Construction and Building Materials 49 (2013) 84–92

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Construction and Building Materials

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Production, microstructure and hydration of sustainable self-compactingconcrete with different types of filler

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.07.107

⇑ Corresponding author at: School of Civil Engineering, Faculty of Engineering,University of Nottingham, University Park, Nottingham NG7 2RD, UK. Tel.: +447427105596.

E-mail addresses: evxmkm@nottingham.ac.uk, mahmoudkh_ani@yahoo.com(M.K. Mohammed).

1 Lecturer at University of Anbar, Faculty of Engineering, Iraq.

Mahmoud Khashaa Mohammed a,b,⇑,1, Andrew Robert Dawson a, Nicholas Howard Thom a

a School of Civil Engineering, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UKb Civil Engineering Department, Faculty of Engineering, University of Anbar, Iraq

h i g h l i g h t s

� Production of economic sustainableSCC with high partial replacement ofcement.� First work to analyse the micro-

characteristics of the interfacialtransition zone (ITZ) of sustainableSCC.� Addition of LP leads to less

homogenous microstructure of inboth the ITZ and the cement paste.� LP gave an indication to the

acceleration effect on the hydration.The analysis approved its inactivity.� Fly ash addition showed its

consistency for the production ofsustainable SCC.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 April 2013Received in revised form 2 July 2013Accepted 21 July 2013Available online 4 September 2013

Keywords:Self-compacting concreteHydrationMicrostructureInterfacial transition zoneFly ashLimestone powder

a b s t r a c t

Production, microstructure and hydration characteristics of sustainable self-compacting concrete (SCC)are investigated with two types of filler having significant differences mainly in chemical compositionand physical properties. The purpose is to show how different fillers at high cement replacement levelscan affect the composition, microstructural and hydration characteristics at early age. Several techniques,comprising X-ray diffraction, scanning electron microscopy (SEM) linked with energy-dispersive X-ray(EDX) analysis, image analysis, mercury intrusion porosimetry and thermo-gravimetric analysis, wereused in order to demonstrate the effect of these two fillers at high replacement proportions.

The two types of sustainable SCC produced had a compressive strength of 50–60 MPa and used thesame water to binder ratio. The replacement rate of both limestone powder (LP) and fly ash (FA) wasabout 33% of the total binder (450 kg/m3).

In spite of the equal water to binder ratio and approximately the same compressive strength grade at28-days, limestone powder self-compacting concrete (LP–SCC) had a different microstructure and hydra-tion products from the fly ash self-compacting concrete (FA–SCC). The results indicate that the fly ash wasthe more suitable for the production of sustainable SCC.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, there has been an increasing interest in usinghigh quantities of fillers as a partial replacement for cement inself-compacting concrete (SCC). This helps to make SCC a moresustainable material [1]. Incorporation of high quantities ofsuperplasticizer (SP) and a large volume of filler material is

Fig. 1. XRD-patterns of (a) LP and (b) FA.

M.K. Mohammed et al. / Construction and Building Materials 49 (2013) 84–92 85

essential to achieve high flowability and sufficient resistance tosegregation. There are many types of filler that have been used suc-cessfully in SCC but the most common fillers are fly ash, limestoneand silica fume [2]. Moreover, the use of mineral admixtures andmicro-fillers as a partial replacement for cement may also reducethe high cost of SCC but the durability of such additives needs tobe studied further [3]. The additions and the absence of vibrationgive self-compacting concrete a distinct microstructure [4]. Thehydration and the microstructure are the most important charac-teristics that can affect the contaminant transport and durabilityproperties of the concrete. This paper aims to investigate themicrostructural characteristics of the cement matrix and the inter-facial transition zone (ITZ) in addition to the hydration propertiesby examining two SCCs with different fillers at a high replacementproportion (33%) of the total binder.

In Europe, 300 kg/m3 cement content for a powder-type self-compacting concrete is commonly used with limestone powder(LP) filler [5]. Generally, in SCC, the fillers can be classified intotwo types: reactive and non-reactive. However, for LP, there is noreliable evidence that it can participate chemically to improvethe structure of the hydrated cement paste [5]. For this reason,fly ash and quartz fillers have also been used as mineral additionsin SCC production in Europe.

The permeability and the water/vapour/contaminant transportproperties of the concrete are highly dependent on the capillarypores and their interconnectivity. In general, the porosity of SCCis lower than that of normal vibrated concrete. This is due to thefilling and dispersal effect of the fillers and the superplasticizer,respectively [6]. However, the type of addition is likely to deter-mine the nature of the porosity of the cement paste (macro or mi-cro) or the connectivity of the pores at high levels of replacementfor the same water to powder ratios [7]. At the microscale, thismight strongly affect the permeation properties such as diffusivityof substances like chloride or carbon dioxide.

Typically, the engineering properties of a concrete are under-stood to result from the combination of the response of the aggre-gate in the concrete, of the filler/fines matrix and of the interface(ITZ) between the two [8]. With regard to the opportunities formovement of water, gas or contaminant through a concrete, theaggregate is considered to be impermeable. Therefore, the compo-sition and microstructure of the ITZ and of the matrix are likely tobe the factors controlling fluid movement through the concrete [9].For SCC, both the ITZ and the matrix are likely to be dense relativeto their characteristics in normal vibrated concrete [1]. However,to date, the effect of different fillers at high level of cement replace-ment on the local porosity and permeability characteristics and thedistribution of atoms in the hydrated phases of the ITZ and the ma-trix of SCC is not very well known. The main objectives of the pres-ent work, therefore, were to study the effect of two common fillerson the production, microstructure and hydration characteristics oftwo types of sustainable SCC.

1.1. Research significance

With the increasing use of SCC, it is becoming increasingly dif-ficult to ignore the influence of its microstructure and hydration onits durability and fluid transport properties. Nowadays, SCC plays avital role in infrastructure across the world, and it is often exposedto external environmental attack. Therefore, this work aims to con-tribute to the understanding of the microstructure and hydrationof sustainable SCC. The paper is one component of a larger researchprogram that is investigating accelerated carbonation and chloridepenetration in relation to the microstructural properties of sustain-able SCC.

2. Experimental program

2.1. Materials

The materials used in this study were:

� Portland cement (CEM I 52.5R);� Local river quartz sand with a maximum particle size of 4.75 mm;� Natural rounded quartz gravel with a nominal maximum size of 10 mm;� Fly ash (FA) type 450-S produced by Cemex Company which was prepared so

that less than 12% was retained on a 45 lm sieve;� Natural local limestone filler (LP) with a grain size less than 65 lm;� Polycarboxylate-based superplasticizer (SP) with 1.08 specific gravity.

Parts of slices (see Section 3.2.1.1 of this paper) of hardened SCC were groundinto a powder and passed through a 75 lm sieve for XRD examination. An XRDscanning speed of 2� per minute and a step of 0.05� were used in the range 10–90� using a Bruker – AXS D8 Advance equipment. The mineralogical phases of theFA and the LP, thereby determined, are given in Fig. 1a and b respectively. FA com-prises a vitreous medium with two main crystalline phases: quartz (SiO2) and mull-ite (Al6Si2O13). Furthermore, the highlighted distinct hump in the XRD curve isindicative of the presence of an amorphous material which is likely to be amor-phous silica, while only calcite was detected in the limestone powder.

Fig. 2a and b shows SEM images of typical particles of used LP and FA at highmagnification. It can be seen that the FA is spherical in shape compared to the LPparticles. This might explain the lower amount of SP needed to maintain the sameworkability properties in the FA–SCC as compared with the LP–SCC mixture as theFA particles are expected to move more easily. In addition, the angular shape of theLP particles may indicate a higher specific surface area for the same particle size andhence greater water demand for surface wetting.

2.2. Mixture design and preparation of specimens

The mixture designs are shown in Table 1. The two mixes were designed to havea compressive strength grade of 50–60 MPa using the same amount of water.Replacement with filler was to about 33% of the total amount of binder. The onlydifference was the type of filler and a small reduction in the coarse aggregate con-tent (35 kg/m3) in the FA mix due to the differences between the specific weights ofFA and LP. For hardened properties, all specimens were cast in one layer, whatever

Fig. 2. Scanning Electron Micrograph (a) LP and (b) FA.

Table 1Mix designs in kg/m3.

Mix type Cement Gravel Sand Fly ash Limestone Water SP% by weight

LP–SCC 300 860 900 – 150 180 2.6FA–SCC 300 825 900 150 – 180 1.83

Fig. 3. Fresh properties of SCC.

86 M.K. Mohammed et al. / Construction and Building Materials 49 (2013) 84–92

the shape of the moulds, without any compaction. The moulds were covered with anylon sheet for 48 h before the samples were demolded and cured in water(20 ± 2 �C) until the date of the test.

3. Experimental program

3.1. Fresh properties and compressive strength tests

A pan type mixer was used to mix the SCC ingredients. The dry mix ingredients(aggregate, cement and filler) were placed in the pan of the mixer, and mixed for2 min to ensure homogeneity. Cement and filler had previously been mixed usinga trowel for 5 min to ensure the dispersion of the filler grains between cement par-ticles. The whole quantity of water was added to the dry mix in the pan and thenthe SP was added gradually while the mixer was running. Mixing continued for2 min. Mixing was paused so as to check the mix before another one minute of mix-ing was applied. For SCC, modified mix procedures are very important so as to takeadvantage of the adsorption of molecules of poly-carboxylic ether, SP, by the mix[10]. Several trial mixes were conducted to obtain the selected dosages of SP.Assessments of slump flow diameter and the time taken to reach a slump diameterof 50 cm, T50, were used to assess the flowability whilst the J-ring and sieve segre-gation tests were conducted to ensure good passing ability and segregation resis-tance. Full details of these tests can be found in the EFNARC European guide forSCC [11]. Fig. 3 shows the tests that were conducted to assess the fresh properties.

Table 2 shows the fresh properties and the compressive strengths for the SCCmixtures. The results indicate that the selected mixes conform to the requirementsof the EFNARC guide. For the compressive strength test, 100 mm cubes were used.The test was conducting according to BS EN 12390-3 [12]. The listed values repre-sent an average of three readings.

3.2. Microstructure and hydration investigations

For microstructural and hydration studies, mortar cubes of 70 mm side lengthwere made according to the same mixing procedures but without coarse aggregate,using a 20-liter Hobart mixer. The water quantity was reduced by about 0.8%

(coarse aggregate absorption) in order to ensure the same water content in the mor-tar. A mortar flow of 240–300 mm was achieved using the same dosage of SP for theconcrete, as shown in Fig. 4.

3.2.1. Microstructure3.2.1.1. SEM investigation. After 28 days curing, the mortar samples were first cutinto slices (15 mm thick) using a diamond saw. From the middle of the mid-slice,a block 20 � 20 mm was cut for SEM examination. Flat-polished epoxy-impreg-nated specimens were used for acquiring back scattered electron (BSE) imagesusing a Philips XL 30 SEM fitted with an Oxford Instruments INCA model spectrom-eter for energy-dispersive X-ray (EDX) analysis. The preparation of such samples isdescribed elsewhere [13]. The microstructural features examined in the currentstudy were:

i. The porosity and the thickness of the ITZ:

An accelerating voltage of 15–25 kV was used. A magnification of 500� waschosen for the ITZ porosity analysis. The original BSE images were digitized usingthe Image J program with a size of 712 � 484 pixels and a scale of 1.18 pixels/mi-cron. The ITZ porosity analysis was conducted for each 10 lm from the surface ofselected aggregate particles using a duplicated image with a size of 374 � 254 pix-els and the same scale. First, the aggregate was removed from the duplicated BSEimage. Then the upper porosity threshold values for the FA and LP pastes were care-fully selected for each analysed image using the overflow method [14]. This wasconducted because of the difficulty in determining the upper porosity threshold va-lue for a blended cement paste from the ordinary frequency histogram of greyscales as compared with a pure cement paste. The average upper porosity thresholdvalue for the FA-paste was 50 while it was 70 for the LP-paste. Eight images wereanalysed for each mix. A T-distribution with a confidence interval of 95% was usedto examine the accuracy of the detected porosity.

ii. The distribution of atomic ratios in the in the aggregate, hydrated phases ofthe ITZ and the bulk cement paste using carbon coated specimens:

X-ray line spectrum analysis using EDX linked with the SEM was adopted forthis purpose. An accelerating voltage of 20 kV was used. The analyses were con-ducted for each 5 lm using a magnification of 3000�. Each line consisted of 14spectra, 4 in the aggregate and the rest in the ITZ and the bulk cement paste. Theresults represent an average value of 3–4 lines scanned in different areas of theITZ. In order to detect the main composition of the hydrated phases in the ITZ regionand the bulk cement paste (C–S–H, CH and AFm), the following criteria were used asreported by Rossignolo [15]:

C� S�H 0:8 6 Ca=Si 6 2:5; ðAlþ FeÞ=Ca 6 0:2

CH Ca=Si P 10; ðAlþ FeÞ=Ca 6 0:04 and S=Ca 6 0:04

AFm Ca=Si P 4:0; ðAlþ FeÞ=Ca > 0:4 and S=Ca > 0:15

Table 2Fresh properties and compressive strength of SCC-mixes.

Mix type Slump flow (±2 mm) T50 (sec) SI (%) (Bj) J-ring (±2 mm) Compressive strength MPa at 28 days

LP–SCC 700 4.5 11.2 6.3 50FA–SCC 720 3.2 9.2 7 56.5EFNARC requirements 650–800 2–5 0–15 0–15 –

T50 = Time to obtain a slump flow of 50 cm.SI = Segregation index (the fraction of about 5 kg of the fresh SCC mix that can pass a 5 mm sieve).Bj = Difference between the average heights of the fresh concrete outside the barrier of the J-ring and the height at the centre of the ring.

Fig. 4. Mini slump flow of SCC mortar.

M.K. Mohammed et al. / Construction and Building Materials 49 (2013) 84–92 87

iii. Observation of the capillary pores and their arrangements and the hydra-tion products in the bulk cement paste using SEM Platinum-coated frac-tured surfaces and BSE images

3.2.1.2. Mercury intrusion porosimetry (MIP). Small pieces weighing 1–2 g, from themiddle of the same slice, were used for the MIP test. In order to stop the hydration,the specimens were dried at 100 �C for 24 h and then they were kept in sealed con-tainers. A Micrometrics Autopore IV mercury porosimeter, which can detect poresdown to 7 nm diameter, with a maximum pressure of 212 MPa, was used for theanalysis of the pore structure.

3.2.2. Hydration investigationsParts of the ground powder prepared for XRD analysis (see Section 2.1) were

used for thermo-gravimetric analysis (TGA) testing using a Perkin Elmer thermo-gravimetric analyser. The samples were heated up from 30 to 950 �C .The rate ofheating was 50 �C per minute up to 300 �C and 20 �C per minute thereafter.

Fig. 5. BSE micrographs (a) LP–SCC and (b) FA–SCC.

4. Results and discussion of micro-scale investigation

4.1. ITZ porosity and thickness

BSE micrographs for the LP and FA self-compacting concretesare shown in Fig. 5a and b respectively. Using the image analysistechnique described earlier, it was possible to detect an ITZ forboth types of SCC. Fig. 6a and b shows the distribution of the de-tected porosity from the aggregate–matrix interface up to a dis-tance of 50 lm in the bulk cement paste for the two types ofSCC. It can be seen that the porosity of the ITZ in the FA–SCC at10 lm was slightly higher than in the LP–SCC. However, a dramaticdecrease was recorded beyond that distance while a more gradualreduction was observed in the LP–SCC. Fig. 6 also shows that theITZ thicknesses were approximately 27.5 and 18.5 lm for LP–SCCand FA–SCC respectively. The exact determination of ITZ is veryhard [14]. In the present work, the ITZ was defined as the regionfrom the aggregate–matrix interface until the detected porositywas the same as that of the bulk cement paste, which was 12%and 13.5% for LP- and FA–SCC respectively. In the case of the

LP–SCC an ‘‘outer ITZ’’ is seen between approximately 30 and50 lm from the aggregate in which greater density than eitherthe inner ITZ or the bulk cement paste is observed. The same fea-ture is seen in the FA–SCC, but not so clearly. It may be that thisrepresents the existence of a CSH-rich zone as proposed by Praveenand Kaushik [16], although if this is the case, the thickness is sub-stantially greater than in their proposal. The porosity of the bulkcement paste was defined as the average porosity of the matrixfrom 50 lm away from the aggregate until the end of the dupli-cated SEM image. The analysis was performed using the T-distribu-tion statistical analysis. Had the genuine ITZ thickness been greaterthan 50 lm this approach would not have been possible; happilythis was not the case. Recently, Leeman et al. [17] demonstratedan ITZ thickness between 37 and 50 lm for SCC containing differ-ent types of cement, whereas the ITZ thicknesses determined in thepresent study are noticeably smaller. However, they defined theITZ as the zone near the aggregate–matrix interface where themeasured porosity is more than 15% greater than the average of

Fig. 6. Distribution of detected porosity and ITZ thickness (a) LP–SCC and (b) FA–SCC.

Fig. 7. ITZ at high magnifications (a) LP–SCC and (b) FA–SCC.

88 M.K. Mohammed et al. / Construction and Building Materials 49 (2013) 84–92

the bulk paste (assessed beyond 70 lm) as well as using differentbinder types.

Fig. 7a and b represent the ITZ’s of the two mixes at high mag-nification, showing the presence of un-reacted grains of LP and FAin the ITZ regions for the two types of SCC. In SCC, the presence ofun-reacted particles of pozzolanic material in the ITZ is not harm-ful compared to that of un-reacted/partially reacted particles of ce-ment [16]. However, from a physical aspect, the agglomeration ofun-reacted coarser and angular LP particles is likely to increasethe entrapped water between these particles due to the dilution ef-fect leading to an increase in the thickness of the ITZ whilst the fineLP grains reduce the local porosity. The finer and more sphericalshape of the small FA particles might reduce this effect. On theother hand, from a chemical aspect, it seems that the presence ofhigh amounts of CH crystals in this region increases the porosityof the ITZ. This was confirmed by the EDX analysis (examples of ra-tios of atoms in the hydrated phases in the ITZ and the bulk cementpaste are given in Fig. 8a and b for the two types of SCC).

4.2. Chemistry of ITZ

According to the criteria used for identifying the main hydrousphases in the ITZ and cement paste, the average Al + Fe/Ca and S/Caratios indicated there was no significant amount of Ettringite(AFm) in the investigated ITZs nor in the cement pastes. Thus,the discussion will be based on the differences between the Ca/Siratios only. It was decided to define the ITZ for the FA–SCC as theregion until the point where the Ca/Si ratio decreased below thevalue of 2.5 signifying the formation of C–S–H gel. However, forLP–SCC, the ITZ thickness was defined from the image analysisalone since the Ca/Si ratio exceeded 2.5 even beyond 50 lm. Thehigher deduced Ca/Si ratio in the ITZ of LP–SCC will also bediscussed.

The average Ca/Si, Al + Fe/Ca and S/Ca ratios in the ITZ regionsfor the two SCCs are listed in Table 3. For the FA–SCC, the averageCa/Si ratio was 3.63 indicating a relatively large amount of C–S–Hin this area while it was 14.43 for the LP–SCC. This suggests thepresence of significant CH and unreacted LP grains in the LP–SCCITZ. It has previously been reported that the orientation of the hex-agonal crystals of CH in the ITZ gives it a more porous structure [8].However, it should be emphasized that the EDX analyzer has realdifficulties in differentiating between Ca(OH)2 and CaCO3. It wasstated by Lawrence [18] that it is difficult to distinguish betweenthese two compounds in EDX analysis since the only differencesbetween them are carbon (atomic weight 12) and hydrogen (atom-ic weight 1). Such lighter elements are less readily detected by theEDX analyzer due to their small atomic weights as compared withoxygen (16). Therefore the analysis can give an over-estimation ofthe Ca/Si ratio resulting in apparently large amounts of CH in thisregion. Consequently, a semi-quantitative EDX analysis could beconsidered for the LP–SCC. However, Fig. 8 also shows that thereis an approximately constant distribution of the Ca/Si ratio in theITZ (between 1.79 at 5 lm and 4.13 at 30 lm). In contrast, theCa/Si ratio varied between 1.77 and 30.8 across the thickness ofthe ITZ (27.5 lm) in the LP–SCC.

4.3. Mercury intrusion porosimetry (MIP) and pore size distribution

Capillary pores between 0.007 lm and 100 lm were detectedby MIP. Fig. 9 illustrates the mercury intrusion curves versus thepore size diameter of the investigated mixtures at 28 and 56 dayswhilst Fig. 10 shows the derivative of these curves. The analysisshowed that both LP and FA produced a refinement of the porestructure such that the percentage of micro-pores (pores smallerthan 100 nm) was greater than 65%. The percentages of micro-pores were 66.7% and 75% at 28 days and 78.5% and 82.8% at

Fig. 8. Atomic ratio distributions of the hydrous phases in the aggregate, ITZ and bulk cement paste (a) LP–SCC and (b) FA–SCC.

Table 3The average Ca/Si, Al + Fe/Ca and S/Ca ratios in the ITZ of SCC-mixes.

Mix type Ca/Si ratio Al + Fe/Ca S/Ca

FA–SCC 3.63 0.436 0.051LP–SCC 14.43 0.120 0.026

M.K. Mohammed et al. / Construction and Building Materials 49 (2013) 84–92 89

56 days for the LP and FA–SCC respectively. Thus the type of fillerdid not significantly alter the nature of the pore structure (micro ormacro) of the SCC. However, for the macro-pores, FA–SCC has aslightly higher concentration of pores in the 10–100 lm range thandoes the LP–SCC at 28 days, whereas in the 0.1–10 lm range theLP–SCC had a higher concentration. On the other hand, there wasa significant difference between the deduced critical micro-porediameters – about 60 and 38 nm for the LP- and FA–SCC respec-tively at 28 days. With further hydration, the larger micro-poresbecome occupied by more and more hydration products leadingto more, smaller micro-pores (see Fig. 10). The critical pore diam-eter, indicated by the inflection point of the cumulative intrusion

curves (Fig. 10), could play an important role in determining themicro-permeation properties of the cement paste. According toCraeye et al. [19], use of different types of filler for SCC at the samewater to binder ratio had no significant effect on the critical porediameter of the microstructure at an age of 120 days. This assertionwas not tested in this study, but the data at 56 days (Figs. 9 and 10)still shows slight differences.

Typical SEM and BSE images captured at 28-days were used toexamine the morphology of the capillary pore-structure in the bulkcement pastes and the hydration products with the results beingpresented in Figs. 11a, b and 12. The images in Fig. 11 clearly showthe differences between the capillary pores and their arrangementsin the bulk cement paste of LP- and FA–SCC. It can be seen that LP–SCC exhibited larger capillary pores as compared with FA–SCC. Inthe same manner as reported by Zhijun [20], the result obtainedfrom BSE imaging of LP–SCC (presented in Fig. 12) revealed a por-ous interface between the limestone grains and the hydrationproducts. In contrast, a dense interface was observed by Zhijun[20] between the unhydrated cement grains and the surroundinghydration product in a high performance cement paste. Thus, the

Fig. 9. Mercury intrusion curves versus pore size diameter of LP–SCC and FA–SCC.

Fig. 10. Derivative of MIP curves of LP–SCC and FA–SCC.

90 M.K. Mohammed et al. / Construction and Building Materials 49 (2013) 84–92

influence of the crooked/rough surface detected by the SEM imageof the LP is observed at the hydrate–limestone powder interface.

5. Results and discussion of hydration investigation

5.1. Thermo-gravimetric analysis (TGA)

The TG curves in Fig. 13a represents the cumulative weight lossof the powder specimens as the temperature increased whereasthe DTG curves in Fig. 13b shows the derivatives. From the DTGcurves, the percentages of CH loss were calculated using Pyris soft-ware 2009 from Perkin Elmer, Inc., Version 10.1.

The total amount of the CH in the powder samples is deter-mined using the following equations [21]:

Amount of CaðOHÞ2% ¼ CH%de-hydroxylation

þ CH%de-carbonation

¼ 74=18Aþ 74=44B

where A = area under the DTG curve corresponding to the total masslost due to the de-hydroxylation of calcium hydroxide at a temper-ature between 420 and 550 �C, i.e.

CaðOHÞ2 ! CaOþH2O½de-hydroxylation of calcium hydroxide�

and B = area under the DTG curve of total mass lost due to the de-carbonation reaction at a temperature between 600 and 780 �C, i.e.

CaCO3 ! CaOþ CO2½de-carbonation of calcium carbonate�

74, 18 and 44 are the molecular weights of CH, H2O and CO2

respectively.The greater de-carbonation reaction of LP–SCC mortars can be

compared to the corresponding ones for FA–SCC in Fig. 13b. Thisis likely to be due to the higher original amount of CaCO3 presentin LP–SCC and this result is consistent with those obtained by Yeet al. [22].

A theoretical correction has been made for the sake of calcula-tion of the total amount of CH in the LP–SCC powder because ofthe de carbonation of the original LP. The total weight of the pow-der sample in the TGA test was 25.99 mg. Therefore, 8.57 mg of thisshould be subtracted, representing the 33% LP content. The calcu-lated total mass loss from the TGA curve was 8.953% between600 and 780 �C. Thus, CH% due to the de-carbonation of the mortarbecomes = 74/44 � (25.99–8.57) 8.953% = 2.62%.

Fig. 11. Capillary pore structure of bulk cement pastes (a) LP–SCC and (b) FA–SCC.

Fig. 12. BSE image showing the porous interface (dark-grey areas) between the LP(light) and the hydration products (mid-grey).

Fig. 13. (a) TG curves and (b) DTG curves.

Table 4Total amount of CH% in SCC-mortars.

Mix type CH de-hydroxylation CH de-carbonation Total amount of CH

LP–SCC 6.3 2.62 8.98FA–SCC 5.7 6.1 11.8

M.K. Mohammed et al. / Construction and Building Materials 49 (2013) 84–92 91

The results of the total CH% are listed in Table 4. These indi-cate that the FA–SCC showed a somewhat lower CH% due tode-hydroxylation than the LP–SCC. This may be due to the con-sumption of the CH due to the higher pozzolanic activity of theFA as compared to that of the LP. However, exposure of the pow-der samples to CO2 in the atmosphere during the preparation is acomplicating factor.

5.2. X-ray diffraction (XRD) analysis

Typical XRD spectra for the LP- and FA–SCC mortar powders arepresented in Fig. 14a and b. Four main crystalline phases wereidentified: Quartz (Q), Portlandite (CH), Calcite (C) and Ettringite(E). The detected pure quartz does not represent one of the hydra-tion products but it is likely to be due to the presence of SiO2 in thesand particles. The main focus of the discussion will be on the pres-ence and intensity of both the CH and C. From the figures, it wasnoticeable that there were intensity peaks of CH in the XRD pat-terns for both the LP- and FA–SCC at 2-theta values of 18.1�,34.2�, 47.2� and 50.9�. However, the intensities of these peaks werehigher in the LP–SCC than those in the FA–SCC, indicating a higheramount of CH in the cement paste. These results are compatiblewith those obtained from the thermo-gravimetric analysis. Thehigher amount of CH in LP–SCC tends to confirm the hypothesisthat the hydration of C3S is enhanced at early ages in the presenceof LP because the hydration of the C3S is known to produce a higheramount of CH in comparison with C2S at early ages [23]. On theother hand, the pozzolanic reaction of the finer particles of FA atearly ages may decrease the amount of CH in the cement paste.The micrograph of the FA–SCC paste in Fig. 11b illustrates threetypes of FA particles: 1-rounded un-reacted, 2-partially reactedand 3-completely reacted particles. X-ray traces of the two typesof SCC mortar also indicate the presence of calcite in the cement

Fig. 14. XRD traces (a) LP–SCC and (b) FA–SCC.

92 M.K. Mohammed et al. / Construction and

pastes. The higher intensity of the calcite peaks in the LP–SCC indi-cates a higher proportion of this phase and this is likely to be due tothe presence of un-reacted LP.

6. Conclusion

The production, microstructure and hydration of sustainableSCC with two different types of filler have been studied. The fol-lowing conclusions can be drawn:

� A successful sustainable and economic SCC with a compressivestrength grade of 50–60 MPa could be produced using a total bin-der content of 450 kg/m3 with approximately 33% replacementby LP or FA filler. Moreover, the FA–SCC required smaller quanti-ties of SP to meet the consistency requirements of the SCC.� Micro-scale porosity was deduced for the matrix of the both

types of SCC. It seems that the filler type had no effect on thenature of the porosity (micro or macro) of the SCC whereas itdid influence the critical pore diameter especially at 28-days.� The microstructural investigation revealed that the FA–SCC had

a relatively dense matrix with an approximately constant distri-bution of the Ca/Si ratio across the aggregate–matrix interface.In comparison, the LP–SCC had a less dense matrix with variedCa/Si ratio across the thickness of the ITZ.� The microstructural and hydration studies indicated the pres-

ence of higher amounts of Ca(OH)2 and CaCO3 for the LP–SCCin both the ITZ and in the bulk cement paste in comparison withFA–SCC. This agrees that LP is a relatively in active mineral.However, the higher amount of Ca(OH)2 in the LP–SCC powderdetected by the TGA test due to the dehydroxylation of CH andthat detected by the XRD analysis may give an indication of theaccelerating effect of LP on the hydration process of C3S.

Acknowledgements

The principal author would like to express his gratitude for hisPhD scholarship sponsored by Higher Committee for EducationDevelopment in Iraq (HCED). The authors would like to thank Dr.Nigel Neate (University of Nottingham – Faculty of Engineering)for his valuable help in conducting the XRD and SEM examinations.Special thanks must also go to Miss Vikki Archibald (AnalyticalTechnician, University of Nottingham – Faculty of Engineering)and Mr. Keith Dinsdale (Chief Experimental Officer, University ofNottingham – Faculty of Engineering) for their help in performingthe thermo-gravimetric and MIP tests.

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