Construction and Building Materials 73 (2014) 754–763

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Construction and Building Materials

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Investigation of using hybrid recycled powder from demolished concretesolids and clay bricks as a pozzolanic supplement for cement

http://dx.doi.org/10.1016/j.conbuildmat.2014.09.0660950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +1 412 624 9899.E-mail address: qiy15@pitt.edu (Q. Yu).

Qiong Liu a,b, Teng Tong a, Shuhua Liu c, Dezhi Yang d, Qiang Yu a,⇑a Department of Civil and Environmental Engineering, University of Pittsburgh, PA 15261, USAb Shanghai Research Institute of Building Science, 200032, Chinac State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, 430072, Chinad Dujiangyan Recycled Construction Material Company, China

h i g h l i g h t s

� The pozzolanic property of hybrid recycled powder is systematically studied.� The unique microstructure morphology of hybrid recycled powder is captured.� Quantitative chemical analysis is carried out for hybrid recycled powder.� Hybrid recycled powder changes the microstructure characteristics of cement paste.� Hybrid recycled powder shows potential of being used as a supplement for cement.

a r t i c l e i n f o

Article history:Received 8 August 2014Received in revised form 23 September2014Accepted 25 September 2014Available online 31 October 2014

Keywords:Construction and demolition wastesSintered clay brickHybrid recycled powderPozzolanic activityAir pollution

a b s t r a c t

During recycling construction and demolition wastes containing both concrete solids and clay bricks, alarge amount of hybrid fine powder is generated. Finding a ‘‘green’’ way to dispose this hybrid recycledpowder not only promotes sustainable construction, but also benefits the current effort to reduce thehuman-produced aerosol, which triggers air pollution. To investigate the use of hybrid recycled powderas a supplementary cementing material, the pozzolanic property of hybrid powder is comprehensivelystudied here by utilizing advanced tools including SEM, AFM, LPS and XRD. Recycled powder of differentconcrete–clay brick ratios is first studied with a focus on its fineness, loss on ignition, strength activityindex and water requirement. Then, the microstructure characteristics and chemical composition ofthe hybrid recycled powder, obtained directly from a dust collection system, are qualitatively and quan-titatively probed. To develop a deeper understanding of the activity mechanism of hybrid powder, nano-scale characterization is employed to scan and analyze the microstructure of cement paste supplementedwith hybrid recycled powder. It is found that the activity mechanism of hybrid powder is strongly cor-related with its unique microstructure morphology and chemical composition. The results show that ifthe proportion of clay brick as well as the replacement percentage is well designed, hybrid powder fromdust collection systems has potentials of being used as a cement supplement for concrete.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The construction and demolition wastes (C&D wastes) generallyconsist of materials used in civil construction, which typicallyinclude concrete, clay brick, mortar, wood, plastic and steel. Asmore and more civil structures are approaching the end of theirexpected lifespan, finding economical and environment-friendlystrategies to manage the disposal of C&D wastes is an essential

aspect of the current progress towards a sustainable builtenvironment.

A promising practice to meet this goal is to utilize C&D wastesto generate recycled materials and then use them again to produce‘‘green’’ construction materials. After metals and organic materialsare removed, C&D wastes can be crushed into recycled aggregates,which are then reused in construction based on their mechanicaland chemical properties. This practice, which significantly reducesthe environmental and economic footprints of civil structures, hasbeen widely adopted around the world and many international andnational specifications and standards [1–5] are developed to guideand improve its implementations in practice.

Table 1Density of recycled clay brick, recycled concrete and natural aggregate.

Claybrick

Concrete Naturalaggregate

Bulk density (kg/m3) of coarse solid 1920 2270 2610Apparent density (kg/m3) of fine

powder2521 2594 2645

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During the recycling process, a large amount of dust, as abyproduct, is generated. The dust obtained from collection systemsis usually called recycled micro-powder and how to reuse it is achallenge attracting increasing interests in recent years. Similarto many other countries, a significant portion of C&D wastes is sin-tered clay brick in China, especially for those collected in townsand villages. Due to the financial burden resulting from the highdemands for labor and time, there are usually no screening proce-dures implemented to separate concrete solids and clay bricksfrom C&D wastes in the current recycling practice. Consequently,the recycled powder obtained in collection systems is a hybrid ofconcrete and clay brick. A recent report shows that the percentageof clay brick can be up to more than 50% in C&D wastes collected inChinese cities [6,7]. This unique composition of C&D wastes leadsto more dust obtained in collection systems, because sintered claybrick has lower strength compared to concrete and thus is easier tobe pulverized during recycling [8,9].

This hybrid recycled powder of a large number of fine particles,if not correctly collected and disposed, can spread and suspend inair to increase the level of human-produced aerosol, which triggersair pollution causing respiratory system diseases and other disor-ders, e.g., asthmas and lung cancer. In many developing countries,the particulate-induced air pollution imposes a severe threat topublic health, which is exemplified by the increasing air qualitywarnings issued in Chinese cities. Therefore, finding a ‘‘green’’way to dispose the waste dust generated in the recycling of C&Dwastes is of great importance for reducing the fine particulates sus-pended in air, and thus contributes to both public health and envi-ronmental sustainability.

An environment-friendly way is to reuse it like fly ash to pro-duce ‘‘green’’ construction materials. It is widely reported that sin-tered clay brick, after comminuted, can be used as a pozzolanicsupplement like fly ash because of the rich pozzolanic ingredientsin its mineral composition [10,11]. Thus, it can be added to con-crete to replace part of cement, as well as to suppress the expan-sion induced by the possible alkali–silica reaction [12]. In arecent research [13], different types of clay bricks from differentEuropean countries were collected and then ground into powderto replace the cement. Chemical tests for pozzolanic activity con-firmed that all the brick types investigated displayed good pozzo-lanic activity and this conclusion was further supported bystrength development data based on the mortar bars [14].

It is found if the proportion of the recycled clay brick powder inthe concrete mix is well designed, the concrete strength is not sig-nificantly compromised. In an investigation by Kartini et al. [15],recycled clay brick powder is used to replace part of cement in con-crete samples. The test results show corresponding to 10%, 20% and30% replacement, the average strength of samples, when comparedwith the benchmark samples containing no recycled clay brickpowder, is 4.4%, 8.4% and 14.9% lower, respectively. In anotherinvestigation, it is reported that no significant effect is found onthe water demand and the setting time even the replacement isup to 25% [16].

The effect of recycled clay brick powder is also investigated inmortar. It is documented that when it replaces 10% of cement,the values of compressive and flexural strength at 90 days are sim-ilar to the companion samples, whose mix does not include anysupplements [17]. Besides proportion, the grain size of brick pow-der shows a significant impact on the mortar strength. A recent teston mortar samples containing recycled clay brick powder gradedby 4 different grain sizes, namely, 0.04, 0.06, 0.1 and 0.3 mm,shows that the grain size of 0.06 mm gives the highest strengthamong all the samples tested [18].

Similar to recycled clay brick powder, recycled concrete powdercan also be used as a supplement in concrete because of its pozzo-lanic property. Both Chen et al. [19] and Sun et al. [20] studied the

recycled micro-powder collected from the dust collection systemsused for recycling demolished concrete solids. In their studies, therecycled micro-powder collected is mainly fine particles ofhardened cement paste, mixed with a very small portion of stonepowder from aggregate. By comparing the strength of samples con-taining recycled concrete powder with those containing none, theyfound that the recycled concrete powder showed about 70% pozzo-lanic activity.

The pozzolanic properties exhibited in both recycled clay brickpowder and recycled concrete powder imply that it is possible touse the hybrid recycled powder obtained from dust collection sys-tems as a cement supplement. Motivated by this, research toexplore the use of the recycled powder from a hybrid of demol-ished concrete and clay brick attracts increasing interests. How-ever, despite the progress has been made, information of theactivity mechanisms of hybrid recycled powder and their correla-tion with concrete macro- and micro-properties is limited and fur-ther systematic study is much needed. To respond to this need, theobjective of this study is to comprehensively investigate hybridpowder with a focus on microstructure characteristics and chemi-cal compositions. A general study on hybrid powder of differentconcrete–clay brick ratios is first carried out to investigate the cor-relations of clay brick content with the properties of hybrid pow-der. Then, mortar samples supplemented with hybrid powderfrom a dust collection system are examined. The result found inthis study provides qualitative and quantitative information todeepen the understanding of the activity mechanisms of hybridrecycled powder, and thus aids the current efforts to developguidelines for its ‘‘green application’’ in practice.

2. Properties of hybrid powder of different concrete–clay brickratios

As demonstrated by the measured bulk density of coarse solids(Table 1), clay brick is weaker than concrete because of its moreporous meso-structure. This leads to lower strength for aggregatesrecycled from clay bricks. However, after ground into powder, claybrick shows an apparent density similar to concrete (Table 1). Thisimplies the disadvantage related to the higher porosity in meso-structure is suppressed after clay brick is ground. Furthermore,after grinding, clay brick powder may show more active pozzolanicproperty because of the increase in surface area.

However, the similarity in porosity does not mean recycled claybrick powder is same as recycled concrete powder when used as apozzolanic supplement to replace cement. This is due to their dif-ference in mechanical property and mineral composition. To sys-tematically characterize the properties of their mix, powdercontaining different weight proportions of concrete and clay brickis examined here. In addition to pure clay brick powder and pureconcrete powder, hybrid powder with the percentage of clay brickincreasing by a 10% interval is mixed. Thus, total 11 concrete–claybrick ratios are used for mixing the hybrid powder. Here the pow-der is collected by using a vacuum equipped with a proper sievewhen the clay bricks and concrete solids are being ground in alab ball mill.

Fig. 1. The fineness and LOI of the hybrid powder for different concrete–clay brickratios.

Fig. 2. The strength activity index and water requirement for different concrete–clay brick ratios.

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2.1. Fineness and Loss on Ignition (LOI)

The powder fineness, characterizing the surface-to-volumeratio, has a significant effect on the cement paste when the powderis added as a supplement. For the hybrid powder used in this study,its fineness is measured based on the standard method recom-mended for fly ash in Chinese specifications [21] and ASTM stan-dard test methods [22]. According to the specifications and ASTMrecommendation, the fineness of powder is characterized by thepercentage of grains of size larger than 45 lm. Obviously, thelower this percentage, the finer is the powder.

The fineness of the hybrid powder of different concrete–claybrick ratios is obtained by tests using negative pressure sievingmethod here. As shown in Fig. 1, the hybrid powder becomes finerwith the growth of the percentage of clay brick. For pure clay brickpowder or hybrid powder containing more than 70% clay brick,almost all the solids are ground into fine grains smaller than45 lm (Fig. 1).

To use supplements (e.g. fly ash) to replace cement in concrete,the carbon content in the powder must be controlled because itdetrimentally influences the properties of concrete [21]. For hybridpowder, it usually contains some organic fine particles, which arefrom the organic materials in C&D wastes, for instance, wood, plas-tic and so on. So, carbon content criterion must be satisfied ifhybrid powder is used as an additive in concrete. Generally, thecarbon content can be characterized by LOI, which is the mass lossafter the material is heated to a high temperature. Since there areno specifications available for hybrid powder, the standards for flyash will be used here as a reference. Based on ASTM, the materialunder test is heated from 105 �C to 750 �C, while in Chinese stan-dards, it is from 105 �C to 950 �C. In this test, the recycled powderis heated to 950 �C. To ensure the concrete quality, LOI must belower than a limit, for example, less than 8% for Class II fly ash inChinese standards [21] and less than 6% for Class F fly ash in ASTMrecommendation [23].

Fig. 1 shows that the value of LOI decreases as the percentage ofclay brick in the hybrid powder increases. This means the recycledclay brick powder contains less carbon ingredients than the recy-cled concrete powder. For the pure concrete powder used in thistest, its LOI is about 11%, beyond the limits allowed in Chineseand ASTM standards respectively. However, if the proportion ofclay brick reaches 20%, the LOI of the hybrid powder drops withinthe limit required by Chinese specifications.

For hybrid powder, one source of LOI is organic materials.Another contribution is the decomposition of calcium carbonate,which decomposes at about 740 �C. As a result, CO2 is releasedand the powder mass decreases [24]. The higher LOI exhibitedin the recycled concrete powder is mainly due to the concrete

carbonation, which produces calcium carbonate in concrete. Thiswill be further verified in the subsequent Thermo-Gravimetricand Differential Thermal Analysis (TG-DTA).

2.2. Strength activity index and water requirement

The effect of additives on compressive strength of concrete isgenerally called strength activity index. Generally, the strengthactivity index of cement supplements is measured by the ratio ofthe compressive strength between benchmark mortars containingno supplements and mortar samples with certain amount ofcement being replaced.

In this study, the compression test is run on a standard MTSloading frame. Following Chinese recommendations [25], for eachtype of hybrid powder, 3 mortar cubes of size 40 mm are cast andthen cured for 28 days before test. The mix proportion for mortarsamples is cement:recycled powder:sand:water = 0.7:0.3:3:0.5, forbenchmark samples, it is cement:sand:water = 1:3:0.5. Note, themix proportion based on Chinese standard is slightly differentfrom that of ASTM, for which 20% replacement is used andwater–binder-ratio is 0.484 for control samples. The test resultsare plotted in Fig. 2. It shows the compressive strength generallyrises up as the percentage of clay brick in the hybrid powderincreases. When the percentage of clay brick reaches 40% in thehybrid powder, the strength activity index reaches 70%, which isthe strength limit required by Chinese specifications for Class IIfly ash [21] (75% is required for Class F fly ash in ASTM [23]). Ifpure recycled clay brick power is used, the strength activity indexjumps to 82%. This is consistent with the tests by Kartini et al.[15].

If part of the cement is replaced by fly ash or other supple-ments, the amount of water required to remain the concreteworkability may differ. In both Chinese specifications and ASTMstandards, the workability of concrete containing powder as anadditive is characterized by the water requirement based onthe mortar flow method. Here the water requirement is the ratioof water quantity to remain 130–140 mm flow in mortar samplescontaining 30% additive powder to that of the benchmark mor-tars without any cement replacement. As shown in Fig. 2, toachieve the same degree of workability, extra water is neededfor all hybrid powder. The extra water does not monotonicallyincrease with the clay brick content. It reaches its peak at 6%when the percentage of clay brick is about 70%. Then it is grad-ually reduced to 4% for pure clay brick powder. This unique trendis closely related to the microstructure morphology of hybridpowder, which will be shown in the subsequent microscopiccharacterization.

Table 2The chemical composition (wt, %).

Hybrid recycled powder Brick powder [13] Recycled powder from concrete [19] Fly ash [20] Cement [13]

SiO2 43.85 66.54 38.61 35.4 21.33Al2O3 12.64 16.88 7.13 18.9 4.99Fe2O3 6.15 6.62 3.19 7.1 3.65CaO 17.86 4.24 41.22 25.9 62.48MgO 2.58 2.73 1.35 1.6 1.16K2O 2.05 2.41 1.57 _ 0.65Na2O 1.10 1.03 2.01 1.1 0.38TiO2 0.67 _ _ _ _SO3 2.01 0.46 1.04 2.4 2.60P2O5 0.22 _ _ _ _Cl 0.032 _ 0.04 _ _

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3. Recycled powder obtained from dust collection systems

Knowing these general properties, one can estimate the perfor-mance of the hybrid recycled powder obtained from dust collectionsystems based on the proportion of clay brick, which can be eval-uated from the fineness and LOI described in Fig. 1. However, todevelop deeper understanding of the activity mechanisms ofhybrid recycled powder, qualitative and quantitative characteriza-tion based on micro-scale probe and high-resolution chemicalanalysis is necessitated because the effects of hybrid recycled pow-der on cement paste are strongly correlated to its microstructureand chemical composition.

In this study, the hybrid recycled powder used for micro-characterization and chemical analysis is obtained directly from adust collection system. The C&D wastes are collected from thedemolished structures in Dujiangyan, a typical Chinese city whereclay brick is widely used together with concrete for buildings. Aninitial investigation based on the weight of bricks and concrete sol-ids picked out from the C&D wastes shows that the weight per-centage of clay brick is about 50%.

3.1. Chemical and mineral composition

The chemical composition of this hybrid recycled powder isquantitatively analyzed with Axios X Ray Fluorescence (XRF),which takes advantage of the emission of the characteristicsecondary X-ray from a material that is excited by high-energyX-ray. During the test, voltage 30–60 kV and current 50–100 mAare employed. The results are shown in Table 2, where the typical

Fig. 3. The mineral composition of the hybrid recycled powder based on XRDspectrum.

chemical compositions of fly ash, cement, brick powder and con-crete powder are also shown for comparison.

According to Table 2, the dominant element in the hybrid pow-der is Si, followed by Ca and Al. These 3 elements count for over70% of the total weight of the hybrid recycled powder. Comparedwith fly ash, cement, brick powder and concrete powder, thehybrid recycled powder displays a wider spectrum of chemicalcomposition.

In addition to the chemical element analysis, the mineral com-pounds are also investigated. X-ray diffraction (XRD) measure-ments are performed to analyze the mineral constituents of thehybrid recycled powder. XRD is a popular tool used for determin-ing the atomic and molecular structure of a crystal whose crystal-line atoms cause a beam of X-ray to diffract into many specificdirections. Here a diffractometer equipped with a graphite mono-chromator is used to scan the hybrid recycled powder with Cu Kradiation under an operation condition of 40 kV and 20 mA. Inorder to achieve high scan resolution, a scan speed of 2 �/min isadopted in this study.

The mineral compounds identified by XRD scan are shown inFig. 3. Based on the diffraction peaks shown on the spectrum, themain mineral compounds in the hybrid powder are Gismondine(Ca(Al2Si2O8)�4H2O), Illite (KAl2Si3AlO10(OH)2), Anorthite (CaS2)-�aSiO2, and CaCO3, which are common in both hardened cementpaste and clay brick. Among them, aSiO2 is found related to pozzo-lanic activity [26], and CaCO3 is reported to participate in the hydra-tion of C3A to generate monocarboaluminate (C3A�CaCO3�11H2O)[27]. In the XRD spectrum, there are some small diffraction peaksshowing the hybrid powder contains rutile (TiO2), which is alsofound in Zheng et al.’s work [18]. The existence of TiO2 is from claybrick. In the process of producing clay brick, TiO2 is commonly usedas colorant to enhance the mechanical properties of brick.

Surprisingly, neither ettringite nor calcium hydroxide from thehardened paste is found in the XRD spectrum. The disappearance

Fig. 4. Grain size distribution of the hybrid recycled powder obtained by LPS.

(a) Magnified by 200 times (b) Magnified by 2000 times

(c) Magnified by 5000 times (d) Magnified by 10000 times

Fig. 5. The SEM images of the hybrid recycled powder.

Fig. 6. Curves obtained based on TG-DTA analysis.

758 Q. Liu et al. / Construction and Building Materials 73 (2014) 754–763

of calcium hydroxide may result from carbonation. After the hard-ened paste is ground into fine particles, calcium hydroxide isreleased from the paste and its direct exposure to CO2 in the airleads to accelerated carbonation [28]. Thus, part of the CaCO3 inthe XRD spectrum is from the carbonate generated after grinding,which increases LOI of hybrid recycled powder. As for the ettring-ite, its absence may stem from its decomposition at temperatureabove 80 �C [29,30]. The hybrid powder is produced in a dry grind-ing system and the local temperature during crushing can easilyrise beyond 80 �C.

Table 3Mix design and compression test for samples with a higher water–binder-ratio.

Mix proportion

Cement (g) Hybrid powder (g) Sand (g) Water (g)

550 0 1650 320385 165 1650 336

3.2. Grain size distribution and microstructure morphology

The grain size distribution of the hybrid powder is quantified bylaser diffraction spectroscopy (LPS), a technology that utilizes dif-fraction patterns of a laser beam passing through any particle rang-ing from nanometers to millimeters in size to quickly measure itsgeometrical dimensions. A Laser Diffraction Particle Size Analyzer(LPS) is used in this study. For this hybrid recycled powderobtained from a dust collection system, its grain size distribution(solid line) and cumulative distribution (dotted line) are displayedin Fig. 4. It can be seen that most of the grains have a size smallerthan 45 lm and the fineness of the hybrid powder is about 8%. Asdemonstrated by the plateau in the size distribution curve (solidline), the distribution of grain size ranging from 6 to 25 lm is rel-atively uniform. Based on the cumulative distribution, the percent-age of particles of size less than 10 lm is about 40%, which benefitsthe strength activity due to the increased surface-volume ratio.

The morphology of the grain particles is then studied by SEMscan. Before test, the hybrid recycled powder is cleaned with ace-tone for better surface exposure. After it dries, the hybrid recycledpowder is sprinkled on a conductive adhesive tape for scanning bya high resolution SEM. During the SEM scan, the general shape andsurface texture of the particles are shown at different degrees ofmagnification; see Fig. 5. It is found that unlike the fly ash, the par-ticles in the hybrid recycled powder are neither spherical norsmooth. Instead, they contain sharp corners and irregular edges,which form slits and V-notches. Under the highest magnification,finer particles are found staying in the slits and V-notches of larger

28-day strength (kN) Strength activity index

37.22 –53.51 70%

Fig. 7. AFM image of Region I.

Fig. 8. AFM image of Region II.

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ones. This unique configuration definitely increases the waterrequirement and hinders the workability of concrete when hybridrecycled powder is used as a supplement. Since clay brick is easierto be ground into finer particles, the amount of smaller particlesattached on larger ones increases with the proportion of clay brick.This explains the growth of water requirement with the content ofclay brick. However, if the percentage of concrete is too small,there are no enough coarse particles in the hybrid recycled powderto host the finer ones. Consequently, the water requirement drops,see Fig. 2.

3.3. Thermo-Gravimetric and Differential Thermal Analysis (TG-DTA)

Thermo-gravimetric (TG) is a method of thermal analysis inwhich the mass loss is measured as a function of temperatureduring heating. Differential thermal analysis (DTA) is a thermo-analytic technique to investigate the energy adsorption andrelease. In DTA, the material under study and an inert referenceare made to undergo identical thermal history and any tempera-ture difference between them is recorded and then compared.The TG-DTA results of the hybrid recycled powder are shown inFig. 6, where the solid line shows TG measurement and the dottedline is DTA measurement. A constant heating rate (10 �C/min) isapplied to the hybrid recycled powder to obtain the mass losscurve. The tests are carried out using approximately 50 mg of pow-der sample in dry nitrogen. Although the temperature is heated to1100 �C, TG curve shows the mass loss of powder is stabilized at12% after 800 �C.

The exothermic peak at 340 �C corresponds with the burning ofcellulose particles. When the waste clay brick is ground into pow-der, the cellulose particles are released. The accelerated drop on TGcurve at 740 �C corresponds to the decomposition of CaCO3, agree-ing with the endothermic valley on the DTA curve. At the sametime, the decomposition of Illite may also contribute to the endo-thermic valley at 740 �C. It can be found that the exothermic peaksand endothermic valleys are small on the DTA curve shown inFig. 6.

According to Chinese specifications and ASTM standards, in LOItest for fly ash, the temperature is raised to 950 �C and 750 �C,respectively. While, for the hybrid recycled powder, the calciumcarbonate decomposes at 740 �C, which can be demonstrated bythe steeper drop of TG curve and the valley of DTA curve around740 �C. Unlike the organic materials, calcium carbonate in thepowder is reported to contribute to the pozzolanic property [27].Since both the TG and DTA curves are shown to be stabilized after800 �C, there should be no further decomposition after it. Thus, forhybrid recycled powder, it may be more appropriate to set the igni-tion temperature from 105 �C to 730 �C, before the decompositionof calcium carbonate.

3.4. Strength activity index

The fineness and LOI tests show that this hybrid powder hasfineness = 8% and LOI = 7.41%. Further standard test finds that itswater requirement is about 105%. According to the measured fine-ness, LOI and water requirement, the percentage of clay brick inthis hybrid recycled powder obtained from a dust collection sys-tem should be about 50%, which agrees with the initial estimationbased on the weight of bricks picked out from the C&D wastesbefore recycling.

For hybrid powder containing 50% clay brick, Fig. 2 shows thestrength activity index is about 75% when water–binder-ratio is0.5 and replacement is 30%. However, to remain same workabilityof concrete, extra water is needed when hybrid powder is used as apozzolanic supplement. As it is well known, high water content hasdeleterious effects on concrete strength and other mechanicalproperties. Therefore, to ensure the higher water requirement ofthis hybrid powder does not compromise its strength activityindex significantly, compressive strength of mortar samples con-taining this hybrid powder with a higher water–binder-ratio istested. Since this hybrid powder from a dust collection systemshows a water requirement of 105%, 5% more water is added inthe mix to achieve the same level of workability. As shown inTable 3, although the water–binder-ratio is higher, the strengthactivity index of the hybrid powder still satisfies the Chinese spec-ifications, 70% (for Class F fly ash, ASTM requires the strength activ-ity index be higher than 75% for 20% replacement).

4. Microstructure of cement paste containing hybrid recycledpowder

After the microstructure morphology and chemical compositionare probed, this hybrid recycled powder obtained from a dust col-lection system is used to replace 30% of cement in casting mortarsamples. The water–binder-ratio is chosen as 0.5. After curing for28 days, the mortar samples with and without hybrid recycledpowder are cut into small blocks. Then the small blocks are castin epoxy resin and polished with care to prepare samples forAFM tests.

In order to study the effect of hybrid recycled powder oncement paste by AFM, a representative region containing thehybrid recycled particles must be identified for the nano-scaleprobe. This can be accomplished by searching for the clay brick

(a) Region Ia (b) Region Ib

(c) Region Ic

Fig. 9. AFM images of C–S–H gels in different regions.

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particles in the hybrid powder, which display a color very differentfrom other constituents in the mortar. Considering that finer con-crete powder prefers to be attached on coarser clay brick particles,it is more appropriate to treat them as hybrid particles here thanjust as pure clay brick particles. In this study, a representativeregion containing 1 hybrid particle and 1 fine aggregate (sand),which is denoted as Region I, is selected (Fig. 7). For comparison,another region containing no hybrid recycled particles and no fineaggregates, denoted as Region II, is also selected to represent thenormal cement paste (Fig. 8). Both Region I and Region II havethe identical scanning size of 30 � 30 lm.

4.1. Quantitative evaluation of microstructure roughness

It is suggested that the intrinsic roughness of cement paste,which is generally expressed by root-mean-squared (RMS)roughness, should be within a range for properly prepared mortarsamples. Based on Trtik et al. [31], the average intrinsic RMS rough-ness of motar paste ranges from 115 to 492 nm, and a RMS out ofthis range may indicate surface damage or artificial polishing.According to Mondal [32], the RMS roughness of motar sample isabout from 40 to 200 nm, corresponding to 10 � 10 lm and50 � 50 lm scanning sizes, respectively. The reason why the valueis lower in Mondal’s test is that special polish method is employedby her [32].

Here the surface of the two selected representative regions isquantitatively scanned by AFM under tapping mode. Fig. 7 showsthe surface topography of Region I, where the fine aggregate isenclosed by a dashed curve and the hybrid particle by a solid curve.

The vertical deviation of the surface is illustrated by the given colorscale, in which dark grade means a valley and bright one means aprotrusion. In Region I the height varies from +2.2 to �2.2 lm.Using the scan results, it can be found that the RMS roughness ofRegion I reaches 595 nm, a relatively high value due to the exis-tence of sand and hybrid recycled particle. However, if one onlyconsiders the cement paste, the RMS roughness drops within therange given by Trtik et al. [31]. The surface topography of RegionII is shown in Fig. 8. In this region, the main constituent is C–S–Hgel. Based on the quantitative measurement, the RMS roughnessof Region II is 470 nm, a value within the limit given by Trtiket al. [31]. Based on the quantitative measurement in Regions Iand II, it seems that although the addition of hybrid recycled pow-der increases the microstructure roughness of cement paste, thechange is still at an acceptable level when compared to normalcement paste.

4.2. Microstructure topography of C–S–H

To investigate the effect of hybrid recycled powder on the forma-tion of C–S–H, 3 sub-regions, enclosed by dashed rectangles, areselected in Region I; see Fig. 7. The sub-regions, labeled as RegionsIa, Ib and Ic respectively, all have a same size of 4.1 � 4.1 lm. Asshown in Fig. 7, Region Ia is selected in the vicinity of the fine aggre-gate, Region Ib close to the hybrid particle, and Region Ic at a loca-tion away from both the sand and the hybrid particle.

The microstructure topography of the cement paste in these 3sub-regions is shown in Fig. 9. Based on the topographical featuresshown in the AFM images, one can estimate the size of C–S–H gel

(a) Region I

(b) Region II

Fig. 10. Nanoindentation results showing elastic modulus at each node of a 7 � 7grid (unit: GPa).

Fig. 11. Fitted probability distributions representing four phases: (a) Region I and(b) Region II.

Q. Liu et al. / Construction and Building Materials 73 (2014) 754–763 761

in these 3 sub-regions. For the C–S–H gels in Region Ia, the size ofC–S–H gels ranges from 200 to 900 nm, in Region Ib, it is from 200to 1200 nm, and in Region Ic, it is from 800 to 1400 nm; see Fig. 9.

The C–S–H gels in Region Ic are more uniform than those inRegions Ia and Ib, which are influenced by the sand an hybrid pow-der respectively. In addition to the size, the boundary between theC–S–H gels is different in these 3 sub-regions. In Region Ic, a dis-crete boundary can hardly be found between the C–S–H gels, whilein Regions Ia and Ib, the boundary between C–S–H gels is conspic-uous (Fig. 9). This indicates a stronger bond of C–S–H gels in RegionIc. According to the AFM images, it transpires that the C–S–H gelsin Region Ia are more uniform than those in Region Ib. This may beinduced by the unique microstructure morphology of the hybridrecycled powder shown in Fig. 5. Considering the locations of these2 sub-regions, the AFM images represent the topographical proper-ties of C–S–H gels in the interfacial transition zones (ITZ) surround-ing fine aggregate and hybrid recycled powder, respectively.

4.3. Modulus based on nanoindentation

The mechanical properties of the C–S–H gels are further quan-tified by nanoindentation based on Hertz model [33]. In order tocharacterize the distribution of elastic modulus, both Regions Iand II are mapped by a 7 � 7 grid, based on which nanoindentationis carried out. The details of calculating elastic modulus can bereferred to the work by Tan et al. [34].

Following the methodology proposed by Mondal [32], fullyhydrated cement paste can be classified by 4 distinct phases inorder of increasing elastic modulus: porous phase, low stiffnessC–S–H, high stiffness C–S–H and calcium hydroxide. For Region I,2 extra phases, namely, recycled particle and sand, exist. The nan-oindentation is carried out at each node of the 7 � 7 grid, and themeasured elastic modulus is labeled at the corresponding location(Fig. 10).

The average modulus of the cement paste in Region II is32.7 GPa, within the range from 10 to 30 GPa as reported by Mon-dal [32]. The average modulus of Region I is 25.6 GPa, here themoduli of sand and hybrid particle are not considered. The lowermodulus in Region I is due to the existence of ITZ around the sandand the recycled particle. In Fig. 10, it shows extremely high mod-uli (60 GPa or higher) at some nodes in the cement paste. This isrelated to the unhydrated cement particles and they are not usedin the analysis.

In Fig. 10a, the nodes close to the ITZs are marked out with a cir-cle. The average modulus of ITZ around sand is about 9.0 GPa, andfor the recycled particle it is about 8.7 GPa. This means that thehybrid recycled particle has a similar effect on the formation ofC–S–H around its interface. The marginal difference may resultfrom the unique morphology of the hybrid powder, in which thefiner particles are attached in the slits and notches of larger onesand thus attract more water during hydration.

As proposed by Mondal [32], the measured modulus at differentnodes can be used to estimate the distribution of different phasesin the cement paste based on statistical analysis. If the interval isset as 5 GPa and Gaussian distribution is assumed, the probability

Table 4Change of elastic modulus and volume fraction of different phases.

Region I Region II

Elastic modulus (GPa) Volume fraction (%) Elastic modulus (GPa) Volume fraction (%)

Porous phase 8.2 ± 2.75 37 8.94 ± 2.8 17Low stiffness C–S–H 18.3 ± 3.16 36 19.0 ± 2.7 24high stiffness C–S–H 28.3 ± 2.16 15 30.0 ± 10.1 48Calcium hydroxide 46.4 ± 3.80 12 53.4 ± 3.89 11

762 Q. Liu et al. / Construction and Building Materials 73 (2014) 754–763

distribution of elastic modulus can be obtained for the Region I andRegion II respectively (solid lines with square symbols in Fig. 11aand b). Taking advantage of peak analysis, the overall probabilitydistribution of elastic modulus can be decomposed into individualprobability distribution for the 4 different phases of cement paste[32], which is represented by the dashed curves in Fig. 11. Thisindividual probability distribution is further validated by the good-ness of fitting based on Chi-square test [35]; see the solid curves inFig. 11a and b.

In Fig. 11, the volume fraction of each phase can be calculatedbased on the area under the dashed curve representing itsindividual probability distribution. The volume fraction of eachphase is presented in Table 4. It can be seen that under the influ-ence of the hybrid recycled powder and sand, there is a substantialchange in the microstructure of cement paste. If compared withRegion II, one can find that in Region I volume fraction of high stiff-ness C–S–H drops from 48% to 15%, while those of porous phaseand low stiffness C–S–H rise from 17% and 24% to 37% and 36%,respectively. Considering the high active surface area and uniquemorphology of the hybrid recycled powder, the locally highwater–binder-ratio contributes significantly to this phase distribu-tion change.

5. Conclusion

In the present study, a comprehensive investigation on theproperties and microstructure characteristics of hybrid recycledpowder is carried out. Based on the qualitative and quantitativeanalysis of its morphology, chemical composition and effects oncement paste, the following conclusions can be drawn for thehybrid recycled powder when it is used as a cement supplement:

1. For C&D wastes containing concrete solids and clay bricks, alarge amount of hybrid recycled powder is collected from dustcollection systems during recycling. Characterized by theconcrete–clay brick ratio, the hybrid recycled powder obtainedfrom dust collection systems displays different levels ofpozzolanic activity. With the increase of clay brick, tests showthat the powder’s LOI drops and fineness and strength activityindex rise. If the proportion of clay brick reaches 40%, thestrength activity index approaches 70% for 30% replacement.

2. Hybrid recycled powder exhibits a unique microstructure mor-phology, which leads to higher water requirement to maintainconcrete workability. The water requirement increases withclay brick until its proportion in the hybrid powder reachesaround 70%. Compression tests based on 30% replacement showthat the extra water reduces strength activity index, but not sig-nificantly. Thus, by controlling the replacement percentage andadjusting the clay brick content, concrete supplemented withhybrid recycled powder can meet both the strength and work-ability requirements.

3. Quantitative chemical analysis finds that hybrid recycled pow-der displays a wider spectrum of chemical composition, primar-ily inherited from both hardened paste and clay brick. Themineral compounds of aSiO2 and CaCO3 found in hybrid recycled

powder contribute to the pozzolanic property. However, due tothe accelerated carbonation and elevated temperature duringgrinding, ettringite and calcium hydroxide in hardened pastedisappear from hybrid recycled powder.

4. AFM scan reveals that hybrid recycled powder affects themicrostructure of cement paste around it. The size andmorphology of C–S–H gels in the vicinity of hybrid powderare similar to those close to sand, but very different from thenormal C–S–H gels. The increased surface roughness and con-spicuous boundary indicates a weaker bond of C–S–H gelsaround hybrid recycle powder.

5. Nanoindentation test shows that a weaker ITZ, with an averagemodulus similar to that surrounding sand, is produced to formthe cement paste-recycled particle interface. The unique micro-structure morphology of hybrid recycled powder is deemed asthe main contributor to this ITZ.

6. Using the elastic modulus at each node of the nanoindentationgrid, the probability distribution of different constituent phasesof the cement paste can be estimated based on statistical anal-ysis. It transpires that the existence of hybrid recycled powdersignificantly changes the volume fractions of the constituents,characterized by a substantial increase of porous phase andlow stiffness C–S–H.

This study reveals that the microstructure morphology andchemical composition of hybrid recycled powder are closelyrelated to its activity mechanism, and thus have a strong effecton the microstructure of cement paste around it. Like fine aggre-gates, similar ITZs form around hybrid recycled particles. Thus, ifthe proportion of clay brick and replacement percentage are wellcontrolled, hybrid recycled powder can be used as a pozzolanicsupplement to replace part of the cement in concrete. Of course,to establish a complete guideline for the ‘‘green’’ use of hybridrecycled powder, deeper understanding of its role in cementhydration is indispensable. Therefore, further investigation isneeded to study its effects on the packing density, chemical com-position and morphology of C–S–H gels. In addition, the increaseof porous phase and low stiffness C–S–H demands research effortson durability (e.g., freeze and thaw) and time-dependent proper-ties (e.g., creep and shrinkage) for concrete supplemented withhybrid recycle powder.

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