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doi:10.1016/j.bu

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Building and Environment 43 (2008) 1318–1325

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Mineralogy of plain Portland and blended cement pastes

Hanifi Binicia, Ismail H. Cagatayb,�, Tahir Shahc, Selim Kapurd

aDepartment of Civil Engineering, K.S. University, Kahramanmaras, TurkeybDepartment of Civil Engineering, Cukurova University, Adana, Turkey

cCentre for Materials Research and Innovation, The University of Bolton, UKdDepartment of Soil Science, Cukurova University, Adana, Turkey

Received 4 July 2006; received in revised form 22 March 2007; accepted 22 March 2007

Abstract

The compressive strength and microstructure of blended cement was investigated in this study. The hydration products of cements

were identified by means of scanning electron microscopy (SEM) and polarising microscopy (thin section). Results indicated that the

blended cement required pozzolanic activity in addition to its cementing property with the addition of 30% material. Aggregations of

belite grains were observed which were surrounded by alite. Blended cement exhibited high early-term strength. However, its 3d-age

strength is lower than ordinary Portland cement. On the other hand, all blended cement mortars fulfil the compressive strength

requirements of TS 24. Hence, it can be said that blended cement can achieve adequate early compressive strength. A reduction in the

amount of CSH gel and the porosity of the matrix in mortars were found to be responsible for the poor compressive strength of the

cement paste in early the age.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Microstructure; SEM; Thin section; Basaltic pumice (BP) and ground granulated blast-furnace slag (GGBS)

1. Introduction

The blended cement pastes appear to be a real challengefor the future of the cement industry. Composite cementsbased on partial replacement of Portland cement by wastematerials has become commonplace because they offer costreduction, energy saving, and arguably superior products.Calcium hydroxide was formed initially in all systems, butin the presence of large amounts of pulverised fuel ash andblast furnace slag (BFS) it did not become such a majorreaction by-product as in the Plain Portland Cementsystem. In particular the calcium hydroxide initially formedin the BFS–cement systems was totally consumed within 6months, indicating the important pozzolanic behaviour ofBFS at such high replacement levels [1]. The bonding isdocumented to be of high quality due to an improvedmechanical interlocking between the aggregate and cement

e front matter r 2007 Elsevier Ltd. All rights reserved.

ildenv.2007.03.012

ing author. Tel.: +90322 3386784; fax: +90 322 3386702.

esses: hbinici@ksu.edu.tr (H. Binici),

rova.edu.tr (I.H. Cagatay), ths1@bolton.ac.uk (T. Shah),

.tr (S. Kapur).

paste. The results indicate that the CSH structure in thestudied pumice and slag can increase. The CSH structurein the intergrinded blends is more than that in the separatesystems when the fineness ratio of the additives iskept constant. However, there is no considerable changein CSH structure when the amount of additive is increased[2]. Due to the rapid economic development and popula-tion growth, the energy consumption has signifi-cantly increased throughout the world in the last decades.Thus, the use of natural materials (pumice and perlite) andwaste have became two of the most popular concreteingredients due to their puzzolanic properties [3]. Fromexamination of the XRD patterns of the three cementpastes, the amount of alite was noted to be significantlyhigher in the ultrafine mixtures than in the Type III cementespecially in the initial aging period. Calcium hydroxidegeneration, however, was lower compared to the Type IIIcement, confirming that effective hydration was yet tocommence. The XRD patterns displayed a considerableamount of alite consumption after 7 days in thesemixes when formation of Ca(OH)2 was also observed [4].Around the world, binary blended cements such as

ARTICLE IN PRESSH. Binici et al. / Building and Environment 43 (2008) 1318–1325 1319

Portland slag cement, pozzolanic cements and limestonefiller Portland cements are categorised according tointernational standards [5]. Types of standard blendedcement available in different countries depend uponthe resources present therefore several combinationscan be found worldwide. The benefits of addition ofactive supplementary materials and fillers to Portlandcements are well documented [6]. Since the 1990s, theuse of cement made with Portland cement and twosupplementary materials, also called blended or compositecements, has increased because it presents more advantagethan some binary cements [7,8]. Blended cements contain-ing the combinations of fly ash–silica fume or slag–silicafume are a common practice [9] and several studieshave been published [10]. The process of optimisingthe composition of the raw materials for productionof Portland cement clinker is still mainly a matter oftrial and error. Some more general guidelines areunderstood regarding relationships between chemistryand the composition of the clinker minerals and theirreactivity, but many questions remain. Studieshave reported the occurrence of either b- or a-type belitebeing important for the early development of theultimate strength of Portland cements. It is generallyacknowledged that a-modification is more reactiveof the two, but the compositions leading only topartial transition are superior [11]. When an expansiveagent was added to high-performance compositecementitious material, a needle-shaped ettringite wasobtained due to the reaction of the expansive agentwith Ca(OH)2, forming a three-dimensional networkstructure, which not only improved the early strength ofthe high-performance composite cementitious paste butalso increased its late strength. However, the ratio ofexpansive agent added should be optimised in order toavoid cracking of the cement paste due to ettringiteexpansion [12].

In this study, we have opted to investigate separateand intergrinding of blended cement and the incorporationof some additives in order to show the effect of the fine-ness of the additives. The fineness of the controlcement (clinker+gypsum) (OPC) was maintained atconstant values of approximately 2800730 and 4800730 cm2/g. The blended cement (clinker+pumice+slag+gypsum), had the same Blaine values. The additionof pumice and slag in small proportion (10% and 30%weight of clinker) was repeated for two different finenessvalues.

The studied pumice is basaltic to rhyolitic in composi-tion, containing volcanic glass and lower amounts ofminerals and rock fragments. The mineral contents includefeldspars, quartz and biotite. Clay minerals occur asalteration products. The chemical compositions of thepuzzolans are consistent with the requirements given inboth the TS 12142 [13] and ASTM C168 [14] standards.The mineralogy was determined by scanning electronmicroscopy (SEM) and thin section.

2. Materials used

The basaltic pumice (BP) studied in this research hasbasalt to rhyolictic compositions. It contains glass shards,mineral phases and small amount of volcanic rock.Essential minerals are feldspar, quartz and biotite. Clayminerals occur as alteration products. The clinker used wasobtained from Adana Cement Plant. The ground granu-lated blast furnace slag (GGBS) specimens were obtainedfrom Iskenderun cement grinding plant. The terminologyfor all specimens is given in Table 1. Chemical, miner-alogical and physical characteristics of materials used aregiven in Table 2.

HM : hydrualic modulus ¼CaO

SiO2 þAl2O3 þ Fe2O3

�,

SM : silicate modulus ¼SiO2

Al2O3 þ Fe2O3,

AM : aluminate modulus ¼Al2O3

Fe2O3,

LM : lime modulus ¼100CaO

2:8SiO2 þ 1:1Al2O3 þ 0:7Fe2O3:

�

3. Methods

3.1. Preparation and testing of samples

The blended cements were prepared using one type ofclinker (4% gypsum), BP and GGBS. Different amounts ofadditives (10% and 30%) were incorporated into theseblends. Cement paste and mortars were prepared usingOPC, two types of grinding systems (inter-grinding andseparate grinding) at two Blaine values (2800730 and4800730 cm2/g). The compressive strengths of the speci-mens were determined by the TS 24 [15]. Experiments werecarried out in OPC standard mortars and GGBS mortars.Compressive strengths values were determined after 3, 7and 28 days. Specimen batches with plain Portland cement(2800730m2/g and 4800730 cm2/g) and prisms (40mm�40mm� 160mm) were prepared.

3.2. Development of compressive strength

Compressive strength tests were carried out on prisms40mm� 40mm� 160mm prismatic specimens, accordingto Turkish Standard 24. In the experiment, moist curing wasapplied to the specimens. The specimens were dried for 24hprior to testing for every mix at the required age, and theaverage strength of three specimens was used as an index.

3.3. Microstructure studies

3.3.1. SEM

In this work, the demolition products were identified bymeans of a SEM. In order to determine the properties of

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Table 1

Composition of studied cements

Cement Composition (% percentages by weight) Blaine (cm2/g)

Clinker Gypsum GGBS BP

A1 96 4 — — 2800

A2 96 4 — — 4800

B1 (separate grinding) 86 4 5 5 2800

B3 (separate grinding) 66 4 15 15 2800

C1 (separate grinding) 86 4 5 5 4800

C3 (separate grinding) 66 4 15 15 4800

D1 (intergrinding) 86 4 5 5 2800

D3 (intergrinding) 66 4 15 15 2800

E1 (intergrinding) 86 4 5 5 4800

E3 (intergrinding) 66 4 15 15 4800

Table 2

Chemical, mineralogical and physical characteristics of materials used

Specimens Oxides (%)

SiO2 Al2O3 Fe2O3 CaO MgO SO3 LOI (loss on ignition)

Clinker 20.2 5.5 3.8 64.7 1.9 0.8 —

A1 ¼ A2 19.4 5.5 3.9 63.4 1.8 2.0 —

Basaltic pumice 51.8 22.1 7.3 6.2 8.3 — 0.4

GGBS 41.6 13.7 7.3 28.2 4.9 1.8 0.01

Specimens Cement modulus Mineral composition (%)

HM SM AM LM C3S C2S C3A C4AF

Clinker 2.1 2.1 1.4 98.2 65.7 8.6 8.2 11.7

A1 ¼ A2 2.1 2.0 1.4 99.7 66.5 5.6 8.1 11.9

Materials Physical properties of materials

Specific gravity (kg/m3) Blaine (cm2/g) Sieve analysis (%)

Residue on 90 mm Residue on 200mm

Basaltic pumice 2960 2800 and 4800 0.3 0.08

GGBFS 2895 2800 and 4800 0.2 0.09

Clinker 3195 2800 and 4800 0.3 0.09

TS 12142 standard requirements for basaltic pumice and GGBFS

SiO2+Al2O3+Fe2O3 SO3 LOI

461 o3.5 o10

H. Binici et al. / Building and Environment 43 (2008) 1318–13251320

the concrete specimens from a microscale view, SEMphotos were taken at the end of the 7 days testing period.Ten millimetre cubic specimens prepared from some mixesselected as having particular microstructure were studiedunder the JEOL JEM 840 model SEM.

3.3.2. Thin sections

Microstructure of the blended cement prism wasdetermined in thin sections by the polarising microscopefor the development of the hydration process.

4. Results

4.1. Development of compressive strength

The compressive strength development of the testedcement is given in Figs. 1 and 2. From these figures, it canbe said that the compressive strengths of the mortars madewith blended cements with separately ground finer speci-mens were higher than those of the OPC at all tested agesafter 28 days. The strength development characteristics ofthe blended cement mortars were affected not only by the

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20

25

30

35

40

45

50

55

3 28

Co

mp

ressiv

e s

tre

ng

th

(MP

a)

A1 B1

B3 D1

D3

Age (days)

7

Fig. 1. Compressive strength development of tested cement (fineness:

2800 cm2/g).

35

40

45

50

55

60

3 28

Age (days)

Co

mp

ressiv

e s

tre

ng

th

(MP

a)

A2 C1

C3 E1

E3

7

Fig. 2. Compressive strength development of tested cement (fineness:

4800 cm2/g).

0.4

0.6

0.8

1

3 28

Age (days)

Re

lative

str

en

gth

ra

tio

B1

B3

D1

D3

A1

7

Fig. 3. Relative strength of tested cements in relation to curing age

(fineness: 2800 cm2/g).

0.6

0.8

1

3 28

Age (days)

Rela

tive s

trength

ratio

C1 C3

E1 E3

A2

7

Fig. 4. Relative strength of tested cements in relation to curing age

(fineness: 4800 cm2/g).

H. Binici et al. / Building and Environment 43 (2008) 1318–1325 1321

finenesses of the cements, but also, in some cases, by thegrinding method. Specimen C3 had the highest compressivestrength at 28 days. However, this value was lower for theother blended cement mortars than the reference mortars.

The relative strength (the ratio of the blended cement tothe strength of OPC) of the blended cements in relation tocuring age is given in Figs. 3 and 4. It can be observed fromFig. 3 that relative strength values of the coarser specimenswere lower at early ages. On the other hand, the relativestrengths of the finer blended cement specimens werehigher at early ages. Compared to those for the coarserspecimens (C1, C3 and E1) this value is even higher thanthose for the reference specimens.

The development of the relative strength of theblended cements in relation to the curing ages is observedto be different. The relative strength ratio values for thecoarser specimens were lower than those for the finerspecimens.

All blended cement mortars fulfil the compressivestrength requirements of TS 24. Hence, it can be said thatblended cement can achieve adequate early compressivestrength.

4.2. Microstructure studies

4.2.1. Thin sections

Some selected thin section images in Figs. 5–7 aresamples that have been hydrated for 7 days, with theexception of the blended cement paste.Some hydration particles are surrounded by rims of

hydration products. The characteristics of samples at 7days were viewed using polarising microscope are shown inFigs. 5–7. It is seen from Fig. 5 that the microstructure ofOPC is very compact and a great deal of plate-shapedcalcium hydroxide is also present. The microstructures ofblended cements are also very compact however, needle-shaped ettringite and plate-shaped calcium hydroxide wereobserved.

4.2.2. SEM

The distribution of CSH, CH and Ettringite crystals inthe microstructure were determined by SEM and someselected ones are presented as backscattered electronimages in Figs. 8–11.All samples were hydrated for 7 days, with the exception

of the blended cement paste. Specimens aged for 7 dayswere viewed using SEM. A large amount of needle-shapedettringite and plate-shaped calcium hydroxide can beobserved in the blended cement. These figures show theformation of Portlandite (P) and CSH in OPC. However,the thin section images of OPC and blended cement pastesshow that the microstructure has newly formed aliteminerals. SEM images of the thin sections of blendedcement show formation of numerous CSH entities. Themain hydration products are cotton-shaped CSH gel and acertain amount of needle-shaped ettringite.

5. Discussion

Results of the study show that blended cement exhibitsexcellent compressive strength characteristics. Thoughblended cement has a high long-term strength, its 3d-agestrength is low. The early-age compressive strength of

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Fig. 5. Thin section of A1 OPC mortar (A: alite, B: belite, P: Portlandite, P: pore).

Fig. 6. Thin section of B3 blended cement mortar (F: ferrite, B: belite, P: Portlandite).

H. Binici et al. / Building and Environment 43 (2008) 1318–13251322

blended cement is determined to a large extent by theproperties of BP and GGBS. It is well known that for agiven replacement level with mineral admixtures, the early-age properties of high-strength concrete are influenced bythe reactivity of the mineral admixtures. The more reactivethe mineral admixtures the higher the early-age strength ofthe concrete. From Figs. 1 and 2, we can see that thecombination of BP and GGBS has a modulus higher thanOPC alone. Therefore, the hydration rate of cementitiousmaterials in BP+GGBS is faster than that in OPC, whichwill give rise to higher early-age strength. It can also beseen from the SEM images that there are numerous un-hydrated BP particles in GGBS (Figs. 5–8) at the age of 7days. Some studies showed that GGBS is slowly reactive, it

reacts with the hydrated lime (CH) of Portland cement,forming a secondary calcium silicate compound. At thesame time, a lot of homogeneous hydration products likeettringite and CSH are also formed, which have largerspecific surface than that of Portland cement. FromFigs. 4–6, we can see that the glass phase of BP is brokendown, and a lot of secondary calcium silicate, ettingite andCSH are produced on the surface of BP and GGBS.The amount of free Ca(OH)2 decreases due to the

pozzolanic reaction. The poor compressive strength ofOPC is due to the presence of large pores and free Ca(OH)2in the concrete (see Fig. 5). Blended cement has poorcompressive strength at the 3d-age. This is due to thepresence of numerous un-hydrated BP particles in the

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Fig. 7. Thin section of C3 blended cement mortar (B: belite, F: ferrite, C: calcite).

Fig. 8. SEM image of A1 OPC sample.

H. Binici et al. / Building and Environment 43 (2008) 1318–1325 1323

mortar, thus the matrix is significantly porous at the earlyage. Furthermore, the amount of CSH gel in OPC formedby cement hydration is less than that in blended cement.The reduction in the amount of CSH gel and the porosityof the matrix in mortars are responsible for the poorcompressive strength in early ages.

The results indicate that the addition of BP and slag canlead to an increase CSH structure. In SEM studies, thestructure of CSH, CH and ettringite crystals wereobserved. The CSH formation in the structure of theblended blend is higher when the fineness is increased at

constant additive ratio. The structure of CSH, CH,ettringite crystals and alite, belite, and other mineralswere observed. Some hydrated particles were observedsurrounded by rims of hydration products. Finenessof blended cement has an effect on the pore size ofconcrete. As the hydration products are distributed morehomogeneously in the space available, a more refinedpore structure develops, as shown in Figs. 5 and 11.Other workers also support these observations [16].Fine ettringite crystals appeared in voids and wereobserved in conjunction with CSH. The results reveal a

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Fig. 9. SEM image of A2 blended cement sample.

Fig. 10. SEM image of B3 blended cement sample.

Fig. 11. SEM image of D3 blended cement sample.

H. Binici et al. / Building and Environment 43 (2008) 1318–13251324

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clear improvement in the performance characteristics of thecement obtained using BP and slag. Most alite grains weresubhedral and most of the alite minerals are fine andangular (see Fig. 5). Abundant cavities were observed onone Portlandite mineral in thin section. Alite crystals inOPC specimens were smaller than those in blended blends.These are most probably derived from the source material.Fig. 6 shows that the blended cement grain contains iron.Smaller alite minerals are most probably responsible forthe high strength in sample C3. As far as hydrated phasesare concerned, densified CSH was the major phase andneedle-shaped ettringite crystals were widely dispersed inthe matrix (see Figs. 5, 9 and 11). As seen in Figs. 9 and 11,poorly formed crystalline CSH coats mineral and aggregatesurfaces as a dominant binder. The use of high proportionsof pumice and slag increases the strength of the cementpaste. Using blended cement, with reduced amount ofCa(OH)2 in the paste, increases the overall volume of CSHgel. The blended cement seems to be the real challenge forthe future of cement technology. An increase in fineness ofthe blended cements induces development of uniform porespaces and products of hydration as seen in Figs. 7 and 11.Other workers also support these observations [17].

6. Conclusions

Results of this study clearly show the presence ofconglomerations of oval and spheroidal belite grains.Microscopic investigations of the various cement samplesshow that there are obvious differences in belite, alite, freeCaO, aluminate and ferrite crystal shapes, sizes anddistribution between the blended cement and OPC. Thinsection micrographs of the cements show the presence ofsome chlorite crystals, which are most probably derivedfrom the source material. Using blended cement withreduced amount of Ca(OH)2 in the paste increases overallvolume of CSH gel. Increase in fineness of blended cementinduces development of uniform pore spaces and productsof hydration. Also, all blended cement mortars fulfill thecompressive strength requirements of TS 24. Therefore,blended cement can achieve adequate early compressivestrength.

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