Hybrid effect of carbon nanotube and nanonext term-clay on phys

Hybrid effect of carbon nanotube and nano

-clay on physico-mechanical properties of cement mortar

M.S. Morsy ^,^a^,, S.H. Alsayed ^a and M. Aqel ^a

^a King Saud University, College of Engineering, Specialty Units for Safety & Preservation of Structures P.O. Box 800, Saudi Arabia

Received 7 March 2010;

revised 5 May 2010;

accepted 19 June 2010.

Available online 15 July 2010.

Abstract

In this work, several nanomaterials have been used in cementitious matrices: multi wall carbon nanotubes (MWCNTs) and nano-clays. The physico-mechanical behavior of these nanomaterials and ordinary Portland cement (OPC) was studied. The nano-clay used in this investigation was nano-kaolin. The metakaolin was prepared by thermal activation of nano-kaolin clay at 750 °C for 2 h. The organic ammonium chloride was used to aid in the exfoliation of the clay platelets. The blended cement used in this investigation consists of ordinary Portland cement, carbon nanotubes and exfoliated nano metakaolin. The OPC was substituted by 6 wt.% of cement by nano metakaolin (NMK) and the carbon nanotube was added by ratios of 0.005, 0.02, 0.05 and 0.1 wt.% of cement. The blended cement: sand ratio used in this investigation was 1:2 wt.%. The blended cement mortar was prepared using water/binder ratio of 0.5 wt.% of cement. The fresh mortar pastes were first cured at 100% relative humidity for 24 h and then cured in water for 28 days. Compressive strength, phase composition and microstructure of blended cement were investigated. The results showed that, the replacement of OPC by 6 wt.% NMK increases the compressive strength of blended mortar by 18% compared to control mix and the combination of 6 wt.% NMK and 0.02 wt.% CNTs increased the compressive strength by 29% than control.

Keywords: Carbon nanotube; Nano-clay; Cement mortar; Thermal analysis; Compressive strength; Microstructure

1. Introduction

The mechanical behavior of concrete materials depends on structural elements and phenomena that occur in a micro and a

nano

scale. As a result, nanotechnology can modify the molecular structure of concrete which leads to improvement in the material’s bulk properties. Nanotechnology can also improve the mechanical performance, volume stability, durability, and sustainability of concrete. The revolutionary effects accompanying nanotechnology allows the development of cost-effective, high-performance, and long-lasting products of cement and concrete which can lead to unprecedented uses of concrete materials.

One of the most desired properties of nanomaterials in the construction sector is their capability to confer a mechanical reinforcement to cement based structural materials. When using nanomaterials three main advantages are considered. The first advantage is the production of high-strength concrete for specific application. The second advantage is to reduce the amount of cement needed in concrete in order to obtain similar strengths and decreasing the cost and the environmental impact of construction materials. The third advantage is reducing the construction periods as nanomaterials can produces high-strength concrete with less curing time.

Carbon nanotubes (CNTs) are hollow tubular channels, formed either by one single walls carbon nanotube (SWCNTs) or malty walls carbon nanotube (MWCNTs) of rolled graphene sheets [1] and [2]. They have received an increasing scientific and industrial interest due to their physical and chemical properties that is suitable for different potential applications ranging from living matter structure to nanometer-sized computer circuits and composites [3] and [4]. Since CNTs exhibit great mechanical properties along with extremely high aspect ratios (length-to-diameter ratio) ranging from 30 to more than many thousands, they are expected to produce significantly stronger and tougher cement composites than traditional reinforcing materials (e.g. glass fibers or carbon fibers). In fact, because of their size (ranging from 1 nm to 10 nm) and aspect ratios, CNTs can be distributed in a much finer scale than common fibers, giving as a result a more efficient crack bridging at the very preliminary stage of crack propagation within composites. However, properties and dimensions of CNTs are strongly depend on the deposition parameters and the nature of the synthesis method, i.e., arc discharge [5], laser ablation [6], or chemical vapor deposition (CVD) [6] and [7]. Carbon nanotubes used in this investigation were produced by arc discharge technique with diameters from 3 up to 8 nm and different length. The high specific strength, chemical resistance, electrical conductivity and thermal conductivity of carbon nanotubes (CNTs) make them attractive for use as reinforcement to develop superior cementitious composites [8] and [9].

Metakaolin has been recently introduced as a highly active and effective pozzolan for the partial replacement of cement in concrete. It is an ultrafine material produced by the dehydroxylation of a kaolin precursor upon heating in the temperature range of 700–800 °C [10]. Metakaolin is a silica-based product that, on reaction with Ca(OH)₂, produces CSH gel at ambient temperature. Metakaolin also contains alumina that reacts with CH to produce additional alumina-containing phases, including C₄AH₁₃, C₂ASH₈, and C₃AH₆ [11] and [12]. Research results have shown that the incorporation of metakaolin in concrete significantly enhances early strength [13]. Metakaolin increases resistance of concrete to alkali-silica reaction [14], and its effect on sulfate resistance increases systematically with increasing the replacement ratio of cement by metakaolin [15]. Energy absorption or toughness of high-performance steel–fiber-reinforced concrete is increase with the introduction of high-reactivity metakaolin into the mix. Therefore, for applications where both enhanced durability and high toughness are required, the use of high-reactivity metakaolin concrete may be advantageous [16]. However, other research has also shown that increasing replacement levels of metakaolin produce increasing water demand, although this can be adjusted by adding a water reducer to maintain the workability or flow properties [17].

Possessing nanoscale dimensions similar to calcium silicate hydrates (C–S–H in cement nomenclature) which is the glue that holds a cementitious matrix together, carbon nano fiber (CNFs) are expected to affect the nanoscale processes that control C–S–H and to create hybrid CNF/cement composites with improved performance. However, considerable research efforts are focusing on CNFs in polymer composites [18] and [19] while limited efforts have focus on their use in cement composites [20] and [21].

This research aimed to investigate the performance of hybrid CNTs/nano-clay cement mortar composites in terms of microstructure, physical, and mechanical properties.

2. Experimental work

2.1. Materials

The materials used in this study were nano-clay of Blaine surface area ≈48 m²/g and dimensions (200 × 100 × 20 nm), ordinary Portland cement (OPC), ASTM Type I [22]; supplied by Yamama Cement Company, Saudi Arabia .

The oxide composition of kaolin and ordinary Portland cement is shown in Table 1. The nano-clay used in this investigation was kaolin clay supplied by middle east mining investments company (MEMCO), Cairo, Egypt. The nano-kaolin was thermally treated at 750 °C for 2 h to produce active amorphous nano metakaolin.

Table 1. The chemical composition of starting material.

Oxide composition	Ordinary Portland cement (%)	Kaolin (%)
CaO	63.85	0.16
SiO₂	19.83	61.24
Al₂O₃	5.29	20.89
Fe₂O₃	3.53	6.38
MgO	0.52	0.38
SO₃	2.43	0.17
Na₂O	0.21	1.61
K₂O	0.07	0.71
TiO₂	–	0.7
P₂O₅	–	0.12
Ignition loss	2.82	13.62

The multi wall carbon nanotubes were synthesis in Egypt by arc discharge technique with sizes diameter from 3 up to 8 nm with different length.

2.2. Mortar preparation and identification

The blended cement used in this investigation was ordinary Portland cement, nano metakaolin and CNTs. The OPC was substituted by 6% of nano metakaolin by weight [23]. Dispersant solution was prepared by adding organic ammonium chloride solution containing 7 mg/g of clay. The nano metakaolin was added to the dispersant solution and mixed to insure homogeneously. The solution is covered and left for 24 h to insure that the clay plates had been exfoliated. The cement, exfoliated clay and CNTs were dry mixed for 5 min until homogeneity was achieved. The mortar was prepared using blended cement: sand ratio of 1:2 and water/binder ratio of 0.5% as illustrated in Table 2.

Table 2. The dry mixes composition of blended cement (mass %).

Mixes	OPC	NMK	CNTs
M0	94	6	0
M1	94	6	0.005
M2	94	6	0.02
M3	94	6	0.05
M4	94	6	0.1

The mortar pastes were molded into 5 cm cubes for compressive strength. The molds were vibrated for 1 min to remove any air bubbles. The samples were kept in molds at 100% relative humidity for 24 h, and then cured in water for 28 days. The hardened cement mortar was removed from water before mechanical tests. The compressive strength was performed on wetted specimens. The crushed samples resulted from compressive strength tests were grounded to be used for thermal and microstructure analysis. The evaporable water of the hydrated crushed samples was removed using the method described elsewhere [24].

2.3. Testing

2.3.1. Compressive strength

The compressive strength tests were performed on a Toni Tech machine using 50 mm cube samples according to ASTM C-109 [25]. Three samples per batch were tested, and the average strength value was reported. The loading rate on the cubes was 0.72 mm/min.

2.3.2. Differential scanning calorimeter

Differential thermal analysis conducted using a Shimadzu DSC 50 thermal analyzer at a heating rate of 20 °C/min. The samples chamber was purged with nitrogen at a flow rate of 30 cc/min.

2.3.3. Scanning electron microscope

The scanning electron microscope (FEL-Spectra) was used for identification of the changes occurred in the microstructure of the formed and/or decomposed phases. The SEM resolution was 4 nm.

3. Results and discussion

Fig. 1 shows the development of the mean values of compressive strength of blended mortar containing 6% exfoliated NMK versus CNTs ratios at 28 days of hydration. Basically, the compressive strength increased with the increase of CNTs until it reaches an optimal amount of 0.02% and then started to drop. Evidently, the replacement of OPC by 6% NMK in blended mortar increases the compressive strength by 18% compare to control mix. The enhancement of compressive strengths of hardened cement mortar due to the addition of NMK can be explained by two mechanisms. The first strengthening mechanism was the packing effect of small NMK acted as filler to fill into the interstitial spaces inside the skeleton of hardened microstructure of cement mortar which leads to increments in strength and density. The second strengthening mechanism was the pozzolanic effect that combines glass-like silicon and alumina elements in NMK with the lime elements of calcium oxide and hydroxide in cement to add the bonding strength and solid volume, resulting in higher compressive strength of hardened cement mortar.

Full-size image (27K)

Fig. 1.

Compressive strength of blended cement mortar containing exfoliated 6% NMK versus CNTs ratios at 28 days of hydration.

Most pozzolanic reaction between the calcium hydroxide and amorphous NMK (silicon and alumina dioxide) normally reacts slowly during a prolonged period of moist curing. Since the platelet particles of NMK have an average dimensions of (200 × 100 × 20 nm) which is about 1000 times finer than average cement particle of 20 μm resulting in an extremely large surface area, the NMK reacts very rapidly with the calcium hydroxide to form calcium silicate in an alkaline environment such as the pore solution of fresh Portland cement paste. Additionally, the improvement of cement mortar strength as CNTs loaded up to 0.02% was attributed to the crosslink of CNTs fiber with hydration product which lead to resist microcracks formation. Furthermore, at higher ratios of CNTs loaded the CNTs were agglomerated around cement grains leading to partial hydration of cement grains and producing hydrated product having weak bond. Also, the fibers may not be wetted properly thus causing fiber pullout resulting to formation and propagation of microcracks.

Fig. 2 illustrates the variations of the differential scanning calorimetry (DSC thermograms of blended cement mortar containing exfoliated 6% NMK versus CNTs ratios at 28 days of hydration. Evidently, there were almost five endothermic peaks. The first endothermic peak located at 95 °C, which was mainly due to the decomposition of calcium silicate hydrates (CSH).

Full-size image (34K)

Fig. 2.

DSC thermograms of blended cement mortar containing exfoliated 6% NMK versus CNTs ratios at 28 days of hydration.

The second endothermic peak observed at 174 °C represents the decomposition of the gehlenite hydrate (C₂ASH₈). The third endothermic peak located at 380 °C, represents the decomposition of hydrogarnet (C₃ASH₆). The fourth endothermic peak observed at 470 °C represents the decomposition of CH. The fifth endothermic peak appeared at 580 °C represents the decomposition of quartz. The mean features of the thermograms were characterized by a consumption of the peak area of CH and an increase of the peak area of CSH, C₂ASH₈ and C₃ASH₆ phases as the NMK loaded. The enthalpy of formed CH during hydration decreases from 35.73 J/g to 33.57 J/g for control and blended mortar containing 6% NMK respectively.

Also the presence of NMK in mortar pastes leads to an increase in the enthalpy of CSH from 90.21 J/g to 113.91 J/g, whereas the enthalpy of C2ASH8 increased from 0.421 J/g to 1.64 J/g and the enthalpy of C3ASH6 increases from 0.078 J/g to 0.283 J/g. Moreover, the addition of NMK leads to the transformation of CH phases from well crystalline to ill-crystalline. Therefore, the increase of phase’s enthalpy indicates the formation of well crystalline phases. Further, crystallizations produced by NMK and CH can fill up the pores and enhance microstructure and mechanical properties of cementitious materials. Furthermore, the loading of CNTs in cement at higher ratios decreases the enthalpy of CSH from 113.9 J/g at 0% to 92.69 J/g at 0.1% CNTs. At higher ratios of CNTs, the cement grains were wrapped by CNTs particles which leads to a partial separation of cement grains from hydration process. Basically, the partially hydrated cement grains decreases the formed hydrates and bond strength of cement pastes. On the other hand at lower CNTs ratios, the formed hydrates were well crystalline and amorphous phases as well as the CNTs bridges the hydration products and resist the formation of microcracks.

Fig. 3 shows the SEM micrographs of control and blended mortar containing NMK and CNTs hydrated for 28 days. Evidently, the microstructure of the OPC mortar displayed the existence of microcrystalline and nearly amorphous, mainly as calcium silicate hydrates (CSH). Furthermore, it can be seen calcium hydroxide (CH) crystals and air voids between the hydrated phases as shown in Fig. 3a. The SEM micrographs obtained for the NMK cement mortar indicated that the hydration products obtained were perfectly dense structure. The calcium hydroxide was appeared as ill-crystals as shown in Fig. 3b. Obviously, the pozzolanic reaction of NMK with calcium hydroxide liberated during hydration produced additional CSH gel and ill-crystals CH which leads to improvement in mechanical properties of blended mortar. The SEM micrographs of mortar containing NMK and CNTs are illustrated in Fig. 3c–f. It can be observed that the CNTs were dispersed uniformly in the cement mortar and there was no obvious aggregation of CNTs. The microscopic observation also reveals that the surface of CNTs was covered by CSH. The micrographs clearly demonstrated the dispersing potential of exfoliated NMK for CNTs in cement mortar. The CNTs were found embedded as individual fibers in the paste and acting as bridges between hydrates and across cracks (Fig. 3c and d). The NMK particles disrupted the fiber–fiber interactions (van der Waals forces) that held the CNTs together as clumps. The small size of the NMK particles compared to that of anhydrous cement particles (ca. 1000 times smaller) allowed them to work their way in-between the individual CNTs during the dry mixing process, causing the CNTs to separate from one another as mixing occurred, resulting in the separation of fibers.

Full-size image (117K)

Fig. 3.

SME micrograph of ordinary Portland cement mortar; (a) control mortar, (b) mortar containing 6% NMK, (c) mortar containing 6% NMK and 0.005% CNTs, (d) mortar containing 6% NMK and 0.02% CNTs, (e) mortar containing 6% NMK and 0.05% CNTs and (f) mortar containing 6% NMK and 0.01% CNTs.

The presence of the small NMK particles, both in the clumps that remained after dry mixing and intermixed with the individual, dispersed fibers was thought to provide a ready source of silicon for the generation of Ca–Si rich phases when combined with the highly mobile Ca²⁺ ions. The CNTs may have provided potential nucleation sites for the self-assembly of the Ca–Si rich phases. Basically, as the CNTs loaded increases in cement mortar the SEM micrograph indicated appearance of microcracks as presented in Fig. 3e and f. Therefore, at higher concentration the CNTs can be re-agglomerate and slide on each other when the microcracks formed which will leads to weak bond in the microstructure and produces lower strength.

4. Conclusions

The following conclusions may be drawn from the obtained experimental data.

• Replacement of OPC by 6% exfoliated NMK in cement mortar increases compressive strength by 18% compare to control mix.

• Due to its small particle size NMK facilitated CNTs dispersion and improved the interfacial interaction between the CNTs and the cement phases. The CNTs were found as well anchored in the hydration products throughout the paste.

• The addition of CNTs (up to 0.02%) to NMK cement mortar improves the compressive strength of the composites. The improvement was11% higher than blended mortar containing 6% NMK but the addition of CNTs by 0.1% leads to decreases the compressive strength.

Acknowledgements

The authors gratefully acknowledge Dr. M. Etman for carbon nanotube synthesis, HBRC, Cairo, Egypt and technical support from the concrete Laboratory of KSU University.

References

[1] K.T. Lau and D. Hui, The revolutionary creation of new advanced materials – carbon nanotube composites, Composites: Part B 33 (2002), pp. 263–277.

[2] N. Grobert, Carbon nanotubes becoming clean, Mater Today 10 (1–2) (2007), pp. 28–35. Article | PDF (4105 K) | View Record in Scopus | Cited By in Scopus (50)

[3] H. Dai, Carbon nanotubes: opportunities and challenges, Surf Sci 500 (2002), pp. 218–241. Article | PDF (1358 K) | View Record in Scopus | Cited By in Scopus (532)

[4] V.N. Popov, Carbon nanotubes: properties and application, Mater Sci Eng R 43 (3) (2004), pp. 61–102. Article | PDF (770 K)

[5] Y. Ando, The preparation of carbon nanotubes, Fuel Sci Technol 2 (2) (1994), pp. 173–180. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (23)

[6] W.K. Maser, A.M. Benito and M.T. Martinez, Production of carbon nanotubes the light approach, Carbon 40 (10) (2002), pp. 1685–1695. Article | PDF (680 K) | View Record in Scopus | Cited By in Scopus (25)

[7] C. Oncel and Y. Yurum, Carbon nanotube synthesis via the catalytic CVD method, a review on the effect of reaction parameters, Fullerene, Nanotubes, Carbon Nanostruct 14 (2006), pp. 17–37. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (20)

[8] Makar JM, Beaudoin JJ. Carbon nanotubes and their applications in the construction industry. In: Proceedings of the first international symposium on nanotechnology in construction, Paisley, Scotland; June 23–25 2003. p. 331–41.

[9] K.P. Chong and E.J. Garboczi, Smart and designer structural material systems, Prog Struct Mater Eng 4 (2002), pp. 417–430. Full Text via CrossRef

[10] D.S. Klimesch and A. Ray, Autoclaved cement–quartz pastes with metakaolin additions, Adv Cem Based Mater 7 (1998), pp. 109–118. Article | PDF (795 K) | View Record in Scopus | Cited By in Scopus (7)

[11] H. Changling, B. Osbaeck and E. Makovicky, Pozzolanic reaction of six principal clay minerals: activation reactivity assessments and technological effects, Cem Concr Res 25 (8) (1995), pp. 1691–1702.

[12] M.H. Zhang and V.M. Malhotra, Characteristics of a thermally activated alumina-silicate pozzolanic material and its use in concrete, Cem Concr Res 25 (8) (1995), pp. 1713–1725. Abstract | Article | PDF (841 K) | View Record in Scopus | Cited By in Scopus (56)

[13] S. Wild, J.M. Khatib and A. Jones, Relative strength, pozzolanic activity and cement hydration in superplasticised metakaolin concrete, Cem Concr Res 26 (10) (1996), pp. 1537–1544. Abstract | Article | PDF (525 K) | View Record in Scopus | Cited By in Scopus (101)

[14] Jones TR, Walters GV, Kostuch JA. Role of metakaolin in suppressing ASR in concrete containing reactive aggregate and exposed to saturated NaCl solution. In: Proc 9th int conf alkali–aggregate react concr, vol. 1; 1992. p. 485–96.

[15] J. Khatib and S. Wild, Sulfate resistance of metakaolin mortar, Cem Concr Res 28 (1) (1998), pp. 83–92. Article | PDF (424 K) | View Record in Scopus | Cited By in Scopus (40)

[16] A. Dubey and N. Banthia, Influence of high-reactivity metakaolin and silica fume on the flexural toughness of high-performance steel fiber reinforced concrete, ACI Mater J 95 (3) (1998), pp. 284–292. View Record in Scopus | Cited By in Scopus (15)

[17] J. Bai, S. Wild, B.B. Sabir and J.M. Kinuthia, Workability of concrete incorporating pulverized fuel ash and metakaolin, Mag Concr Res 51 (3) (1999), pp. 207–216. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (21)

[18] E. Hammel, X. Tang, M. Trampert, T. Schmitt, K. Mauthner and A. Eder, Carbon nanofibers for composite applications, Carbon 42 (5–6) (2004), pp. 1153–1158. Article | PDF (667 K) | View Record in Scopus | Cited By in Scopus (151)

[19] S. Kumar, T. Rath, R.N. Mahaling, C.S. Reddy, C.K. Das and K.N. Pandey, Study on mechanical, morphological and electrical properties of carbon nanofiber/polyetherimide composites, Mater Sci Eng B 141 (1–2) (2007), pp. 61–70. Article | PDF (800 K) | Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (29)

[20] Makar JM, Margeson J, Luh J. Carbon nanotube/cement composites – early results and potential applications. In: The third international conference on construction materials: performance, innovations and structural implications, Vancouver, BC; August 22–24 2005. p. 1–10.

[21] D.D.L. Chung, Comparison of submicron-diameter carbon filaments and conventional carbon fibers as fillers in composite materials, Carbon 39 (8) (2001), pp. 1119–1125. Article | PDF (51 K) | View Record in Scopus | Cited By in Scopus (56)

[22] ASTM C 150-04. Standard specification for Portland cement.

[23] M.S. Morsy, S.H. Alsayed and M. Aqel, Effect of nano-clay on mechanical properties and microstructure of ordinary Portland cement mortar, Int J Civ Environ Eng IJCEE-IJENS 10 (01) (2010), pp. 23–27.

[24] O. Henning, H. El-Didamony, K.M. Hanna and Z. Wiss, Hochsch Archit, Weimar, Banwes (1972).

[25] ASTM C 109/C 109M-02. Standard test method for compressive strength of hydraulic cement mortars (using 2-in. or [50-mm] cube specimens).

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