ASTM C1723-16(2022)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C1723 − 16 (Reapproved 2022)
Standard Guide for
Examination of Hardened Concrete Using Scanning Electron
Microscopy
This standard is issued under the fixed designation C1723; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope can be used to qualitatively or quantitatively determine the
elemental composition of very small volumes intersecting the
1.1 This guide provides information for the examination of
surface of the observed specimen (for example, 1-10 cubic
hardened concrete using scanning electron microscopy (SEM)
microns) and those measured compositional determinations
combined with energy-dispersive X-ray spectroscopy (EDX or
can be correlated with specific features observed in the SEM
EDS).Sincethe1960s,SEMhasbeenusedfortheexamination
image. See Note 1.
of concrete and has proved to be an insightful tool for the
NOTE 1—An electronic document consisting of electron micrographs
microstructural analysis of concrete and its components.There
and EDX (EDS) spectra illustrating the materials, reaction products, and
are no standardized procedures for the SEM analysis of
phenomena discussed below is available at http://netfiles.uiuc.edu/dlange/
concrete. SEM supplements techniques of light microscopy,
www/CML/index.html.
which are described in Practice C856/C856M, and, when
1.5 Performance of SEM and EDX (EDS) analyses on
applicable, techniques described in Practice C856/C856M
hardened concrete specimens can, in some cases, present
should be consulted for SEM analysis. For further study, see
unique challenges not normally encountered with other mate-
the bibliography at the end of this guide.
rials analyzed using the same techniques.
1.2 This guide is intended to provide a general introduction
to the application of SEM/EDS analytical techniques for the
1.6 This guide can be used to assist a concrete petrographer
examination and analysis of concrete. It is meant to be useful
inperformingorinterpretingSEMandEDX(EDS)analysesin
toengineersandscientistswhowanttostudyconcreteandwho
a manner that maximizes the usefulness of these techniques in
are familiar with, but not expert in, the operation and applica-
conducting petrographic examinations of concrete and other
tion of SEM/EDS technology. The guide is not intended to
cementitious materials, such as mortar and stucco. For a more
provide explicit instructions concerning the operation of this
in-depth, comprehensive tutorial on scanning electron micros-
technology or interpretation of information obtained through
copyorthepetrographicexaminationofconcreteandconcrete-
SEM/EDS.
related materials, the reader is directed to the additional
1.3 It is critical that petrographer or operator or both be
publications referenced in the bibliography section of this
familiar with the SEM/EDX (EDS) equipment, specimen
guide.
preparation procedures, and the use of other appropriate
1.7 Units—The values stated in SI units are to be regarded
procedures for this purpose. This guide does not discuss data
asstandard.Nootherunitsofmeasurementareincludedinthis
interpretation. Proper data interpretation is best done by
standard.
individuals knowledgeable about the significance and limita-
tions of SEM/EDX (EDS) and the materials being evaluated.
1.8 This standard does not purport to address all of the
safety concerns, if any, associated with the use of electron
1.4 The SEM provides images that can range in scale from
microscopes, X-ray spectrometers, chemicals, and equipment
alowmagnification(forexample,15×)toahighmagnification
used to prepare samples for electron microscopy. It is the
(for example, 50 000× or greater) of concrete specimens such
as fragments, polished surfaces, or powders.These images can responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
provide information indicating compositional or topographical
variations in the observed specimen. The EDX (EDS) system mine the applicability of regulatory limitations prior to use.
1.9 This international standard was developed in accor-
1 dance with internationally recognized principles on standard-
This guide is under the jurisdiction ofASTM Committee C09 on Concrete and
Concrete Aggregates and is the direct responsibility of Subcommittee C09.65 on
ization established in the Decision on Principles for the
Petrography.
Development of International Standards, Guides and Recom-
Current edition approved Oct. 1, 2022. Published October 2022. Originally
mendations issued by the World Trade Organization Technical
approved in 2010. Last previous edition approved in 2016 as C1723 – 16. DOI:
10.1520/C1723-16R22. Barriers to Trade (TBT) Committee.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1723 − 16 (2022)
2. Referenced Documents 3.1.12 stage, n—platform upon which the specimen is
2 placedwithinthevacuumchamberthatcanberemotelymoved
2.1 ASTM Standards:
in various directions.
C125Terminology Relating to Concrete and Concrete Ag-
3.1.13 working distance, n—thedistancebetweenthedetec-
gregates
tor and the sample. Each SEM will have an optimun distance
C294Descriptive Nomenclature for Constituents of Con-
in which X-rays can be collected for EDX (EDS).
crete Aggregates
C295/C295MGuide for Petrographic Examination of Ag-
3.1.14 X-ray detector, n—also known as EDX (EDS) sys-
gregates for Concrete
tem.
C457/C457MTest Method for Microscopical Determination
of Parameters of the Air-Void System in Hardened Con-
4. Description of Equipment
crete
4.1 The principles of the electron system of the scanning
C856/C856MPractice for Petrographic Examination of
electron microscope, the interactions of the electron beam and
Hardened Concrete
the specimen under examination, and the detection systems
C1356 Test Method for Quantitative Determination of
used for the examination are based on concepts that need
Phases in Portland Cement Clinker by Microscopical
understanding if the resulting image and other analytical
Point-Count Procedure
information obtained are to be best resolved and understood.
An abbreviated discussion is provided here. A more compre-
3. Terminology
hensive understanding can be obtained from texts devoted to
3.1 Definitions of Terms Specific to This Standard: 3
this subject (1,2).
3.1.1 BSE, n—backscatter electrons; these are high-energy
4.1.1 SEM Optics:
electronsemittedbackfromthespecimensurface.Elementsof
4.1.1.1 An electron beam is generated in a column consist-
higheratomicnumberwillhavestrongeremissionsandappear
ing of an electron gun and multiple electromagnetic lenses and
brighter.
apertures.Theelectronbeamisgeneratedbyheatingafilament
3.1.2 brightness, n—the amount of energy used to produce
so that it emits electrons. The most common filament for
an X-ray.
general SEM work is tungsten, but other filaments can be used
for increased brightness.The electrons are accelerated towards
3.1.3 charging, n—thebuildupofelectronsonthespecimen
the specimen by an applied potential and then focused by
at the point where the beam impacts the sample. Charging can
lenses and apertures. The energy of the electron beam influ-
alter the normal contrast of the image (usually becomes
ences resolution, image quality, and quantitative and qualita-
brighter)andmaydeflectthebeam.Coatingthespecimenwith
tive X-ray microanalyses.
athinlayerofconductivematerial(suchasgoldorcarbon)can
4.1.1.2 Theelectronbeamisfinelyfocusedthroughelectro-
minimize this effect.
magnetic lenses and apertures. A smaller beam size improves
3.1.4 contrast, n—the difference in intensity of the energy
resolution, but decreases signal intensity.
produced by varying elements when excited.
4.1.1.3 Electron systems operate under vacuum. Specimens
3.1.5 dead-time, n—the time of finite processing during
should be prepared to minimize alteration or damage when
which the circuit is “dead” and unable to accept a new pulse
they are exposed to the vacuum (See 5.1.4). Variable pressure
from the X-rays.
scanning electron microscopes, low vacuum scanning electron
3.1.6 EDX (EDS) (energy-dispersive X-ray spectroscopy),
microscopes (LVSEM), and environmental scanning electron
n—the interaction of the electron beam with atoms in the
microscopes (ESEM) permit the examination of samples con-
sample produces characteristic X-rays having energies and
taining some moisture under low vacuum. The ESEM also
wavelengths unique to atoms.
allows analysis of organic materials. Even in an ESEM,
however, some drying occurs.
3.1.7 live-time, n—howtheacquistionofX-raydataistimed
4.1.2 Signal Generation and Detection:
when the rate of X-ray events between measurements are
4.1.2.1 Theinteractionoftheelectronbeamwiththesample
compared. Opposite of dead-time.
generates several types of signals that can be utilized for
3.1.8 K, L, or M peaks, n—characteristic X-ray intensities
imaging and X-ray microanalysis. The intensities of these
detected for elements.
signals are measured by detectors. The signals allow the
3.1.9 raster, n—to scan as when the beam from the filament
examination and determination of properties such as surface
sweeps back and forth over the sample
topography, elemental composition, and spatial distribution of
3.1.10 SE, n—secondary electrons; these are low-energy
components.Signalintensitiesaregenerallyusedtoprovidean
electrons emitted when the specimen is hit with the beam and
image on a screen.
associated with the topography of the same.
4.1.2.2 The signals that are produced when the electron
beam strikes the specimen surface are secondary electrons
3.1.11 SEM, n—scanning electron microscope.
(SE), backscattered electrons (BSE), and X-rays.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The boldface numbers in parentheses refer to a list of references at the end of
the ASTM website. this standard.
C1723 − 16 (2022)
4.1.2.3 To generate an image, the electron beam is moved concrete may also contain supplementary cementitious
repeatedly across the specimen to form a raster. The magnifi- materials,organic,inorganic,andmetallicfibers,andentrained
cation is the ratio between the size of the raster and that of the air voids.
screen image.
5.1.3 Cement Paste—The cement paste contains residual
cement,frequentlysupplementarypozzolanicandcementitious
4.1.2.4 Images produced by secondary electrons are most
materials, and various hydration products that together have a
commonlyutilizedfortopographicalimaging.TheSEintensity
complex and porous microstructure. The paste is initially a
depends mainly on the angles between the electron beam and
mixtureofindividualgrainsofcementiousmaterialsandwater,
thespecimensurfaceandbetweenthespecimensurfaceandthe
and may also contain chemical admixtures. Over time, hydra-
detector. The SE intensity is relatively insensitive to the
tion reactions consume the cement and produce various hydra-
specimen composition.
tion products, some of which grow on the surface of cement
4.1.2.5 Images produced by backscattered electrons are
grains, while progressively filling the initial water-filled space.
often used for elemental contrast imaging. The BSE image is
5.1.3.1 Residualportlandcementparticlesappeardenseand
useful for identifying different chemical constituents in con-
angulartosubangular.Aliteusuallyhasatleastonecrystalface
crete. The BSE intensity depends on the average atomic
while belite is usually rounded and sometimes striated. In a
number and density of each phase. The BSE intensity also
BSE image, residual portland cement particles occur as rela-
depends on the angles between the electron beam and the
tively bright objects in a matrix of gray cement hydration
specimen surface and between the specimen surface and the
products.
detector.Therefore,someBSEdetectorscanbemanipulatedto
5.1.3.2 Calcium-silicate-hydrate is the major hydration
observe the sample topography.
product of portland cement and is usually amorphous or very
4.1.2.6 The interaction of the electron beam with atoms in
poorly crystalline. Its morphology varies depending on the
the sample produces characteristic X-rays having energies and
calcium to silica ratio, water to cementitious materials ratio,
wavelengths unique to atoms. Chemical analysis (or micro-
curing conditions, degree of cement hydration, and chemical
analysis) is performed using an X-ray spectrometer that mea-
admixtures. At high magnifications, the morphology of
sures the energies and intensities of the X-rays.The intensities
calcium-silicate-hydrate varies from very fine fibrous growths,
of X-rays depend upon many factors, including electron beam
to sheet-like units, to irregular massive grains.
currents and accelerating voltages, as well as chemical com-
5.1.3.3 Portlandite (calcium hydroxide) is a major phase of
position of the specimen interacting with the electron beam.
cement hydration and occurs in variable sizes and shapes
4.1.2.7 One important parameter for image quality is the
including platy hexagonal crystals and sheet-like masses,
working distance, the distance between specimen surface and
depending on the orientation. Calcium hydroxide is normally
the point where the electron beam exits the electron optics.
observedthroughoutthecementpasteandsometimesdevelops
Small working distances maximize BSE collection efficiency
along paste-aggregate interfaces. It also sometimes occurs as
and improve the image resolution. Long working distances
secondary deposits in voids and cracks.
improve image depth of field for topographical images but
5.1.3.4 Ettringite is a primary product of the reactions
decrease image resolution. The working distance generally
between calcium aluminates and the sulfate phases in cement.
must be within a predetermined range to perform X-ray
It has a characteristic acicular shape. Ettringite often also
microanalysis.
appears as a secondary deposit. Secondary deposits of ettring-
itearecommonlyfoundinvoidsandcracks.X-raymicroanaly-
5. Materials and Features
sis is sometimes required for its identification. A compound
5.1 Important microstructural features include the size and
that has similar morphology is thaumasite (See 5.1.7 on
shape of individual constituents (including pores), the spatial
secondary deposits). These two compounds can be distin-
relationships between these constituents (what constituents are
guished by elemental analysis. In polished sections (See 6.1)
touching or associated with each other), and the volume
ettringite may sometimes appear microcrystalline and high
fractionofeachconstituent.Constituentsaredescribedinmore
magnification (for example, 50,000X) may be needed to see
detail by Taylor (3).
individualcrystals.Insomecasesettringitecrystalsmaybetoo
5.1.1 In order to study these microstructural features, it is small to be identified.
necessary to recognize the individual phases which are usually
5.1.3.5 Calcium monosulfoaluminate usually forms platy
recognized by their size, shape, association, backscatter inten-
crystals.Elementalanalysis(EDXorEDS)mayberequiredfor
sity and elemental composition (See Note 1 for examples). its identification.
These characteristics may sometimes be insufficient to conclu-
5.1.4 Aggregates—Descriptive Nomenclature C294 and
sively identify a phase, or to differentiate between two phases,
Guide C295/C295M outlines methods and information rel-
such as chert and quartz (SeeTerminology C125). In this case,
evant to the identification and classification of aggregates.
othertechniquesmustbeused,suchasXRDorpolarizinglight
Microstructural features of individual constituents within the
microscopy. Additional information can be found in Practice
aggregate can be studied using BSE images of polished
C856/C856M.
specimens. The elemental compositions of aggregates can be
5.1.2 Concrete—Hardened concrete consists of aggregate, determined using SEM with EDX (EDS) and sometimes the
hydration products of pozzolanic and cementitious materials, generic rock type (such as limestone, sandstone, dolomite, and
residual cement particles, capillary pores and voids. Some granite)maybeinferred.Microstructuralpropertiessuchasthe
C1723 − 16 (2022)
amount of each phase and crystal or particle size may be C856M. Caution must be used in evaluating secondary depos-
determined using semiquantitative BSE techniques. Cath- its because they can be artifacts of the sample preparation.
odoluminesence and electron backscatter diffraction may be 5.1.7.1 Alkali-silicagelisaproductofalkali-silicareaction.
used to further understand composition, grain structure and The appearance of the gel varies depending on its composition
crystallographic texture. Other information about aggregates and where it occurs. It can appear as foliated to massive or
that can be obtained includes porosity, surface contaminants, spongy and have a grainy texture. Alkali-silica gel always
and inclusions. contains silicon plus varying combinations and concentrations
of calcium, potassium, and sodium.
5.1.4.1 Coarse and fine aggregate particles cannot be iden-
5.1.7.2 Thaumasite is a secondary deposit that may form
tified based on size. In cross sectioned samples, most particles
during sulfate attack. It occurs as acicular crystals similar in
arenotintersectedbytheplanesurfaceattheirlargestdiameter
morphology to ettringite. The identification of thaumasite is
so that some intersected coarse aggregate particles may appear
aided by elemental analysis and usually requires the use of
torepresentfineaggregate.Asidefromsize,fineaggregatecan
petrographic microscopy and other techniques.
only be identified when the lithology and mineralogy are
5.1.7.3 Secondary deposits such as gypsum and thenardite
different from the coarse aggregate. The shape and surface
usually cannot be positively identified by morphology in the
texture of aggregate particles vary greatly depending on the
SEM. Elemental analysis is needed and identification may still
mineralogy of the aggregate. The distribution of the aggregate
be ambiguous when mixtures of phases are present. Petro-
and its potential effect on the durability of the concrete should
graphic microscopy or X-ray diffraction may be used to
be evaluated. Positive mineralogical identification can be done
identify many deposits.
using methods such as optical microscopy (See Guide C295/
5.1.8 Cracks and Voids—Using the SEM, cracks and voids
C295M).
in aggregates and cement paste can be observed. The size,
5.1.5 Paste/Aggregate Interfacial Zone—The paste-
shape, and configuration of voids can also be observed and
aggregate interfacial transition zone, which is typically about
their volume estimated using image analysis techniques. The
50 µm thick, is an important area to examine because it affects
sample size, however, may be too small to meet the require-
somephysicalpropertiesofconcrete.Theinterfacialzonemay
mentsofTestMethodC457/C457Mandtherearenostandard-
account for increased porosity and may be enriched with
ized procedures for this using SEM. Caution must be used in
ettringite and portlandite. Some pozzolanic admixtures greatly
evaluating the cause of cracks because they may be artifacts
reduce the thickness and porosity of the interfacial zone.
introduced during specimen preparation (drying), or formed
Secondary compounds such as alkali silica gel may be depos-
due to dehydration of the sample during evacuation of the
ited in this area.
specimen chamber. Secondary reaction products may be ob-
5.1.6 Supplementary Cementitious Materials—
served in cracks and voids.
Supplementary cementitious materials, including natural
pozzolans, ground granulated blast-furnace slag, silica fume,
6. Specimen Preparation and Procedures
and fly ash, may be identified in concrete.
NOTE 2—Specimen preparation for scanning electron microscopy is
5.1.6.1 Fly ash usually contains a major amount of spheres
discussed by Echlin (4).
of glasses, and to a lesser extent, irregularly shaped particles.
6.1 Specimens—Various types of specimens (for example,
Particle sizes are similar to those of portland cement. Fly ash
fractured surfaces, sectioned or polished surfaces, thin
contains different phases and the chemistry of individual
sections, or powders) can be examined using SEM. For
particles can be quite variable.
hardened concrete, it is often desirable to examine large
5.1.6.2 Groundgranulatedblast-furnaceslagparticlesoccur
specimens or a number of small specimens concurrently, and
as bright, angular particles in a BSE image. Hydration rims
chambers are available that can accommodate specimen sizes
may be present when hydration is incomplete. It is sometimes
up to 150 mm.
difficult to conclusively differentiate between granulated blast
6.1.1 Guidelines for the selection of samples are given in
furnace slag and residual portland cement particles.
Practice C856/C856M.
5.1.6.3 Silica fume occurs as spherical particles that are at
6.1.2 Theexaminationofas-receivedsurfacesismademore
least an order of magnitude smaller than fly ash and portland
complex by carbonation and material adhering to surfaces.
cementparticles,anditthereforemaynotbedistinguishablein
Contamination can occur during in-service use, specimen
concrete. Densified (nodular) silica fume that has not been
handling, or from debris produced during sample acquisition.
well-dispersed appears as agglomerates of various shapes and
Most contamination can be removed by careful cleaning.
sizes, which may be detectable in SE or BSE images.
6.2 Visual Examination of As-Received Specimens—Visual
5.1.7 Secondary Reaction Products—The reaction products
and stereo-optical inspections are important first steps and
formed during normal hydration of cementitious materials are
should be done before SEM analysis. Examination of as-
considered to be primary. Sometimes these primary products
received samples can help to characterize a variety of surface
react with other constituents to form secondary products. The
phenomenaincludingefflorescence,pop-outs,carbonation,and
type,location,andamountofsecondarydepositsareimportant
cracking.
indicators of the condition of the concrete. Some primary
cement hydration products, such as portlandite and ettringite, 6.3 Sample Drying—Sample drying is needed for conven-
may also occur as secondary deposits. Possible secondary tional high vacuum SEM examinations. Variou
...