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Cement Portland Microscopy by ono method

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  • MICROSCOPICALEXAMINATION ANDINTERPRETATION OFPORTLAND CEMENTAND CLINKER

    by Donald H. Campbell, Ph.D.

    SP030

    P O R T L A N D C E M E N T A S S O C I A T I O N

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    Microscopical Examination and Interpretation of Portland Cement and Clinker

    Microscopical Examination and Interpretationof Portland Cement and Clinker

    Second Edition

    Donald H. Campbell, Ph.D.

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    PCA SP030

    Authored by:Donald H. Campbell, Ph.D.President, Campbell Petrographic Services4001 Berg RoadDodgeville, WI 53533-8508Phone: (608)623-2387Fax: (608)623-2594

    Edited by:Natalie C. Holz, Associate EditorPortland Cement Association

    Published by:Portland Cement Association5420 Old Orchard Rd.Skokie, IL 60077-1083USAPhone: (847) 966-6200Fax: (847) 966-8389Website: www.portcement.org

    Print history:First edition 1986Second edition 1999

    1999 Portland Cement AssociationAll rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmittedin any form or by any means, electronic, mechanical,photocopying, recording, or otherwise, without theprior permission of the copyright owner.

    Printed in the United States of America

    This publication is based on the facts, tests, and authorities statedherein. It is intended for the use of professional personnel compe-tent to evaluate the significance and limitations of the reportedfindings and who will accept responsibility for the application ofthe material it contains. The Portland Cement Association disclaimsany and all responsibility for application of stated principles or forthe accuracy of any of the sources other than work performed orinformation developed by the Association.

    Manufacturers and products are listed for reference or to assist inlocating various products. This does not imply Portland CementAssociation endorsement or approval.

    Warning: Contact with wet (unhardened) concrete, mortar,cement, or cement mixtures can cause SKIN IRRITATION,SEVERE CHEMICAL BURNS (THIRD-DEGREE), or SERIOUSEYE DAMAGE. Frequent exposure may be associated withirritant and/or allergic contact dermatitis. Wear waterproofgloves, a long-sleeved shirt, full-length trousers, and proper eyeprotection when working with these materials. If you have tostand in wet concrete, use waterproof boots that are highenough to keep concrete from flowing into them. Wash wetconcrete, mortar, cement, or cement mixtures from your skinimmediately. Flush eyes with clean water immediately aftercontact. Indirect contact through clothing can be as serious asdirect contact, so promptly rinse out wet concrete, mortar,cement, or cement mixtures from clothing. Seek immediatemedical attention if you have persistent or severe discomfort.

    Library of Congress Catalog Card Number 85-63563ISBN-0-89312-084-7

    SP030.02T PCA R&D Serial No. 1754

    Cover Photo:Upper left: Polished section of portland cementclinker at 400X(see also page 79). (S#A6636)Lower left: Feed particles in thin section(see also page 120). (S#A6715)Right: Polished section of cement in epoxy(see also page 68). (S#A6622)

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    Microscopical Examination and Interpretation of Portland Cement and Clinker

    Table of ContentsPreface to the First Edition ........................................................................................................vPreface to the Second Edition .................................................................................................. viAcknowledgments ................................................................................................................... viiiIntroduction ................................................................................................................................ ixChapter 1 History of Clinker Microscopy ................................................................................. 1

    Photomicrographs of Aspdin Paste .................................................................................. 1Chapter 2 Sampling and Sample Storage ................................................................................ 7

    Sampling .......................................................................................................................... 7Sample Storage................................................................................................................ 8Storage of Prepared Specimens ...................................................................................... 8

    Chapter 3 Stains and Etches ................................................................................................... 11Aluminates and Free Lime ............................................................................................. 11Silicates .......................................................................................................................... 12Calcium Fluoroaluminate ................................................................................................ 14Examination of Stained Cement ..................................................................................... 15Photomicrographs of Effects of Stains and Etches ........................................................ 16

    Chapter 4 Preparation of Polished Sections,Thin Sections, and Particle Mounts ....................................................................................... 19

    Basic Steps for Rapid Polished Section Preparation ..................................................... 20Encapsulation, Impregnation, and Particle Mounting ..................................................... 21

    Encapsulation and Impregnation .............................................................................. 21Sawing, Grinding, and Polishing .................................................................................... 22

    Isomet and Minimet Method .............................................................................. 22Use of Horizontal Rotary Grinder/Polisher ................................................................ 23Harriss Technique .................................................................................................... 24

    Thin Sections.................................................................................................................. 25Techniques with Hyrax and Meltmount ................................................................... 26Particle Mounts on Thin Epoxy Film ............................................................................... 27

    Chapter 5 Microscopic Characteristics of Clinker Phases .................................................. 29Alite ................................................................................................................................ 30Belite .............................................................................................................................. 32

    Comments on Belite Classification and Polymorphic Varieties ................................ 34Tricalcium Aluminate ...................................................................................................... 36Alkali Aluminate .............................................................................................................. 37Ferrite ............................................................................................................................. 37Free Lime ....................................................................................................................... 38Periclase ......................................................................................................................... 39Alkali Sulfates ................................................................................................................. 39Miscellaneous Phases.................................................................................................... 40

    Chapter 6 Onos MethodHistory, Explanation, and Practice ............................................ 43History of Onos Theories of Kiln Control Through Microscopy ...................................... 43The Ono Method ............................................................................................................ 46

    Alite Size ................................................................................................................... 47Photomicrographs Illustrating Onos Method ............................................................ 48Alite Birefringence ..................................................................................................... 50

    iii

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    PCA SP030

    iv

    Belite Size ................................................................................................................. 52Belite Color ............................................................................................................... 52Use of Onos Table to Interpret Kiln Conditions andFormula to Predict 28-day Mortar-Cube Strength ..................................................... 52

    Additional Comments on the Ono Method and Recent Research.................................. 55Alite Birefringence ..................................................................................................... 55Alite Size ................................................................................................................... 57Belite Color ............................................................................................................... 59

    Chapter 7 Microscopical Interpretation of Clinkers .............................................................. 63Photomicrographs of General Features of Clinkers ....................................................... 68Photomicrographs of Alite .............................................................................................. 79Photomicrographs of Belite ............................................................................................ 88Photomicrographs Illustrating the Matrix ...................................................................... 104Photomicrographs of Free Lime ................................................................................... 110Photomicrographs of Periclase .................................................................................... 114Photomicrographs of Miscellaneous Phases ............................................................... 117

    Chapter 8 Misinterpretations in Clinker Microscopy .......................................................... 121Photomicrographs of Artifacts ...................................................................................... 122

    Chapter 9 Scanning Electron Microscopy ........................................................................... 127Scanning Electron Microscopy ..................................................................................... 129

    Chapter 10 Microscopical Examination of Portland Cement Raw Materials .................... 139Selected Literature Review .......................................................................................... 139Raw Material Examination ............................................................................................ 142

    Petrographic Identification of Raw Feed Constituents ............................................ 142Feed Particle Classification .................................................................................... 143Application of F. L. Smidths Burnability Equations ................................................ 144Sample Preparation and Method of Counting......................................................... 147 Sample Preparation ........................................................................................... 147 Insoluble Residues ............................................................................................ 147 Counting Method ............................................................................................... 147Thin-Section and Half-Section Methods for Raw Feeds ......................................... 148 Half-Sections ..................................................................................................... 150Organic and Inorganic Stains for Raw Feed Mineral Identification ......................... 150 Stain Technique No. 1 ....................................................................................... 150 Stain Technique No. 2 ....................................................................................... 151 Stain Technique No. 3 ....................................................................................... 151Photomicrographs of Portland Cement Raw Materials ........................................... 153

    Chapter 11 Recommended Formats and Materials ............................................................. 163Suggested Format for Detailed Clinker Examination ................................................... 163Extraction Techniques for Concentration of Clinker Silicates and Matrix ..................... 166Quantitative Microscopy ............................................................................................... 167Microscopical Equipment, Supplies, and Thin Section Services .................................. 169

    Chapter 12 Conclusions ........................................................................................................ 173References .............................................................................................................................. 177Glossary .................................................................................................................................. 193Author Index ........................................................................................................................... 195Subject Index .......................................................................................................................... 197

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    Microscopical Examination and Interpretation of Portland Cement and Clinker

    PREFACE TO THE FIRST EDITION

    The aim of this handbook is to improve economicalproduction and quality control of portland cement.Samples of clinker, cement, and raw materials can beprepared for microscopical examination with relativeease and rapidity. Virtually immediate improvementsin the production process can result, quickly justifyingthe costs of optical equipment and personnel training.Use of the microscope, therefore, readily translatesinto energy savings and production of a competitivecement while facilitating control of cement quality.The underlying variables in the equation of cementquality and performance are essentially those of manyother chemical (mineral) industries: nature of the rawmaterials, efficiency of treatment of those raw materi-als during product manufacture, and the proper use ofthe product. Consequently, the answer to the questionHow can we improve the quality of portlandcement? lies, to a great extent, in the sciences ofmineralogy and chemistry.

    The primary purposes, therefore, of this publica-tion are1. To describe the methods of sample preparation

    for microscopical study and to recommend theuse of certain methods of analysis and micro-chemical techniques

    2. To describe the common phases in portlandcement clinker

    3. To present a set of microscopical observations(illustrated with photomicrographs where pos-sible) with corresponding genetic interpreta-tions drawn, for the most part, from publishedsources.An effort has been made to present information

    valuable in day-to-day cement manufacture and toseparate microscopical observation from interpreta-tion. Even though some interpretations may be some-what contradictory from author to author, such contra-dictions point out directions for further research. Thecompilation of optical data and interpretations is there-fore considered preliminary and should serve as thebasis for continued study of clinker phases, preferablywith statistical methods.

    The publication is not meant to cover the theory oflight transmission in solid crystalline and noncrystal-line media or optical mineralogy. These subjects arediscussed by Midgley * in Taylor (1964), Wahlstrom(1969), and Kerr (1977). The readers working knowl-edge of polarized-light and reflected-light microscopyis assumed. College or industrial courses or privatestudy and experience in light microscopy are requiredto derive optimum benefit from this material, which,for the most part, evolved from a course in cement andclinker microscopy given for several years at the Port-land Cement Association (PCA), Skokie, Illinois.Consequently, this handbook was written for the prac-ticing microscopist in the cement plant or in theresearch laboratory.

    Most of the photomicrographs were taken by thewriter as part of a PCA research project (HR-1404,Microscopical Analysis of Clinker) in which samplesof raw feed, clinker, and cement from approximately51 North American kilns were studied and interpreted.

    One should not assume, however, that interpre-tive cement microscopy has an unalterable foundationin optical fact, for much research remains to be done indescribing and defining the correlations between mi-croscopical observation and the production regime.Extensive systematic research is needed on the natureof portland cement phases (in particular, the polymor-phic varieties) discerned through combined observa-tions utilizing transmitted- and reflected-light micro-scopes, scanning and transmission electron micro-scopes, electron microprobe, and X-ray diffraction. Anappreciation of the techniques, problems, and applica-bility of these complementary modes of analysis addsimmeasurably to the depth of ones competence inclinker interpretation and consequently increases onesvalue in the economics of cement production. Modernmethods of cement production, therefore, require mod-ern techniques of microscopy and chemical analysis.

    v

    ________________

    * See references at the back of this publication.

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    PCA SP030

    PREFACE TO THE SECOND EDITION

    Many reports concerning raw feed, clinker, and cementmicroscopy have been published since the first editionof this book in 1986. Most of the publications are in theannual Reviews of the General Meetings of the JapaneseCement Association (JCA), the monthly journal ofZement-Kalk-Gips (ZKG), the Proceedings of the Inter-national Cement Microscopy Association (ICMA), and afew other journals. Thus considerable space, describingsome of the salient results of research that have applica-tion to or involve microscopy, is required to bring therevised edition of this book up to date. Selected informa-tion from these publications has been inserted into therelevant contexts throughout the second edition. Timeand the requirements of other projects, unfortunately,have not permitted a review of all the available literatureand, regrettably, some probably very informative ar-ticles have been unintentionally omitted.

    The strong influence of the meritorious work ofYoshio Ono of Chichibu Onoda Cement Company isseen not only in the Japanese literature but also in thebasic and practical research from workers in othercountries, some of whom have challenged Ono whileothers have defended and, to some degree, verifiedOnos broad interpretations of kiln conditions in labo-ratory and plant studies. Ono recently summarizedmuch of his more than 40 years of industrial research ina Chichibu Onoda publication (1995), Onos Method,Fundamental Microscopy of Portland Cement Clin-ker, in which he emphasized the use of polished sec-tions and etching degree to evaluate clinker. Onos kilninterpretations, based largely on transmitted- and re-flected-light characteristics of the clinker silicates, ap-pears to be of optimum use in cement plants character-ized by relative uniformity of the major pyroprocessingvariables, and during start up.

    Illustrating the complexity of clinker phase crystalchemistry and microscopy, the basic research workof Iwao Maki at the Nagoya Institute of Technology,Nagoya, Japan, is especially illuminating anddefinitive.

    Recognition of the profound effects of raw feedparticle size, mineralogy, and homogeneity in control-ling many clinker silicate characteristics has come tothe forefront in clinker interpretations in recent years.As the reader will undoubtedly observe in this book,the separation of raw feed and clinker phase micros-copy and interpretation is exceedingly difficult be-cause of their many complex relationships. Thus, onemight expect to find discussions of alite crystal size interms of nodulization, feed mineralogy/particle size,SO3, etc. Indeed this complexity makes for continuinginterest. Consequently, the microscopy of raw feed isgiven major emphasis in the Second Edition, forming anewly added Chapter 10. Most of the added references,observations, and interpretations in the second editiondeal with correlations of raw feed characteristics withclinker microscopy. A new classification of belite, basedon internal microstructure, and a classification of ma-trix crystal size are proposed. A few of the previouslypublished clinker photographs have been eliminated,improved, or replaced, and many photomicrographsof raw feed particles have been added.

    As we look to the future, we see an increasingapplication of electronic controls in clinker and cementproduction, expensive automated systems that, theo-retically, eventually provide a higher-quality productat a reasonable price. The essential value and use ofmicroscopy in the cement industry, however, have notchanged. The light microscope remains an economical,practical, easily applied means of material quality controlfrom the quarry to the construction. It should be a comple-mentary tool amidst other equally valued instrumentsof analysis. But, as in mastery of the piano, the virtuosomust practice, practice, practice. One can alwaysmake better observations, tighter correlations, and moreexplanatory interpretations.

    Thus it is to my fellow microscopical practitioners,my friends and colleagues, those who recognize thetremendous value of the microscope, that I dedicatethis book.

    iiivi

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    Microscopical Examination and Interpretation of Portland Cement and Clinker

    For this second edition, I am particularly indebtedto Mr. Steven H. Kosmatka of the Portland CementAssociation in Skokie, Illinois (USA), for his conge-nial, editorial thoroughness and tenacity, to DianeVanderlinde who masterfully re-keyed the entiremanuscript, to Natalie Holz for her meritorious edito-rial efforts, and to the staff at Construction Technol-ogy Laboratories, particularly F. M. Miller and FulvioTang who ably assisted me on numerous occasions inthe pursuit of answers. Gratitude is also extended toWalt Rowe (Centex Construction Products), HungChen (Southdown Inc.), and Paul Tennis (PCA) fortheir thorough thoughtful reviews.

    Donald H. Campbell, Ph.D.

    vii

    Conversion factorskg/cm2 (14.22) = psipsi (0.006894) = MPakg/cm2 (0.09807) = MPa

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    PCA SP030

    viii

    ACKNOWLEDGMENTS

    The writer is particularly grateful to the late George J.Vanisko of PCA who introduced the author to thesubject of clinker and cement microscopy and whoparticipated in the teaching of that subject in a coursegiven at the PCA laboratories in Skokie, Illinois. Vaniskowas particularly fortunate to have had instruction fromYoshio Ono and persevered in the mastery of what helearned.

    The writer is indebted to Stewart Tresouthick, pastdirector, Chemical-Physical Research Department, CTL,and Jack Prout, St. Marys Cement Company, Toronto,Ontario. Gratitude is also extended to G. R. Long of theBlue Circle Research Laboratories in Greenhithe, En-gland, for assistance at numerous times, especially forinformation on the calcium silicosulfates, and to Dr.Peter Hawkins, California Portland Cement Company,for procedure utilizing the Babinet compensator todetermine alite birefringence. Yoshio Ono (formerly ofChichibuOnoda Cement Company, Tokyo), Rong FarLee (Taiwan Cement Corporation, Taipei), and IwaoMaki (Nagoya Institute of Technology, Japan) havebeen particularly helpful through correspondence onseveral occasions. I am grateful to Hugh Love for valu-able assistance in the scanning electron microscopy andJean Randolph, for aid in typing many of the observa-tions and interpretations. My wife, Karen, kindly pro-vided expertise on text and photograph formats, andassisted in editing and checking references.

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    Microscopical Examination and Interpretation of Portland Cement and Clinker

    The fundamental use of the microscope in portlandcement clinker analysis is to bring to the observer avisual appreciation of phase identities, sizes, condi-tions, and mutual relationships. With only a basicassemblage of equipment, microscopical analysis canbe easily performed, in many cases within a fewminutes. The rapidity with which potentially energy-saving information can be acquired clearly rendersthe analysis economically justifiable, especially in rou-tine quality-control and trouble-shooting situations.In addition, the microscope has obvious value inscientific research in the manufacturing process.

    Study of the polished section or thin section ofportland cement clinker, for example, quickly revealsseveral details of crystal size, morphology, abundance,and distribution, leading almost intuitively to inter-pretations relating these data to certain features of theraw material and burning conditions. The microscopi-cal method of analysis, using polished sections or thinsections of clinkers, is uniquely advantageous be-cause the investigator can see individual crystals,virtually undisturbed, in their place of origin, and caninterpret these observations in terms of the microenvi-ronment developed in that clinker nodule. These ob-servations are related to characteristics of the raw feedparticles and the burning conditions in the kiln. Forexample, nests of tightly packed belite crystals form insilica-rich areas of the clinker and suggest the possibil-ity of coarse quartz grains in the raw feed. Alite crystalsizes of 10 to 15 m may indicate an undesirably rapidrate of temperature rise in the clinker as it passesthrough the kiln. Large clusters of free lime suggestcoarse limestone particles.

    Following are some of the many aspects of port-land cement production in which microscopy canplay an analytical and quality-controlling role:

    1. Analysis of Raw MaterialsA. Quarry rock analysis

    (1) Areal and volume distribution of rocktypes

    (2) Mineralogy and chemistry(3) Potential grindability

    B. Raw-mix analysis(1) Mineralogy and chemistry of size frac-

    tions and individual phases(2) Efficiency of grinding and homogeni-

    zation processes(3) Estimation of burnability

    2. Clinker and Cement ExaminationA. Phase changes and phase concentrations

    at various stages in the pyroprocessingsystem (including buildups, rings, coat-ings, and clinker-refractory reactions)

    B. Temperature profileburning efficiencyrelationships in the calcining and burn-ing zones of the kiln(1) Rate of heating (rate of temperature

    change in the kiln feed through theapproximate range of 1200C to1600C)

    (2) Maximum clinker temperature (aboveapproximately 1450C)

    (3) Time of clinker retention at high tem-perature (length of time above ap-proximately 1400C)

    (4) Rate of clinker cooling (rate of tem-perature change from maximum toapproximately 1200C)

    C. Grinding and storage(1) Prediction of clinker grindability(2) Efficiency of clinker-grinding process

    (mineralogy of size fractions, estimateof Blaine surface area)

    (3) Clinker weathering during storageD. Prediction of cement performance

    (1) Hydration characteristics

    ix

    INTRODUCTION

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    PCA SP030

    (2) Strength gain(3) Sulfate resistance

    3. Analysis of Other MaterialsA. Dust mineralogy and chemistry

    (1) Stack emission(2) Bag-house collection

    B. Coal(1) Mineralogy(2) Fineness(3) Grindability

    C. Constitution of coal ash and slag(1) As blended material in cement(2) As a raw material for kiln feed

    D. Gypsum and other sulfates(1) Purity (byproduct or natural deposit)(2) Size distribution (grinding efficiency)(3) Alterations in silo storage and grind-

    ing effectsE. Metallography

    (1) Kiln chain examination(2) Grinding-ball examination

    Optimum use of the microscope requires cer-tain skills of the microscopist. Above all, one mustbe patient in the proper preparation of samplesand diligent in perfecting those analytical tech-niques that give reliable data. Of prime impor-tance is the microscopists ability to quickly recog-

    nize many phases that are routinely investigatedwithout resort to the time-consuming process ofgathering large amounts of optical data. In otherwords, sight identification of phases with a mini-mum of data is clearly an asset. With accumulatedexperience, most of which comprises long hours atthe microscope, an ability for sight identificationof the common phases is attained, interpretationsare refined, knowledge is acquired, and the mi-croscopist can confidently state the results of his orher analysis. A critical eye, an appreciation ofoptical mineralogy, and a knowledge of the chemi-cal nature of the portland cement production pro-cess, therefore, are the primary requirements foroptimum use of the microscope in the cementindustry.

    Up-to-date photographic or electronic equip-ment to provide a permanent record is practicallymandatory. A video or photographic camera at-tached to the microscope can be quite helpful inpresenting microscopical data to others, especiallyin an instructional and recordkeeping context.Complementary use of X-ray diffraction and thescanning electron microscope (with microprobe)add to the versatility of the microscopist, provid-ing structural and compositional details not other-wise available, thus strengthening and wideningthe interpretations.

    x

  • 1PCA SP030

    Although it is not clear whether LeChatelier ex-amined cement made by Joseph Aspdin, who pat-ented portland cement in England in 1824, a fewcomments on the nature of the Aspdin cement appearrelevant to the history of clinker microscopy.

    In 1978 a sample of hardened paste was given tothe writer by Norman Gregg of R. H. Harry Stanger,Ltd., Hertfordshire, United Kingdom. Gregg reportedthat the paste represented several barrels of cement(made by William Aspdin, son of Joseph Aspdin) thathad been aboard a ship that sank in the River Thamesin 1848 near Sheerness, Kent, England. The story ofthese barrels of cement and other early cements is toldby Blezard (1984).

    A polished thin section of the hardened Aspdinpaste (Photographs 1-1 through 1-4) was examined bythe writer and found to contain approximately 10percent unhydrated portland cement clinker (UPC)

    CHAPTER 1History of Clinker Microscopy

    Microscopical descriptions of clinker phases had theirorigins in 1887 with the work of the French chemistLeChatelier. Following the methods of microscopicalanalysis of rocks developed by the English geologistH.C. Sorby, founder of petrography and metallogra-phy, LeChatelier reported the presence of the follow-ing constituents in a portland cement clinker thinsection:1. Clear, colorless, angular crystals with a low

    birefringence, identified as tricalcium silicate2. Rounded, turbid, yellowish crystals with moder-

    ate birefringence, identified as dicalcium silicate3. A dark brown intermediate substance of irregu-

    lar and ragged form with a lime-iron-aluminatecomposition (later shown to be calciumaluminoferrite)

    4. Another material, which, he inferred chemi-cally, should be tricalcium aluminate.

    PHOTOMICROGRAPHS OF ASPDIN PASTE

    Photograph 1-1 Portland cement clinkerparticle in Aspdin paste. Subhedral to euhedralpale-green alite; raggedy, round multicoloredbelite; coarsely crystalline brightly reflecting ferrite;and gray aluminate (left center). Edge of particleshows pseudomorphic hydration effects.(S#A6606)

    Polished section*KOH followed by nital etchFD (Field Dimensions) = 0.21x0.21 mm

    * Polished section photomicrographs were taken in reflectedlight unless otherwise indicated.

  • 2Microscopical Examination and Interpretation of Portland Cement and Clinker

    PHOTOMICROGRAPHS OF ASPDIN PASTE

    Photograph 1-2 Unhydrated portland cementclinker (UPC) in Aspdin paste. Large, blue-green,angular alite; small, tan-orange round belite (Type II,Insley); brightly reflecting ferrite; and pinkish-grayaluminate, presumably C3A. (S#A6607)

    Polished sectionNital etchFD = 0.21x0.21 mm

    Photograph 1-3 Unusually large belite in UPC inAspdin paste. Note prominent lamellar extensions intoferrite matrix. Probably an effect of CaO resorptionduring slow cooling. (S#A6608)

    Polished sectionNital etchFD = 0.21x0.21 mm

    Photograph 1-4 UPC in Aspdin paste. Water etchreveals dark-blue, coarsely crystalline aluminate,presumably C3A. (S#A6609)

    Polished sectionFD = 0.21x0.21 mm

  • 3PCA SP030

    particles. Although the UPCs are far from identical tothose of modern production, they clearly contain,among other phases, the four principal phases typicalof portland cement (alite, belite, aluminate, and fer-rite). Glassy particles were also observed in the Aspdinpaste and appear similar to those described by Idornand Thaulow (1983), who described some of the mi-croscopic characteristics of a precast concrete wallplaced in front of Portland Hall, Gravesend, Kent,England, in 1847. The wall is said to have been built forWilliam Aspdin. Further discussion of this wall andthe nature of the UPCs is given by Blezard (1981 and1984), who shows photomicrographs suggesting acoarsely crystalline clinker that was slowly heatedand slowly cooled. Cements similar to this Aspdincement may have comprised some of the samplesstudied by LeChatelier and other early workers inclinker microscopy.

    Scrivener (1988) studied the Aspdin paste withbackscattered electron imaging (BSE), showing clearlythe development of hydration products pseudomor-phic after the original clinker crystals and drawingattention to the occurrence of layers of hydrationproduct (inner product).

    In 1897 Trnebohm, a Swedish investigator, pos-sibly realizing that because of compositional variationmineral names might be better suited for clinker phasesthan chemical formulas, clearly described the opticalfeatures of the principal clinker phases in thin sectionsand powder mounts and coined the terms alite, belite,celite, felite, and also glassy residue. Trnebohm statedthat belite has two or three sets of cross striations andfelite has one set of parallel striations formed at lowtemperature. Trnebohm related microscopical datato burning conditions, stating:

    1. Well-burned clinkers are less porous and con-tain better-crystallized colorless alite and dirty-green to muddy belite.

    2. Brownish-orange celite functions as a flux, pro-moting the development of the silicates.

    3. Underburned clinker disintegrates because ofatmospheric moisture combining with residuallime.

    Trnebohm also made notable contributions tothe microscopical understanding of cement hydra-tion, a topic that must be left for future discussion.

    Richardson (1903-1905) summarized theories onthe chemistry of portland cement and demonstratedthe use of a polarized-light microscope in the predic-tion of cement quality from clinker examinations.Richardson stated: If the structure is coarser and theelements are more segregated, the cement from such

    a clinker will be less reliable. Most of Richardsonswork, however, was in the laboratory where, withnumerous sintering experiments, he made frequentuse of powder mounts and thin sections to study theproducts. Richardson, undoubtedly, developed anextensive and systematic body of knowledge thatformed foundation for later work by others.

    Bates in 1912, describing some of the cementchemistry work at the National Bureau of Standards,stated (p. 369) It was recognized from the first, that inorder that the studies, which were to be made, mightbe complete, a petrographer with a complete outfit forpetrographic studies must be installed. All burns wouldthen be examined for their constitution according tothe most approved and exacting methods.

    Rankin and Wright (1915), although they werenot particularly concerned with the interpretation ofburning conditions, firmly established the optical prop-erties of pure compounds and the principal phases incommercial cements. These authors systematicallyinvestigated approximately 1000 combinations of lime,alumina, and silica with fully 7000 heat treatmentsand microscopical examinations.

    Using Trnebohms classification, Guttmann andGille in 1928 tabulated the basic optical properties ofclinker phases and the common hydration products.In 1931, Guttmann and Gille summarized the 50-yearcontroversy over the nature of alite and demonstratedconclusively that alite is C3S.

    *

    According to Insley (1936), polished section ex-amination of portland cement clinker was reported byStern (1908) and by Wetzel (1913); but, largely due topoor technique, the metallographic method was aban-doned until Tavascis very detailed paper in 1934 inwhich reflected-light microscopy was combined withthat from transmitted light.

    Tavasci (1934) believed that clinker was com-posed primarily of alite, belite (alpha and beta),and celite (a fine mixture of 3CaOAl2O3 and4CaOAl2O3Fe2O3), with free lime as a frequent addi-tional phase. With a series of etches, including nital,oxalic acid, hydrofluoric acid, water, and others,Tavasci carefully described the various effects of thesesolutions on clinker phases and other synthetic com-pounds. Tavasci presented rather meticulous descrip-tions of the forms of belite, suggesting a martensite-type separation in the transformation of alpha to beta.

    Tavasci classified belite into three morphologicaltypes: I, II, and III. Belite I was said to show striations,sometimes like twinning, prevalently in two direc-tions. The striae were described as being relatively

    * An abbreviated chemical symbolism in which C = CaO, S =SiO2, A = Al2O3, F = Fe2O3, K = K2O, and N = Na2O.

  • 4Microscopical Examination and Interpretation of Portland Cement and Clinker

    thick but not very fitte (a term believed by the presentwriter to mean etching resistance). Upon etching withalcoholic nitric acid (nital) the striae were hollowed outwith respect to the rest of the grain. Belite II crystalswere generally large, containing very fitte striae, thedark striae showing relatively less attack by alcoholicnitric acid than the bright striae. Belite III was compara-tively small and appeared to be an external zone over acore formed by belite I or II. Coarse striation did notoccur in belite III, but fine parallel striations were ob-served like those in belite II. Belite III was said to containa kind of veining formed by inclusions which at highmagnification appear white and in strong relief.

    In 1936 Insley (about whom much more is saidlater) clearly showed that (1) alite is tricalcium silicate(C3S), (2) two different habits of dicalcium silicate (C2S)comprise belite and felite, and (3) celite is tetracalciumaluminoferrite (C4AF). Insleys descriptions and illus-trations of clinker phases remain the basis for much ofsucceeding publications by others.

    Among the many historically important contribu-tors to the microscopy of portland cement clinker, LeviS. Brown deserves special recognition for his observa-tional skills and interpretive acumen. Brown workedfor Lone Star Research Laboratory in Hudson, NewYork, in the 1930s and in 1940 joined the research staffat the Portland Cement Association, where he spentapproximately 25 years in cement and concrete inves-tigations. Most of his scientific efforts were dedicated tothe microscopical interpretation of clinker burning,cement hydration, and concrete deterioration. An un-published report (Brown, 1936) contains the followinginteresting observations:

    1. C3A and C3S were discriminated in thin sectionsand powders mounted in Hyrax, * a synthetic resinwith index of refraction ** of approximately 1.710.

    2. Differences in optical properties of C3S were de-fined and birefringence and morphology wereobserved to show wide ranges; crystal zoningwas not clearly understood.

    3. Optical characteristics of C2S, particularly thediscrimination between polymorphic varieties(alpha, beta, and gamma), were described. Thebetter burned clinkers were said to containrelatively clear crystals.

    4. Optical characteristics of C4AF, especially thecolor variations, were related to burning condi-tions, magnesium oxide content, and a reducingenvironment, the latter indicated by a honey-brown C4AF color and weak pleochroism. Thedarkening and strong pleochroism of C4AF werecorrectly thought to be due to incorporation ofmagnesium oxide.

    5. The morphologic and volumetric changes in thetransformation of calcite to lime in a portlandcement raw mix were described.

    6. Large crystals of periclase were described andexplained as an effect of annealing of commer-cial clinkers.

    7. Gehlenite, found sparingly in practically all port-land cement clinkers, was detected by examina-tion of floating particles that have a uniaxialcharacter and perfect basal cleavage in an oil ofapproximately 1.71 refractive index. Gehlenitewas said to be suggestive of underburning.

    8. Sulfate minerals in clinkers, observed as floatingparticles in refractive index oil, were said tooccur abundantly in underburned clinker. Opti-cal characteristics of clinker sulfates comparedwith sulfate phases formed in the laboratory ledto the conclusion that the low-index mineral inclinker is an alkali sulfate with a variable butsmall amount of calcium sulfate held in solidsolution.

    9. Free lime was seen to increase with raw feedparticle size and decrease with increasing burn-ing time (flame length).

    Brown and Swayze in 1938 published a paperdescribing the application of the microscope to auto-clave problems, namely, free lime and magnesia inportland cement. Three forms of free lime were de-fined: (a) light-burned (quicklime), (b) hard-burned,and (c) air-slaked. The latter type was described as aheretofore unidentified form of calcium hydroxidehaving optical properties different from normal cal-cium hydroxide (portlandite) and thought to be theEpezit which was defined by Guttmann and Gille in1928a and 1928b. Epezite was said to differ fromportlandite in optical sign and refractive indices asfollows:

    Epezite PortlanditeUniaxial (+) Uniaxial (-)Epsilon = 1.55 - 1.56 Epsilon = 1.545Omega = 1.54 - 1.55 Omega = 1.574

    Epezite typically forms tiny popcornlike crystals.Portlandite crystallizes in pore spaces as tablets andplatelets. The growth of epezite was thought to beresponsible for clinker disintegration in open storage(even in supposedly tightly sealed containers on the

    * Hyrax (no longer available) is briefly described in Chapter 4under the heading Techniques with Hyrax.

    ** Unless otherwise stated, the indices of refraction given in thisbook refer to sodium light.

  • 5PCA SP030

    laboratory shelf) due to the 97% volume change whenfree lime combines with atmospheric moisture, hencethe term air-slaked.

    Browns most widely known published work ishis Microscopical Study of Clinkers (1948) in which 21different lots of clinkers were microscopically studiedat the Portland Cement Association laboratories inorder to correlate mineral composition with whatBrown termed the degrees of burning in the cementkiln and concrete durability. Although Browns de-scription and interpretation of what he termed theglass and dark prismatic phase may be question-able in the light of recent research, his work in clinkerand concrete microscopy was seminal. Brown contrib-uted significantly to the discussion of clinker phases ina book by Insley and Frchette (1955).

    Brown summarized his philosophy of micros-copy in 1959 when he discussed the two primarymodes of specimen examination (transmitted and re-flected light); the phase rule in relation to the micro-scope; the nature of cement hydration and its effectson strength, water-cement ratio, dimensional stabil-ity, durability, and other concrete properties.

    Tavasci (1978) elaborated on the three forms ofbelite (I, II, III), relating them to C2S polymorphs(alpha, alpha prime, beta, and gamma), and attemptedto show the analogy with the austenite-martensiteconversion in high-carbon, hardened steel. Belite Iwas said to contain alpha lamellae (etching relativelylight colored in nital) and alpha prime lamellae (nar-row and etching dark in nital). Belite II was said tocontain alpha and beta; the alpha prime, having origi-nally formed from alpha, was transformed to betaupon further cooling. Alpha remained as an includedphase. Belite III differed from belite II in having sharperand less-regular separation of the alpha inclusions.

    Also among the many major historical contribu-tions in clinker microscopy are the works of Parkerand Nurse (1939); Taylor (1943); Gille (1955); Krmer(1960); Nurse, Midgley, and Welch (1961); Midgley(1964); Butt and Timashev (1974); and others. Most ofthese authors are mentioned again in Chapter 7,Microscopical Interpretation of Clinkers.

    Three publications of European origin are consid-ered required reading for cement microscopists:1. Mikroskopie des Zementklinkers, Bilderatlas, F. Gille,

    I. Dreizler, K. Grade, H. Krmer, and E. Woermann(1965, Verein Deutscher Zementwerke),

    2. Microstructure of Portland Cement Clinker,Friedrich Hofmnner (1973, Holderbank), and

    3. Microscopy of Cement Raw Mix and Clinker, ErlingFundal (1980, F.L. Smidth).As will be evident, the present writer has drawn

    heavily on the above three publications, plus severalJapanese reports, particularly the work of Yoshio Ono(1995), whose detailed studies demonstrate the prac-ticality of transmitted-light microscopy in the cementplant. Onos Method is discussed in Chapter 6.

    A commendable effort to bring cement and con-crete microscopists together for the purposes of shar-ing knowledge and promoting the use of the micro-scope in the construction industry is seen in the found-ing of the International Cement Microscopy Associa-tion (ICMA) in 1980. * Published proceedings of theirannual meetings have helped immeasurably in spread-ing knowledge of various microscopical methods andhave generally stimulated growth in cement qualitycontrol through microscopy in North America.

    Illustrating quality-control methods in well ce-ments, polarized-light microscopy and fluorescentmicroscopy have been applied to the analysis of oil-well cement blends containing pozzolans, bentonite,potassium chloride, friction reducer, modified poz-zolan, fluid-loss addition, silica flour, and other mate-rials (Reeves, Bailey, and Caveny, 1983). Examinationof cement polished sections has shown a relationshipof oil-well cement thickening times and retardationrates (Caveny, Weigand, and Bailey, 1983). Cavenyand Weigand (1985) described a good oil-well cementas having well-formed alite (40 to 50 microns), nosurficial deterioration of silicates, low free lime (lessthan 0.5%), and being free of metallic iron.

    Relatively recent contributions to oil-well cementmicroscopy include Polkowski (1987) who concludedthat four cements with less than ideal microscopicalcharacteristics still performed acceptably with differ-ent loadings of admixtures.

    Carruthers, Livesay, and Wells (1994) describesome of the burning conditions required for produc-tion of a Class H (HSR) oil-well cement: (1) hot burn-ing zone and long retention time (dendritic belite), (2)high burning zone temperature and long burningzone (cannibalistic alite, wrap-around belite), (3)lengthening of burning zone and increasing tempera-ture (belite beginning to disperse, silicate enlarge-ment, and clarification of matrix), (4) dust recircula-tion (zoning in alite), slow cooling from extension ofburning zone farther back in the kiln (ragged belite),and others. Desirable properties of the Class H (HSR)cement include a dead burned clinker with large alite,cannibalistic alite, amoeboidal belite, wrap-aroundbelite, and finely crystalline C3A.

    * 1206 Coventry Lane, Duncanville, Texas 75137 U.S.A.e-mail: [email protected] address: www.cemmicro.org

  • 6Microscopical Examination and Interpretation of Portland Cement and Clinker

    The desirable characteristics of Class H well ce-ment were listed by Arbelaez (1990): free lime levelsless than 0.5% with a uniform distribution, C3A lessthan 6.5%, no weathered clinker, using only the 12.7-to 38.1-mm clinker fraction for the cement, relativelyhot burning without production of cannibalistic alite,and avoidance of ragged belite by rapid cooling.

    The subject of clinker grindability also has micro-scopical aspects and the most complete literature sur-vey, to date, is that of Hills (1995) who enumerated mostof the prevailing agreed-upon relations (such as de-creasing alite crystal size increasing grindability). Othervariables on which the interpretations were not as clearcut (such as percent liquid phase) were also listed.

    Tachihata, Kotani, and Jyo (1981), in a laboratorystudy of the relationships between rate of heating, rawmeal fineness, and other factors, concluded that clin-kers with large crystal sizes in a narrow size rangeshowed unfavorable grindability, and that crackswithin the crystals and at the boundaries were some ofthe most important factors in grindability.

    Viggh (1994) studied clinker grindability and otherrelated cement characteristics, concluding, amongother things, that better grindability results with in-crease in liquid and alite percentages, and a decreasein alite crystal size. Poorer grindability resulted whenbelite percentage and crystal size increased. A de-crease in setting time and improvement in strengthdevelopment follow from better grindability.Cement flowage was said to be dependent on theamount of gypsum.

    Theisen (1993) described a rapid method of mi-croscopical determination of alite and belite size andapproximation of visible pore space by recording thenumber of intercepts along a line of traverse in succes-sive fields of view. The intercept numbers were usedwith Bogue calculations and related to power con-sumption (kwh/t) in grinding. Data can be gatheredin less than an hour.

    Many additional recent publications linking mi-croscopy to a wide range of performance-related prop-erties of cement are given in the following chapters.

  • 7PCA SP030

    CHAPTER 2Sampling and Sample Storage

    ticles, from which a few particles are randomly selectedor riffled (1) for encapsulation in epoxy for reflected-light examination and (2) for further crushing to apowder for immersion in oil on a microscope slide forexamination in transmitted polarized light. Severalencapsulations can be made, thereby increasing theprobability of studying particles representing most ofthe original clinker sizes. One should be aware thatdifferent size fractions of crushed clinkers may havedifferent phase abundances (some alite-rich, othersbelite-rich).

    J.D. Dorn (personal communication, 1985) statedthat clinkers less than approximately 25 mm are virtu-ally the same and that larger clinkers exhibit effects ofdifferent cooling rates. Dorn recommended passing aliter of clinker through a crusher, producing particlesof approximately 5-mm diameter, followed by rifflingto a volume of 1/4 liter and pulverizing to less than0.59 mm. The 0.59- to 0.30-mm (No. 30- to 50-mesh)fraction is used for a polished section.

    Centurione (1993) recommends an initial 15-kgclinker sample, which is then quartered to 2.5 kg andsieved. The sieved fractions are crushed, sieved into2.4-, 0.6-, and 0.3-mm fractions, and blended. A 50-gram sample is taken for microscopy, XRF, and chemi-cal determination of free lime.

    One problem with the crushing of clinker prior toexamination is that microcracks seen in polished-sec-tion or thin-section study are ambiguously interpreted.Microcracks that are not artifacts of sample prepara-tion may, in some investigations, be related to straincaused by thermal stress (Hornain and Regourd, 1980),crystal reorganization, hydration, and expansion.

    If the clinker is extremely sandy or dusty, crush-ing prior to sieving may not be necessary. A randomspoonful taken from a well-mixed sample will likelybe adequate.

    Other workers prefer to sieve the clinker sample,after which representative portions of arbitrarily

    SAMPLINGTaking the clinker sample for microscopical examina-tion has, as yet, no formally accepted procedure andseveral techniques are currently used, largely depen-dent on the purpose of the investigation. Because of timeconstraints during clinker analysis, the clinker samplemust necessarily be small and, therefore, the conclu-sions must be cautiously drawn. A grab sample is pref-erable to composite samples for most investigations.

    Hofmnner (1973) recommends the following sam-pling technique:

    1. At intervals of five minutes or less take three 2-kgsamples, mix, and quarter down to 500 g.

    2. Crush the 500-g sample to 5-mm particles.3. Quarter until a sufficient amount remains for

    encapsulation with resin in a 25-mm-diametercup. Two encapsulations are recommended toget a representative average of the randomsample.

    Ono (1981) recommends a grab sample every eight-hour shift during clinker production; hourly samplesare taken during kiln startup.

    Hicks and Dorn (1982) recommend the Ono test(except birefringence) once per day and every time achange is made in the burning process and, once perweek, a polished-section examination of the 0.84- to0.59-mm (No. 20- to 30-mesh) granulated clinker.

    For determination of the phase content of clinker,Chromy (1983) utilized polished sections made fromthe quartered residue from 0.5 kg of clinker ground toa particle size passing a 1.0-mm sieve. A 20-mm-diam-eter polished section of particles embedded in epoxywas prepared.

    One of the most popular methods involves crush-ing a random clinker sample of roughly a liter volume(1 to 2 kg) to approximately 2- to 4-mm-diameter par-

  • 8Microscopical Examination and Interpretation of Portland Cement and Clinker

    defined coarse, medium, and fine fractions are selectedfor analysis. Whole or crushed clinkers are encapsu-lated in epoxy and polished sections are prepared.Powders for study in transmitted light can be madefrom representative portions of the same sieve fractions.

    Long (1982a) stated that the sampling techniquemust be dependent on the kind of problem underinvestigation. A constant cement-quality problem mightbe studied with a clinker grab sample. However, foranalysis of process variations of several days, for ex-ample, a combination of several clinker samples to forma composite might provide an abundance of informa-tion, particularly if the clinker shows variability. Hourlysamples may also be studied as kiln modifications takeplace. Long recommends taking a 15-kg sample, crush-ing it to less than 6 mm, then riffling or quartering andseparating the 2- to 4-mm fraction for microscopicalstudy. Dusty or sandy clinker should be sieved into acoarse fraction (greater than 2 mm) and a fine fraction(less than 2 mm). The coarse fraction is then crushed tosupply the 2- to 4-mm-size material for microscopicalstudy as a companion to the fine fraction. Whole clinkernodules can also be studied. These should proportion-ally represent the sizes of the nodules in the grab sampleand typically number 10 to 12.

    The sampling method normally followed by theauthor is to restrict the microscopical investigation toclinkers from only a broadly defined modal-size classfrom which a number of clinker nodules (at least 30) arerandomly selected and crushed to 1.0 to 2.0 mm, somefragments for encapsulation in epoxy and others fur-ther crushed and treated with KOH-sugar solution forpowder-mount examination and X-ray diffraction (seeChapters 4 and 5). The broadly defined modal class ispresumed to represent that part of the clinker sizepopulation that volumetrically supplies most of thecement and, therefore, has the dominant influence onthe cements hydraulic characteristics. Thus, by ne-glecting the largest and smallest clinkers, one studiesthe most common clinker sizes that perhaps moreaccurately reflect the burning conditions and the na-ture of the raw mix. Sampling just downstream fromthe cooler is also recommended because the clinkersrepresent a relatively narrow range of kiln conditions,simplifying the interpretation.

    The sampling of cements appears to present nomajor problems. Care should be taken, however, toavoid bias from samples unduly rich in coarse or fineparticles, or samples representing areas that might beaffected by moisture condensationunless incipienthydration is the object of the investigation.

    In conclusion, sample volumes and sampling tech-niques appear to be largely the arbitrary choice of the

    microscopist, with objectivity and relevance to theaim of the investigation as the primary considerations.A standard practice for sampling and sample prepara-tion is needed for routine microscopy. For certainstudies, clinker nodules can be halved, one half formicroscopy, the other half for chemistry and X-raydiffraction (XRD). Only one kiln should be repre-sented in a single clinker or cement sample. A compos-ite clinker sample can be somewhat confusing due tothe possible variety of burning conditions represented.

    Systematic microscopical analyses of the clinkerwith its corresponding raw mix and cement are highlyrecommended. It is not uncommon for the writer toplace a portion of the greater than 45-m cement andraw mix in the same cup with the clinkers for epoxyimpregnation and polished thin-section examination.

    SAMPLE STORAGEPreventing atmospheric hydration and carbonation ofcement and clinker is a difficult but, for most micro-scopical studies, not an insurmountable problem.Sample contact with water, atmospheric or otherwise,should be minimized. For long-term storage, glass jarsor vials with corks or screwtops that have been sealedwith molten wax appear to be moderately effective.During routine examinations, the author stores ce-ment or crushed clinker sieve fractions (after wetsieving with an isopropyl alcohol spray) in 15-mLscrewtop glass vials. Only the less than 75-m size(No. 200-mesh sieve) is retained. To help preventhydration, the vials can be stored over DrieriteTM orsimilar hydrophilic material in a vacuum jar. Varioustypes of plastic bags with sealable tops are availableand may suffice for temporary storage. However,pinholes produced by abrasion are not uncommon ifthe samples have been subjected to jostling or othertypes of rough handling. Metal cans with tight-fittinglids (the type in which paint is supplied) are alsorelatively satisfactory for sample storage.

    Regardless of the type of clinker storage con-tainer, if a significant quantity of free lime is present inthe clinker, disintegration of the clinker nodules willprobably occur as a result of lime hydration (air slak-ing) forming calcium hydroxide. A dry (humidity-controlled) storage room or cabinet is recommended.

    STORAGE OF PREPARED SPECIMENSPolished sections and thin sections can be protectedduring storage by mounting the cover glass with adrop of epoxy (without hardener) on the preparedsection surface. A small dropper bottle containing

  • 9PCA SP030

    epoxy resin (without hardener) is kept at approxi-mately 40C on the slide warmer for the purpose ofmounting cover glasses. Keeping the resin at thistemperature in the bottle seems to minimize the crys-tallization that may occur at room temperature. Thecover glass can be easily removed for several months,but even the epoxy (without hardener) will eventuallybond the cover glass to the section. Then the problembecomes one of trying to remove the cover slip with arazor blade or by grinding. The polished surface canbe protected with an acrylic spray, which can beremoved by gentle rubbing with an acetone- or xy-lene-soaked rag. An acrylic spray eventually cracks,however, and does not prevent hydration of free limeexposed on the section surface.

    Immersion of epoxy-encapsulated materials inpolished sections in an anhydrous lightweight oil(preferably odorless) in a wide-mouth glass jar with ascrewtop lid effectively minimizes, but does not elimi-nate, hydration. If the specimen is re-examined micro-scopically, the oil appearing on the polished surfacecan be removed with a sonic cleaner containing iso-propyl alcohol, followed by a forceful isopropyl alco-hol spray. Dorn and Adams (1983) used Freon in asonic cleaner to remove residual oil on polished-sec-tion surfaces. In the writer's experience, oil droplets ona polished section can be removed with a brief appli-cation of acetone, followed by an alcohol spray wash,and blow drying.

  • 10

    Microscopical Examination and Interpretation of Portland Cement and Clinker

  • 11

    PCA SP030

    CHAPTER 3Stains and Etches

    unless stated otherwise, the tests are carried out atroom temperature. It will be obvious that the effects ofvarious etches and stains are also functions of timeand clinker phase composition.

    Relative reactivities of silicates among severalclinker samples, or comparison of the phase percent-ages of clinkers from different daily productions ordifferent cement companies, can be determined byetching and staining several polished sections simul-taneously at the same temperature. To facilitate thistechnique, one can combine several polished sectionswith a rubber band, immersing the assemblage in theetchant for the required length of time. Thus all sec-tions are exposed simultaneously for the same lengthof time, at the same temperature, and relative rates ofreaction can be evaluated according to the colorsproduced. Similar tests can be performed with 0.2%nital and 0.01% aqueous ammonium chloride. CDTAin successive 15-second applications with examina-tions after each is particularly good to evaluate therelative rates of silicate reactivities in a suite of samplesetched simultaneously. Reaction rates can be increasedby heating the polished section with the hair dryer fora few seconds prior to application of the etchant.

    Another helpful procedure in polished sectionexamination is to immerse only one half of the polishedsurface in water, for example, holding the section witha pair of forceps, spray wash the sample with isopropylalcohol, dry, and then rotate the sample 90 immersinghalf of the section in nital. Thus the surface is dividedby this procedure into quarters: one quarter with onlywater, one quarter with water plus nital, one quarterwith only nital, and a quarter remaining with no etch.

    ALUMINATES AND FREE LIMEA. Potassium hydroxideethyl alcohol solution

    (5%) is placed in contact with the polished sec-

    The techniques of imparting color to various crystallinephases preferentially are well known in geology (seeCarver, 1971, and Hutchison, 1974). Stain differentiationbetween plagioclase and potash feldspars and betweenvarious carbonate minerals is commonplace, using par-ticles, thin sections, and polished slabs. Stains and etchesare those liquids or vapors that, when applied to thepolished cross section of a clinker or to a sample ofportland cement, preferentially color or dissolve certainphases observed in reflected or transmitted light. Thecolors mainly result from the refraction, reflection, andinterference of light within the thin layer of reactionproduct formed on the clinker phases. Stains and etchesare used to bring out microstructural details of indi-vidual crystals. Both stains and etches can be related tothe relative reactivities of various clinker phases. Photo-graphs 3-1 through 3-6 illustrate some of the effects of afew stains and etches.

    Perhaps the most thorough treatment of the sub-ject of stains and etches is the work of Ellson andWeymouth of Australia (1968). Their paper lists ap-proximately 43 reagent solutions and their effects onportland cement and blast furnace slag phases in termsof (a) reaction type (stain or structural etch), (b) timerequired for the desired effect, (c) recommended tem-peratures, and (d) concentrations. Futing summarizedthe application of many varieties of etches in 1986.

    Much of the information given in this chapter wasextracted from the work of John Marlin of the Okla-homa Cement Corporation (now a subsidiary of LoneStar Cement Company, Greenwich, Connecticut).Many of his recipes and results (1978 and 1979) arereproduced in this chapter with only slight modifica-tion but only a few have been tried by the presentwriter. Marlin recommends making fresh solutionsevery two months for most of these stains and etches.Most of the solutions described in the following pageshave simultaneous staining and etching effects, and,

  • 12

    Microscopical Examination and Interpretation of Portland Cement and Clinker

    tion for no more than 20 seconds. Wash thesection surface in a 1:1 ethyl alcohol-water solu-tion followed by a wash in isopropyl alcohol,and buff for approximately 15 seconds onMicroclothTM *,** wetted with isopropyl alcohol.Wash with isopropyl alcohol. C3A turns blue.

    B. Sodium hydroxideethyl alcohol solution isprepared with 2.5 g of sodium hydroxide plus 40mL of water plus 10 mL of ethyl alcohol. If thecontact of the polished surface with the solutionis more than roughly 20 seconds, a deposit froma reaction between hydroxide and aluminateforms that buffing will not remove. C3A turnsblue. If determination of alkali sulfates is desired,stain only one time for approximately 10 seconds,washing with 1:1 ethyl alcohol-water solution,followed by isopropyl alcohol. Do not buff. Thistreatment will darken alkali sulfates slightly andwith prolonged treatment (as for C3A) will dis-solve the alkali sulfate, producing a dark void.

    C. Potassium hydroxide solution (0.1 molar aque-ous) can be applied in single drop fashion or ina small puddle on a polished surface for 30seconds. Rinse with an isopropyl alcohol sprayand dry with forced warm air. C3A and alkali-aluminate stain blue-brown, alkali sulfate dark-ens, and free lime turns brown.

    D. Boiling sodium hydroxide solution (10% bymass) will turn calcium aluminate blue or brownin 20 seconds in a high-alumina cement. Etching30 seconds with a 1% borax solution turns C12A7gray (Long, 1983).

    E. Warm distilled water (40C) in 5 to 10 secondsturns aluminates blue to brown, alite light tan,free lime multicolored, and does not affect belite.

    SILICATESA. Dilute salicylic acid stain is mixed as follows:

    0.2 g salicylic acid plus 25 mL of ethyl alcoholplus 25 mL of water. After a 20- to 30-secondimmersion, followed by an alcohol spray wash,alite and belite are blue-green. A modification ofthis stain is 0.2 g of salicylic acid plus 25 mL ofisopropyl alcohol plus 25 mL of water, which,after 20 to 30 seconds, reveals that alite stains 50percent faster than belite and which, therefore,can be used to distinguish the two phases. Aprecise immersion time for a series of samplesaids in their comparison.

    B. Salicylic acid etchant is made by dissolving 0.5g of salicylic acid in 50 mL of methyl alcohol.After a 45-second etch the alite and belite areclearly seen, the latter showing its lamellar struc-

    ture. Longer contact with the solution degradesbelite lamellae. Alite is more strongly etchedthan belite. This etchant can be used prior toammonium nitrate for alite-belite differentia-tion with very little effect on the matrix phases.Reaction of salicylic acid in ethyl alcohol is 50percent that of methyl alcohol and attacks aliteabout twice as fast as belite. With isopropylalcohol, however, the reaction is less than 25percent that of methyl alcohol, and alite is in-tensely and rapidly attacked, with belite almostnonreactive.

    C. Nital is perhaps the most common etchant andstain for silicates and improves with age. Nital is1.5 mL of nitric acid (HNO3) in 100 mL of ethyl,methyl, isopropyl, or amyl alcohol. The authorroutinely uses a solution of 1 mL of HNO3 and 99mL of anhydrous isopropyl alcohol. The solu-tion quickly reacts in 6 to 10 seconds with aliteand belite. At a 0.05% dilution the reaction timeis 20 to 40 seconds. Ono (1995) relates alitereactivity to color produced with 0.2% nital.Depending on the relative reactivity of silicates,alite normally turns blue to green, belite is brownto blueboth silicates showing details of inter-nal structure. Nital superimposed on a 20-sec-ond potassium hydroxide etch turns C3A lightbrown and colors the silicates.

    D. Acetone-water solution (in a 1:1 proportion)can be used as a rinse because it reacts slowly onsilicates. A 120-second stain time reveals well-stained alite and belite. C3A is also visible.

    E. Isopropyl alcohol solution (10%) is an easilymade stain (10 mL of isopropyl alcohol plus 90mL of water) that reacts strongly with alite andweakly with belite in 30 seconds to 2 minutes.C3A exhibits a weak reaction. Compare with HFvapor.

    F. Maleic acid attacks alite and belite at aboutequal rates and a little faster than salicylic acid.When followed by NH4NO3, it does not givecolor distinction to alite and belite.

    G. Ammonium chloride (saturated, aqueous) col-ors a hexagonal section of alite (perpendicular tothe threefold crystallographic axis) light yellow.The slender hexagonal section of alite (parallelto the c axis) is colored blue. Zoned crystals in theslender hexagonal section show light-blue coresand dark-blue rims. Ono (1995) recommends an

    * MicroclothTM is a tough, feltlike, rayon polishing cloth with alow nap marketed by Buehler Ltd., of Lake Bluff, Illinois.

    ** Manufacturers and products are listed for reference or to assistin locating various products; this does not imply PortlandCement Association endorsement or approval.

  • 13

    PCA SP030

    aqueous ammonium chloride solution (0.2 to2.0%) for etching of polished sections. He relatedthe thickness of the film produced by etching tothe color of the resulting reflected light with theequation R = 2d(n), where R is retardation, d isthe thickness of the thin film of etching product,and n is approximately 1.5. Thus R = approxi-mately 3d. A table of etch colors is presented inrelation to different values of R and d, using awell-burnt clinker and 0.5% ammonium chlo-ride. Likewise, alite etch colors produced with0.2% HNO3-alcohol are presented in relation tolocation in the clinker, R, and d. Many of Onosphotomicrographs, however, indicate etching for20 seconds with water followed by 5 secondswith 2% aqueous ammonium chloride. Uchikawa (1992) summarized the quality-controltechniques for cement and concrete and presenteda numerical etch-color scale from 0 to 16, relatingeach clinker phase reactivity to etch color, using0.01% aqueous ammonium chloride. The interpretedreactivities were said to be relevant to the initial andearly stages of hydration, as well as the sinteringconditions. Alite was reported to be more easilyetched with the increase in heating rate, the de-crease in burning temperature, the coarsening ofthe particles of raw materials, and the burningatmosphere approaching reducing. Interstitial mi-crostructure (ferrite and aluminate crystal sizes,and ferrite crystal shape factor) and the etch colorsof alite were correlated with heat of hydration,mortar flow, and setting time. Relatively slowlycooled matrix was hydraulically more reactive butled to lower, more variable, mortar flow and lowerfluidity. The more easily a clinker was etched, theshorter the initial setting time, which was also short-ened by 40 minutes when free lime was increasedby only 0.5%. Slowly cooled belite (Type IIIb varietyshowing extended lamellae and remelting) wasshown to be colorless and, on a color basis, indistin-guishable from quickly quenched belite. Alite withhigh amounts of impurities and high Al2O3/Fe2O3ratio correlated with low 28-day strength. Underreducing conditions, triclinic alite and partial trans-formation of belite to the gamma polymorph wereproduced, along with smaller alite, larger belite,and lower strength development.

    Dorn and Adams (1983) have described the vari-ous etch rates of alite and belite in relation tohydraulic activity. A blue color on alite after a 15-second nital etch was said to represent an activealite.

    H. Another variety of the ammonium chloride stainis made as follows:

    1 g NH4Cl + 20 mL H2O+ 20 mL ethyl alcohol + 10 mL acetone+ 150 mL isopropyl alcohol

    Effects of this stain are very similar to those ofNH4NO3 except the NH4Cl stain is approximately25% faster. Alite turns brown in 10 to 20 seconds;belite is unaffected. This stain can be used directlyas a belite indicator by extending the submersiontime to 30 to 45 seconds. Alite turns yellow toyellowish green and belite to brown. The effects ofthis NH4Cl solution are not as clear for belitelamellae as NH4NO3 following salicylic acid.

    I. Ammonium nitrate solution is composed of thefollowing ingredients:

    1 g NH4NO3 + 20 mL H2O + 20 mL ethyl alcohol+ 10 mL acetone + 150 mL isopropyl alcohol

    Alite is colored in 25 to 30 seconds. With increas-ing treatment time, the colors on the silicatesprogressively range from light brown to brownto purplish brown to blue to blue-green to greento yellow-green. Normally, when alite is stainedyellow-green, belite will be brown. This solutioncan be applied following the salicylic acid stain toshow alite and belite with an approximately 30-second submersion time.

    J. Hydrofluoric acid (HF) vapor, used to etch andstain a polished clinker, has been a very informa-tive technique (Long, 1982a). Almost all the clin-ker phases can be differentiated with an HF vaporetch. The HF is kept at a temperature of 20C to22C. A finely polished surface is held for 5 to 10seconds in HF vapor and, after waiting a minute ortwo for the excess HF fumes to leave the polishedsurface, the section is examined in reflected light.Belite turns blue and alite is brown. With practiceat varying the etch times one can develop reliableHF-vapor etch criteria for other phases such as thealkali sulfates. Prout reported (personal commu-nication, 1984) that a temperature differential be-tween fume and specimen enhances etching. Thespecimen can be cooled or the HF warmed. Inci-dentally, C2AS (melilite) is colored with HF vaporand occurs in high-alumina cement (Long, 1983).NOTE: Care must be taken to avoid damaging themicroscope objective lens with HF vapors ema-nating from a freshly etched polished section.Waiting a few minutes before examination isrecommended. Because of the extreme danger inskin contact with HF, suitable precautions withgloves and ventilated hood are strongly advised.

    K. Distilled water was described by Brown (1948)as an etch that enabled one to discriminate nineclinker phases after a relief polish. With the use ofpresent-day materials and equipment, Browns

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    Microscopical Examination and Interpretation of Portland Cement and Clinker

    procedure is as follows: (1) Final polish onMicrocloth or nylon with 0.05 m alumina. (2) Aremoval etch, using distilled water at pH 6.8 to7.0, is developed by holding the polished surfacewith moderate pressure on a rotating saturatedMicrocloth for two to three seconds while thedistilled water is poured onto the Microcloth.(3) Wash quickly with isopropyl alcohol and drywith forced warm air. Periclase remains topo-graphically high due to its relative hardness. Freelime etches dark to iridescent green and blue.C3A turns dark blue. What Brown called darkprismatic (actually, alkali aluminate) and raggeddark interstitial material turn faint blue. Alitebecomes brown, and belite is recognized mor-phologically. Alkali sulfates are dark.

    L. Dimethyl ammonium citrate (DAC) solution isprepared by dissolving 192.6 g of citric acid in 1 Lof warm water. The solution is cooled and broughtto 2 L by adding 891 mL of aqueous dimethylammonium solution (33 percent). A 5- to 10-second application of DAC on a polished surfacestructurally etches alite strongly and belite slightly.An optional preparatory etch with water for fiveseconds will aid in the identification of aluminate.

    M. Borax solution is used for etching pleochroite(approximately C22A13F3S4). This mineral occursin some high-alumina cements and characteristi-cally has a bladelike habit. It is etched by boilingin a 1-percent borax solution (Long, 1983).

    N. Cyclohexanediaminetetraacetic acid disodiumsalt, Hexaver Chelant* (CDTA) solution is mixedas follows: 5 g CDTA in 100 mL distilled pure waterplus 100 mL denatured ethyl alcohol. The polishedsection is covered with the etchant and two dropsof etchant are added every 10 seconds until 60seconds have elapsed. The surface is rinsed withethyl alcohol. Alite is blue, green, pastels, and othercolors; belite is not highly colored; ferrite remainsbrightly reflecting; aluminate appears as gray flecksor spots; free lime is high colored (Caveny andWeigand, 1985). Dorn (1985) stated that lime-richalite with a CDTA-type etchant (30 seconds) quicklyturns blue; an average lime-rich belite burns bluishgray. Blue, relatively lime-rich belite crystals occuron the periphery of some belite nests.

    The writer has found that etching and exami-nation with CDTA at successive 15-second inter-vals reveals information about relative rates of alitereactivity, for example, when comparing clinkersfrom different production periods. The polishedsections are bound together with a rubber band andetched simultaneously, or the clinkers can be en-capsulated in a multichambered container.

    CALCIUM FLUOROALUMINATEA staining procedure for calcium fluoroaluminate(rare in normal clinker) was developed by microsco-pists in the 1960s at the PCA laboratories. It is based onthe slightly different activities of C3A and C11A7CaF2.A polished surface of clinker, etched for 3 seconds indistilled water with a pH of 6.5 to 7.0, reveals C3A as abluish color. The surface is then given a second polishand a 30-second etch with a 0.1-molar potassiumhydroxide solution that reveals C11A7CaF2 as a deepbrownish-purple hue. Comments on each of theseetches follow.

    A. Water etch (distilled water) in the pH range of6.5 to 7.0 reacts rapidly with C3A to form aninterference film on the C3A that produces abluish color when viewed through a reflected-light microscope. The procedure must be fol-lowed closely because other colors may appearwith shorter etch times or slightly different acidi-ties. Although the fluoroaluminate compoundsometimes also reacts to produce a faintly vis-ible brownish purple hue, this particular reac-tion is not used for positive identification.

    B. Potassium hydroxide is used for detection offluoroaluminate. The section surface should berepolished after the water etch. The freshly pol-ished clinker surface is then exposed to 0.1-molar potassium hydroxide solution for 30 sec-onds. The fluoroaluminate compound isidentified by the definitive brownish purpleinterference color that is deeper in hue andsharper in outline than the one that, as men-tioned, is sometimes visible after the 30-seconddistilled water etch.

    The 30-second period of etching with the potas-sium hydroxide solution apparently is not criticalsince similar results have been obtained with etchperiods of 25 to 35 seconds or longer. Any reactionproduct of C3A with potassium hydroxide, if present,will not interfere. C3A is more reactive in basic solu-tions than the fluoroaluminate, and the 30-secondreaction time will produce a relatively thick and veryirregular orange-colored reaction product on the C3A.This reaction product does not have a uniform inter-ference color; much of the reflected light is irregularlyscattered to produce a generally nondescript area ofboth positive and negative relief, often giving theappearance of a void in the clinker.

    Experimental work on the microscopical stainingmethod also reveals that fluorine-modified alite could

    * CDTA is available from Hach, Inc., Loveland, Colorado, USA.

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    PCA SP030

    be identified with the 30-second potassium hydroxideetch. In that case, the characteristic pseudohexagonaloutlines of individual alite crystals would becomeragged and indefinite and the crystals would assumea faint brownish purple hue similar to that seen on theternary compound after the 30-second distilled-wateretch. In all cases thus far, little or no fluoroaluminatecompound has been seen when the fluorine-modifiedalite was present.

    It should be pointed out that the potassium hy-droxide method should be applied to materials repre-senting only the compositional field where the usualportland cement phases and fluoroaluminate can oc-cur, as identification does not rely on unique opticalproperties of the various phases.

    EXAMINATION OF STAINED CEMENTA stain technique proposed by Poole and Thomas(1975) for detecting sulfates in aggregates has beenmodified and found to be quite appropriate for gyp-sum, plaster, and anhydrite in portland cement and, tosome extent, alkali sulfates in clinker. The 6% stainsolution is made from an aqueous mixture of BaCl2and KMnO4 in a 2:1 ratio.

    A few milligrams of cement or crushed clinker areimmersed in a puddle of the stain solution in a smallbeaker for 1 minute, after which the mixture is washedwith isopropyl alcohol into a 75-m (No. 200) sieve,and finally into a watch glass. Excess alcohol in thewatch glass can be drawn off with a paper towel. Theresidue is slowly dried in the watch glass at a tempera-ture of approximately 40C, under a heat lamp or onthe slide warmer, and examined with oblique or trans-mitted light on the stage of a polarized-light micro-scope. Oblique light illuminates the particles from apoint beside the microscope objective. Sulfate miner-als retain a prominent pink to red color. A white paperbackground accentuates the color contrast while view-ing the stained particles with oblique lighting. Refrac-tive-index oils as mounting media can be used withtransmitted light; however, the alteration of somesulfates during the staining may interfere with therefractive-index determinations.

    The following easy method for concentratingsome of the sulfates in cement or crushed clinker formicroscopical examination has been developed.(1) Place a small portion of sample into a watch glassand flood with isopropyl alcohol. Swirl the mixture inthe watch glass for a few seconds to concentrate thewhite-to-clear sulfate particles in the center. (2) Drawthe liquid off with a paper towel and dry the remain-ing powder in the watch glass under a heat lamp or onthe slide warmer (no hotter than 40C). (3) Using a thinmetal spatula, scrape off the top-central area of theresidue in the watch glass and place it on a microscopeslide for examination in oil with a selected index ofrefraction (n).

    Although the mount is impure (contains severalphases), in an oil with a refractive index of 1.54,gypsum (n = 1.52 to 1.53) and plaster (n = 1.55 to 1.57)can be distinguished. Gypsum has inclined extinction,but the extinction of plaster is straight (Lea, 1970).Anhydrite (n = 1.57 to 1.61) also shows straight extinc-tion, cleavages at right angles, and relatively highbirefringence. Extinction angles to differentiate gyp-sum from plaster, however, are difficult to apply dueto the very finely microcrystalline structure commonin gypsum and plaster particles. This microstructureappears to be a product of recrystallization due torelatively high temperature, grinding stress, or both,which convert the previously continuous atomic struc-ture into a myriad of minute polygonal crystallineunits. Dorn (personal communication, 1985) reportedthat sulfates such as gypsum and plaster float inrefractive-index oil (n = 1.71) and can be skimmedaside for study in a powder mount; anhydrite maysink in the liquid. A simple heavy liquid-centrifugemethod appears to be an efficient technique for sepa-ration of cement sulfates for microscopical examina-tions and x-ray diffraction, leading, perhaps, to aquantitative determination of phase abundance.

    Gypsum, plaster, alkali sulfates, and epezite floatin refractive index liquid (n=1.715) and accumulatejust beneath the cover glass.

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    Microscopical Examination and Interpretation of Portland Cement and Clinker

    PHOTOMICROGRAPHS OF EFFECTS OFSTAINS AND ETCHES

    Photograph 3-1 Coarsely crystalline clinker fromcoarse seashell feed. Angular subhedral to euhedralblue alite, round tan-to-brown belite with widelamellae, and matrix of dark aluminate (C3A) andbrightly reflecting ferrite (C4AF). Coal- and coke-fired,semidry process kiln, 1850 tons/day. High maximumtemperature, long burning time, slow heating rate,quick to moderately quick cooling, 44.1 MPa.(S#A6610)

    Polished sectionNital on KOH etchField dimensions = 0.21x0.21 mm

    Photograph 3-2 Blue coloration on alite with anotherwise uniform tan color on alite crystals. Possibleexplanations include differences in crystallographicorientation, chemical composition, structural state (forexample, monoclinic versus triclinic), or perhapscombinations of these. (S#A6612)

    Polished sectionNital etchField dimensions = 0.21x0.21 mm

    Photograph 3-3 Differential coloration by nital etchon angular alite and round belite. Coarsely crystallineclinker from coarse raw feed. Coal- and coke-fired,semidry process kiln, 1850 tons/day. High maximumtemperature, long burning time, slow heating rate,quick to moderately quick cooling. (S#A6613)

    Polished sectionField dimensions = 0.21x0.21 mm

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    PHOTOMICROGRAPHS OF EFFECTS OFSTAINS AND ETCHES (CONTINUED)

    Photograph 3-4 Aqueous NH4Cl etch (30 to 40seconds). Sections perpendicular to the ccrystallographic (three-fold axis) of alite are brown.Sections parallel to c are blue. (S#A6614)

    Polished sectionField dimensions = 0.12x0.12 mm

    Photograph 3-5 Large, round, slightly raggedbelite; blue subhedral to anhedral alite; and matrix ofaluminate (C3A, arrow) and ferrite. Superimposed nitalover KOH produces increased clarity in matrix phasedefinition. (S#A6615)

    Polished sectionReflected light with stopField dimensions = 0.21x0.21 mm

    Photograph 3-6 Well-differentiated, finelymicrocrystalline matrix of aluminate (C3A) and ferrite(C4AF) in nital-etched polished section. (S#A6611)

    Oil immersionReflected lightField dimensions = 0.10x0.10 mm

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    Microscopical Examination and Interpretation of Portland Cement and Clinker

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    PCA SP030

    CHAPTER 4Preparation of Polished Sections,Thin Sections, and Particle Mounts

    Table 4-1. Recommended Methods of Microscopical ExaminationTHIN SECTION POLISHED SECTION POWDER MOUNTS

    Transmitted Light Reflected Light Transmitted LightClinker and Cement

    Porosity ++ ++ oAlkali Sulfate o ++ oPericlase o ++ oFree Lime o ++ ++(epezite)*Tricalcium Aluminate o ++ oAlkali Aluminate + ++ +Ferrite + ++ +Alite ++ ++ ++*Belite ++ ++ ++*Metallic Iron + ++ ++(magnetic)Calcium Sulfates + ++Portlandite ++ ++ ++Raw Feed** ++ o +Quarry Rocks** ++ o +Pozzolans ++ ++ ++

    o Not recommended.+ Recommended, but somewhat difficult, requires more training.++ Recommended, relatively easy.Note: A well-made polished, ultra-thin section, allowing the combination of transmitted- and reflected-light observations, sometimes

    simultaneously, is probably the best method for all phases, particularly for fly ash, quarry rocks, and clinkers.

    * KOH-sugar treated sample** Optional staining for mineral identifications

    trates the principal types of sample preparations.Table 4-1 gives the recommended techniques for vari-ous kinds of materials to be examined. This chaptersummarizes sample preparation procedures for clinkerand cement studies using the so-called petrographicmicroscope with transmitted polarized light, the so-called metallographic microscope with reflected light,and the scanning electron microscope (SEM).

    Methods of sample preparation for the microscopicalinvestigation of clinker and cement are essentially theclassic geologic techniques of optical mineralogy andpetrography, with minor variations. In this chapterthe use of diamond-rimmed cutoff saws and variousgrinding and polishing equipment is given principalemphasis in the production of polished sections, thinsections, and polished thin sections. Figure 4-1 illus-

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    Microscopical Examination and Interpretation of Portland Cement and