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Scanning electron microscopy imaging of hydraulic cementmicrostructure
Paul StutzmanBuilding and Fire Research Laboratory
National Institute of Standards and TechnologyGaithersburg, MD 20899 USA
Reprinted from Cement and Concrete Composites, Vol. 26, No. 8, 957-966 pp., November2004.
NOTE: This paper is a contribution of the National Institute of Standards andTechnology and is not subject to copyright.
Available online at www.sciencedirect.com
Cement &ConcreteCompositesELSEVIER Cement & Concrete Composites 26 (2004) 957-966
Scanning electron microscopy imaging of hydrauliccement microstructure
Use of the scanning electron microscope (SEM) with X-ray microanalysis allows study of clinker and cements; permittingmeasuring bulk phase abundance and surface areas of the phases, as well as bulk chemistry of constituent phases can be carried out.Direct imaging of hydraulic cements by SEM yields a more complete picture of both bulk and surface phase compositions. Masspercentages obtained by SEM imaging are in good agreement with percentages based upon QXRD and may differ significantly fromthose estimated by the Bogue calculations. The finer-grained phases (gypsum, tricalcium aluminate, and ferrite) show much highersurface areas per unit mass than the coarser-grained phases such as alite and belite. Such data are being applied to develop betterrelationships between the cement material properties and performance properties and to provide starting images for a cementhydration simulation model being developed at NIST.@ 2004 Elsevier Ltd. All rights reserved.
Cement; Clinker; Image processing; Image analysis; Microstructure; Phase; Scanning electron microscopy; X-ray powder diffraction
Hydraulic cements comprise about 20% by volume ofmost concretes: their hydration products serve as abinder for the aggregate. Cement composition andfineness, along with other concrete constituents, influ-ence fresh concrete properties such as rheology, heatevol\j.tion, setting, rates of strength development, ulti-mate strength, and color. The hydration products ofcement (and on occasion, residual cement phases) affectconcrete durability.
Cement clinker is manufactured from a finely-ground, homogenized blend of limestone, shale and ironore sintered in a rotary kiln to temperatures ofapproximately 1450 DC: clinker nodules 3-20 mm indiameter are normally produced. The nodules are sub-sequently ground with gypsum, which serves to controlsetting, to a fine powder 45 ~) to produce cement.Fig. 1 shows a representative clinker nodule.
LeChatlier  first examined thin sections of clinkerusing the then new petrographic microscope. About adecade later, petrographic investigations of A.E. Tor-
nebohm named four crystalline phases alite, belite, cel-ite, felite, in addition to a supposed glassy phase.Subsequent studies of clinker have utilized both trans-mitted
light and reflected light on polished, etched sur-faces and have sought to understand the connectionsbetween microstructure and performance properties.Reflected light microscopy is still widely used today andASTM standard test method C 1356 is used for quan-titative phase abundance determination .
Phase composition and texture (crystal size, abun-dance, and distribution) of clinker result from complexinteractions of raw feed particle size, feed homogeniza-tion, and the heating and cooling regime. Mill grindingaffects the cement microstructure through fracturing ofthe calcium silicates and interstitial phase crystals and,depending upon conditions, it may alter the form ofcalcium sulfate added at this stage to control cementsetting. These features in turn influence the cement'shydration characteristics.
Phase abundance estimates are commonly derivedfrom the bulk chemistry using the Bogue equation. Er-rors in phase abundance estimates were recognized earlyand derive in part from the variability of clinker phasechemistry relative to the assumed compositions, andfrom not accounting for minor constituents [4,5]. WhileE-mail address: email@example.com (p. Stutzman).
0958-9465/$ -see front matter @ 2004 Elsevier Ltd. All rights reserved.doi: 10.10 16/j.cemconcomp.2004.02.043
Building and Fire Research Laboratory, 100 Bureau Drive, Mail Stop 8621, National Institute of Standards and Technology,Gaithersburg, MD 20899, USA
958 P. Stutzman I Cement & Concrete Composites 26 (2004) 957-966
Fig. 1. Nodule of portland cement clinker with a cut and polishedface.
this approach has served the industry well, it does notconsider the textural attributes of the clinker, i.e. fea-tures such as crystal size, phase distribution, and surfacearea, which will influence performance. In contrast,microscopy provides a direct means to observe andquantify microstructure features. Detailed informationon clinker microscopy may be found in [6,7].
2. Phases in clinker and cement
prising up to 5%. Light microscopy of lamellar struc-tures on etched specimens and X-ray powder diffractiondata are useful for distinguishing the belite forms. Beliteis less reactive than alite but does contribute to later-agestrengths (>28 d). Upon hydration, both alite and beliteform a poorly-crystallized calcium silicate hydrate (C-S-H) and well-crystallized calcium hydroxide.
Tricalcium aluminate, Ca3Al2O6, comprises 1-15% ofa clinker occurring as small 1-60 JlIn crystals exhibitingirregular to lath-like habit, filling the area between theferrite crystals . Tricalcium aluminate may occur ascubic or orthorhombic forms, and may contain up to 7%minor oxides. It is highly reactive with water.
Ferrite (tetracalcium aluminoferrite, Ca2(Al,Fe)Os),comprises between 0% and 18% of a clinker with crystalhabits as dendritic, prismatic, and massive. Minor oxidecontent within ferrites may be up to 10% and ferritesexhibit variable reactivity with water. The ferrite andaluminate phases are sometimes referred to as theinterstitial or matrix phases as they occur between, andappear to bind the silicate crystals.
Phases in lesser quantities, but still influential toperformance, include periclase (MgO) and free lime(CaO). Periclase: may exhibit a dendritic or equantcrystal habit both within and between the other clinkerconstituents, ranging in size up to 30 JlIn. Free lime mayoccur as isolated rounded crystals br in masses withvariable crystal size.
Alkali sulfates and calcium sulfates may also occur inclinker and are of interest since they have been found toaffect hydration rates and strength development [10,11].Increased alkali levels in clinker are considered poten-tially deleterious if alkali-susceptible aggregates are usedin the concrete.
Miller and Tang  found arcanite (K2SO4), aph-thitalite Na,KhS04), calcium langbeinite (K2SO4.2CaS04), and soluble anhydrite (CaS04) in a samplingof North American clinkers, although the anhydrite wasthought to be a byproduct of the laboratory extractionprocess. Taylor  lists additional alkali sulfate andsulfate phases such as thenardite (Na2S04) in high Na/Kclinkers and possibly anhydrite (CaS04) in some high-S03 clinkers. These phases form late in the clinkering
Alite (tricalcium silicate, Ca3SiOs) forms the bulk of aclinker (40-70%, by mass) with crystal sizes up to about150 ~. Alite generally displays a hexagonal crystalhabit in cross section, with forms ranging from euhedral(clearly-defined crystal faces), to subhedral (poorly-de-fined crystal faces), to anhedral (no crystal faces). Minoroxide contents may range from about 3% to 4% by mass.Alite reacts rapidly with water and is responsible formuch of the early-age (28 d) strength development [5,9](Table 1).
Belite (dicalcium silicate, CazSiO4) forms 15-45% of aclinker, and displays a rounded habit with crystal sizesranging from 5 to 40 ~. Normally the p polymorph isfound in clinkers, although smaller amounts of, IX, IX'H'and IX; polymorphs may occur with minor oxides com-
Table ITypical chemical compositions for the primary phases in cement clinkers. From 
Alite 0.1Belite 0.1Aluminate 1.0
(cubic)Ferrite 0.1Aluminate 0.6
(orthorhombic)Aluminate (low Fe) 0.4Ferrite (low AI) 0.4
P. Stutzman I Cement & Concrete Composites 26 (2004) 957-966 959
process and generally are found along crystal perimeterswithin the voids . Cements have one or more forms ofcalcium sulfate added upon grinding to control settingand, depending on its classification, may also containcalcite, slag, fly ash, or silica fume.
While it is important to know the bulk phase com-position of a clinker or cement, the surface area of eachphase will also affect cement performance, especially atearly ages. The surface area of a constituent is influencedby the clinker texture and grinding characteristics, whichmay be studied by microscopy. A wide range of texturesand compositions are possible in cement clinker, asillustrated
by the three NIST Standard ReferenceClinkers (Figs. 2-4). These clinkers were selected asrepresentative
of a wide range of North American clin-ker production with respect to phase abundance, crystalsize
and distribution. The images were obtained bybackscattered electron imaging in a scanning electronmicroscope.
4. NIST SRM 2688, SEM backscattered electron image.
Clinker 2686 (Fig. 2) is intermediate in crystal sizeand exhibits more heterogeneous phase dis