The ITZ in concrete with natural and
recycled aggregates: Study of
microstructures based on image and SEM
analysis
G. Bonifazi
a, G. Capobianco
a, S. Serranti
a,⇤, M. Eggimann
bE. Wagner
c, F. Di Maio
d, S. Lotfi
daDepartment of Chemical Engineering Materials & Environment, Sapienza University of Rome, Rome, Italy bHolcim Technology Ltd., Innovation, 5113 Holderbank, Switzerland
cHeidelberg Cement AG, Germany
dSec. of Materials and Environment, Fac. of Civil Engineering and Geosciences, Delft University of Tech., The Netherlands
⇤silvia.serranti@uniroma1.it Abstract
Aim of this work was to investigate the microstructure of the Interfacial Transition Zone (ITZ) between cement paste and aggregate in concrete utilizing Scanning Electron Microscope (SEM) in order to identify possible effects on the ITZ related to different recipes and production parameters. SEM is an important tool to carry out concrete, cement and aggregate microstructure characterization. SEM imaging facilitates identification of elements in concrete. Thanks to the higher spatial resolution, and the analytical capabilities, it allows to perform a materials-imaging-related analysis. Many information can thus be extracted from these data, such as composition, phase abundance and distribution. In particular, during the last years, backscattered electron imaging of polished surfaces became an important method for the study of cement and concrete microstructures. The technique has many advantages, as the possibility to visualize representative cross-sections over a wide range of magnifications and to correlate image contrast with materials composition (i.e. atomic number). Comparing the information acquired starting from SEM imaging and X-ray microanalysis, the complex microstructure of concrete can be analyzed and images with sub-micrometer definition acquired and processed, obtaining compositional, morphological and morphometric information allowing to understand and evaluate the mechanism affecting the interaction of the aggregates with the cement paste. A common image representative of the ITZ in concrete involves approximately a domain of about 30 µm surrounding each aggregate and showing, theoretically, an increase of the porosity at the aggregate interface. In order to study this transition zone, 12 different types of concrete mixtures were analyzed, characterized by different combination of cement and virgin/recycled aggregates. The influence of these aggregates (i.e. virgin and recycled) on the resulting concrete microstructure was studied. The significant morphological and overall textural characteristics (i.e. area with higher porosity around aggregate, micro-cracks, size of the crystals in the ITZ) were detected by SEM analysis. Image analysis and SEM investigation, obtained from different concrete samples, provided very interesting results due to the different porosity and the possible interactions between the aggregates and the cement paste.
Keywords: Recycled aggregates, concrete, Interfacial Transition Zone (ITZ), SEM, image analysis
I. Introduction
The possibility to reutilize aggregates, resulting from recycling of End-Of-Life (EOL) concrete, repre-sents a challenge in the sector. In these last years, more and more recycled coarse aggregates have been proven to be commercially and technically sound for both non-structural and structural appli-cations.
Recycled Aggregate Concrete (RAC) behavior and characteristics are strongly conditioned and influ-enced by the "source" waste aggregates, the adopted
concrete mix design, the operative conditions to re-alize the mix, and the intrinsic RAC attributes (i.e. mineralogical composition, surface status, morpho-logical and morphometrical properties). A preemi-nent role in final concrete characteristics is played by the cohesive interface properties resulting from RAC utilization, properties strongly linked to Inter-facial Transition Zone (ITZ) characteristics, old mor-tar quality and the old mormor-tar content of the origi-nal concrete (Xia et al., 2013).
The ITZ, that is the region between aggregate and cement paste, is usually characterized by a
Figure 1: Polished sections of the investigated samples, as resulting after specimen preparation, finalized to perform SEM analyses. The details about the adopted samples labelling is reported in Table 1.
different microstructure than the bulk paste. ITZ microstructure characterization is usually carried out analyzing Backscattered Electron Images (BEI) ob-tained by Scanning Electron Microscopy (SEM) based investigations. Following this approach ITZ higher porosity and lower content of un-hydrated cement grains, in respect of the bulk paste, is usually de-tected. These characteristics change according to aggregate surface distance up to reach those of the bulk paste. This distance is usually defined as ITZ thickness (Elsharief et al., 2003). Water-cement ratio (w/c) has been considered to have only small effect on the ITZ microstructure (Scrivener, 1999).
Mortar and concrete porosity in the ITZ is higher compared to the bulk of the hardened paste. Fresh
concrete subjected to compaction shows, around the areas of the granules, a greater size of the aggregate and a water film (of hundreds of microns) collecting bleeding water. As result a higher water/cement ratio occurs in the ITZ in respect of the cement matrix, this fact determines, as a consequence, a higher porosity in the ITZ.
The formation of the first-crystalline germs
of calcium hydroxide (Ca(OH)2) and ettringite
(3CaO.Al2O3.3CaSO4.32H2O), occurring through-out the cementitious matrix (i.e. cement silicates and aluminates hydration), is followed by a greater crystal growth of these products in the transition area and a corresponding production of higher porosity due to the "local" existence of higher water-Table 1: Concrete samples labelling according to utilized aggregates and cement based recipes C1 and C2 (see water-Table 2).
Sample Description ADR feed rate
[t/h]
Autogenous mill tilt angle[degrees]
C1R-REF Concrete produced with natural roundish aggregates -
-C1-D15-100
Concrete produced with 100% recycled aggregates adopting recipe C1
15 5
C1-C50-100 50 5
C1-A100-100 100 7
C1-B100-100 100 5
C1-ADR-100 Concrete produced with 100% recycled aggregates without preliminary
autogenous milling 100
-C1C-REF Concrete produced with natural crushed aggregates -
-C2-REF Concrete produced with natural roundish aggregates -
-C2-D15-100
Concrete produced with 100% recycled aggregates adopting recipe C2
15 5
C2-C50-100 50 5
C2-A100-100 100 7
solid ratio. This latter effect can be attributed to the "wall effect" leading to a relatively low con-centration of large cement particles in the ITZ pro-ducing, as a consequence, an increased porosity. During cement hydration the pores in the ITZ are partially filled, according to a redistribution of cal-cium hydroxide and calcal-cium-silicate-hydrates (C-S-H) (Elsharief et al., 2003). The ITZ of individual aggregates can be interconnected depending on their width and the content of aggregates. Such connectivity, or percolation, is considered as an important characteristic that can influence and/or condition the permeability and transport properties of mortar and concrete (Leemann et al., 2006). Ex-tra pore space, originally associated with the wall effect in ordinary concrete, is thus mostly filled up by deposits of calcium hydroxide and by deposits of (C-S-H) passing through solution, leading to the retention of only a relatively low residual extra av-erage pore content within the ITZ (Diamond and Huang, 2001).
Historically, recycled aggregates properties were studied through the evaluation of mix workability and "final product" compressive strength. The long-term properties evaluation/comparison of normal and recycled based aggregates concretes, studying shrinkage and creep phenomena, was also carried out. However, few studies have been conducted on exploring the microstructure of recycled aggregate concrete and interactions with cement paste (Poon et al., 2004).
ITZ characteristics can be qualitatively revealed via SEM performing a BSE analysis. A BSE image, in fact, depends on the mean atomic number of the concrete constituents. The grey scale of a BSE image is thus linked to the composition and textu-ral attributes of the investigated regions (Hussina and Poleb, 2011). Following this strategy porosity and individuals (i.e. cement particles and/or crys-tals) are clearly evidenced (i.e. epoxy impregnated pores are seen as black). Furthermore the possi-bility to perform a digital processing of the image could allow to quantify several topological and mor-phometrical attributes ITZ related. Combining the information derived from the BSE images and those obtained by EDX analysis (i.e. maps of elements) on the same ITZ areas, it is thus possible to acquire a huge amount of information allowing to evalu-ate the interaction existing between aggregevalu-ates and cement paste, in respect of different recipes and aggregates origin: virgin or recycled.
The results reported in this paper are re-lated to some ITZ studies performed inside a project financed by the European Union (EU): Collaborative Project n.265189, C2CA: "Ad-vanced Technologies for the Production of Cement and Clean Aggregates from Construction and Demo-lition Waste"
(http://www.c2ca.eu/activities/the-c2ca-project/) whose main target is to compare the interaction of cement/aggregate between nat-ural and recycled aggregates. The C2CA project, in fact, aims to develop a cost-effective approach for recycling EOL concrete high-volume streams into prime-grade aggregates and cement. To reach this goal an innovative low-cost combined com-minution/classification, called Advanced Dry Recov-ery (ADR) was applied to EOL concrete in order to produce secondary cleaned aggregates, removing at the same time the presence of fines and light con-taminants, in size class interval ranging between 1 and 4 mm. The utilization of the ADR approach allows to break the bonds existing between by mois-ture and fine particles and to realize a materials classification independent from their moisture con-tent. After breaking up the material into a jet, the fine particles are separated from the coarse particles. ADR separation produces as results the aggregates concentrations into a coarse aggregate product and the production of fine fraction constituted by ce-ment paste and contaminants (i.e. wood, plastics and foams) (Lotfi et al., 2014).
II. Materials and methods
The study was performed with reference to 12 dif-ferent types of concrete mixtures (Figure 1) realized utilizing different aggregates and cement. The se-lected aggregates were constituted by virgin ones and as resulting from recycling according to dif-ferent sequential autogenous and ADR based com-minution strategies (Table 1). EOL concrete was preliminary milled by autogenous milling, then the -16 cm fraction was fed to ADR. Two series of con-crete, identified as C1 and C2 were produced by Holcim and HeidelbergCement AG, respectively, utilizing different cements and mix designs. Their characteristics are outlined in Table 2.
All the analyses have been carried out by a SEM equipped with an Energy Dispersive X-ray Spec-troscopy (EDS) analytical unit.
Cast samples were cut in slices of about 1 cm thickness. From each slice a prismatic sample of about 2x1x1cm (Figure 2) was realized and utilized for polished samples preparation (Figure 3). To reach this goal, concrete "prismatic specimens" were immersed in ethanol and impregnated with epoxy resin. Samples were then cut by diamond-bladed saw and polished, utilizing a series of abrasive pa-pers (i.e. 240, 600, 1000 and 2000 grit) for about 2 min each, with white spirit. Finally, a diamond paste of 3 µm on a low-relief polishing cloth was utilized for a further surface specimen polishing. Polishing was manually carried out for about 2 min. Samples were examined by a S2500 Hitachi SEM Microscope equipped with Kevex 8000 for X-ray microanalysis. Polished samples were examined
Table 2: Composition of C1 and C2 concrete mixtures.
Recipes Cement [kg/m3] w/c ratio Water [kg/m3] Plasticizer Air entrainer Consistency Class Aggregate grading
C1 300 CEM III 0.60 180 none none S3 DIN A-B 16 mm
C2 340 CEM I 52.5R 0.45 153 For slump none S3 DIN A-B 16 mm
both under secondary electronic imaging and BEI. Detection was performed with a low acceleration voltage of 20 kV. Six magnifications, i.e. 50x, 100x, 150x, 200x and 400x were adopted. Images were "randomly" acquired around aggregates in order to statistically guarantee the representative of the analyses.
SEM analyses have been carried out in order to identify and quantitatively assess, the most signif-icant morphological, morphometrical and overall textural characteristics of the ITZ, that is: i) pres-ence of regions of porosity around aggregates, ii) presence of micro-cracks, iii) presence of multi-layer oriented crystals around the aggregates and iv) size of the crystals detected in the ITZ. Micro-cracks around the ITZ have been observed at low mag-nification (50x). ITZ morphological and porosity characteristics have been identified adopting an in-termediate magnification (from 200x to 300x). Crys-tal presence (i.e. ettringite or hydroxide of calcium) and size have been detected at high magnification (from 400x to 2000x). X-ray mapping was systemat-ically utilized (400x) to identify the possible pres-ence of the ITZ in respect of cementiceous matrix.
III. Results
The C1C-REF sample (Figure 4a), constituted by vir-gin crushed aggregates, shows a variable porosity around greater aggregates. The average dimension of the pore, detectable in the ITZ. is about 30 µm. The effect of water bleeding is quite evident (Fig-ure 4b). A clear separation between aggregates and cement matrix exists. Oriented multi-layer crystals structures can be also identified in the areas char-acterized by a higher porosity (i.e. water bleeding
effect) around bigger aggregates (Figure 4c). The C1R-REF sample (Figure 5a), composed with roundish aggregates, shows an ITZ characterized by a lower porosity in comparison of that detected in C1C-REF sample. A complete detachment of the aggregate from cement matrix can be observed (Figure 5c). The average thickness of ITZ is about 40-50 µm. X-ray mapping shows a higher calcium concentration around the biggest aggregates and a lower, as found in C1C-REF sample, silicate content (Figure 5b). These phenomena can be associated to the presence of calcium hydroxide along the ITZ.
The C2-REF matrix is well linked with the aggre-gates. Micro-cracks at low magnification (Figure 6a) are practically undetectable. ITZ average area is below 5 µm in this sample (Figure 6b) changes of matrix cement structure and characteristics around aggregates are not detected. Micro-cracks can be observed only at higher magnification (Figure 6c). Micro-cracks, for their characteristics, can be associ-ated to cement shrinkage and not to point of weak-ness around aggregate. Making a comparison with the results obtained for C1R-REF it results clear as C2-REF samples present better ITZ characteristics than C1R-REF. This difference can be mainly at-tributed to the morphological and morphometrical characteristics of the aggregates.
Cement matrix of C2-B100-100 sample is well linked with the aggregates. The presence of micro-cracks is not significantly detectable (Figure 7a) both at low and high magnification. Around the aggregates no significant variation in the cement matrix is observed. The presence of oriented ag-gregates, due to bleeding effect, is also evidenced (Figure 7b). The ITZ is very small with an average dimension ranging between 6 and 8 µm (Figure 7c).
Figure 2: Example of quartering performed with reference to C2 C50 100 sample (2a) and a slice samples (about 2x1x1cm) utilized to prepare the polished sections analyzed by SEM (2b).
Figure 3: Samples preparation sequence. Vacuum system and impregnated specimens (3a) , diamond-bladed saw (3b),polishing treatment (3c) and diamond pastes utilized to perfom hand made surface finishing (3d).
Figure 4: Backscattered Electron Image (BEI) based acquisition of C1C-REF concrete sample at 50x (4a), 200x (4b) and 400x (4c) magnification, respectively.
Figure 5: BEI based image acquisition of concrete C1R-REF sample at 50x (5a) magnification, maps of element of Si, O , Ca (5b), and BEI image at 400x (5c) magnification.
Figure 6: BEI based image acquisition of concrete C2-REF sample at 50x (6a), 200x (6b) and 400x (6c) magnification, respectively.
Micro-fractures are almost absent. ITZ character-istics are in good agreement with the operative con-ditions adopted for the EOL concrete comminution and the adopted low water/cement (W/C) ratio in concrete mix design.
The results obtained for C2-B100-100 sample also fit with those of C2-D15-100 sample. Based on their analysis, in fact, it is clear as EOL concrete feed rate to ADR is not so important with respect to the milling actions carried out and the adopted concrete mix design.
C1-B100-100 sample shows an ITZ extremely complex, with a high porosity and a large pres-ence of micro-fractures. Furthermore the BEI shows as a marked difference between matrix and cement and as the ITZ, surrounding the aggregates, shows the presence of oriented crystal aggregates due to water bleeding effects. The visualized transition zone is characterized by a width around 65 µm (Fig-ure 8a). The analyses carried out by EDS produce an elements mapping confirming the probable pres-ence, in the ITZ, of calcium hydroxide (Figure 8b). The pores of larger dimension are characterized by the presence of clearly visible great size crystals of ettringite and calcium hydroxide (Figure 8c).
C1-C50-100 sample shows an area around greater
aggregates, characterized by high porosity. In this area (ITZ) the presence of micro-cracks is evident (Figure 9a). Comparing the elements map (Fig-ure 9b), resulting from EDS analysis, is possible to detect the presence all around the aggregates of Ca. The thickness of the area showing the presence of Ca is of about 30 µm). BEI determinations show the presence of an ITZ of 50 µm average thickness. At higher magnification (400x), it is possible to de-tect, inside the ITZ, the presence of dark regions, representative of a marked porosity. The presence of micro-aggregates, ranging between 5 and 20 µm, is noticeable and the texture that they originates can be clearly attributed to water bleeding effects (Figure 9c).
C2-C50-100 sample shows the presence of old ce-ment paste attached to the aggregates. Analyses show that the old cement is characterized by a dif-ferent matrix, embedding small calcium carbonate individuals and/or aggregates. The presence of micro-cracks is low (Figure 10a). Apparently, any marked discontinuity seems to exists, between the old and new concrete (Figure 10b). The dimension of ITZ is not relevant, being the detected thickness always below 10 µm (Figure 10c).
These results are quite interesting, evidencing
Figure 7: C2-B100-100 concrete sample. BEI image fields acquired at different magnification: 50x (7a), 200x (7b) and 400x (7c) magnification.
Figure 8: C1-B100-100 concrete sample. BEI image field acquired at 200x (8a). Si, O and Ca, X-ray mapping (8b). BEI image field acquired at 400x (8c).
as ITZ characteristics are not particularly influ-enced by the utilization of recycled aggregates, on condition that correct operative conditions to recy-cled aggregates and proper concrete manufacturing recipes are adopted.
C1-D15-100 sample is characterized by a high porosity, clearly identified by the presence of darker color regions around the aggregates, mainly around the big ones (Figure 11a). The presence of micro-cracks, detectable inside the ITZ (Figure 11b), is also noticeable. ITZ thickness ranges in average, be-tween 30-40 µm and below. ITZ texture, pictorially evidenced by grey tone variations, is clearly due to a marked bleeding water effect (Figure 11c).
C2-D15-100 sample does not show a high poros-ity around the ITZ. The presence of the old cement matrix is quite evident (Figure 12a). Any marked discontinuity linked to the old (i.e. presence of indi-viduals, constituted by cement and aggregates, be-longing to the utilized EOL concrete ) and new con-crete (Figure 12b) seems not to be present, porosity is low and material composition around aggregates does not present significant variations (Figure 12c). Making a comparison with the corresponding sample (C1-D15-100) obtained with a different mix
design, it seems quite clear the role that these pa-rameters play in respect of the final attributes of concrete, as well as ITZ characteristics.
Analyzing the C1-A100-100 sample (Figure 13a), the presence of micro-cracks inside the cementitious matrix is clear (Figure 13a). The presence of old cement elements, around the bigger aggregates, is also detected as well as the presence of micro-cracks all around the ITZ of new concrete and inside the old one (Figure 13b). At higher magnification (400x), darker areas, representative of porosities and/or physical discontinuities inside the sample, can be detected. The ITZ average thickness is estimated to be 30 µm (Figure 13c).
The C2-A100-100 sample does not show the pres-ence of a significant amount of micro-cracks inside the cementitious matrix (Figure 14a). The maps of element confirm the presence of calcium hydroxide in the ITZ (Figure 14b). ITZ average thickness, de-tected at higher magnification (400x), is about 12
µm (Figure 14c). Also for the detected textural
char-acteristics of samples C1-A100-100 and C2-A100-100 the considerations already done for samples C1-D15-100 and C2-D15-100 are valid; these are the influence of water water/cement (w/c) ratio and
Figure 9: C1-C50-100 concrete sample. BEI image field acquired at 50x (9a). Si, O and Ca, X-ray mapping (9b). BEI image field acquired at 400x (9c).
Figure 10: C2-C50-100 concrete sample. BEI image field acquired at 50x (10a), 200x (10b) and 400x (10c) magnifi-cation, respectively.
Figure 11: C1-D15-100 concrete sample. BEI image field acquired at 50x (11a), 200x (11b) and 400x (11c) magnifi-cation, respectively.
Figure 12: C2-D15-100 concrete sample. BEI image field acquired at 50x (12a), 200x (12b) and 400x (12c) magnifi-cation, respectively..
Figure 13: C1-A100-100 concrete sample. Secondary electron image acquisition of concrete at 50x (13a). BEI image field acquired at 200x (13b) and 400x (13c) magnification, respectively.
Figure 14: C2-A100-100 concrete sample. BEI image field acquired at 50x (14a), X-ray mapping of Si, O and Ca elements (14b). BEI image field acquired at 400x (14c).
Figure 15: C1-ADR-100 concrete sample. BEI image field acquired at 50x (15a), X-ray mapping of Si, O and Ca elements (15b). BEI image field acquired at 400x (15c).
concrete mix design.
Making a comparison between the two pairs of C1-A100-100, A100-100 and C1-D15-100, C2-D15-100 samples, it is also quite clear the influence that autogenous mill tilt angle set-up and feed rate play on ITZ.
C1-ADR-100 is characterized, in respect of the other samples, by the marked presence of micro-cracks all around the greater aggregates. Further-more the ITZ can be identified also at lower
mag-nification (Figure 15a). The map of the elements referred to calcium, silicon and oxygen confirms the probable presence of calcium hydroxide in the transition zone between matrix and aggregate (Fig-ure 15b). ITZ also shows a marked porosity, its thickness varies between 50 and 70 µm and the presence of oriented crystals, determined by wa-ter bleeding effect (Figure 15c), can also be clearly detected.
IV. Conclusion
The micro-structure and the attributes of the ITZ in different concrete sample, realized with natural and recycled aggregates of different characteristics, according to different comminution strategies (i.e. adoption of autogenous milling and different ADR feeding conditions) and adopting different concrete mix designs was carried out.
According to the achieved results, it is possible to affirm as concrete samples produced by natural roundish aggregates (i.e. C2-REF samples) and those produced with recycled aggregates (i.e. C2-C50-100), obtained after ADR processing with a medium feed rate (i.e. 50 t/h), adopting the same concrete C2 mix design (i.e. low water/cement ratio), seems to be characterized by the presence of an ITZ with an average thickness of about 2
µm. This result is in good agreement with the
operative conditions adopted to produce aggregates (i.e. lower values of ADR feed produce a higher degree of liberation of aggregates form the matrix) and concrete mix design (C1 and C2).
Samples C2-D15-100 and C2-B100-100 show an ITZ characterized by an average thickness ranging between 2-5 µm and 5-12 µm, respectively. These results are clearly influenced by ADR feeding set up. For sample C2-A100-100 the values of ITZ increase in agreement with the different autogenous mill operative conditions, in respect of those adopted for samples C2-D15-100 and C2-B100-100, that is higher tilt angle and, as a consequence, lower residence time of the material in the mill before to be fed to ADR.
Comparing ITZ characteristics detected for the concrete obtained with natural roundish (i.e. C1R-REF) and crushed (i.e. C1C-C1R-REF) aggregates, adopt-ing the same C1 mix design (i.e. high water/cement ratio), it is possible to see as the first sample shows lower ITZ thickness values (about 50 µm) than the second one (about 60 µm). Such behavior can prob-ably due to the different aggregates morphological and morphometrical attributes. On the contrary, comparing the ITZ concrete samples characteris-tics as resulting from the utilization of recycled ag-gregates not preliminary milled, but directly feed to ADR (C1-ADR-100), the results, independently from recipes, show the highest ITZ thickness, the highest porosity and the larger presence of micro-cracks.
The study of the ITZ for all the investigated sam-ples shows that:
• autogenous milling-ADR based combined com-minution influence aggregates characteristics (i.e. aggregate degree of liberation in respect of cementitious matrix,
• the water/cement (w/c) ratio in mix concrete
is influenced by the aggregates degree of liber-ation and, as a consequence, the ITZ character-istics will change,
• a clear correlation can be established among recycled aggregates production strategies, se-lected concrete manufacturing mix designs, ITZ characteristics, concrete mechanical behav-ior and durability.
Further studies will be addressed to validate the obtained results, analyzing more concrete samples produced with the same mix designs and to assess ITZ attributes adopting imaging based strategies.
Acknowledgement
This research is funded by the European Commis-sion in the framework of the FP7 Collaborative project "Advanced Technologies for the Production of Cement and Clean Aggregates from Construction and Demolition Waste (C2CA)". Grant Agreement No. 265189.
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