JOURNAL OF MATERIALS SCIENCE35 (2000 )3607– 3613

Acoustic microscopy of portland cement mortar

aggregate/paste interfaces

M. PRASAD∗, M. H. MANGHNANI, Y. WANG, P. ZININSchool of Ocean and Earth Science and Technology, University of Hawaii, Honolulu,HI 9682, USAE-mail: manika@pangea.stanford.edu

R. A. LIVINGSTONFederal Highway Administration, Turner-Fairbank Highway Research Center, McLean,VA 22101, USA

We have demonstrated the use of acoustic microscopy to characterize hydrated Portlandcement microstructure. High frequency (1 GHz) studies identified reaction zones aroundbubbles, ettringite formation, and effects of aggregate composition. Lower frequencyimages (200–400 MHz) have been used to characterize effects of aggregate composition oncement/grain interface. The information obtained from acoustic microscopy complementsoptical and scanning electron microscopy. We demonstrate the importance of acousticmicroscopy as an important nondestructive in situ technique for characterization ofconcrete. A major, unique advantage of the scanning acoustic microscope technique is itscapability of penetrating into the material, which allows us to study cracks, defects andother zones of weakness that might be hidden below the surface. C© 2000 Kluwer AcademicPublishers

1. IntroductionPortland cement concrete is one of the most commonlyused man-made materials. Its mechanical strength anddurability are functions of the various chemical pro-cesses occurring during the mixing and curing pro-cesses [1]. A major factor determining the strength ofthe concrete is the bond at the interface of the matrixand the aggregates [2]. The interfacial zones betweenthe cement paste and aggregates are considered to beabout 25–70µm thick and are characterized by highporosity and abundance of hydrated crystalline prod-ucts such as portlandite and ettringite [3]. They formthe weakest part in the concrete structure where crackscan propagate more easily [3, 4]. Microstructural andmicromechanical characterization of these interfacesyields important insight in the development of the con-crete properties, in particular their degradation withtime. It is also invaluable in devising techniques to im-prove the strength and durability of concrete.

Recently, it has been shown that scanning acousticmicroscopy (SAM) can provide additional information[5, 6] that complements classical methods of interfacecharacterisation, such as optical microscopy [3, 7] andscanning electron microscopy (SEM) [3, 8]. Acous-tic waves can propagate through optically opaque ma-terials, which makes acoustic mapping an importanttool for in situ characterisation of elastic and strengthproperties of concrete. Micro-and macrostructural vari-ations in the elastic properties can be imaged using

∗ Now at SRB Project, Geophysics Department, Stanford University, Stanford, CA 94305, USA.

acoustic waves at different frequencies. Since the wavepropagation is extremely sensitive to local variationsin elastic properties (elastic moduli and density), thistechnique is ideally suited for characterizing the inter-facial and microstructural properties of concrete. Moststudies of the mechanical structure of concrete havebeen done in the low (MHz) frequency range [9–11].In this range only the large voids or cracks (several cen-timeters) can be studied. In order to detect features thatmight be associated with degradation, the elastic prop-erties should be mapped at a micrometer scale, that is, atthe scale of the transition zone between the cement andaggregates, which is responsible for the deterioration ofconcrete [2]. A preliminary study conducted with a highfrequency (0.8–2 GHz) microscope on a Portland ce-ment mortar with granitic aggregates [6] demonstratedfor the first time that the mechanical structure of in-terfaces between cement matrix and aggregate can beexamined with the desired micron resolution.

In this paper we study the interfaces between ag-gregates and paste, bubble and paste using acousticmicroscopy. We describe applications of SAM to thefollowing relevant problems on concrete characteri-zations: acoustical contrast of the aggregate and ce-ment matrix interfaces, visualization of the subsur-face interfaces, imaging of structure within the cementmatrix and imaging of the ettringite formation. Wepresent representative acoustic images of these featuresalong with scanning electron microscopy (SEM) and

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reflected-light optical microscopy images to support theresults. A complete interpretation of obtained resultshas been made by combining high and low-frequencyacoustic microscopy, SEM, reflected-light optical mi-croscopy and energy dispersive (X-Ray) spectroscopy(EDS).

2. Experimental methodThe high frequency acoustic microscope with a spatialresolution of about 1µm is the acoustic equivalent ofan optical microscope [12]. In the acoustic microscopea monochromatic acoustical signal is focused onto thespecimen through the sapphire rod and the couplant(usually water) between the lens and the sample. Acous-tic images are obtained when the acoustic microscopemechanically scans the sample in a plane parallel to thesample surface. Variation of the mechanical propertieswith depth can be studied by scanning the sample atvarious lens positions. The acoustic wave propagatesthrough the coupling fluid to the sample surface andis reflected by the surface of the sample with the re-flectance functionR(θ ). When the acoustic wave aredefocused into the sample by a distancez, they experi-ence a phase delay of 2z kcosθ . The total output signalV(z) received at the transducer is a function of the depthof defocusz and is calculated by integrating the wave-front over the surface of the transducer. Assuming thatthe material is isotropic and that the transfer function ofthe lens is independent from the azimutal angle, [13, 14]

V(z) =∫ α

0P(θ )R(θ ) e−i 2zkcosθ sinθ cosθ dθ, (1)

whereα is the half-aperture of the lens,k is wavenumber,P(θ ) is the combined pupil function [12], andR(θ ) is the reflection coefficient [15]

R(θ ) = Ztot− ZW

Ztot+ ZW. (2)

HereZ are the acoustic impedances defined by

ZL = ρSVL

cosθL, ZT = ρSVT

cosθT, ZW = ρWVW

cosθ,

Ztot = ZL cos2(2θT)+ ZT cos2(2θT), (3)

and whereρS, VL, VT are respectively the density,longitudinal wave velocity and shear wave velocity ofthe specimen; andρW, VW are respectively the densityand the longitudinal wave velocity of water. AnglesθLandθT are determined from relations

sinθL = VW

VL, sinθT = VW

VT. (4)

As can be seen from (1), contrast in the acousticmicroscope represents the variation of the acousticalproperties of the sample and is dependent of the de-focusing distancez. Penetrating power of the acousticwaves into the specimen is a function of the frequency

of the acoustic waves. Low frequency waves penetratedeeper into the material. In the present study we usedan Ernst Leitz scanning acoustic microscope (ELSAM)with a high frequency lens (about 1 GHz) to investigatefine structure of the interfaces and low frequency lenses(100 MHz–400 MHz) to investigate deep structures inconcrete.

3. Sample preparationConcrete samples investigated in this study were pre-pared by casting plain cement paste or silica fume pastewith different types of aggregates (andesite, granite).Detailed description of the samples can be found else-where [8]. One sample used in this study was obtainedfrom a failed concrete block, presumably due to ettrin-gite formation. This sample was investigated to identifyettringite formation.

To obtain a high quality acoustic image at high (GHz)frequency range, the surface of the sample must be pol-ished to make roughness smaller than 1/10 microns.The concrete samples were embedded in epoxy resin,ground on each side, and then polished to a final fin-ish of 0.1µm. The two sides were parallel to about1.0µm. Due to variable stiffness of the cement matrixand aggregates, polishing required special care in or-der to prevent topographic effects between grains andmatrix.

4. Results and discussion4.1. Aggregate and cement matrix

interfacesThe contact zone between aggregate and cement pasteis of major interest in concrete characterization becausethis zone forms a weak link in concrete where crackscan easily develop. Usually aggregates are consideredas inert parts in the concrete formation [3]. Recently,Uchikawa [7] has shown that the aggregate type andits surface structure play an important role in deter-mining the properties of the contact zone. Analysis ofhigh frequency acoustical micrographs demonstratesthat different aggregate compositions produce differ-ent microstructures of the contact zone. Fig. 1 showsacoustic and SEM images of a granitic aggregate in ce-ment matrix. The various mineral compositions wereidentified by EDS analyses.

The acoustic image in Fig. 1 demonstrates varioustypes of aggregate grain boundaries with the matrix.The quartz, and to some extent plagioclase, grains havesharp and well defined grain boundaries. The smoothboundaries of the quartz and plagioclase grains implychemical inertness of the aggregates. In contrast theboundaries of both the albite and hornblende grainsare very irregular. The frayed boundaries of the albitegrain suggest extensive chemical reactions between thegrain and the cement matrix. The hornblende grain hasalmost completely reacted. The relative reactivity ofthe minerals appears to follow the trends observed inmetamorphic petrology. Amphiboles, i.e., hornblende,would be the farthest from equilibrium with an aqueoussolution. At Earth surface conditions, a highly alkaline

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Figure 1 Acoustic (a) and SEM (b) images of concrete sample made with granitic aggregate grains and Portland cement paste. The acoustic imagewas made at 400 MHz,z= 0.

Figure 2 Acoustic images of a concrete mortar sample made with andesitic aggregate grains and with Portland cement paste. The acoustic image wasmade at 1 GHz,z= 0. The box in (a) marks position of the high magnification acoustic image (b).

solution typical of concrete pore solutions would bemost reactive and would react with hornblende first.In contrast, quartz would be the closest to equilibriumand the feldspars would be intermediate. Note that thecalcium rich plagioclase shows less reactivity with thecalcium-containing matrix than with the sodium-richalbite. Fig. 2 also shows some gel at the lower end ofthe quartz grain. This gel does not appear to have beenderived from the quartz, but possibly from some otherunidentified grain that is not located in the plane of thisimage. A complex microstructure can be seen in thecement paste regions (Fig. 1), reflecting local variationsin the amounts of crystalline calcium hydroxide and

amorphous C-S-H. This could provide the basis for aquantitative estimate of the ratio of calcium hydroxideto C-S-H.

Fig. 2 shows the cement paste-aggregate interactionfor another type of aggregate in this case, andesite.This igneous rock has a chemical composition simi-lar to granite. However, it is of volcanic origin and asa result, does not have the well developed crystallinestructure. The fine-grained andesite would be likely tobe more reactive. The effect of reaction on the aggregateboundaries is clearly seen in Fig. 2. There are finger-like growths along the edges, which continue into thecement paste.

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Figure 3 Acoustical micrographs of a concrete specimen made of andesitic aggregate grains in Portland cement paste with silica fume. (a): frequency400 MHz; defocusZ= 0µm. (b): frequency 200 MHz; defocusZ=−100µm.

4.2. Imaging of subsurface interfacesOne of the unique features of the acoustic microscopyis the ability to visualize subsurface microstructuresnot observed by optical microscopy [14]. A compari-son of acoustic, reflected-light, and scanning electronmicroscopy images of a cement mortar specimen withandesitic aggregates and silica fume is shown in Figs 3and 4. Acoustic micrographs made at 200 MHz and at400 MHz are presented in Fig. 3a and b, respectively.Since the attenuation of sound is very high at 1 GHz,images of subsurface interfaces (e.g., Fig. 3b at defo-cusz=−100µm) were taken only at a low frequencyof 200 MHz. A better resolution image of the aggre-gate surface was made at higher (400 MHz) frequency(Fig. 3a). In all four images (acoustic in Fig. 3, opticaland SEM in Fig. 4) made of the same area, the bubblestructure is seen clearly on the right hand side. Surface(Fig. 3a) and subsurface (Fig. 3b) images of aggregateexhibit excellent contrast of aggregate boundaries. Asdefocus increases, the boundary of aggregate appearscloser to the bubble (Fig. 3b). Numerous black spots,most likely representative of voids, are observed in thesurface acoustic image in Fig. 3a. They hamper inves-tigations of the interfaces within concrete, when theacoustic waves are focussed at the surface. These effectscan be eliminated by defocusing the acoustic waves be-low the surface as shown in Fig. 3b. The reflected-lightoptical and SEM images (Fig. 4a and b, respectively)are quite distinct from the acoustic images. Aggregateboundaries are only faintly visible in the SEM imagesand not seen at all in the optical images. Furthermore,the SEM image shows numerous cracks. These cracksare formed typically in samples, which have not beenfully cured. During SEM, the high vacuum leads to wa-ter evaporation from the matrix. This water evaporationfrom the sample causes a change in volume of the pasteand leads to formation of cracks. Such cracks were ob-served in most SEM investigations performed duringthis study. These comparative examples of acoustic,

Figure 4 (a) reflected light optic, and (b) SEM images of of same areaof the specimen shown in Fig. 3. The magnification of the optical imageis 50*1.25.

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Figure 5 Bubble in cement mortar; (a) acoustical image made at 400 MHz; (b) SEM image.

reflected-light optical and scanning electron mi-croscopy demonstrate the strength of SAM over the tra-ditional methods of microstructural characterizations.

4.3. Bubble and cement matrix interfacesVoids are typical features found in cement pastes.A long standing question in concrete technology isthe nature of the bubble/matrix interface. 400 MHzSAM and SEM images of a bubble structure in cementpaste are shown in Fig. 5a and b, respectively. Underthe scanning electron microscope, a relatively broadreaction rim can barely be seen (Fig. 5b). In the SAMimage, a thin layer is revealed within the reaction rim.Below the Rayleigh angle the brightness of contrastof the SAM image in focus (see Equations 1 and 2) ismainly determined by difference between impedancesof liquid and of solid. High contrast of the layeraround the bubble indicates a high impedance andhence a relatively high hardness. As mentioned ear-lier, SEM images showed numerous major cracks in thesample.

4.4. Ettringite formationFormation and presence of ettringite in hardened con-crete is often undesirable. Ettringite is a complex min-eral of hydrated calcium aluminosulfate [2] with thechemical formula Ca6[Al(OH)6]2(SO4)3xH2O wherex is around 26. It is usually associated with structuraldamage and causes reduction of the concrete strength.We have conducted high frequency (0.9 GHz) SAM,SEM, and optical microscopy on a failed concrete sam-ple from a precasting plant which has failed, possi-bly due to ettringite formation. These analyses wereaimed at locating and identifying ettringite and study-ing its immediate surroundings for changes. At highfrequency, SAM [16] is extremely successful in distin-

guishing between crystalline, “proto-crystalline”, andglassy textures in basalt glasses owing to their largeimpedance differences. Since a large impedance differ-ence can be expected between various features associ-ated with ettringite formation, we used SAM to mapthese various features in the failed concrete sample.The acoustic C-scan and SEM images of this sampleare shown in Fig. 6. The acoustic image on the leftshows a dark colored aggregate in the upper left cornersurrounded by cement matrix with colors varying fromblack to light grey. A bubble is seen in the lower partof the image. The bright area is the void space seenhere as filled with epoxy. Between the void space andbubble rim, elongated structures are observed whichappear to have grown into the void. This needle-shapedform of ettringite is also clearly seen in the SEM image(Fig. 6b). Apart from their distinct shape, these crystalswere identified as ettringite by X-ray microanalyses.Comparing the acoustic and SEM images (Fig. 6), a cir-cular rim is observed in the acoustical image (Fig. 6a)around the bubble in which the ettringite crystals werefound. This rim is not observed in all SEM images. Al-though the bubble area might be completely filled withettringite crystals, the impedance change is detectable.Numerous cracks are formed in the SEM analyses.

The needle-shaped crystal form observed in SAMand in SEM, along with EDS of Ca-Al-S ratios,confirms the existence of the ettringite in the sample.The EDS analyses of the various components ofFig. 6a are given in Fig 7. The positions taken werenear the reaction rim (columns 1 to 5), at increasingdistance from rim, in matrix (columns 6 to 11), andin aggregate (columns 12 to 13). The Ca-Al-S ratioscalculated from the these values near the reaction rimcompare very well with those for ettringite 3 : 1 :1.5 (column 0 in Fig. 7a). Outside the reaction rim,Ca/Al ratio increases sufficiently. It is around 60 at posi-tion 10.

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Figure 6 Acoustic C-scan (a) and SEM (b) images of the damaged concrete sample from a precasting plant. The image clearly shows a rim aroundthe bubble. Elongated texture within the rim is due to crystals of ettringite. The acoustic image was made at 1 GHz. In SEM image, cracks have beenformed due during the process of coating and putting the sample under vacuum.

Figure 7 EDS analyses of the various components shown in Fig. 7a. Columns 1 to 5 contain data of positions in the reaction rim, columns 6 to 11at increasing distance from the rim, data in column 12 and 13 was collected from aggregate grains, and column 0 shows EDS analyses of a standardettringite sample (Ca : Al : S= 3 : 1 :1.5).

5. ConclusionsWe have shown the importance of scanning acousticmicroscopy as an important nondestructive in situ tech-nique for analyzing concrete. Applications of SAMshown here are characterizations of aggregate-matrixand bubble-matrix interfaces. In a majority of observa-tions of bubbles, we find a rim of material with dif-ferent impedance from the surrounding cement ma-trix. The interface between aggregate and matrix isstrongly dependent on the mineralogy of the aggre-gate grain. In the case of non reactive minerals suchas quartz, the grain boundaries are sharp and well de-fined. The more reactive grains show frayed edges.

We have also compared SAM results with conven-tional reflected-light optical microscopy and SEManalyses. The major advantage of SAM is that it is anin situ technique with very high resolution. AlthoughSEM has a much higher resolution power than SAM, itproduced cracks in all the uncured samples examined

here. Reflected-light optical microscopy lacks the im-age contrast, and transmitted-light microscopy has thedisadvantage of requiring thin sections of the samples.A major, unique advantage of the SAM technique isits capability of penetrating into the material. Acousticwaves reflected from interfaces within the materialcarry information not only of the surface but alsoof subsurface characteristics. Thus, subsurface cracks,defects and other zones of weakness that affect thedurability and strength of concrete can be mapped non-destructively with high resolution.

AcknowledgementsThe authors are thankful to Sidney Mindess for pro-viding the samples for this study. We also thank JohnBalogh for technical help and Mollie Ebersbach forsample preparation. MP thanks the Stanford Rockphys-ics and Borehole Project at Stanford University for

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support during completion of this manuscript. Theresearch was supported by the Office of AdvanceResearch, Federal Highway Administration, un-der grant number DTFH61-94-X-00020. It is SOESTcontribution.

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Received 25 Februaryand accepted 16 August 1999

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