, there is a relationship between clinker microstructure, the kiln feed, and burning conditions.

Kiln feed-The primary step in the process of cement manufacture is the combination of silica with calcium to produce hydraulic compounds. In order to produce these compounds, the cement plant chemist needs to choose the raw mix components carefully; often several components are required. The chosen raw materials must then be ground into a fineness adequate to produce burnable kiln feed, but variances in grindability and burnability of the materials offer many challenges. For instance, calcite is much easier to grind and burn than quartz. If quartz particles are coarse in the feed, they will make the feed harder to burn and may leave a cluster of silicate crystals in the clinker, leading to decreased clinker grindability.

The chemical composition and fineness of the feed components will influence the amount of the compounds formed, their distribution, and size. These characteristics will also be influenced by the burning conditions.

Pyroprocessing-Burning conditions in the kiln develop the desired hydraulic compounds from the raw materials. The many factors that constitute burning conditions all relate to some aspect of the formed microstructure. Heating rate, cooling rate, kiln atmosphere, maximum temperature, and other factors will determine the size, morphology, and abundance of various compounds.

Clinker microscopy and cement performance How can microscopy reveal so much information about cement performance? Certain phases react quickly with water and are important for early strength and setting time. The abundance of these phases, as well as their crystal size, morphology, and distribution will determine how these phases hydrate, therefore determining the cement's early performance. Other phases react more slowly; the characteristics of these phases will dictate how the cement performs at later ages.

Formation and identification of clinker phases Raw materials and burning conditions produce the various clinker phases. The most abundant phases are termed "alite" and "belite." Alite is impure tricalcium silicate (C3S). Belite is impure dicalcium silicate, generally termed C2S. Combined, these two phases constitute roughly 75% to 85% of ordinary ASTM C 150 Type I/II cement. In typical clinker, the other two most common phases are comprised of aluminate (C3A) and ferrite (C4AF), and are referred to as the matrix, interstitial, or liquid phase (Photos 1 and 2).

Alite, C3S These are normally observed as six-sided angular crystals, 25- to 50-um long. Alite crystallizes between 1,200degrees and 1,400degrees C from the melt. Crystal size indicates rate of temperature rise in sintering zone and fineness of raw feed. A quick-heating rate and fine raw feed promote smaller alite crystals. Alite is quick to react, therefore, its properties (abundance, size, reactivity) affect early strength.

Belite, C2S Belite is typically observed as 25- to 40-um rounded crystals with multidirectional lamellae (or striations). Crystal size indicates residence time in burning zone (above 1,400degrees C). Long burning time produces larger crystals, whereas a shorter burning time will produce smaller belite crystals. Belite reacts slower than alite; belite hydration primarily has an effect on later-age compressive strength.

Tricalcium Aluminate, C3A These are observed in polished and etched clinker as blue-to-gray angular crystals in the interstitial. Size is a function of cooling rate: crystals increase in size with slow cooling. A high alumina ratio will produce greater amount of aluminate over ferrite. Alkali-modified aluminate (alkali aluminate) is observed in etched clinker as prismatic or cigar shaped. The presence of alkali aluminate indicates excess alkalies and possibly reducing conditions in the kiln.

Ferrite, C4AF The interstitial counterpart of the C3A, it is more reflective than C3A in an etched polished surface. Dull reflectivity of C4AF in nital-etched polished clinker indicates reducing conditions. The presence of metallic iron indicates severe reducing conditions. A low alumina ratio will produce greater amount of ferrite over aluminate.

Other phases that can form in the clinker are:

Free Lime These are observed in water-etched polished sections as brightly colored drop-like phases. Free lime is a result of coarse feed, high lime saturation factor, and/or improperly burned clinker. Tightly packed clusters or nests of free lime generally indicate coarse feed. Excessive free lime results in increased autoclave expansion.

Periclase, MgO Crystals are observed as colorless to light pink with a positive relief in a non-etched polished surface. The presence of periclase crystals is caused by more magnesium than can be incorporated in other phases. Large crystals generally indicate slow cooling, whereas dendritic crystals (a branching pattern of crystals) indicate quick cooling and possibly overburning.

Alkali Sulfate These are usually observed as darker phases associated with voids or as deposits on alite crystals in negative relief in a non-etched, polished surface. The presence of alkali sulfate is caused by excess sulfate over equivalent alkalies. The sulfur can originate from the raw materials or the fuel source.

There are other miscellaneous phases that can be present, such as gehlenite (2Ca2SiO4.CaCO3), spurrite (2Ca2SiO4.CaCO3), and calcium sulfide (CaS), in addition to subcategories of the phases described above. For example, alite can be further divided into its polymorphs; belite can be classified into four types depending on the number of lamellae present.

Case Study #1: Coarse and/or non-uniform feed Observation: belite nest (below left) and belite streaking (below right).

Caused by: Nests, especially when there is an observed pore in the center, are generally caused by coarse quartz grains in the raw feed. Belite streaking is generally due to coarse and/or non-uniform feed, and possibly coal ash absorption.

Effect on clinker: Coarse feed is harder to burn. Non-uniform distribution of belite crystals in nests or clusters can decrease clinker grindability, especially when the belite crystals are tightly packed (little liquid phase).

Effect on cement properties: Belite reactivity may be decreased and therefore 28-day strength may be affected.

Case study #2: Overburning Observation: "amoeboid" (left) and dendritic (below) belite crystals.

Caused by: Overburning (high temperature for long burn time).

Effect on clinker: There will be a decrease in clinker grindability.

Effect on cement properties: Belite reactivity may be decreased and therefore 28-day strength may be affected.

Case study #3: Slow cooling Observation: "ragged" belite crystals, coarse C3A, belite fringing on alite (below).

Caused by: Slow cooling.

Effect on clinker: There will be a decrease in clinker grindability due to coating on alite and ragged belite.

Effect on cement properties: Coating on alite will decrease its reactivity. Reactivity of ragged belite will also be decreased.

* The International Cement Microscopy Association (ICMA) hosts a free sample exchange program. Three clinker samples per year are mailed along with a report form to interested participants, who examine the clinker and provide results. The results from all participants are compiled and sent to those who submitted their samples (individuals are not identified). Those interested can join or drop the program at any time by contacting the ICMA by fax: (+1) 618-524-7291, or through its web site (www.cemmicro.org).

* The National Institute of Standards and Technology (NIST) has three Reference Material (RM) clinkers intended for use in training and calibration. These RMs are provided with information on the abundance of major cement clinker phases they each contain based on microscopic point counting using reflected light microscopy. The samples and the Report of Investigation, which includes the point count results, can be obtained by contacting NIST at (+1) 301-975-6776; fax: (+1) 301-948-3730; or on the web: http://ts.nist.gov/srm

Those interested in finding out more on the subject should contact the International Cement Microscopy Association (ICMA), which is devoted specifically to microscopy of cement and clinker. ICMA holds an annual conference with 30 or more presentations on various cement microscopy topics. The conference attracts microscopists and cement plant quality-control personnel from around the world, and conference proceedings are available on ICMA's web site. Contact the ICMA by fax: (+1) 618-524-7291, or on the web at: www.cemmicro.org

Other microscopy resources include:

* Microscopical Examination and Interpretation of Portland Cement and Clinker by Dr. Don Campbell, published by Portland Cement Association (PCA), 1999. Campbell discusses preparation methods and evaluation techniques, and clinker microscopic observations and interpretation are supported by 176 color photomicrographs. Raw feed microscopy is also described. To order, contact PCA Customer Service at (+1) 800-868-6733 or order online at: www.portcement.org

* The Effect of Clinker Microstructure on Grindability: Literature Review Database by Linda Hills, 1995, also available from PCA. This publication provides a compilation of literature on the relationship between clinker microstructure and grindability. Information is summarized and presented in table format by categories of phase size, morphology, and distribution.

* Ono's Method: Fundamental Microscopy of Portland Cement Clinker, Dr. Yoshio Ono, published by the Chichibu Onoda Cement Corp., 1995. Ono's most recent book is not only a good guide to the Ono method, but also includes a detailed description of clinker formation. Nearly 200 color photomicrographs demonstrate clinker phases produced from varying burning conditions. To order a copy of Dr. Ono's book, contact the librarian at Taiheiyo Cement Co. (formerly Chichibu Onoda Cement Corp.) at (+81) 43-498-3816, or via the company's web site: www.taiheiyo-cement.co.jp.