Exploration of fracture characteristics , nanoscale properties and nanostructure of cementitious matrices with carbon nanotubes and carbon nanofibers

This research investigates changes in the fracture mechanics characteristics and nanoscale properties of cement paste materials reinforced with nanofibers, such as multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs). The effect of the nanofibers on the microstructure of cement paste was also investigated. It was found that MWCNTs and CNFs reinforce cementitious materials by controlling cracks at the nanoscale level and improving the Young’s modulus and the flexural strength of the matrix. The use of MWCNTs was also found to improve the nanomechanical properties of cement matrix. In particular, nanoindentation results have shown that the incorporation of MWCNTs led to the reduction of the nanoporosity of the matrix and significantly increased the amount of high stiffness C-S-H gel. Additionally, the nanocomposites reinforced with CNFs showed improved fracture behavior when compared to the samples with MWCNTs. Exploration of fracture characteristics, nanoscale properties and nanostructure of cementitious matrices with carbon nanotubes and carbon nanofibers S.P. Shah Center for Advanced Ceme t Based Materials, Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208, USA M.S. Ko sta-Gdoutos & Z.S. Metaxa Department of Civil Engineering, School of Engineering, Democritus University of Thrace, Xanthi, Greece Fracture Mechanics of Concrete and Concrete Structures Recent Advances in Fracture Mechanics of Concrete B. H. Oh, et al.(eds) c 2010 Korea Concrete Institute, Seoul, ISBN 978-89-5708-180-8 develop high performance nanofiber/cement nanocomposites homogeneous dispersion of nanofibers in the cementitious matrix must be accomplished (Xie et al. 2005). In this study, the dispersion method described in Konsta-Gdoutos et al. (2008) was used. Based on this method, MWCNTs and CNFs were dispersed in an aqueous surfactant solution by applying ultrasonic energy using a 500 W cup-horn high intensity ultrasonic processor. After dispersion, OPC was added to the nanofiber suspensions at a water to cement ratio of 0.5 (w/c=0.5). The materials were mixed using a standard Hobart mixer, following the procedure outlined by ASTM 305. After mixing, the material was cast in 20 × 20 × 80 mm molds. 2.2 Fracture mechanics tests The mechanical properties of the mixes were measured by three-point bending tests of beams with a 6 mm notch cut at the midspan. The beams were tested at the age of 3, 7 and 28 days. An average value of three specimens was used for each curing age, based on ASTM C 348. The tests were performed with a closed-loop MTS servo-hydraulic testing machine with an 89 kN capacity. Crack mouth opening displacement was used to control the test and was advanced at a rate of 0.012 mm/min. Load versus CMOD graphs were created from the test results. Young’s modulus was then calculated from these graphs using the two-parameter fracture model by Jenq & Shah (Shah et al. 1995). Strength was calculated using the net specimen depth. 2.3 Nanoindentation tests The nanomechanical properties of CNTs nanocomposites were investigated using a Hysitron Triboindenter. Prismatic specimens of 25.4 × 6.35 × 6.35 mm were prepared and cured in water saturated with lime for 28 days. Before testing, thin slides of approximately 5 mm were cut out of the specimens. The surfaces were polished with silicon carbide paper discs and diamond lapping films in order to obtain a very smooth and flat surface. A Berkovich tip with a total included angle of 142.3 degrees was used for indentation. Multiple cycles of partial loading and unloading were used to make each indention, eliminating creep and size effects (Nemecek 2009). Nanoindentation was performed in a 12×12 grid (10 μm between adjacent grid points). This procedure was repeated in at least two different areas on each sample. The Oliver & Pharr (1992) method was used to determine the mechanical properties, where the indentation modulus is calculated from the final unloading curve. 2.4 Scanning electron microscopy An ultra-high resolution field emission scanning electron microscope (LEO Gemini 1525) was used to examine the morphology and microstructure of the fracture surfaces of the nanocomposites. Specimens of 25.4 × 6.35 × 6.35mm were prepared for each mix. Prior to their observation, the fracture surface of the specimens was sputter-coated using the Denton Desk III system. Due to the roughness of the surface, a 20 to 25 nm thick layer of gold-palladium (Au/Pd) was used to eliminate charging effects caused by insufficient coating. The SEM machine was operated at 3 to 5 kV. Secondary electron (SE) imaging was employed to obtain clear images at medium to high magnifications (10,000× to 150,000×). 3 RESULTS AND DISCUSSION The flexural strength rate of the nanocomposites incorporating either MWCNTs or CNFs up to the age of 28 days of hydration is shown in Figure 1(a). The samples reinforced with nanofibers at all ages exhibit higher flexural strength than plain cement paste. In particular, an increase up to 25% is achieved when MWCNTs are utilized. The use of CNFs results in an increase of the flexural strength up to 45%. Comparing the two nanocomposites it is observed that despite the fact that MWCNTs exhibit a higher aspect ratio due to their smaller diameter and larger amounts of nanotubes is reinforcing the cement matrix since the concentration of the fibers is constant, CNFs provide the matrix with the ability to carry higher flexural loads at lower strains. A possible explanation could be that the bonding between the CNFs and the matrix is enhanced due to the unusual outer surface texture of the carbon nanofibers. The CNFs used in this study exhibit graphite planes which extend beyond the diameter of the nanofiber and are present along the circumference of the fiber. These edges probably help anchor the fiber in the matrix, preventing interfacial slip and enable more sufficient load transfer across nanocracks and pores. The Young’s modulus of the samples reinforced with either MWCNTs or CNFs at the age of 3, 7 and 28 days is illustrated in Figure 1(b). Similar to the flexural strength results, samples reinforced with nanofibers clearly exhibit improved Young’s modulus over OPC specimens. Specifically, an increase of the Young’s modulus of 44% to 50% over plain cement specimens is achieved with the use of MWCNTs. In the case of the samples with CNFs, an increase of at least 50% is observed. Proceedings of FraMCoS-7, May 23-28, 2010 h T h D ∇ − = ) , ( J (1) The proportionality coefficient D(h,T) is called moisture permeability and it is a nonlinear function of the relative humidity h and temperature T (Bažant & Najjar 1972). The moisture mass balance requires that the variation in time of the ter mass per unit volume of concrete (water content w) b equal to the divergence of the moisture flux J J • ∇ = ∂ ∂ − t w (2) The water content w can be expressed as the sum of the evaporable water we (capillary water, water vapor, and adsorbed water) and the non-evaporable (chemically bound) water wn (Mills 1966, Pantazopoulo & Mills 1995). It is reasonable to assume that the evaporable water is a function of relative humidity, h, degree of hydration, αc, and degree of silica fum reactio , αs, i.e. we= (h,αc,αs) = ag -dependent sorption/desorption isoth rm (Norling Mjon ll 1997). Under this assumption an by substituting Equation 1 into Equation 2 one obtains