CARBON NANOTUBES AS A NEW REINFORCEMENT MATERIAL FOR MODERN CEMENT-BASED BINDERS

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1 CARBON NANOTUBES AS A NEW REINFORCEMENT MATERIAL FOR MODERN CEMENT-BASED BINDERS Xin Jiang (1), Torsten L. Kowald (2), Thorsten Staedler (1) and Reinhard H. F. Trettin (2) (1) Institute of Materials Engineering, University of Siegen, Germany (2) Institute for Building and Materials Chemistry, University of Siegen, Germany Abstract The use of carbon-based nano-structures (CNSs) to reinforce cement-based binders leads to an improvement in the mechanical characteristics of these nano-composites [1, 2, 3, 4]. To achieve even better mechanical properties of the composite the dispersion of the CNSs and the linkage between the CNSs and the concrete matrix has to be optimized. On the other hand detailed information about the influence of the CNSs on the hydration of the binder is necessary. With respect to the complexity of this task a cooperation between the institute of materials engineering and the institute for building and materials chemistry has been initiated at the University of Siegen. Due to the cooperation and exchange of information between the institute of materials engineering and the institute for building chemistry a synthesis of CNSs that fits best for the incorporation into the inorganic binders can be developed. Keywords: carbon nanotubes, composite, mechanical strength, hydration 1. INTRODUCTION The mechanical characteristics of the composite concrete can be significantly improved by the use of micro- and nanoscale volume-filling additives. Due to their outstanding mechanical properties, carbon-based nanostructures (CNSs) have been utilized to reinforce concrete [1, 2] and ultra-high-performance concrete [3, 4]. Multi-walled nanotubes (MWNTs) are the most promising CNSs with respect to the reinforcement of building materials, as they show more surface defects than any other CNSs. These defects lead to an improved linkage between the MWNTs and the cement matrix [2]. By the use of MWNTs within pure cement pastes the 14 d compressive strength could be improved by 30% with respect to the plain cement paste whereas the incorporation of singlewalled nanotubes (SWNTs) only lead to an increase of 6% [2]. Additionally the defects allow a functionalization of the nanotube s surface. There are different possibilities of modifying the inert surfaces of the MWNTs. Without changing the nanotube s surface itself, polymers can be wrapped around these and enhance

the dispersibility of the carbon nanotubes (CNTs) in water [5]. By means of oxidation it is possible to add surface defects and functional groups to the inert surfaces of the MWNTs [6].

2 the dispersibility of the carbon nanotubes (CNTs) in water [5]. By means of oxidation it is possible to add surface defects and functional groups to the inert surfaces of the MWNTs [6]. The modifications simplify the dispersion of the CNSs within the building material and improve the linkage between CNSs and cement matrix. Figure 1 shows a sample containing oxidized MWNTs exhibiting large pores which are frequently filled by MWNTs. These pores are mainly in the vicinity of coarse particles. For dense MWNTs composites the use of finely ground binders is very important, otherwise an agglomeration of the MWNTs between coarse particles will occur. Figure 1: SEM photo of a sample containing 1 wt % MWNTs To get more information about the influence of different CNSs on the mechanical properties of the composite and the hydration of the binder the very complex portland cement was reduced to the main phase C 3 S. This model-system was examined by mechanical tests, in situ x-ray powder diffraction (XRPD) and differential heat-flow calorimetry. 2. EXPERIMENTAL During the experiments commercial MWNTs and non-commercial CNSs were used. The commercial ones were purchased from Sun Nanotech Co Ltd. They were grown by chemical vapour deposition (CVD) method having a purity of >80%. Their diameter is between 10 and 30 nm and their length between 1 and 10 µm. Also non-commercial carbon-nano-coils (CNCs) with a length of some ten microns were used. Due to Van-der-Waals-Forces a bundling of the individual nano-fibres occurs. For a homogeneous distribution of the nano-particles within the composite these bundles must be broken into the individual fibres. This has been done by sonification of the CNSs within the mixing water. After sonification the superplasticizer (SP) should wrap around the dispersed CNSs thus preventing a bundling after the sonification is stopped. To enhance the surface defect concentration the MWNTs had been oxidized. 500 mg of the nanostructures were refluxed in 160 ml of nitric acid for 2 h. Afterwards the acid was distilled off and the MWNTs were repeatedly washed with distilled water and centrifuged at 3000 rpm until a ph-value of 7 had been reached. For the experiments plain mixtures or mixtures containing CNSs were used. The system consisted of C 3 S (d50 = 3.5 µm), a polycarboxylate-based SP and the mixing water. The

3 water to binder ratio of 0.22 had been chosen to decrease the amount of pores in the samples. The SP is needed to assure the workability of the pastes at the low water to binder ratio and for dispersing the MWNTs. Untreated or oxidized MWNTs were dispersed in the mixing water using a 400 W ultrasonic-finger for 15 minutes. Afterwards the SP was added and the mixture sonificated in addition for 2 minutes. After mixing the pastes were moulded into prism-shaped forms (8 mm x 10 mm x 30 mm) and compacted by applying a pressure of 125 MPa for 30 minutes. The prisms were demoulded after 2 days of storage at a relative humidity of >90% and 20 C. Than the prisms were cured under water at 20 C until the mechanical strength tests took place after 7 d. The in situ x-ray powder diffraction experiments were done using a Panalytical X Pert Pro diffractometer using Cu-K α radiation and an X Celerator detector. The scan-range in 2 theta was 5-65 which results in a scan time of 136 sec for a single scan. To prevent the sample from drying during the hydration process the paste was measured in a special fluid cell were the sample was covered by a foil. 3. RESULTS 3.1 Mechanical Properties The influence of polymer-wrapped MWNTs, oxidized MWNTs and CNCs on the mechanical properties of the composite was observed by compressive and flexural strength tests. The addition of 1 wt % MWNTs lead to an improvement in the compressive strength of prisms without CNSs from 238 N/mm² to 250 N/mm². The compressive strength after 7 d of prisms containing 1 wt % of oxidized MWNTs showed values of 263 N/mm² and the addition of 1 wt % Coils results in 259 N/mm² (Figure 2). The reference system without CNSs had a flexural strength of 17 N/mm². Here also an improvement due to the addition of the different CNSs could be observed. In the case of the untreated MWNTs values of 20 N/mm² could be observed, whereas the addition of the oxidized MWNTs lead to 25 N/mm² and the incorporation of the CNCs resulted in 22 N/mm². Figure 2: Compressive Strength after 7 d Figure 3: Flexural Strength after 7 d 3.2 Hydration The influence of MWNTs on the hydration of C 3 S was observed by in situ XRPD and by differential heat-flow calorimetry. As in indicator for the hydration of the pastes during the XRPD-experiments the (001)-reflex of the hydration product Ca(OH) 2 has been taken. Figure 4

$For better clearness only a small part of the collected diffraction patterns are shown in Figure 4 and Figure 5.$

4 shows the evolution of the portlandite-peak at during the first 24 h of hydration. For better clearness only a small part of the collected diffraction patterns are shown in Figure 4 and Figure 5. After a hydration time of 5 hours the portlandite-peak could be observed in the sample without MWNTs. Figure 4: 24 h in situ XRPD of hydrating C 3 S Figure 5: 48 h in situ XRPD of hydrating C 3 S with 1 wt % MWNTs Figure 5 shows the evolution of crystalline portlandite during the first 48 h of hydration. Here the first appearance of Ca(OH) 2 could be observed after 25 h. Besides the delay in the appearance of crystalline portlandite the broadening of the peak is remarkable. This point to much smaller portlandite crystals than in the sample without MWNTs. Complementary to the information about the effect of the MWNTs on the phase formation which we got from the XRPD-experiments, differential heat-flow calorimetry experiments show the influence on the chemical processes during the hydration of the samples. Figure 6: 72 h differential heat-flow calorimetry of hydrating C 3 S Figure 6 shows the heat evolution during the first 72 h of a plain C 3 S sample and one containing 1 wt % MWNTs. A clearly visible shift to shorter hydration times could be found.

5 Together with the XRPD-experiments we can say that the addition of MWNTs to a C 3 S superplasticizer water system has an influence on the reaction kinetics as well as on the formation of the hydration product portlandite. 4. CONCLUSIONS We showed that the addition of small amounts (1 wt %) of CNSs can improve the mechanical properties of samples consisting of the main portland cement phase C 3 S, SP and water. The oxidized MWNTs showed the best improvements both in compressive strength (+ 25 N/mm 2 ) and flexural strength (+ 8 N/mm 2 ) compared to the reference samples without the reinforcement (figures 2 and 3). The high defect concentration on the surface of the oxidized MWNTs could have led to a better linkage between the nanostructures and the binder thus improving the mechanical properties of the composite. The CNCs showed the second best improvement in the mechanical properties which are + 21 N/mm 2 for the compressive strength and + 5 N/mm 2 for the flexural strength. This may be due to the greater length of the CNCs compared to the commercial MWNTs. The observation of the phase formation and the heat evolution by in situ XRPD and differential heat-flow calorimetry showed that there are also influences on the hydration products and the hydration kinetics. The results of the differential heat flow experiments showed an acceleration of the hydration and the results from the XRPD experiments points to the formation of smaller portlandite crystals when MWNTs are used within the C 3 S pastes. REFERENCES [1] Makar, J.M. and Beaudoin, J.J. Carbon Nanotubes and their Application in the Construction Industry, in Proceedings of the 1 st International Symposium on Nanotechnology in Construction, Paisley, 2003, [2] Campillo, I., Dolado, J.S. and Porro, A., High-Performance Nanostructured Materials for Construction, in Proceedings of the 1 st International Symposium on Nanotechnology in Construction, Paisley, 2003, [3] Kowald, T., and Trettin, R., Influence of surface-modified Carbon Nanotubes on Ultra-High Performance Concrete, in Proceedings of the International Symposium on Ultra High Performance Concrete, Kassel, 2004, [4] Kowald, T., and Trettin, R., Kohlenstoffbasierte Nanostrukturen in modernen anorganischen Bindemitteln, in Tagung Bauchemie, GDCh-Monographie 32, Erlangen, October, 2004, [5] O Connell, M.J., Boul, P., Ericson, L.M., et al., Reversible water-solubilization of single-walled carbon nanotubes by polymer wrapping, Chemical Physics Letters 342 (2001) [6] Ebbesen, T.W., Hiura, H., Bisher, M.E., Treacy, M.M.J., Shreeve-Keyer, J.L. and Haushalter, R.C., Decoration of Carbon Nanotubes, Advanced Materials 8 (1996)

Nano-modification of Building Materials for Sustainable Construction

Nano-modification of Building Materials for Sustainable Construction M. Kutschera, T. Breiner, H. Wiese, M. Leitl, and M. Bräu 1 Abstract. Nanostructured products or products which were developed by means