Preparation of size controlled silica nano particles and itsfunctional role in cementitious systemLok P. Singh , Sriman K.Bhattacharyya SaurabhAhalawat, ,Journal of Advanced Concrete Technology, volume ( ), pp.10 2012 345-352

XRD/Rietveld analysis of the hydration and strength development of slag and limestone blended cementSeiichi Hoshino, KazuoYamada Hiroshi Hirao,Journal of Advanced Concrete Technology, volume ( ), pp.4 2006 357-367

Relation between the shape of silica fume and the fluidity of cement paste at low water to powder ratiosEtsuo Sakai, YasuoKakinuma Kenji Sakamoto, , Masaki DaimonJournal of Advanced Concrete Technology, volume ( ), pp.7 2009 13-20

Effect of the methylene blue value of manufactured sand on performances of concreteBeixing Li, Mingkai Zhou Jiliang Wang,Journal of Advanced Concrete Technology, volume ( ), pp.9 2011 127-132

Journal of Advanced Concrete Technology Vol. 10, 345-352, November 2012 / Copyright © 2012 Japan Concrete Institute 345

Scientific paper

Preparation of Size Controlled Silica Nano Particles and Its Functional Role in Cementitious System Lok P. Singh1, Sriman K. Bhattacharyya1 and Saurabh Ahalawat1

Received 17 January 2012, accepted 28 October 2012 doi:10.3151/jact.10.345

Abstract Dispersed, spherical particles of nano silica (NS) with controllable size have been synthesized using a metal alkoxide, tetraethoxysilane, as starting material and ammonia as base catalyst using sol-gel method. The size of particles and dis-persivity were controlled by varying the solvent and temperature conditions of the reaction mixture. NS was synthesized using four different alcohols as solvent viz., methanol, ethanol, propanol and butanol at different reaction temperatures 25°C, 50°C, 70°C and 90°C. The results showed that the size of NS became larger and more uniform with the increase of the reaction temperature. The particle size of NS gradually increased with increasing carbon chain length of the sol-vent. Further, the compressive strength and setting time of cement pastes have been examined by addition of NS (~ 100 nm) of varied contents (0.25 %, 0.5%, 1.0%, 2.5% and 5.0%). The experimental results of these studies showed that the compressive strength of fresh cement paste was increased by increasing the content of NS up to 5% in cement paste. The setting time of fresh cement paste was decreased by increasing the content of NS. Characterization of synthesized NS particles were carried out using scanning electron microscopy (SEM) powder x-ray diffraction (XRD), 29Si MAS NMR, infra-red spectroscopy (IR) and microstructure of cement paste incorporated with NS were analyzed using SEM, XRD and thermogravimetric analysis (TGA) for morphological and mineralogical attributes.

1. Introduction

Nanotechnology is gaining widespread attention and being applied in many fields to formulate materials with novel functions due to their unique physical and chemi-cal properties. In the construction sector, nanotechnol-ogy is being used in various ways to produce innovative materials. Using nanotechnology as a tool, it is possible to modify the nano/basic structure of the materials to improve the material's bulk properties such as mechani-cal performance, volume stability, durability and sus-tainability. Recently, researchers have shown enormous interest to investigate the properties of building materi-als such as mechanical, temperature & strain sensing, durability etc. with nano materials (Bjornstrom et al. 2002). There are other functionalities that can be ob-tained with the addition of nanomaterials to cementi-tious system. Self-cleaning facade with the addition of titania nanoparticles to the mortar (Vittoria et al. 2008), self sensing cementitious composites (Ying et al. 2007) and electromagnetic interference (EMI) shielding (Chung 2001) by the addition of conductive nanomate-rials viz., carbon nanotubes, carbon nanofibers or car-bon black have been explored. Use of nano-sized ingre-dients such as alumina and silica particles and under-standing of the hydration behavior are the upcoming research areas dealing with cement and concrete materi-als (Sorelli et al. 2008; Nazari et al. 2010; Li et al. 2004; Kim et al. 2007; Jo et al. 2007; Temiz and Kara-

keci 2002). Self-Compacting Concretes were manufac-tured by using ultrafine amorphous colloidal silica (Collepardi et al. 2002). The mechanical behavior of concrete depends on the nano-sized calcium-silicate-hydrate (C-S-H) gel produced as a result of hydration process in the cement. The partial replacement of ce-ment with nanophase particles improves the mechanical properties of cementitious system. The effect of nano silica to reduce the calcium leaching and refinement in pore structure of cement paste has been studied (Gaitero et al. 2008). The addition of nanosilica improved the performance of cementitious system by controlling the C-S-H degradation under aggressive condition of cal-cium leaching process.

The pores within the hydrated phases have a diameter ranging from nano level to micro level and strongly influence the properties, viz. strength, shrinkage, creep, permeability diffusion etc. (Bentz 2006; Aligizaki 2006). Problem in cementitious materials is mainly due to chlo-ride ingress and sulphate attack which occurs by the diffusion of water and gases through the pores leading to corrosion of reinforcement. Since the size of pores present in cement paste, ranges from few nanometers to micrometer, it is desirable to understand the hydration behavior of the cement based materials with the addition of nano particles in regard to mineralogical and morpho-logical attributes for better understanding of microstruc-ture of hydrated cementitious materials.

In the present work, we have reported the preparation of dispersed, stable and dry powder form of NS particles synthesized by simplified sol-gel method and the effect of addition of these nanoparticles in cement pastes for mineralogical and morphological attributes.

1CSIR-Central Building Research Institute (CBRI)Roorkee – 247 667 (Uttarakhand), India. E-mail: lpsingh@cbri.in

L. P. Singh, S. K. Bhattacharyya and S. Ahalawat / Journal of Advanced Concrete Technology Vol. 10, 345-352, 2012 346

2. Experimantal

2.1 Materials In the present studies, ordinary Portland cement (OPC), 43 grade, type I conforming to IS: 8112 was used. The chemical and mineralogical characteristics of the OPC are given in Table 1(a&b). Tetraethoxysilane (TEOS) (99.9% Alfa Aesar, UK), absolute ethanol, methanol, propanol and butanol (99.9% Merck, India), ammonia (25% Thomas Baker, India) were used without any fur-ther purification. Deionized water was used throughout the experiments. Chemical composition and physical properties of silica fume (SF) was shown in Table 2.

Most of the commercially available nanoparticles are either available in colloidal form or agglomerated dry powder. Therefore, recent interest in nanoparticles syn-thesis is to prepare dispersed, stable NS particles in dry powder form (Gaitero et al. 2008). In the present studies, sol-gel process was used as a method of producing nano silica particles with controlled size. The sol–gel process has been extensively used as a versatile method to syn-thesis of NS directly from molecular precursors under mild conditions (Martinez et al. 2006; Pinto et al. 2006). This well-established route involves base or acid cata-lyzed hydrolysis and condensation reactions of mono-meric alkoxide precursors in aqueous systems. This chemical synthesis technique also provides the particles of high purity and uniformity. Dispersed uniform sized approximately 100nm NS were prepared by hydrolysis of TEOS in ethanol in the presence of ammonium hy-droxide. For each experiments, 10 ml ethanol, 1 ml TEOS and 3 ml deionized water were added and stirred for 30 minutes. Ammonium hydroxide (1 ml) was then added drop wise to the mixture as a catalyst to promote the condensation reaction. Stirring was continued for a further 2h to get a white turbid suspension. The result-ing white powder was dried overnight and then calcined in air for a period of 2h at 800°C. The reaction was also performed at different temperature and using various solvents for controlling the particle size and dispersity of NS.

The consistency and setting time of cement pastes with and without NS were tested according to IS 4031. Consistency was ascertained by putting the paste in a mould consisting of a steel ring (40 mm in height) on a sheet of glass and by determining the penetration depth of a plunger applied to the top surface of the paste specimen. The initial and the final setting times were determined with the Vicat apparatus. The fresh cement pastes with varying composition of NS (0.25, 0.5, 1.0, 2.5, and 5.0%) were casted with moulds (25 x 25 x 25 mm) to prepare specimens for the measurement of com-pressive strength. After being demoulded at the age of 24 h, all specimens were cured in water and three cubes were tested at a given age (1, 3, 7 & 28 days) using HEICO make UTM.

Morphological attributes of calcined NS and hydrated cement paste were measured using Scanning Electron

Microscope (LEO-438 VP) at an accelerating voltage of 20 kV. The samples were analyzed under Variable Pres-sure (VP) mode by depositing on a sample holder with a double stick conducting carbon tape. Powder X-ray dif-fractions were measured at 30kV and 40 mA on a Ri-gaku D-Max 2200 X-ray Diffractometer using Cu-Kα radiation at a scanning rate of 1˚/min. TGA studies were performed on a Perkin Elmer (TG/DTA) analyzer at a heating rate of 20 °C/ min. 29Si MAS measurements were carried out on a Bruker AV 300 WB NMR spec-trometer operating at 59.63 MHz for 29Si using a 4mm double resonance probe. The samples were packed in a 4 mm zirconia rotor and were spun at 10 kHz and the data were acquired with a spectral width of 30 KHz, 5 sec relaxation delay using a single pulse excitation se-quence (45° flip angle) for MAS experiments. A contact time of 5 MSec was employed for MAS experiments. Approximately 3000 to 5000 transients were acquired and the raw data were processed with 25 Hz line broad-ening prior to Fourier Transformation. The chemical shifts were referred to an external sample of DSS (4,4-dimethyl-4-silapentane-1-sulfonic acid). Various chemi-cal bonding of silica nanoparticles were studied with Thermo Nicolet NEXUS FTIR. Each IR spectra were collected from 400-4000 cm-1. Specific surface area of synthesized NS was measured by Brunauer Emmett and Teller (BET) (Microtrac SAA). For the preparation of sample for SSA, the sample is vacuum dried at 105 °C for 2h. Nitrogen gas was used to determine the SSA of

Table 1(a) Chemical properties of OPC. Items Content (%)

SiO2 22.3 Al2O3 6.1 Fe2O3 2.2 CaO 61.1 MgO 3.1 SO3 2.1 LOI 2.3

Table 1(b) Mineralogical properties of OPC.

Phases Content (%) C3S 49.4 C2S 27.2 C3A 10.9 C4AF 11.3

Table 2 Chemical composition and physical properties of SF.

Items Content (%) SiO2 92.70 Al2O3 2.04 Fe2O3 1.08 CaO 0.25 MgO 0.38 SO3 0.24

Average diameter ~500 nm Specific surface area 2900 m2Kg-1

L. P. Singh, S. K. Bhattacharyya and S. Ahalawat / Journal of Advanced Concrete Technology Vol. 10, 345-352, 2012 347

dry powder. As nitrogen gas is passed over the powder a monolayer of Nitrogen molecules is adsorbed onto the surface of the particles. Adsorbed nitrogen on sample surface is measured by dynamic chromatograph. Through this Nitrogen Adsorption Method (Dynamic Flow) the surface area measured.

3. Results and discussion

3.1 Preparation of silica nano particles (NS) The size and morphology of silica particles via sol-gel method were controlled by parameters such as concen-tration of silica precursor, catalyst, solvent, reaction temperature, aging time, admixture etc. At the same concentration of all reagents, the effect of temperature on the particle size and dispersivity of NS was investi-gated. SEM micrographs of NS synthesized at different temperature 25°C, 50°C, 70°C and 90°C are illustrated in Fig. 1. Particle size becomes larger and more uniform with the increase of the temperature as silica particles are stabilized by electrostatic repulsion. The charge originates from silanol groups, which are relatively acidic and dissociate in the presence of ammonia. Under basic conditions, TEOS undergoes hydrolysis and poly-condensation reaction which results in the formation of

dispersed spherical particles of amorphous silica. The base catalyst hydrolysis occurs by a nucleophilic

substitution mechanism, where the hydroxyl anion at-tacks the silicon atom (Aelion et al. 1950; IIer 1979). The rate of TEOS hydrolysis is affected by the nature of the solvent and this has been attributed to both steric effect and hydrogen bonding (Byers et al. 1987; Artaki et al. 1985). The hydrolysis rate is faster in longer chain alcohols such as butanol than when using short chain alcohols such as methanol. The strong hydrogen bond-ing existing between the water molecules and the lower chain alcohols reduces the effective availability of water molecules and thus the hydrolysis rate, probably by re-ducing the mobility of OH- ions. At the same concentra-tion of all reagents, the effect of various alcohols as sol-vent such as MeOH, EtOH, PrOH and BuOH on the particle size and dispersivity of NS was studied. Effects of various alcohols as solvents on silica particle size and distribution are shown in Fig. 2. As the chain length of alcohol increased from MeOH to BuOH, the average particle size of silica nano particles increased. Generally, the observed particle size distribution was narrow, ex-cept for BuOH, whose silica particles were distributed bimodally with sizes of 250 nm and 600 nm. The alco-hol has a function of co-solvent to help mixing silicon

A

B

C

D

Fig. 1 SEM micrograph of NS particles (100-250nm) prepared at reaction temperature of 25°C (A), 50°C (B), 70°C (C) and 90°C (D).

L. P. Singh, S. K. Bhattacharyya and S. Ahalawat / Journal of Advanced Concrete Technology Vol. 10, 345-352, 2012 348

alkoxide and water. The used alcohols except BuOH are well miscible with water, however, BuOH is less misci-ble and that’s why the reaction system using BuOH is heterogeneous and gives bimodal particles of sizes 250 and 600nm.

The specific surface area (SSA) of these NS particles was obtained by BET method. Data presented in Table 3 clearly indicates that at NS particles prepared at 25°C and using MeOH as solvent exhibit higher surface area (~10,600 m2/Kg) and particle size is approximately 50 nm. Figure 3 shows the 29Si MAS NMR spectra of syn-thesized NS. The 29Si MAS NMR spectra of silica

nanoparticles show the T and Q sites. The signals at approx -113.0 and -103.0 ppm arise from the Si species Q4 (Si(OSi)4) and Q3 (Si(OH)(OSi)3), respectively. A low-intensity peak at -94.20 ppm arises from chemical shift correlation and relaxation data to germinal-hydroxyl silanol sites (Beganskiene et. al. 2004). A rep-resentative IR spectrum of synthesized silica nanoparti-cles is shown in Fig. 4 and spectral assignments of the NS are shown in Table 4. Presence of adsorbed water and free surface silanol groups as well as siloxane link-

MeOH

EtOH

PrOH

BuOH

Fig. 2 SEM micrographs of NS particles prepared using different solvents MeOH, EtOH, PrOH and BuOH, respectively.

Table 3 Specific surface area (SSA) of NS synthesized different temperature and solvents.

Nanosilica SSA (m2Kg-1) Particle size (nm) NS (25 °C) 10,500 ~ 80 NS (50 °C) 6,200 ~ 100 NS (70 °C) 5,100 ~ 150 NS (90 °C) 3,800 ~ 250 NS (MeOH) 10,600 ~ 50 NS (EtOH) 10,500 ~ 80 NS (PrOH) 7,300 ~ 120 NS (BuOH) 6,600 ~ 280

Fig. 3 Si (29) MAS NMR spectra of synthesized NS.

L. P. Singh, S. K. Bhattacharyya and S. Ahalawat / Journal of Advanced Concrete Technology Vol. 10, 345-352, 2012 349

ages can easily be conceived from the IR spectra of NS in the range 400-4000 cm-1. In the spectra the broad band located in the range 3000-4000 cm-1 corresponds to the fundamental stretching vibration of different hy-droxyl groups. The broadness of this band suggests dif-ferent local environments of the OH groups. The band at 1631 cm-1 is assigned to the deformation mode of water molecules. Bands located at about 462, 802 and 1101 cm-1 are the bond rocking, bond bending and bond stretching vibrations of the Si-O-Si units.

3.2 The effect of NS on cement paste SEM micrographs of SF and NS were presented in Fig. 5. The effect of NS (100 nm) addition on the properties of cement pastes is presented in Table 5. The setting time of cement pastes containing NS was slightly accel-

erated but the time difference between the initial and the final setting time decreased with increasing the NS con-tent. Replacing cement with high specific surface area particles increases the wettable surface area and the amount of water adsorbed. Thus, the final water re-quirement will depend on which of the two above-mentioned factors will be superior. It is the surface ef-fects of nano-structured particles that cause the superi-orities of cement paste. NS provides much more nuclea-tion sites for hydration products at early ages and ex-hibit higher pozzolanic activity. Thus, NS accelerate the cement setting time and hydration process.

The two silicate phases of cement, tricalcium silicate

A

B

Fig. 5 SEM micrographs of SF (A) and NS (B).

Table 5 Mix proportions, compressive strengths and setting time of cement pastes. S.No. NS (~100nm) in

cement (%w/w) compressive strength

(kg/cm2) water - cement

ratio (w/c) Setting time

1d 3d 7d 28d Initial Final 1. 0.0 % 244 392 417 548 0.30 2h29m 3h49m 2. 0.2 % 269 433 482 585 0.30 2h26m 3h30m 3. 0.5 % 358 436 527 589 0.31 2h20m 3h20m 4. 1.0 % 364 459 535 592 0.31 2h16m 3h12m 5. 2.5 % 371 465 562 680 0.32 2h13m 3h08m 6. 5.0 % 401 528 581 741 0.33 2h09m 2h58m

Fig 4 IR spectra of synthesized NS.

Table 4 Assignment of IR spectral data of synthesized NS. Wave number (cm−1)

Position assignment Literature value (cm-1)

462 Si–O bond rocking 475–465 800 OH bending (silanol) 870–800

980–970 Si–OH bond str. 980–935 1102 Asymmetric Si-O-Si

stretching in SiO4 tetrahedron 1115–1050

1630 O-H bending (molecular water) 1625

3000–4000 O-H str. and adsorbed water 3000–3800

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and dicalcium silicate, give C-S-H and calcium hydrox-ide (CH) as hydration products, which constitute ap-proximately 50-70% and 20-25% of the total volume of the hydrated product, respectively. The C-S-H gel being the main component of cement hydration is responsible for the strength development and microstructure of the cement paste. Porosity has a major effect on the durabil-ity of cementitious materials as the pores inside the hy-dration products accumulate between the liquid phase and the anhydrous cement grains. When superfine or extremely small particles are added to cement or con-crete, it acts as the micro-filler, which reduce the amount of water filled in the void of the blending mate-rials. The large surface area of nanoparticles makes them more reactive, facilitates the chemical reactions with CH and produces a denser cement matrix by form-ing an additional C-S-H gel. The mechanical properties of cement are enhanced by the addition of NS as com-pressive strengths after 1, 3, 7 and 28 days are substan-tially enhanced (Table 5). The nanoparticles have high surface energy and atoms at the surface have a high ac-tivity, leading the atoms to react rapidly. The compres-sive strength of cement paste containing 5% NS is 64% higher at 1 day & 35% at 28 days than that of control cement paste. The difference in the strength develop-ment of the paste is attributed to the pozzolanic reaction of NS with CH and forming additional C-S-H. To verify this mechanism of increased compressive strength by additional formation of C-S-H, SEM, XRD and TGA studies were carried out for the morphological and min-eralogical attributes of cement pastes.

SEM micrographs of plain cement paste and with NS (5%) at 1, 3, 7 and 28 days are shown in Fig. 6. It was observed that in the microstructure of the plain cement paste, the C-S-H gel existed along with needle and plate shaped hydrates of CH. The deposited CH around the C-S-H gel is uniformly distributed among the entire ce-ment phase (Fig. 6). However, the microstructure of the cement paste with the addition of NS revealed that the formation of hydration products was denser, becomes significantly different and showing absence of the hex-agonal plate shaped crystals of CH. After 1 day of hy-dration, plain cement paste shows some formation of C-S-H, whereas 5% NS incorporated paste shows rela-tively larger amount of C-S-H gel. Small plate shape crystals of CH are also observed after 1 day of hydration in plain cement paste. After 3 and 7 days of hydration, the microstructure of cement paste becomes denser due to the formation of additional C-S-H gel, however nee-dle and plates like structure of CH are also clearly pre-sent. Further, at 28 days of hydration SEM micrographs clearly show the presence of needle type crystal of CH throughout plain cement paste, whereas the NS incorpo-rated cement paste shows a denser and packed C-S-H gel as compared to pure cement paste.

CH amount in cement system was determined by measuring the mass loss between 405-515 °C in TGA curves. The amount of CH in the specimen is calculated

using the following equation:

CHCH

H

MWCH(%) WL (%)

MW= ×

where WLCH corresponds to the weight loss attributable to CH dehydration and MWCH and MWH are the mo-lecular weights of CH (74.01 g/mol) and water (18 g/mol), respectively. CH content in various cement paste during the hydration process is shown in Table 6. At early stage of hydration plain cement paste has 4.1% of CH whereas SF incorporated cement paste has 2.1% and NS incorporated paste has only 0.4% CH content. Dur-ing the hydration, CH forms and at 3, 7 and 28 days it amounts to 7.4%, 12.3% and 20%, respectively in plain cement paste. Whereas the CH content, in SF incorpo-rated cement paste is up to 15.9% at 28 days. NS have much significant effect as compared to plain and SF incorporated cement paste and at 28 days of hydration only 8.2% CH content was observed.

XRD analyses were conducted to investigate the min-eralogical composition of cement paste after 1 and 28 days of hydration. For comparison, the peak of CH at 18° (2θ) has been selected. As shown in Fig. 7 (a&b), a sharp peak of CH is observed in the control cement paste representing the CH. Evidently, the intensity of the CH peak is decreased by adding silica fume and signifi-cantly reduced when NS is added, which reflects the consumption of CH by pozzolanic reaction. The poz-zolanic reactivity of material depends on the particle size. Nano-silica has a pozzolanic reactivity which is much higher compared to silica fume due to the high surface to volume ratio of ultra-fine particles. Therefore, the pozzolanic reactivity of NS at early stage of hydra-tion is significantly high and accelerated strength devel-opment at early ages, thereby enhancing the durability and mechanical properties of the cementitious materials.

4. Conclusions

Dispersed, amorphous, spherical silica nano particles (~50-300 nm) can be prepared by the hydrolysis reac-tion of TEOS in ethanol using ammonia as base catalyst by sol-gel method.

The particle size of NS can be controlled by using different alcohols as solvent and varying the reaction temperature. The results showed that the NS became larger and more uniform in size with the increase of the

Table 6 Calcium hydroxide content (%) in cement pastes.

CH content * (%) at

1 day 3 days 7days 28 daysPlain cement paste 4.1 7.4 12.3 20.0 Cement + SF (5%) 2.1 5.7 9.4 15.9 Cement + NS (5%) 0.4 3.3 4.9 8.2 * Standard Deviation ±0.30

L. P. Singh, S. K. Bhattacharyya and S. Ahalawat / Journal of Advanced Concrete Technology Vol. 10, 345-352, 2012 351

temperature. The particle size of NS gradually de-creased with increasing carbon chain length of alcohol MeOH to BuOH.

The compressive strength of cement paste containing 5% NS is 64% higher at 1 day & 35% at 28 days than that of control cement paste.

It was observed from SEM, XRD and TGA studies that addition of NS to cement reduced CH leaching by reacting at early stage of hydration and forming addi-tional C-S-H gel. Therefore, addition of nanoparticles

significantly improves the mechanical properties of the cementitious materials.

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Fig. 7a XRD profiles of cement pastes at 1 day of hydration. Fig. 7b XRD profiles of cement pastes at 28 days of hydration.