Construction and Building Materials 35 (2012) 347–360

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Construction and Building Materials

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Influence of colloidal nanosilica on ultrafine cement hydration: Physicochemicaland microstructural characterization

F. Kontoleontos a, P.E. Tsakiridis b,⇑, A. Marinos a, V. Kaloidas c, M. Katsioti a

a National Technical University of Athens, School of Chemical Engineering, 9 Heroon Polytechniou St., 15780 Athens, Greeceb National Technical University of Athens, School of Mining and Metallurgical Engineering, 9 Heroon Polytechniou St., 15780 Athens, Greecec EKET Central Laboratory, Heracles Group, 19 K. Pateli, 14123 Lykovrissi, Attica, Greece

h i g h l i g h t s

" Colloidal nanosilica addition in ultrafine cement (�10.700 cm2/g) did not lead to immediate mechanical gain." At 28 days an increase of 11 MPa was observed." More packed and refined microstructure was observed." Total porosity and average pore diameter were decreased leading to a denser microstructure.

a r t i c l e i n f o

Article history:Received 25 January 2012Received in revised form 26 March 2012Accepted 25 April 2012Available online 23 May 2012

Keywords:Ultrafine cementNanosilicaHydrationMicrostructure

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.04.022

⇑ Corresponding author. Tel.: +30 210 7722179; faxE-mail address: ptsakiri@central.ntua.gr (P.E. Tsak

a b s t r a c t

The influence of colloidal nanosilica addition on an ultrafine cement have been studied in terms of phys-icomechanical and microstructure properties. Primarily, experiments were carried out to produce anultrafine cement (UF) with a Blaine specific surface area greater than 10.500 cm2/g. Nanosilica was addedin amounts of 2% and 4% on UF cement basis. All cements were tested for initial and final setting times,consistency of standard paste, flow of normal mortar and compressive strengths after 1, 2, 7 and 28 days.The hydration products were determined by X-ray diffraction analysis and by Fourier transform infraredspectroscopy, at 1, 2, 7 and 28 days. The microstructure of the hardened cement pastes and their morpho-logical characteristics were examined by scanning electron microscopy, whereas porosity and pore sizedistribution were evaluated by mercury intrusion porosimetry.

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1. Introduction

The production of special and custom-made cements is becom-ing one of the powerful trends of a modern market. Ultra-High Per-formance Concretes (UHPCs) is a type of building material withvery high strength and durability. Its high strength and ductilitymake it suitable for bridge decks, thin shell structures, nuclearpower plants and defensive facilities that may experience impactloads [1,2].

The performance of an ordinary Portland cement is a direct con-sequence of the clinker chemistry and mineralogy. However, it iswell known that the properties of cement are also affected by itsfineness and particle size distribution. During grinding, the clinkerparticles are substantially reduced in size to generate a certain le-vel of fineness as this has a direct influence on several performancecharacteristics of the final product, such as rate of hydration, water

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demand and strength development. Micro or ultrafine cementshave been proposed to be used in oil well cementing technology[3]. They are also used extensively in the pre-grouting (and post-grouting) of rock tunnels for consolidation and water tighteningof the rock. On the other hand, it is well known that the utilizationof various types of by-product and waste material such as fly ash,slag, silica fume, and rice husk as additives results in High Perfor-mance Cements (HPCs), in terms of better chemical resistance,higher strength or better durability [4,5].

Siliceous mineral admixtures all contain mostly SiO2 in theirchemical composition. The degree of SiO2 crystallinity and its mor-phology vary depending on the temperatures and pressures towhich it is subjected, giving rise to non-pozzolanic structures suchas quartz, tridimite and cristobalite. There is, moreover, a widevariety of microcrystalline, but especially amorphous SiO2, suchas silica fumes and nanosilica. Among ultra-dispersed silicas, thebest characterized and most extensively used is silica fume (micro-silica), a by-product of gas-phase condensation in smelting of sili-con alloys. However, the potential application of hydro-chemically

348 F. Kontoleontos et al. / Construction and Building Materials 35 (2012) 347–360

synthesized ultra-dispersed silicas (precipitated silicas obtainedfrom solutions of sodium silicates) and colloidal silicas, as addi-tions to cements and concretes were less extensively studied [6].

Colloidal silica (CS) denotes small particles consisting of anamorphous SiO2 core with a hydroxylated surface, which are insol-uble in water. The size of the particles can be varied between 1 and500 nm, hence they are small enough to remain suspended in afluid medium without settling. Parameters such as specific surfacearea, size and size distribution can be controlled by the synthesistechnique [7].

The nano-scale size of particles can result in dramatically im-proved properties from conventional grain-size materials of thesame chemical composition. Nanosilica is commonly used for rein-forcement of polymer to increase the hardness, modulus, weather-ability and flammability [8,9]. However, the use of ultra-dispersedactive mineral additions rich in non-crystalline silica makes it pos-sible to prepare and successfully apply materials possessing high(55–80 MPa) and ultrahigh (>80 MPa) strength, low permeability,and enhanced corrosion resistance.

The performance enhancing properties of nanosilica areachieved through two mechanisms. The ultrafine particles are ableto fill the microscopic voids between the cement particles improv-ing ‘‘packing’’ and creating a less permeable structure. In the curingprocess, the nanosilica also reacts with the Ca(OH)2 produced withthe cement paste to form additional calcium silicate hydrate [10].Furthermore, the well-dispersed nanoparticles could act as centersof nucleation for cement hydrates, a fact which would acceleratethe hydration. The mechanism of this working principle is relatedto the high surface area of nanosilica, which works as nucleationsite for the precipitation of CSH-gel. The nanoparticles favor theformation of small-sized crystals (such as calcium hydroxide andAFm) and small sized uniform clusters of C–S–H. Finally, they couldimprove the structure of the aggregate contact zone, which resultsin a better bond between aggregates and cement paste [11]. Due toresults of the filling effect (reduced porosity) and the pozzolanicreaction (consuming of calcium hydroxides for CSH formation),the nanosilica could be used as admixtures for producing HPC withenhancing strength and abrasion resistance along with reducingpermeability and dry shrinkage [12]. However, the compressivestrength of ordinary Portland cement increases with the increaseof the amount of nanosilica, until it reaches an optimal amountof (�0.5%) and then drops to lower values, at higher addition. Be-cause nanosilica presents high specific surface areas with high sur-face energies, agglomeration has been observed in highersubstitution, a fact that prevents uniform distribution of thenano-SiO2 particles within the mortar. Thus, the enhancement ofthe compressive strength (and pore structure) is subjected to a cer-tain limit [8]. Furthermore, when nanosilica is incorporated intothe mortar in the fresh state it has a direct influence on the wateramount required in the mixture, a fact that could lead to a consid-erable decrease of setting times and the reduction of mortar flow,due to the gain in cohesiveness of the paste.

Although, there are few reports on mixing nano-particles in ce-ment-based building materials, the so far published literature hasgiven little attention to the influence of nano-SiO2 in ultrafine ce-ments and its addition marks a novelty in the high performance ce-ments technology, which is expected to improve the physical andmechanical properties of the final product. Jo et al. studied theproperties of cement mortars, comparing nano-SiO2 and silicafume [13]. Their experimental results showed that the compressivestrengths of mortars with nano-SiO2 particles were all higher thanthose of mortars containing silica fume at 7 and 28 days. It wasdemonstrated that the nano-particles are more valuable in enhanc-ing strength than silica fume. Shih et al. examined the effect ofnanosilica on characterization of Portland cement composite [8].Their experimental results showed that the Portland cement

composite with 0.60% of added nanosilica by weight of cement pre-sented an optimum compressive strength, in which the increase ofcompressive strength was about 43.8%.

The influence of nano-SiO2 addition on properties of hardenedcement paste, in relation with silica fume, has been also studiedby Qing et al. [14]. Their results indicated that the use of nano-SiO2 accelerated the cement hydration process and the compres-sive strengths were obviously higher than those incorporating sil-ica fume, especially at early ages. Senff et al. examined the effect ofnanosilica and microsilica on microstructure and hardened proper-ties of cement pastes and mortars [15]. A decrease in the calciumhydroxide contents in later curing ages indicated their pozzolaniceffect. Li et al. experimentally studied the mechanical propertiesof nano-SiO2 cement mortars [16]. The experimental resultsshowed that the compressive and flexural strengths measured atthe 7th day and 28th day of the cement mortars mixed with thenano-particles were higher than that of a plain cement mortar.The SEM study of the microstructures showed that nano-SiO2 filledup the pores and reduced Ca(OH)2 compound among the hydrates.

Today, nanosilica, because of its high cost, is applied in high per-formance concretes and self compacting concretes mainly as ananti-bleeding agent. It is also added to increase the cohesivenessof concrete and to reduce the segregation tendency.

The aim of the present research work was the evaluation of theinfluence of colloidal silica addition in a ultrafine cement (with aspecific surface area greater than 10,500 cm2/g), produced byintensive grinding of a commercial CEM I 42.5N cement. All ce-ments were tested for setting times, consistency of standard paste,flow of normal mortar and compressive strengths after 1, 2, 7 and28 days. X-ray diffraction (XRD), Fourier transform infrared spec-troscopy (FT-IR), scanning electron microscopy (SEM) and mercuryintrusion porosimetry (MIP) tests were conducted to study thehydration products.

2. Experimental

Preliminary experiments were carried out in order to produce an ultrafine (UF)cement with a Blaine specific surface area greater than 10,000 cm2/g. A Portland ce-ment CEM I 42.5N, fabricated by Heracles General Cement Company of Greece wasused as the starting material. Its mineralogical phases, which were determined byXRD analysis, using a Bruker D8-Focus diffractometer with nickel-filtered Cu Ka1

radiation (=1.5405 Å, 40 kV and 40 mA), are given in Fig. 1.Grinding was carried out in a laboratory rotating ball mill, using steel balls as

grinding medium. Grinding was stopped at predefined time intervals to determinethe specific surface area, according to the Blaine air permeability method [17]. Thephysical and chemical properties of the produced ultafine cement, together withthe as received CEM I 42.5N cement, are presented in Table 1. Their particle size dis-tributions were measured by a CILAS-Model 1064 laser scattering particle size dis-tribution analyzer.

Colloidal nanosilica (NS) was commercially available as an aqueous dispersion.It contained about 40% SiO2 corresponding to a density of 1.3 g/mL. Its chemical andphysical properties, as provided by the manufacturer, are given in Table 2. X-ray dif-fraction (XRD) diagram and scanning electron microscopy (SEM) micrograph of thedry material are shown in Figs. 2 and 3. Two different dosages (based on prelimin-ary experiments) of colloidal nanosilica 2 and 4 wt.% of cement, were added to theultrafine cement, with the mixing water (the water in the addition was taken intoaccount). Higher amount of substitution led to significant decrease of setting times.The above dosages correspond to 0.8 and 1.6 by wt% of cement. The mixtures aredesignated as UF-NS2 and UF-NS4 respectively.

The standard consistency and the setting times of the cement pastes weredetermined according to the European Standard EN 196-3 [18]. The determinationof the normal mortar flow was carried out for w/c = 0.5, according to ASTM C1437[19]. Compressive strength measurements were conducted at the ages of 1, 2, 7 and28 days on mortar prisms (dimensions 40 � 40 � 160 mm), prepared and tested inaccordance with EN 196-1 [20].

The study of the hydrated products was carried out on cement pastes ratherthan on mortars. Cement pastes were prepared with a water to binder (cementand dry silica) ratio equal to 0.4 by mixing 300 g of ground mixtures with 120 mlof water equal to 0.4. The pastes were cured in tap water at a temperature20 ± 2 �C. At the ages of 1, 2, 7 and 28 days, the hydration was stopped by meansof acetone and ether extraction. The hydration products were mineralogicallydetermined by X-ray diffraction, using a Bruker D8-Focus diffractometer. They were

Table 1Chemical analyses and physical characteristics of the examined cements.

Oxides Chemical analysis (%)

CEM I 42.5N UF

SiO2 20.11 20.05Al2O3 5.15 5.15Fe2O3 3.35 3.35CaO 63.00 63.10MgO 2.65 2.62K2O 0.62 0.61Na2O 0.29 0.29SO3 2.44 2.40TiO2 0.263 0.251P2O5 0.169 0.153Cr2O3 0.049 0.050LOI 1.39 1.40

Physical propertiesSpecific surface area (Blaine-cm2/g) 3870 10,725Specific gravity (g/cm2) 3.26 3.25

Table 2Technical characteristics of nanosilica (liquid form).

Property Value

Color WhiteDensity (20 �C) 1.3 g/cm3

pH (20 �C) 10.20Viscosity (20 �C) �10 mPa sThermal stability 5–35 �CChlorite <0.1%SiO2 content 40 ± 1%

0

100

200

300

400

500

600

700

5 10 15 20 25 30 35 40 45

Inte

nsti

ty (

cps)

2

2

1. C3S2. C2S3. C3A4. C4AF5. CaOf6. MgO7. CaSO4 -2H2O -Gypsum8. CaSO4 0.5H2O -Bassanite

1

1,2

4

1 42

4

1

1,2

1,2

6

1,2

5

3

1

1

8

21,2

1 1,21,7

7

1 118

8

1

7

7

Fig. 1. X-ray diffraction of the cement CEM I 42.5N.

F. Kontoleontos et al. / Construction and Building Materials 35 (2012) 347–360 349

also characterized by a Spectrum GX (Perkin Elmer) Fourier transform spectropho-tometer, in the range of 4000–400 cm�1, using the KBr pellet technique. The pelletswere prepared by pressing a mixture of the sample and dried KBr (ratio, about1:200) at 8 tons/cm2.

In order to get an idea of their morphology, the hydration products were alsoexamined by scanning electron microscopy (SEM) using a Jeol 6380 LV ScanningElectron Microscope. Experimental conditions involved 15 kV accelerating voltage.Microanalysis of the cement pastes was performed by an Oxford INCA Energy Dis-persive Spectrometer (EDS) connected to the SEM. Finally, mercury intrusion poros-imetry was used for determining basic structural characteristics, such as pore sizedistribution, specific surface area and pore volume. A Carlo Erba 4000 mercuryporosimeter was used for porosimetry measurements, having a pressure range from1 to 4000 bar.

3. Results and discussion

3.1. Cements physical and chemical properties

The chemical compositions of the as received CEM I 42.5N andUF cements are given in Table 1. The quantitative determination ofthe principal cement anhydrous phases was carried by Rietveldanalysis technique and the results are given in Table 3. Colloidalnanosilica (NS) was used as aqueous dispersion of nano-sized silicasols, with a 500 m2/g specific surface area. The characteristic dif-fraction broad peak, centered on 22� (2h), confirmed its nanocrys-talline–amorphous nature (Fig. 2). According to the scanningelectron microscopy analysis, the dry nanosilica appeared withthe form of aggregates (50–100 nm). Higher magnifications re-vealed that the primary aggregates were formed by the agglomer-ation of smaller of spherical nano-particles with size of about 15–20 nm.

The particle size distribution data of the CEM I 42.5N is com-pared with the corresponding of the UF cement, as it is shown inFig. 4. The mean and median sizes of the cements PSD curve, their45 lm percent passing value, size and spread factors (n, Xo) of theRosin–Rammler distribution and Blaine specific surface area val-ues, which are all indicators of the material’s fineness, are summa-rized in Table 4. Grinding resulted in a reasonably smooth andcontinuous shift of the particle size distributions to finer sizes.The spread factor ‘‘n’’, which is a measure of the width of the sizedistribution (the higher ‘‘n’’ value means a narrower PSD), rangebetween 0.873 and 0.876. The size factor ‘‘Xo’’, which representsthe particle size for which 36.8% of the particles are coarser indi-cates how fine the powder is. UF cement presented the highestspecific surface area, lower values of the size distribution parame-ters and higher values of the 45 lm percent passing. It was foundthat 23% of it was below 1 lm, whereas the majority (90%) of theparticles were below 10 lm. This particle size diminution can bealso observed in the scanning electron microscopy micrographsshown in Fig. 5.

3.2. Physical and mechanical properties

Table 5 presents the cement water demand, the setting timeand the mortar flow (for w/c = 0.5) of the tested samples. According

Fig. 3. Scanning electron microscopy micrographs of colloidal nano-SiO2 used, after drying.

Fig. 2. X-ray diffraction analysis of nano-SiO2 used, after drying.

350 F. Kontoleontos et al. / Construction and Building Materials 35 (2012) 347–360

to the results, the setting times slightly decreases as the colloidalnanosilica content increases. The initial and final setting timesreached at 25 and 70 min, respectively in case of 4% substitution.This is at least 5–10 min earlier than those with no nanosilica

Table 3Phase composition by Rietveld analysis.

Phases wt.%

CEM I 42.5N UF

Alite – C3S monoclinic 50.8 50.7Belite – b-C2S 21.2 21.1Aluminates (cubic) (orthorhombic) 6.3 6.3

0.4 0.4Ferrite – C2Fe2�xAlxO5 8.8 8.9Periclase (MgO) 1.9 1.9Lime (CaOf) 1.1 1Arcanite (K2SO4) 0 0.1Apthitalite ((Na, K)2SO4) 0.6 0.6Gypsum 2.2 2.2Hemihydrate 1.1 1Anhydrite 0.1 0.2Portlandite 0.6 0.6Calcite 4.1 4Dolomite 0.8 1Quartz (SiO2) 0 0

added. As the hydration products increase rapidly, the slurry fromthe suspending state starts to be agglomerated faster (setting timeis shortened). Smaller contents did not affect much the settingtimes of cement paste.

The consistency of cement depends on the type and its fineness.As the nanosilica is finer than the cement, the specific surface in-creases as the NS content increases. When fineness increases, thesurface area contacting with water increases and the rate of hydra-tion reactions accelerates, since the hydration reaction occurs atthe interface with water. The standard consistency of the ultra finecement paste was 45.31%, while at 4% NS, the consistency was52.4%.

The flow tests confirmed the above results and showed that fora given w/c, UF-NS4 cement presented the lowest mortar workabil-ity. The mortar flow is connected with cement particles, which,especially in high Blaine values, are agglomerated in water suspen-sions. When particle size gets finer it needs more water to facilitatethe movement of particles on each other, a fact that can lead to theviscosity increase. Furthermore, a part of the water is entrapped inthe pores of the agglomerates and does not contribute to theflowability.

The mortars were tested for compressive strengths after 1, 2, 7and 28 days of curing. The obtained results are shown in Table 6

Fig. 4. Particle size distributions (cumulative passing and particle distribution).

Table 4PSD, Rosin Rammler and Blaine specific surface area results.

Sample PSD Rosin Rammler Specific surface area (Blaine) (cm2/g)

Mean (lm) Median (lm) 45 lm Passing (%) n (–) Xo (lm)

CEM I 42.5 N 18.83 13.5 92.90 0.876 17.7 3870UF 1.85 2.6 100 0.873 4.1 10,725

Fig. 5. SEM micrographs of the cements powders: (a) CEM I 42.5N and (b) UF cement.

F. Kontoleontos et al. / Construction and Building Materials 35 (2012) 347–360 351

Table 5Physical properties of the cements.

Sample Water of normalconsistency (%w/w)

Setting times(min)

Flow of mortar for w/c = 0.5 (%)

Initial Final

CEM I 42.5N 27.1 150200

83.0UF 45.3 30 80 14.0UF-NS2 46.2 28 75 14UF-NS4 52.4 25 70 6

Table 6Mechanical properties of the cements.

Sample Compressive strengths (MPa)

1 day 2 days 7 days 28 days

CEM I 42.5N 15.1 22.1 37.5 48.5UF 48.1 51.7 62.5 68.7UF-NS2 49.0 51.8 63.4 71.6UF-NS4 49.5 52.1 67.8 79.8

352 F. Kontoleontos et al. / Construction and Building Materials 35 (2012) 347–360

and Fig. 6. The relative strengths of tested cements in relation tocuring age are given in Fig. 7. Relative strength is the ratio of thestrength of the fine cement to the strength of the reference one,at each particular curing time. The effects of greater fineness oncompressive strengths are shown principally at early ages. Afterthe first 24 h of hydration, the compressive strengths of the UF ce-ment are about 3 times higher (over 48 MPa) than that of CEM I42.5N (15 MPa). At 28 days the rate of strength gain, in case ofUF cement, was also found to be higher than the rate of the coarserone. The value for CEM I 42.5N was 48.5 MPa, whereas the valuesfor the UF cements were varied from 65.5 to 68.5 MPa.

Although, the compressive strengths were enhanced in mortarscontaining nano-SiO2 particles in each case, its additions did notlead to an immediate mechanical gain. As it was mentioned before,in case of the UF cement, the compressive strengths increment atthe early ages was due to the hydration acceleration. The diameterof its largest particle was about 15 lm, while its mean diameter

Fig. 6. Strength development

was 1.85 lm. The particles of this size hydrate quickly, leading tothe fast increase of the initial strength.

On the other hand, the addition of nanosilica slightly increasesthe strength value of the UF cement at the early ages, mainly be-cause of the packing effect. It actually acted as filler material,which filled into the interstitial spaces and pores, inside the matrixof hardened cement paste, increasing its density as well as itsstrength [8]. At later ages of curing, the strength enhancementwere attributed to the reduction in the content of Ca(OH)2 withsimultaneously secondary C–S–H production, to the pore sizerefinement and to the matrix densification, observation that arepresented below. Nano-SiO2 is very effective pozzolanic materialand the pozzolanic reaction with calcium hydroxide (released fromfast hydration of the UF cement) is proportional to the amount ofsurface area available for reaction.

3.3. Mineralogical analysis of the hydration products by X-raydiffraction

XRD patterns of the hydration products of the examined ce-ments shown after 1 day (Fig. 8) and after 28 days of curing(Fig. 9). The XRD patterns of the reference CEM I 42.5N cementshowed the expected hydration products, including portlandite,ettringite, poorly crystallized C–S–H and unreacted clinker phases(mainly calcium silicate phases). The presence of calcium carbon-ate was attributed to the partial carbonation of portlandite. Gyp-sum disappeared within 1 day hydration. After 1 and 28 days ofhardening the intensity of ettringite main peaks in the samplesof the finest cements, in relation with the corresponding of CEM I42.5N, is higher. Although, a considerable amount of Ca(OH)2 is ob-served in both cases, after 28 days of hardening, Ca(OH)2 crystalli-zation in the UF cement is significant from the very beginning, andcontinues to increase as a function of time. Regarding anhydrousclinker phases, after 28 days of hydration, only C2S (and a smallamount of C3S) have not completely reacted.

In case of UF-NS cements, the amount of portlandite increasescontinuously at the first days of hydration due to the hydrationacceleration of the finest cement. However, after 28 days of curing,the rate-production of crystalline Ca(OH)2 seems to be diminished.It is known that amorphous silica reacts with lime, produced

of the examined cements.

Fig. 7. Relative strength of the cements in relation to curing age.

F. Kontoleontos et al. / Construction and Building Materials 35 (2012) 347–360 353

during the early hydration of alite, to form C–S–H gel, which con-tributes to the strength of the cement paste. As a result, the de-crease on the evolution of portlandite is directly related to thepozzolanic reactions with colloidal nanosilica. Moreover, the peaksassociated with C–S–H intensify with the increase of nanosilica.

3.4. Hydration products analysis by Fourier transform infra redspectroscopy

The FT-IR spectra of the CEM I 42.5N, UF and UF-NS4 cements,after 1 and 28 days of hydration are given in Figs. 10 and 11.

All samples indicated characteristic bands at 1643–1645 cm�1,due to the H–O–H bending (v2) vibration of water and at 3641–3642 cm�1, due to OH stretching mode of Portlandite, which seems

0

50

100

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nsit

y (c

ps)

UF

UF-NS4

CEM I42.5N

1

1

2

2 22 2

2,3

4 45

9 8

5

12

UF-NS2

Fig. 8. X-ray diffraction of tested

to be relatively larger in case of UF-II cement, after 28 days ofhardening.

The SO2�4 stretching vibrations (v3) are found in the range be-

tween 1100 and 1170 cm�1. Ettringite was clearly identified, espe-cially in case of the UF and UF-NS4 cements (at both ages), by thesharp absorption band in the range 1116–1118 cm�1. The absorp-tion in this range for the pastes of CEM I 42.5N is lower.

The progress of hydration was evident by the polymerization ofthe SiO4 units during the hydration of calcium silicate phases. Ashift of the Si–O asymmetric stretching vibration (v3) was observedto higher wave numbers (926–980 cm�1). The appearance of abroad absorption hump at 970–1100 cm�1, after 1 day of curing,is due to polymeric silica. It is correlated with the developmentof water bending vibration bands (1500�1 to 700 cm�1). This

30 35 40 45 50 55

2

1. Ca(OH)22. Ca6Al2(SO4)3(OH)2 26H2O3. CaCO34. C2S, C3S5. Ca2(Al,Fe)2O56. Ca3Al2O6 7. MgO8.Ca4Al2O6CO3 11H2O9. Ca4Al2O6(CO3)0.5(OH) 11.5 H2O10. Ca1.5 SiO3.5 xH2O11. Ca2SiO4 0.35H2O12. Ca2SiO4 xH2O

1

125 1 1

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cements hydrated at 1 day.

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CEM I42.5N

1. Ca(OH)22. Ca6Al2(SO4)3(OH)2 26H2O3. CaCO34. C2S, C3S5. Ca1.5SiO3.5 xH2O6. Ca2SiO4 0.35H2O7. MgO8.Ca4Al2O6CO3 11H2O9. Ca4Al2O6(CO3)0.5(OH) 11.5 H2O10. Ca2SiO4 xH2O11. Ca3(Si3O8(OH)2 )

1

1

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2 22

3,4

2,3

11

4,5

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85

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443 3

1

Fig. 9. X-ray diffraction of tested cements hydrated at 28 days.

354 F. Kontoleontos et al. / Construction and Building Materials 35 (2012) 347–360

implies the formation of calcium silicate hydrate (C–S–H). The rel-ative intensities of the Si–O bending vibrations (�460 cm�1) indi-cate also significant changes in the nature of calcium silicatephases, as a result of polymerization of the SiO4. After 28 days ofcuring, in case of CEM I 42.5N and UF cements, the intensity peakof Ca(OH)2 seems to have been increased. However, the portlanditeband was diminished in case of UF-NS4, because of its consumptionby the colloidal nanosilica, which reacts with calcium hydroxide,produced by the hydrolysis of alite and belite.

The characteristic bands of calcium carbonate could be found inthe range of 1424–1430 cm�1 (v2 + v3 band) and 874–885 cm�1 (v4

band), especially after 28 days of hydration. As was mentionedabove, carbonation of samples is supposed to occur during thestoring of samples previous to perform the hydration productscharacterization.

Fig. 10. FT-IR spectra of cements

3.5. Microstructure observations of the hydration products by SEM

Figs. 12 and 13 show backscattered electron images of pastespolished sections, after 1 and 28 days of hydration, for the UFand UF-NS4 cements, respectively. It is possible to distinguish thefollowing five phases. (a) Unreacted anhydrous cement grains,which are the brightest phases, (b) Ca(OH)2, which is slightly dar-ker than anhydrous grains, (c) Inner C–S–H, which are observed asrim around the anhydrous grains [21], (d) Outer C–S–H, the hydra-tion products which fills the cementitious matrix, and (e) porosity(the darkest phase).

Ca3SiO5 was detected mainly at 1 day samples, whereas after 28of curing only Ca2SiO4 was observed. The dark gray regions formedthe matrix, in which the unreacted clinker phases were embedded,were mainly consisted of hydration products. The nanoparticles lo-

pastes after 1 day of curing.

Fig. 11. FT-IR spectra of cements pastes after 28 days of curing.

(a) Alite (Ca3SiO5) (b) Belite (Ca2SiO4)

Fig. 12. BSE micrographs of UF cement pastes at (a): 1 day and (b): 28 days of curing.

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cated in cement paste promote cement hydration due to their highactivity. This makes the cement matrix more homogeneous andcompact, whereas, the pore structure is improved. At 28 days, asthe hydration reaction has proceeded considerably, a reduction inthe amount of anhydrous cement grains and the pore contentwas observed. The anhydrous cement reacts to give hydratedphases (C–S–H and portlandite), which fill the pores. C–S–H nearthe cement grains is much denser and stronger, while the densityof the C–S–H is much more uniform.

In case of UF-NS4 sample, it should be noticed the detection ofsecondary hydration products between portlandite, produced from

the calcium silicate phases hydration, and colloidal nanosilica,which seems to be bounded in the Ca(OH)2 matrix. The presenceof nano-SiO2 led to the filling of the pores and to the consumingof Ca(OH)2 (pozzolanic reaction-CSH formation), thus improvingthe mechanical properties of the mortars.

The typical microstructures (SEM) of fracture surfaces of UF andUF-NS4 cements pastes, at 1 and 28 days of curing, are shown inFigs. 14 and 15. The main hydration products are amorphous, fi-broid and lumpy products. The C–S–H consists of an irregular corewith characteristic peripheral fibrous outgrowth. In case of UF ce-ment typical hexagonal plates of Ca(OH)2 can be observed inside

(a) Hydration of calcium silicate grain (b) Secondary hydration products (portlandite with silica)

Fig. 13. BSE micrographs of UF-NS4 cement pastes at (a): 1 day and (b): 28 days of curing.

(a) Needle-like ettringite (b) Flaky C-S-H

Fig. 14. SEM micrographs of UF cement pastes at (a): 1 day and (b): 28 days of curing.

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(a) Needle-like ettringite (b) Secondary CSH formation inside the pores

Fig. 15. SEM micrographs of UF-NS4 cement pastes at (a): 1 day and (b): 28 days of curing.

Table 7Results from mercury intrusion porosimetry.

Property Testedcement

CEM I42.5N

UF UF-NS4

Days ofcuring

Total pore volume (mm3/g) 1 95 93 9028 67 50 45

Pore volume in radius of 2–150 nm(mm3/g)

1 77.6 87.1 80.528 64.9 47.2 40.8

Total specific surface area (mm2/g) 1 14 16 12.528 13 12.5 10.5

Specific surface area in radius rangeof 2–150 nm (mm2/g)

1 13.9 16 11.928 13 12.5 10.5

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the capillary pores. At 1 day, the formation of ettringite is still inprogress. Very thin and long needle-like ettringite was detectedmainly inside the pores. The C–S–H gel has formed a dense net-work structure. The finer particles together with hydration prod-ucts are used to fill the voids among coarse particles and to forma denser packing structure. The increase of fineness led to the pro-duction of more fine particles, making the cementing gel denser,the porosity lowered and pore structure modified, and hence themechanical strength improved.

The presence of colloidal nanosilica, which is dispersed amongcement fine grains, seems to act as ‘‘nucleus’’ for the hydrationproducts development [16]. At early ages, the main hydration com-ponent is C3S. This is accomplished by the increased of Ca(OH)2

crystallization. The main role of nanosilica on this stage is thatthe previously dissolved surface of SF or the previously formedgel-like, silica-rich, calcium-poor intermediate phase providesnucleating sites for the precipitation of hydration products, espe-cially lime. The microstructure appears more homogeneous than

in the pastes without nanosilica, mainly due to its pozzolanic reac-tion with portlandite and the production of C–S–H. The hydrationproducts are precipitated on the nano-particles, which afterwardsreact with them around their transition zone, leading to a densermicrostructure. At 28 days of curing most of the cement grains ap-pear to have hydrated and the porosity has been decreased. Most ofthe large pores were completely or partially filled with hydrationproducts.

Nano-SiO2 particles act as nano-fillers and fill the empty poresamong particles of C–S–H gel.

The pozzolanic reaction led to the consumption of portlanditefrom nano-SiO2, a process which is contributing to secondary C–S–H generation.

3.6. Mercury intrusion porosimetry

The results of total cumulative pore volume and specific surfacearea are summarized in Table 7. Figs. 16–18 present the cumula-tive and the differential pore size distributions of CEM I 42.5N,UF and UF-NS4 cements, after 1 and 28 days of curing. In all cases,the differential curves for pastes cured at early ages tend to exhibita sharply defined initial peak, indicating a unimodal distribution ofpore sizes. As curing time increases, a second peak appears at smal-ler pore sizes thus suggesting a bimodal distribution. The first peakis related to the lowest size of pore necks connecting a continuoussystem. The second peak seems to correspond to the pressure re-quired to break through the blockages formed by the hydrationproducts which isolate the interior pore space.

After 24 h of hydration, it is possible to observe that pore sizedistribution curves of CEM I 42.5N pastes typically exhibit at leasttwo peaks. The first one lies approximately at the 5 nm, while thesecond and sharpest one corresponds to a pore radius around 30–75 nm. In case of UF and UF-NS4 cements the peaks are shifted to

Fig. 16. Cumulative and differential pore size distribution curves for CEM I 42.5N pastes at 1 and 28 days.

Fig. 17. Cumulative and differential pore size distribution curves for UF pastes at 1 and 28 days.

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smaller pore radius. After 28 days of curing, in CEM I 42.5N pastes,there was a larger penetration of mercury in the pore range of 100–200 nm, corresponding to the mean pore diameter of 150 nm. Onthe other hand, the pore size distribution curve for UF shows thathigher finesse led to the increase of the portion of smaller pores.The cement with finer particles made more contact surfaces andpresented smaller hydraulic radius, than the cement with thecoarse grains.

The addition of colloidal nanosilica led to the decrease in totalporosity, as well as to the decrease in the average pore diameter.As the hydration proceeded, the larger pores were converted intosmaller ones. They gradually fill with secondary calcium–silicatehydration products, due to the portlandite–nanosilica reaction.The transformation of large pores into finer pores, i.e., pore sizerefinement as a result of pozzalanic reaction led to the enhance-ment of compressive strengths. The presence of nanosilica cement

Fig. 18. Cumulative and differential pore size distribution curves for UF-NS4 pastes at 1 and 28 days.

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caused considerable reduction in the volume of large pores at allages and is therefore instrumental in enhancing the compressivestrength. In case of UF cement, the larger pores are located in ra-dius range of 30–100 nm, whereas the corresponding values forUF-NS4 were 15–30 nm.

4. Conclusions

The addition of colloidal nano-SiO2 in ultrafine cements marks anovelty in the high performance cements technology. Colloidalnanosilica behaved not only as a filler to improve cement micro-structure (porosity decrease), but also as a promoter of pozzolanicreaction by transforming portlandite into C–S–H gel. Among fourages and three sets of mix proportions, the optimal mix proportionis the set of 4% NS addition, which presented the highest compres-sive strength of 78.9 MPa, after 28 days of curing. The addition ofcolloidal nanosilica did not lead to an immediate mechanical gain.At early ages, it slightly increased the strength of the ultra fine ce-ment, mainly because of the packing effect. The enhancement ofthe compressive strength at later ages was due to the consumingof Ca(OH)2 by nanosilica (hydration evolution). On the other hand,initial and final setting times and workability decreased with theincrease of SF, mainly because of the rapid agglomeration of theslurry from the suspending state. Nano-silica improved the packingof particles, decreasing the volume among them (free water de-crease), leading to an internal friction increase between solid par-ticles, which contributes to the growth of paste viscosity (waterdemand increase).

Nano-SiO2 was proved an agent that improves the microstruc-ture of the ultra fine cement paste. The cements with nanosilicapresented a denser microstructure. At early ages, the main hydra-tion component was C3S, whereas silica nanoparticles providednucleating sites for the precipitation of hydration products (espe-cially Ca(OH)2). At later ages, nanosilica modified the internalstructure of the C–S–H gel, increasing the average chain length ofthe silicate chains, leading to a denser structure. Ca(OH)2 evolutionwas diminished due to the pozzolanic reaction, whereas the large

pores were partially or completely filled with hydration products(especially secondary C–S–H).

Both the total porosity and the average pore diameter, deductedby mercury intrusion porosimetry also confirm a denser micro-structure for the hardened cement paste with nanosilica. The poresize refinement at later ages, as a result of the pozzalanic reaction,also led to a significant enhancement of the compressive strengths.

Acknowledgements

This work was done in cooperation with technical support ofEKET – Central Laboratory of Heracles GCC. The authors expresstheir warmest thanks to Mr. J. Marinos, Director in Quality Assur-ance and R&D Manager of Heracles GCC for his constant advice.

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