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Cement and Concrete Composites 85 (2018) 56e66

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Cement and Concrete Composites

journal homepage: www.elsevier .com/locate /cemconcomp

A combined SPM/NI/EDS method to quantify properties of inner andouter C-S-H in OPC and slag-blended cement pastes

Y. Wei a, b, *, X. Gao a, S. Liang a

a Department of Civil Engineering, Tsinghua University, Beijing, 100084, Chinab Key Laboratory of Civil Engineering Safety and Durability of China Education Ministry, Department of Civil Engineering, Tsinghua University, Beijing,100084, China

a r t i c l e i n f o

Article history:Received 11 February 2017Received in revised form30 May 2017Accepted 29 September 2017Available online 3 October 2017

Keywords:Indentation modulusInner C-S-H thicknessInterface transition zoneModulus mapping-based microstructuremorphologyStorage modulus

* Corresponding author. Key Laboratory of Civil Engof China Education Ministry, Department of Civil EngiBeijing, 100084, China.

E-mail address: yawei@tsinghua.edu.cn (Y. Wei).

https://doi.org/10.1016/j.cemconcomp.2017.09.0170958-9465/© 2017 Elsevier Ltd. All rights reserved.

a b s t r a c t

To identify the distinct microstructural features and to provide insight into the mechanism by which thephases in hardened paste possess, this study adopts the coupled techniques of quantitative modulusmapping in the form of Scanning Probe Microscopy (SPM) images, nanoindentation (NI), and energy-dispersive X-ray spectroscopy (EDS) for comprehensive investigation on the chemical-mechanical-morphological properties of C-S-H gel in both ordinary Portland cement (OPC) and slag-blendedcement pastes. The thickness of the inner C-S-H (IP) layer is precisely measured for the first time bymodulus mapping, it varies with the types of unreacted cores as well as the addition of the supple-mentary cementitious materials. An interface transition zone (ITZ) is found between the unreacted C3Sgrain and the surrounding inner C-S-H layer. The mechanical properties of the five types of C-S-H in OPCand the slag-blended pastes are not significantly affected by their chemical compositions. A good cor-relation between the storage modulus and the indentation modulus of the individual phases is found.The results indicate the significance of SPM-based modulus mapping technique as a powerful tool tocharacterize the phase in cementitious materials with more attractive features of higher spatialresolution.

© 2017 Elsevier Ltd. All rights reserved.

1. Introduction

Many macroscopic behaviors of concrete originate from thenano-scale properties of cementitious materials. It is fundamentalrequirement to understand the mechanism at the nano-scale levelfor further material tailoring to achieve better concrete. Differenttechniques have been used to characterize cementitousmaterials atnano- and micro-scales. Static nanoindentation technique was firstapplied to concrete by Acker [1] to characterize the mechanicalproperty of clinker. With successful surface preparation to mini-mize surface roughness, meaningful and reliable information aboutthe mechanical behavior of cement pastes at the nanoscale can beprovided by nanoindentation technique [2]. And it has become anincreasingly used technique in the past 16 years to investigate theelastic and creeping properties of cementitious materials [1e7].

ineering Safety and Durabilityneering, Tsinghua University,

Although many investigations have been conducted to study thephysical properties of cementitious composites, the knowledge ofconcrete materials, especially the concretes containing industrialby-products such as fly ash and blast-furnace slag are far frombeing well established due to their complexity. For example, re-searches on the microscale physical and mechanical properties ofcementitious materials incorporating slag are found rare [8e10].Zadeh et al. [8] reported that the nanoscale mechanical propertiesof concrete containing slag matches those of conventional con-cretes through nanoindentation. Hu et al. [9] found that the phys-ical and mechanical properties of hydration products incementitious materials incorporating slag are quite different fromthose of pure cement pastes through multiple technique. Wei et al.[7] suggested that a nanoindentation-based methodology can beused to calculate the degree of hydration of Portland cement andslag-blended cement pastes.

However, for characterizing multi-phase or composite mate-rials, quantitative modulus mapping in the form of Scanning ProbeMicroscopy (SPM) technique has been found effective over staticnanoindentation [10e13]. Similar to static nanoindentation,modulus mapping is a nano-mechanical probe-based method but

Y. Wei et al. / Cement and Concrete Composites 85 (2018) 56e66 57

characterized as a nondestructive test with contact stiffness mea-surement at a much smaller contact force than that used in nano-indentation tests [10,11]. This modulus mapping technique with insitu SPM imaging has been widely used to characterize the fre-quency response of polymer materials [14,15]. In recent years, it hasbeen used to evaluate the microstructural features and viscoelasticbehavior of solid, such as human teeth [11,16]. In addition tomeasuring the mechanical properties of materials in a nonde-structive manner, another advantage of forcemodulation is that thecontact stiffness can be directly obtained continuously duringindentation, thus forming a modulus mapping, which could be anideal technique for multiphase and composite materials. However,the application of SPM-based modulus mapping technique tocementitious materials is found scarce [13,17]. Its powerful func-tions have not been fully explored for characterizing cementitiousmaterials with multi-scale and multi-phase features.

SEM-EDS has become a feasible approach to study the chemicalcomposition of different materials over the past years [18,19]. Withthe information within the microvolumes probed by SEM-EDS, onecould analyze the chemical composition of cementitious materials.Chen et al. [20] reported the existence of CeSeH/Ca(OH)2 nano-composites in cement paste at the nanometerscale through utiliz-ing coupled SEM-EDS, nanoindentation, and micromechanics.Durdzi�nski et al. [21] has also proposed a new quantificationmethod based on SEM-EDS to assess fly ash composition and studythe reaction of its individual components in hydrating cementpaste.

To identify the distinct microstructural properties and to pro-vide insight into the mechanism that the phases in hardened pastepossess, this study utilizes the coupled techniques of SPM, nano-indentation, and the energy-dispersive X-ray spectroscopy (EDS)for the comprehensive chemical-mechanical-structural analysis forthe ordinary Portland cement (OPC) paste and the slag-blendedcement paste at nano scale. The following has been investigated:modulus mapping-based microstructure morphology of OPC andslag-blended cement pastes; thickness of inner product (IP) layersurrounding different types of grains; the mechanical propertiesand the chemical composition of the five types of C-S-H gels in OPCand slag-blended pastes; the relationship between the indentationmodulus and the storage modulus of different phases.

2. Materials and methods

2.1. Materials

Ordinary Portland cement (OPC) supplied by China UnitedCement Co. and S95 GGBS slag supplied by Sanhe Tianlong NewBuilding Materials Co., Ltd., China were used as cementitious ma-terials. The chemical composition in terms of oxide mass percent-age and the physical properties of eachmaterial are listed in Table 1.Two types of pastes were investigated: OPC paste (denoted as O) vs.slag-blended paste (denoted as S). In slag-blended paste, the OPCwas replaced by slag at a level of 50% by mass of the total cemen-titious materials. The water/cementitious (w/cm) ratio of the pastesis 0.3. All the pastes were prepared according to the procedure ofASTM C305. After mixing, the fresh paste was filled into a plastictube with diameter of 1 cm, and the tubes were then sealed and

Table 1Chemical compositions and physical properties of cementitious materials.

Materials SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) N

Portland cement 20.55 4.59 3.27 62.5 2.61 2.93 0Slag 34.55 14.36 0.45 33.94 11.16 1.94 0

placed in a curing room with temperature controlled at 20 �C. Thetubes were rotated manually during the setting period to preventexcessive bleeding. After setting, the paste-filled tubes were storedin the curing room until the age of testing.

2.2. Sample preparation

The 10-mm thick disc samples were cut from the tube sample atthe age of 180 days. The disc samples were then processed by resinembedding, grinding, polishing, and ultrasonic cleaning to obtainthe final surface for testing. The grinding process was done first byusing silicon carbide papers following the sequence of 180, 400 and1200 grit. In view of the strict requirement of surface roughness formodulus mapping and nanoindentation tests, the samples werethen polished by using polishing pastes containing diamond withparticle size of 9, 3, and 1 mm. The samples were polished for 30minunder each diamond particle size. To avoid further hydration ofcement, oil-based liquid was used as lubricant during polishingprocess, and the samples were cleaned in an ultrasonic bath filledwith alcohol for 2 min to remove the debris on sample surface fromgrinding and polishing. The surface roughness of the polishedsamples was then measured by using atomic force microscope(AFM). The root-mean-square roughness of the polished surfacewas controlled through the above delicate polishing to below40 nm within a 35 � 35 mm area, which is considered sufficientlylow for nanoindentation and modulus mapping.

2.3. SPM modulus mapping

Quantitative modulus mapping in the form of Scanning ProbeMicroscopy (SPM) images was conducted using the direct forcemodulation operating model of a Tribo nanoindenter (Hysitron)equipped with Berkovitch tip. SPM is a general term for variousprobes that are used for imaging and measuring surfaces on a finescale. This technique is capable of mapping the local variation of thenanomechanical properties without causing plastic deformation tothe material. This testing system can be modeled as a physicalsystem with a force applied to a mass that is attached to the twofixed Voigt elements. The two elements represent the stiffness anddamping of the transducer and the contact, respectively.

The storage modulus (E0) and the loss modulus (E

00) of the con-

tact can be calculated as:

E0 ¼ Kc

2a(1)

E00 ¼ uDc

2a(2)

a ¼ffiffiffiffiffiffiffiffi3FR2Kc

s(3)

where, Kc is the stiffness of the contact; u is the angular frequencyof the applied dynamic load; Dc is the damping coefficient of thecontact; F is the nominal contact force; R is the tip radius of theindent, which is determined as 400 nm by the standard nano-indentation protocol conducted in this study. Storage modulus

a2O (%) K2O (%) Ignition loss (%) Specific gravity Blaine fineness (cm2/g)

.53 0.83 1.77 3500

.26 0.56 0.7 2.88 3820

Y. Wei et al. / Cement and Concrete Composites 85 (2018) 56e6658

represents the material capacity to store energy, large storagemodulus means greater elastic property of the material. Lossmodulus represents the material capacity to dissipate energy asheat, and large loss modulus represents more viscoelastic.

In this study, the modulus mapping was done on the randomlyselected testing spots on the polished samples by applying a si-nusoidal force of 4 ± 3.5 mN at the frequency of 200 Hz to theBerkovitch tip. The characteristic amplitude of the probingdisplacement is 0.5 ~ 1 nm. Inmodulusmapping, each scanned areaincludes 256 lines with 256 data points in each line. Therefore, total256 � 256 pixels were generated for each scanned area. The forceand displacement were monitored at each of the pixels.

2.4. Nanoindentation test

The quasi-static force modulation of Triboindenter (Hysitron)was used for the discrete nanoindentation test to obtain theindentation modulus and hardness of each indents made on theindividual phases. The loading pattern was set as first loading to amaximum load of 2 mN within 10 s followed by a holding period of5 s at the maximum load, and then it was unloaded within 10 s tozero load. The geometry of an indentation test and the discrete in-dents made on the hydration product (HP) and the unreacted slaggrain in a paste sample are shown in Fig. 1a and b. It can be seen thatthe size of indent made on the unreacted grain is smaller than thatmade on the hydration product, which is reasonable because of theless stiff feature of the hydration product compared to the unreactedgrain. This feature is also reflected on the load-indentation depthcurve as shown in Fig. 1c, greater indentation depth is observed forless stiff phase. By analyzing the initial part of the unloading curve,the indentation modulusM and hardness H can be obtained as [22]:

M ¼ffiffiffip

p2b

SffiffiffiffiffiAc

p (4)

H ¼ Pmax

Ac(5)

where, S ¼�dPdh

�h¼hmax

is the contact stiffness and determined from

the slope of the initial part of the unloading curve; hmax is themaximum indentation depth; b is the geometrical correction factor,for the Berkovich tip used in this study, b ¼ 1.034; Pmax is themaximum load; Ac is the projected area of contact, it is a function ofthe equivalent half cone angel qeq¼70.32� and the contact depth hc:

Ac ¼ p,�tan qeq,hc

�2 (6)

Fig. 1. Nanoindentation (a) geometry, (b) indents made on hydration product (HP) and the uclinker, composite, and C-S-H.

hchmax

¼ 1� 0:75Pmax

Shmax(7)

Eqs. (6) and (7) are derived based on the material homogeneity,they have been applied to and proven to be appropriate for theheterogeneous cementitious materials as well [23].

2.5. BSE-EDS test

It should be noted that the phases with different stoichiometrybut similar mechanical response may manifest as a single micro-structure component, which could bias the analysis, as suggestedby Ref. [24]. To avoid such problem, the polished samples were alsostudied using FEI QUANTA 200 scanning electronmicroscopy (SEM)with backscattered electron (BSE) and energy-dispersive X-rayspectroscopy (EDS) detector for the coupled analysis of chemical-structure properties. The accelerating voltage of 15 kV was usedduring EDSmeasurement. This chemical analysis of all the indentedareas by SEM-EDS allows to distinguishingmechanically similar butchemically distinct phase.

3. Results and analysis

3.1. Method of individual phase identification

In this study, the mechanical properties and the chemicalcompositions will be measured on individual phase in both OPCand slag-blended pastes. Identification of individual phase is thefirst step to perform the above measurements. The following pro-cedure is proposed in this study to effectively identify the indi-vidual phase in each measurement:

Step 1 Identify unreacted clinker and slag grains

The unreacted clinker and slag grains in hardened paste canbe easily identified by the optical microscope equipped in theTriboindenter.

Step 2 Identify inner product (IP) and outer product (OP)

The SPM is conducted to map areas including the identifiedgrains from Step 1, and then the IP and OP can be distinguished bytheir mapping color as well as their distance from the unreactedgrains. And then, the discrete nanoindentation can be performedon the individual phase desired.

nreacted slag core of a polished sample, and (c) typical load-indentation depth curves of

Y. Wei et al. / Cement and Concrete Composites 85 (2018) 56e66 59

Step 3 Chemical composition measurement

After done with SPM and nanoindentation tests, the BSE-EDS isconducted to measure the chemical composition of the individualphase. The identified phase by SPM is verified based on the EDS linescanning by their chemical composition.

3.2. Modulus mapping-based microstructure morphology ofhardened paste

The IP and OP are generally known as high density (HD) C-S-Hand low density (LD) C-S-H, respectively [25]. Though a third C-S-Hphase with the highest indentation modulus and hardness wasproposed to be a nanocomposite mixture of high-density C-S-Hwith nanoportlandite [3,20], it is not visible from both modulusmapping-based microstructure morphology and BSE images in thisstudy as shown in Fig. 2.

The microstructure morphology obtained from the modulusmapping corresponds well to the SEM image in terms of phasemorphology and size distribution. The mapping colour appearswhite (it was intentionally coloured with black for better contrastwith the background), red, blue, and green for storage modulusvarying from low to high. Both modulus mapping and SEM imageshow that the IP layer surrounding a clinker (C3S) core in OPC paste

Fig. 2. Modulus mapping-based morphology of OPC (a)e(f) and slag-blended (a’)-(f’) pastes(b’) SPM images of pore or weak phases with the lowest storage modulus; (c) and (c’) SPMimages of inner product or other hydrates with high storage modulus; (e) and (e’) SPM imagpastes, respectively; and (g) schematic illustration of thickness of inner C-S-H layer in OPC

is much thicker than the IP layer surrounding a slag core in slag-blended paste. Though such difference was also observed byRef. [26] as demonstrated in Fig. 2g, the thickness of the IP layer hasnot been quantified precisely yet. The methodology of quantifyingthe thickness of the individual phase in hardened paste bymodulusmapping will be demonstrated in Section 3.3.

3.3. Measuring thickness of individual phase by modulus mapping

A modulus mapping-based method is proposed in this study toassess the thickness of the individual phase in hardened cementpastes. The scanned area by modulus mapping is small with size ofup to 35 � 35 mm due to the high resolution of this technique. It isdifficult to cover many unreacted cores and their surroundingmatrix in one scanned area. Therefore, typical surfaces covering atleast one or two unreacted cores within such small area werecarefully selected for modulus mapping, such as areas with C3S inOPC paste (Fig. 3a), C2S and C3S in OPC paste (Fig. 3b), C3S and slagin slag-blended paste (Fig. 3c), and C2S and slag in slag-blendedpaste (Fig. 3d).

The following was done to quantify the phase thickness: first,modulus mapping in terms of storage modulus was conducted byscanning the selected areas to obtain the SPM image; then, lineswere drawn on the image by passing through the unreacted core

; (a) and (a’) SEM images of OPC and slag-blended cement pastes, respectively; (b) andimages of outer product or other hydrates with low storage modulus; (d) and (d’) SPMes of the unreacted cores; (f) and (f’) the overall SPM images of OPC and slag-blendedand slag-blended pastes after Gruskovnjak et al. [26].

Fig. 3. The thickness of inner product (IP) layer in OPC and slag-blended pastes.

Y. Wei et al. / Cement and Concrete Composites 85 (2018) 56e6660

Y. Wei et al. / Cement and Concrete Composites 85 (2018) 56e66 61

and its surrounding matrix of interest. The lines should be normalto the grain surface to obtain the precise thickness of the individualphase. The storage modulus along the lines was plotted vs. themeasuring distance as shown in Fig. 3; and then the border of eachphase can be identified by the color change in the SPM image andthe abrupt drop or increase of storage modulus; finally, the thick-ness of each phases was calculated as: pixel spacing � number ofpixels on the line segment located in each phase. The pixel spacingis defined as the measured length divided by the number of themeasured points within this length. For a scanned area with size of35� 35 mm, the pixel spacing is 35mm/256¼ 137 nm. Themeasuredthickness of different layers around C3S and C2S grains in both OPCand slag-blended pastes as well as slag grain in slag-blended pasteare labelled in Fig. 3aed.

3.4. Variation of chemical composition by EDS line scanning acrossthe featured phases

To evaluate the chemical composition of the featured phases,EDS line scanning was performed on both OPC and slag-blendedcement pastes. The results are illustrated in Fig. 4. In OPC paste(Fig. 4a), EDS line was scanned starting from a C3S grain to thematrix; in slag-blended paste (Fig. 4c), the EDS line was scannedstarting from a slag grain across the matrix reaching a C3S grain.The element concentrations of Si, Ca, Al, and Mg were measuredalong the lines. In addition, the storagemodulus along the lines wasalso measured bymodulus mapping prior to EDS line scanning. Thechemical composition in terms of atom percentage of Ca and Mg isplotted along with the storage modulus as shown in Fig. 4b andd for OPC and slag-blended pastes, respectively.

Fig. 4. (a) EDS line scanning across an unreacted C3S grain in OPC paste, (b) variation of calci(c) EDS line scanning across an unreacted slag and C3S grains in slag-blended paste and (d)line in slag-blended paste.

The variation of the chemical composition corresponds well tothe phase changes indicated by the storage modulus variation. Thechemical composition is plotted on the left y axis and the storagemodulus is plotted on the right y axis as shown in Fig. 4b and d. InOPC paste (Fig. 4b), the Ca content distinguishes the phase well, asit varies in consistent with the variation of the storage moduluswhich distinguishes the unreacted clinker and the hydrationproduct. In slag-blended paste (Fig. 4d), the Mg content distin-guishes the phasemore precisely than that based on the Ca content,as there is a big difference on Mg content between the slag and theclinker grains as well as between the IP and the OP of slag.

For Ca content, it is roughly remaining above 22% for both C3Sand the C-S-Hmatrix in OPC paste, whereas it is below 22% for bothslag grain and the C-S-H matrix surrounding slag grain in slag-blended paste. It should be noted that the EDS line scanning hasshown a Ca-rich (up to 60%) zone which coincides with the locationof ITZ between C3S and its IP layer in OPC paste as shown in Fig. 4b.

For Mg content, it is constantly below 4% in OPC paste (Fig. 4b),and no phases can be distinguished based on theMg content in OPCpaste. However, the Mg content is found highly variable in slag-blended paste depending on the phase type (Fig. 4d). It is high inslag grain and the inner product of slag (IP-S). In contrast, thecontent of Mg in the outer product of slag (OP-S) is significantlylower than that in IP-S.

In between the two OP-S zones (Fig. 4d), there is a zone withhigher Mg content and storage modulus than that of the sur-rounding OP-S zones. This zone should be the inner product of acompletely reacted small slag grain. Because there is no visibleunreacted slag core in this zone judged from the grey level contrastin the BSE image (Fig. 4c), this zone is suspected to be the inner

um and magnesium contents and storage modulus along the scanned line in OPC paste;variation of calcium and magnesium contents and storage modulus along the scanned

Y. Wei et al. / Cement and Concrete Composites 85 (2018) 56e6662

product of a completely reacted small slag grain, which is labelledas IP-S.

A thin layer surrounding C3S grain in slag-blended paste isrecognized as the inner C-S-H layer of the C3S grain and is labelledas IP-O [S] (Fig. 4d). IP-O [S] shows similar Mg content to C3S grainbut with lower storage modulus and Ca content than that of C3S. Noouter C-S-H is found for this C3S grain based on the chemicalcomposition and the storage modulus. It is suspected that the re-action of the small slag grain (less than 2 mm) is so fast [26] that itshydration products (especially the OP-S) fill the narrow interstitialspace between the C3S and the slag grains, such that the OP of C3Scould not be fully produced.

3.5. Measuring indentation modulus and storage modulus of singlephases

The indentation modulus from nanoindentation test representsthe elastic behavior of material under destructive load, while thestorage modulus from modulus mapping relates to the elasticbehavior of materials without causing damage. Obtaining the me-chanical properties of the single phase in hardened paste isimportant for homogenization and upscaling to predict themacroscopic properties.

Total 100 discrete nano indents were made directly on OPC andthe slag-blended pastes. Some of the indent locations were inten-tionally selected on the unreacted clinker and slag grains. Otherindents were made at locations with various distances from thegrains, such that the mechanical properties of both inner and outerproducts can be measured. The measured indentation modulus isplotted for different phase as the data points shown in Fig. 5. It canbe seen that the indentation modulus (M) ranges 15e115 GPa. Theunreacted cores of C3S, C2S and slag have highest indentationmodulus ranging 75e115 GPa. The indentation modulus of clinkersis a bit lower than that measured in Ref. [27] with modulus ranging125.3 ± 9.9 GPa. And the indentation moduli of inner and outerproducts are in good agreement with those summarized inRef. [27]. The indentation modulus of inner product of clinker (IP-Oand IP-O[S]) and slag (IP-S) is slightly higher than that of the outerproduct. This mechanical difference between IP and OP is explainedby the packing density of the solid globules which are the funda-mental building block of C-S-H [25]. No significant difference isfound in either IPs or OPs between the OPC and the slag-blendedpastes.

The storage modulus (E0) measured by modulus mapping is

plotted as the two lines for each phase as shown in Fig. 5. The upperline represents the maximum storage modulus measured and thelower line represents the minimum storage modulus of a singlephase. It can be seen that the indentation modulus is roughly

Fig. 5. Distribution of storage modulus (with the upper and lower lines representingthe measured maximum and minimum value, respectively) and the indentationmodulus (100 data points) for each phases in OPC and slag-blended pastes.

located within the range of storage modulus, and the variation ofindentation modulus is proportional to the variation of the storagemodulus. The ranges of storage modulus are greater than theindentation modulus, especially for clinkers and slag. This can alsobe observed from Fig. 3 that the unreacted grains show muchscattered storage modulus values. The reason might be attributedto the shallower sensing depth of modulus mapping in stiff grainphase, and thus the large variation of the measured modulus.The average values of the indentation modulus and the storagemodulus are summarized in Table 2.

4. Discussions

4.1. Thickness of inner C-S-H layers in OPC and slag-blended pastes

Different thickness of C-S-H layer indicates different volumefraction of that layer and consequently different contribution to theglobal property of the hardened cement paste. Fig. 3 shows thephase thickness in OPC and slag-blended pastes. To be morerepresentative, total 46 measurements on 31 SPM images havebeen analysed following the same method shown in Fig. 3 toquantify the thickness of the typical phases, namely IP of C3S andC2S in OPC and slag-blended pastes, and IP of slag grain in slag-blended paste. The results of total 46 measurements are summa-rized in Fig. 6. The average thickness value of each phase is plottedas the column and the measured single value is plotted as the datapoint for both OPC and the slag-blended pastes.

It can be observed that the thickness of inner C-S-H layer de-pends on the type of the unreacted cores as well as the supple-mentary cementitious material added. The average thickness of IPlayer surrounding C3S in OPC paste is 6.3 mm, which is thicker thanany other types of IP layer for the samples used in this study. Thisagrees with the previous finding measured by using the BSE tech-nique that the IP thickness is 6 mm around a partially hydrated C3Sin OPC system [28].

For thickness of IP layer surrounding C2S grain, there is no muchdifference between OPC and the slag-blended pastes. In OPC paste,the average thickness of IP-C2S is 2.6 mm. In slag-blended paste, it is1.5 mm. It can be deduced that the IP layer of C2S might not be amajor cause of the difference in global property between the OPCand the slag-blended systems because of the relative similar vol-ume fraction.

The thickness of IP layer surrounding slag grain is smallcompared to any other types of IP layer. The average thickness of IP-slag is 1.0 mm. This IP-slag thickness is much less than that of IP-C3Sin OPC paste. This might be one of the causes for different prop-erties between OPC and the slag-blended systems. Research hasbeen conducted in alkali-activated slag (AAS) system by Grus-kovnjak et al. [26]. They compared OPC and the alkali-activated slag(AAS) systems, and they concluded that the unhydrated cores of thecement grains in OPC system are protected by a thick shell of innerproduct, while the protective layers of slag grain in the AAS systemare very thin. The schematic illustration of this difference is given inFig. 2g. However, precise measurements on the thickness of IPlayers in hardened OPC and AAS pastes were not performed inRef. [26].

4.2. Chemical composition of IP and OP in OPC and slag-blendedsystems

Frommodulusmapping and the EDS results (Figs. 3 and 4), thereare five types of C-S-H produced in OPC (indicated as OP-O and IP-O) and slag-blended (indicated as OP-S, IP-S, and IP-O[S]) pastes. Toinvestigate the chemical composition of the five C-S-H gels, 82discrete EDS testing points have been intentionally made on

Table 2Measured indentation modulus and storage modulus of individual phase.

Five types of C-S-H C2S C3S Slag

OP-O IP-O IP-S OP-S IP-O[S]

# of indents 23 17 7 25 15 4 5 4Indentation modulus (GPa) 25.5 ± 4.3 34.8 ± 5.1 36.3 ± 4.5 24.6 ± 3.3 32.1 ± 2.9 83.3 ± 4.5 89.9 ± 11.9 92.0 ± 15.5Storage modulus (GPa) 24.3 ± 3.6 33.0 ± 3.5 31.2 ± 4.7 23.9 ± 4.0 31.2 ± 3.6 82.8 ± 5.3 88.2 ± 13.0 86.3 ± 6.9

Note: OP-O denotes outer product around clinker in OPC paste.IP-O denotes inner product in OPC paste.IP-S denotes outer product around slag in cement-slag paste.OP-S denotes outer product in cement-slag paste.IP-O[S] denotes outer product around clinker in cement-slag paste.

Fig. 6. The thickness of inner product (IP) layers around C3S, C2S, and slag grains inOPC and slag-blended pastes based on 46 measurements on 31 SPM images.

Y. Wei et al. / Cement and Concrete Composites 85 (2018) 56e66 63

different C-S-H phases prior to nanoindentation tests conducted atthe same locations to obtain their corresponding mechanicalproperties. The chemical composition of each phase was examinedin terms of the Ca, Si, Al, Mg atom contents, the Ca/Si ratio, and theCa/(Si þ Al) ratio.

The five types of C-S-H produced in OPC and the slag-blendedpastes possess different chemical composition (Fig. 7a and b). Themain hydration product in slag-blended system is still C-S-H gelsimilar to that found in OPC paste but with different chemicalcompositions. In additions to the lower content of Ca, C-S-H gel inslag-blended paste shows much greater content of Al (3e5%) than

Fig. 7. Difference on (a) atomic percentage, and (b) Ca/Si and Ca/(Si þ Al) ratios of inner anderror bars represent one standard deviation from 82 measurements).

that in the OPC paste (1.5e2%). The Al content is highest in IP of theslag cores, as shown in Fig. 7a. This is because the alumina-richSCMs increase the Al uptake in C-S-H [29], the main reactionproduct in slag-blended paste is a highly Al substituted calciumsilicate hydrate (C-A-S-H) type gel in which aluminum substitutesfor silicon [30e34]. In addition, Mg content is significantly greaterin IP-S than in OP-S as has been discussed in Section 3.4, which iscapable of distinguishing the inner and outer C-S-H produced inslag-blended paste. The chemical composition of the inner C-S-H ofclinker in slag-blended paste (IP-O[S]) is roughly in between that ofIP-O and IP-S.

Since the main reaction product in slag-blended system is ahighly Al substituted calcium silicate hydrate (C-A-S-H) type gel,the Ca/(Siþ Al) ratio is calculated for the five C-S-H gel in additionto Ca/Si ratio (Fig. 7b). In OPC paste, the Ca/Si and Ca/(Si þ Al)atomic ratios remain stable for both inner and outer C-S-H,whereas the Ca/Si and Ca/(Si þ Al) ratios vary with the types ofgrains in slag-blended paste. OPC paste contains C-S-H gel with aCa/Si ratio equal to 1.6e1.7, while it is 1.0e1.3 in slag-blendedpaste. A lower Ca/Si molar ratio of 0.7 has been found in Alkali-activated slag matrix catalyzed by using potassium hydroxide[34].

The Ca/(Si þ Al) atomic ratio of the C-S-H gel in slag-blendedsystem is much lower than that found in OPC system. The Ca/(Si þ Al) ratio of C-S-H in the 50% slag matrix investigated in thisstudy ranges 0.8e1.1, while it is 1.5 for OPC matrix. Kocaba [31]found the Ca/(Si þ Al) atomic ratio is 1.8e1.9 for C-S-H in OPCpaste. The Ca/(Si þ Al) ratio of C-S-H in water active cement-slagpastes (0e100% slag) varies from 0.7 to 2.4 [30].

outer C-S-H in OPC (IP-O, OP-O) and slag-blended (IP-S, IP-O[S], and OP-S) pastes (the

Fig. 8. Difference on mechanical properties of inner and outer C-S-H in OPC (IP-O, OP-O) and slag-blended (IP-S, IP-O[S], and OP-S) pastes.

Y. Wei et al. / Cement and Concrete Composites 85 (2018) 56e6664

4.3. Thickness and chemical composition of ITZ between C3S andthe matrix

An interface transition zone (ITZ) between the unreacted C3Sgrain and its IP layer has been identified in both OPC (Fig. 3a and b)and slag-blended (Fig. 3c) pastes by modulus mapping technique.This finding is based on the different colours in SPM image as wellas the much lower storage modulus of ITZ than that of its sur-rounding IP and C3S (Fig. 3a). It can be seen that the thickness of ITZbetween C3S and thematrix is roughly ranging between 1 and 2 mmin both OPC and the slag-blended pastes. Xu et al. [17] investigatedthe influence of adding nanoSiO2 on the properties of the interfacebetween C-S-H gel and the cement grains, the width of the inter-face between the unreacted core and the surrounding C-S-H gelwas measured as 200 nm by modulus mapping. This value is lessthan the value of 1e2 mm found in this study. Moreover, themineralogy of the cement grain was not provided in Ref. [17], it isnot clear whether the grain is C3S. Scrivener [28] also found a gap of

Fig. 9. Relationship between the indentation modulus and the storage modulus for

around 1 mm between the large grain and the surrounding C-S-Hlayer. However, this gap was found normally at early ages and itdisappears after about 7e14 days due to the formation of thedenser C-S-H inside the gap [28].

Similar to the ITZ between aggregate and the matrix in concrete,the ITZ investigated in this study might induce a weak bond be-tween C3S and the surrounding matrix due to its lower mechanicalproperty in terms of storage modulus. Such defect at the microscale level might serve as the small defect that contributes to theinitiation of mechanical failure under external loads. No ITZ isobserved surrounding the C2S and the slag grains in both OPC andslag-blended paste, suggesting the stronger bond between thesegrains and the matrix. The reason why ITZ exists only between C3Sand the matrix is still a mystery to the authors.

From EDS line scanning results, this ITZ is featured by a high Cacontent reaching 60% (Fig. 4b). It is suspected that the Ca-richphenomenon is related to the formation of calcium hydroxide inthe ITZ in between C3S and the IP layer, similar to the case of ITZbetween aggregate and the paste matrix in concrete where thecalcium hydroxide crystals are formed. However, there is nocalcium-rich zone found in slag-blended pastes as shown in Fig. 4d.This might be due to the pozzolanic reaction which consumesportlandite in slag-blended system.

4.4. Mechanical property of IP and OP in OPC and slag-blendedsystems

The mechanical properties of the five types of C-S-H arecompared in terms of their indentation modulus (M) and hardness(H). As shown in Fig. 8, the hardness increases with the increasingmodulus. The mechanical properties of IP are greater than that ofOP, no matter it is in slag-blended or OPC pastes. The mechanicalproperties of IPs (IP-O, IP-S, IP-O[S]) are similar, and the mechanicalproperties of OPs (OP-O, OP-S) are very close as well. Consideringthe difference in chemical composition of IPs (IP-O, IP-S, IP-O[S])(Fig. 7a and b), it is clear that the mechanical properties are notsignificantly affected by their chemical compositions (Fig. 8).Rather, the differences of mechanical property rely on the packingdensity of C-S-H which reveals as inner and outer C-S-H. Thesimulation-based results by Ref. [35] also showed that substitutingAl for Si had no significant effect on elastic properties of C-S-H.

4.5. Linking storage modulus to nanoindentation modulus

Storage modulus represents elastic property of materials,and thus is practically considered equivalent to the indentation

hydration products and the unreacted grains in OPC and slag-blended pastes.

Y. Wei et al. / Cement and Concrete Composites 85 (2018) 56e66 65

modulus [11]. In this study, the relationship between the storagemodulus E

0and the indentation modulus M is evaluated. Modulus

mapping and the discrete nanoindentationwere both conducted onthe same location of the samples to establish the relationship be-tween the two micro-mechanical properties. As the sample surfacemight be damaged by the large static nanoindentation force,modulus mapping was always conducted prior to the nano-indentation test. After modulus mapping test, the same testinglocation was found for the nanoindentation test by the techniqueshown in Fig. 9a. During static indentation test, the indents weremade on desired phases to measure the indentation modulus(Fig. 9a). The indent locations were then traced back to themodulusmapping image by the position lines intentionally drawn on theimages to facilitate locating, and thus an overlapping image wasobtained with the position lines on the same coordinate. A smallarea with size of 0.5� 0.5 mm covering the indent was identified onthe mapping image. The storage modulus was calculated as theaverage storage modulus values within this area, which is used tocorrelate with the indentation modulus.

The relationship between the indentation modulus and thestorage modulus of each phase is finally plotted in Fig. 9b for bothOPC and the slag-blended pastes. It can be seen that the storagemodulus measured by modulus mapping is generally consistentwith the indentation modulus measured by nanoindentation. Thisagrees with findings by Li et al. [36]. A good linear correlation isfound, though the overall storage modulus value is slightly lessthan that of indentation modulus. The higher indentation moduluswas attributed to the interaction between the multiple phases inthe indented area, because the indent depth of nanoindentation ismuch greater than the probing depth in modulus mapping test. Thelinear relationship agrees well with the previous findings on glassymaterials with storage modulus of E

0z 1 GPa, whereas it was

concluded that there was no good correlation between the inden-tation modulus and the storage modulus for materials with lowerstorage modulus E

0z 1 MPa [37,38]. The well correlation between

the storage modulus and the indentation modulus demonstratesthat the modulus mapping is capable of characterizing the me-chanical property and distinguishing phase as the static nano-indentation technique does.

5. Conclusions

The chemical-mechanical-morphological properties of the fivetypes of C-S-H gels (OP-O, IP-O, OP-S, IP-S, and IP-O[S]) in bothordinary Portland cement (OPC) and slag-blended cement pasteswere investigated by using the coupled techniques of quantitativemodulus mapping, nanoindentation, and the energy-dispersive X-ray spectroscopy. The major findings are:

1. The different phases in hardened paste can be clearly distin-guished by SPM modulus mapping due to its capability ofmapping the local variation of nanomechanical properties. Thethickness of the inner C-S-H (IP) layers in hardened OPC andslag-blended pastes was quantified for the first time by usingthe SPM technique from the abrupt changes of the continuouslymeasured storage modulus. The inner and the outer C-S-H canbe separated meaningfully. The thickness of the IP layer varieswith the types of the unreacted cores as well as the addition ofthe supplementary cementitious materials. The average thick-ness of the IP layer surrounding C3S in OPC paste is 6 mm, thickerthan any other types of IP layer for the samples used in thisstudy.

2. An interface transition zone (ITZ) of about 1e2 mm thick is foundbetween the unreacted C3S core and the surrounding inner C-S-H layer in both OPC and slag-blended systems. The storage

modulus of this ITZ is significantly lower than the other phases,indicating a weak bonding in hardened cement paste. This ITZhas slightly greater Ca content than that of C3S. There is no ITZbetween the matrix and the unreacted C2S and the slag cores inboth OPC and slag-blended systems.

3. The mechanical properties (M and H) of the five types of C-S-Hgels are not significantly affected by their chemical composi-tions, instead the inner or outer products matter. The slag-blended pastes possess distinct chemical compositions whichdistinguish the inner and outer C-S-H.Mg content can be used todistinguish the inner and outer C-S-H gel (IP-S and OP-S) pro-duced in slag-blended system, as the Mg content is significantlygreater in IP-S than in OP-S.

4. The well correlation between the storage modulus and theindentation modulus demonstrates that the modulus mappingtechnique is capable of characterizing the mechanical prop-erty and distinguishing phase as the static nanoindentationtechnique does, but with more attractive features of higherspatial resolution to quantify the phase size of multiphasematerials in nanometer scale level under a nondestructivetesting conditions.

Acknowledgements

The authors wish to thank the supports from National NaturalScience Foundation of China under Grant No. 51578316 and No.51778331.

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