Advanced cement based nanocomposites reinforced with MWCNTs and CNFs

Abstract

Cementitious materials reinforced with well dispersed multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) at the nanoscale were fabricated and tested. The MWCNTs and CNFs were dispersed by the application of ultrasonic energy and the use of a superplasticizer. Mechanical and fracture properties including flexural strength, Young’s modulus, flexural and fracture toughness were measured and compared with similarly processed reference cement based mixes without the nano-reinforcement. The MWCNTs and CNFs reinforced mortars exhibited superior properties demonstrated by a significant improvement in flexural strength (106%), Young’s modulus (95%), flexural toughness (105%), effective crack length (30%) and fracture toughness (120%).

Keywords

multi-walled carbon nanotubes / carbon nanofibers / mortars / toughness / Young’s modulus

Introduction

A major drawback of cementitious materials is their low tensile strength and fracture toughness resulting to the formation of nanocracks under relatively low tensile loads. Reinforcing materials like multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) makes it possible to produce cement based nanocomposites with enhanced mechanical and fracture properties. These materials have aspect ratios greater than 1,000, Young’s modulus around 1 TPa [ 1], tensile strength of 65–93 GPa, and maximum strain of 10–15 %. MWCNTs form agglomerates or bundles and adhere together with strong van der Waals forces which make it difficult to separate. Early attempts to add MWCNTs to cementitious materials have failed due to poor adhesion and difficulties in separating the fibers. A revolutionary method for effective dispersion of MWCNTs in cement pastes, mortars and concrete was developed by Konsta-Gdoutos and coworkers [ 2, 3]. Different lengths of MWCNTs and CNFs were dispersed in water by applying ultrasonic energy and using a surfactant. The same authors have developed multifunctional cement paste nanocomposites with high strength and toughness, and advanced strain detection ability that can be used as a novel self-sensing nanoreinforced cementitious structural material. The incorporation of highly dispersed MWCNTs and/or CNFs at low concentrations has been shown to effectively arrest the cracks at the nanoscale, imposing significant improvements in the mechanical properties of the nanocomposites [ 3– 8]. Even though a few studies on the mechanical strength and stiffness of cementitious materials containing MWCNTs and CNFs exist, there have not been any studies on the fracture performance of MWCNT and CNF mortar nanocomposites. Inclusion of fibers at the nanoscale and their influence on the interfaces and the pore structure of the cementitious matrix are likely to result to significant improvements in the fracture response of the nanoreinforced mortars as compared to conventional Ordinary Portland Cement (OPC) systems.

It is the objective of this study to investigate the mechanical and fracture properties of MWCNT and CNF reinforced mortars. Mortar nanocomposites reinforced with well dispersed MWCNTs and CNFs at an amount of 0.1 wt% of cement were fabricated and tested by conducting three-point bending and fracture mechanics experiments. Results obtained in the present study show that the incorporation of MWCNTs and CNFs in the mortar matrix significantly improves the mechanical and fracture properties of the mortar matrix.

Experimental

Materials and specimen preparation

The material investigated was a mortar composite reinforced by MWCNTs or CNFs. Characteristic properties of MWCNTs and CNFs are shown in Table 1. Values of an estimated number of MWCNTs or CNFs per unit volume of the cementitious matrix are also included in the Table 1, expressed as Fiber count. Specimens were prepared at a water to cement ratio (w/c) of 0.485 and standard sand according to EN 196-1 at a sand to cement ratio (s/c) of €2.75. Commercially available Type I OPC was used for all mixes. Suspensions were prepared by adding the MWCNTs or CNFs in an aqueous surfactant solution and applying ultra sonication energy, following the method described in Konsta-Gdoutos and coworkers [ 1, 2]. The materials were mixed according to ASTM 305. Two types of specimens were prepared: 40 mm×40 mm ×160 mm prisms for the three point bending tests; and 20 mm×20 mm×80 mm notched beams for the determination of the fracture properties. Following demolding, the samples were cured in lime-saturated water for 3, 7, and 28 d.

Mechanical and fracture testing

The mechanical and fracture properties of the nanocomposites was determined by three point bending and fracture mechanics tests, respectively. Three point bending tests were conducted on 40 mm × 40 mm × 160 mm beams at the age of 3, 7 and 28 d, using a closed-loop MTS servo hydraulic testing machine with a 25 kN capacity. The rate of displacement was kept as 0.1 mm/min. The average values of the flexural strength, Young’s modulus and flexural toughness were determined according to ASTM C348. The flexural strength, s_f, is the highest bending stress attained in a three point bending test of a nanoreinforced mortar specimen. The flexural toughness, T, is the amount of energy that the cementitious material can absorb before rupturing and can be measured from the area under the load-deflection curve (Fig. 1). The Young’s modulus, E, was calculated from the three point bending experiment by the following equation:

(1) E={\displaystyle \frac{L^3{C}_i}{{4bd}^3}}

where L, b and d are specimens’ dimensions (Fig. 2) and C_i (N/mm) is the slope of the initial straight-line portion of the load-deflection curve (Fig. 1).

Fracture mechanics tests were conducted on notched 20 mm×20 mm×80 mm prismatic specimens with a 6 mm notch at the age of 3, 7 and 28 d, by the aforementioned experimental procedure. The notch was introduced into the specimens using a water-cooled band saw machine. The crack mouth opening displacement (CMOD) was measured using a pair of knife edges attached at the two sides of a notch on the lower surface of the specimen. The crack mouth opening displacement (CMOD) at the notch, was set at a rate of 0.08 mm/min, and used as the feedback control signal for running the test. Fracture toughness values were calculated using both the loading and unloading compliances according to the two-parameter fracture model (TPFM) developed by Jenq and Shah [9,10].

The fracture parameters were determined using the load versus crack mouth opening displacement (CMOD) curve for a loading-unloading cycle of a three-point bend specimen. To measure the crack mouth opening displacement (CMOD) a pair of knife edges was attached at the two sides of a performed notch on the lower surface of the specimen.

The modulus of elasticity, E, is calculated by:

(2) E={\displaystyle \frac{6S{a}_0{g}_2\left(a_0\right)}{C_i{b}^{2}t}}

where

C_i€=the compliance of the loading part of the load–CMOD curve,

α₀€=(a₀+HO)/(b+HO),

a₀€=crack length,

S=span length, b=specimen depth,

t=specimen thickness,

HO=length defined in Fig. 3,

g₂(α₀)€=geometric function defined by

(3) g_2{(a}_0\mathrm{)=0}\mathrm{.76-2}{\mathrm{.28}a}_0\mathrm{+3}{\mathrm{.87}a}_0^2\mathrm{-2}{\mathrm{.04}a}_0^3+{\displaystyle \frac{0\mathrm{.66}}{{{\mathrm{(1-}a}_0)}^2}} .

The modulus of elasticity E is calculated from the compliance of the unloading part of the load-CMOD curve as

(4) E={\displaystyle \frac{6S{a}_{\mathrm{c}}{g}_2\left(a_{\mathrm{c}}\right)}{C_{\mathrm{u}}{b}^{2}t}} ,

where

C_u =the compliance of the unloading part of the load–CMOD curve,

a_c=the effective crack length, α_c = (a_c + HO)/(b+ HO)

The unloading compliance is taken within 95% of the peak load calculated from the load-CMOD curve. The value of the effective crack length a_c is calculated by

(5) a_{\mathrm{c}}{=a}_0{\displaystyle \frac{C_{\mathrm{u}}}{C_i}}{\displaystyle \frac{g_2{(a}_0)}{g_2{(a}_{\mathrm{c}})}} .

Equation (2.5) is solved numerically for the determination of the critical crack length a_c.

The critical stress intensity factor is calculated by the equation

(6) K_{\mathrm{IC}}^S=3\left(P_{\mathrm{cr}}{+W}_{\mathrm{h}}\right){\displaystyle \frac{S\sqrt{{\mathrm{\pi }a}_{\mathrm{c}}}{g}_1\left({\displaystyle \frac{a_{\mathrm{c}}}{b}}\right)}{{2b}^{2}t}} ,

where

P_cr = the peak load,

W_h = W_hoS/L,

W_ho = the self weight of the beam, and

g_1\left({\displaystyle \frac{a_{\mathrm{c}}}{b}}\right)=

(7) {\displaystyle \frac{1\mathrm{.99-}\left(a_{\mathrm{c}}/b\right)\left(\mathrm{1-}{a}_{\mathrm{c}}/b\right)\left[2\mathrm{.15-3}\mathrm{.93}\left(a_{\mathrm{c}}/b\right)\mathrm{+2}\mathrm{.70}{\left(a_{\mathrm{c}}/b\right)}^2\right]}{\sqrt{\mathrm{\pi }}\left(\mathrm{1+2}{a}_{\mathrm{c}}/b\right){\left(\mathrm{1-}{a}_{\mathrm{c}}/b\right)}^{3/2}}}

Results and discussion

Results of the flexural strength of neat mortars and mortars reinforced with MWCNTs or CNFs at an amount of 0.1 wt% by weight of cement at the age of 28 d are presented in Fig. 4. Table 2 presents values of Young’s modulus of neat mortars and mortars nanomodified with 0.1 wt% MWCNTs and CNFs at 3, 7 and 28 d. Note that in the nanoreinforced composites both the flexural strength and Young’s modulus increase greatly, at all ages. More specifically, the 28-d nanocomposites reinforced with MWCNTs or CNFs at amounts of 0.1 wt% exhibit a 87% and 106% increase in flexural strength and 93% and 95% increase in Young’s modulus, respectively. Previous results of the authors of this work [ 4, 7, 8] confirmed that the increase in flexural strength and Young’s modulus of cement paste nanocomposites reinforced with CNTs and CNFs at amount of 0.048% wt (with the fiber count of this amount of CNTs and CNFs approximately corresponding to the fiber count of the 0.1% wt amount of CNTs and CNFs in mortars) is about 40% and 50%, respectively. Results in the literature report either a decrease, or occasionally only a marginal increase in the flexural strength and Young’s modulus of nanoreinforced mortars [ 11‒ 14]. Values of Young’s modulus determined by the three point bending tests are consistent with the values determined by the fracture mechanics tests on notched specimens (Table 2). Note that, the flexural strength of the CNF nanocomposites is slightly higher than that of the MWCNT mixes, despite the fact that the CNF fiber count is much lower than the MWCNT. This can be explained by the CNFs’ complex nanostructure. Their outer surface usually consists of conically shaped graphite planes canted with respect to the longitudinal fiber axis as shown in the SEM picture of a CNF in Fig. 5 [ 8, 15, 16]. These edges, which are present along the circumference of the fiber, can be used to help anchor the fiber in the matrix and prevent interfacial slip. Additionally, these edges provide the CNFs with a higher reinforcing efficiency. As a result the CNF cementitious nanocomposites have higher load transfer efficiency, which is the key element that determines the mechanical response of the fiber nanocomposites.

Figure 6 presents the flexural toughness of 28-d old neat mortar and 0.1 wt% MWCNTs and CNFs/mortar nanocomposites. Note that all mortar nanocomposites exhibit higher flexural toughness compared to the neat mortar. Specifically, the 28-d nanocomposites reinforced with MWCNTs and CNFs at an amount of 0.1 wt% exhibit 77% and 105% increase in flexural toughness, respectively. The flexural toughness represents the energy required for the material to fail. Addition of MWCNTs and CNFs significantly increases the toughness of the nanoreinforced mortars. This is the effect of an enhanced crack arresting at the nanoscale, which leads to a controlled coalescing process of cracking at the nano- and micro scale and allows the material to greatly improve its toughness and mechanical performance.

Typical load-CMOD curves (following the linear elastic fracture mechanics) of OPC mortar specimens and specimens reinforced with MWCNTs and CNFs at amount of 0.1 wt% of cement, are shown in Fig. 7. The load-CMOD response of the nanocomposite mortars revealed an increase of the peak load as much as 65% for the MWCNT and 89% for the CNF reinforced mortars compared to the neat mortar. Figure 8 shows the TPFM loading and unloading compliance curves for the neat mortar and mortar nanocomposites that are used to determine the effective crack length a_c and fracture toughness K_IC.

The critical value of the stress intensity factor is calculated from the peak load and the effective crack length using TPFM. The effective crack length is the sum of the initial notch plus the length of the fracture process zone at the peak load. Results of the calculated values of the effective crack length a_c, at 28 d for the plain, 0.1 wt% MWCNT and CNF reinforced mortars are shown in Fig. 9. A small increase of ~1.7% was observed for the 0.1% CNF nanoreinforced mortar over the 0.1% MWCNT one.

The fracture toughness is the most important parameter for the characterization of the resistance of a material to the growth of existing cracks. Figure 10 presents the critical values of stress intensity factor of neat mortars and mortars nanomodified with 0.1 wt% MWCNTs and CNFs at 3, 7 and 28 d. From Fig. 10 it is observed that the K_IC values of both the MWCNT and CNF nanocomposites are higher than that of the neat mortar at all ages. Compared to the neat mortar the 28-d MWCNT and CNF nanocomposites exhibit a 87% and 120% increase in fracture toughness values, respectively. Interestingly, and despite the fact that the estimated fiber count of the MWCNT reinforced mortars is 17 times higher than this of the CNF mortars (4.0×10¹⁰) (see Table 1), the 28-d stress intensity factor of the CNF mortar is ~23% higher than the MWCNT one. Nevertheless, several parameters besides dispersion are considered to improve the fracture toughness of the nanocomposites. These parameters include the diameter and the length of the carbon fibers at the nanoscale and the strength of MWCNT/CNF-mortar matrix interfacial characteristics [ 17, 18]. Taking into account that the load bearing capacity of both MWCNTs and CNFs is large enough, it is possible that the bigger diameter and length of the CNFs, along with the extended graphite planes that exist in the outer CNF surface [8], create a stronger interfacial bonding between the mortar matrix and the carbon nanofibers that enhances the observed toughening effect.

Conclusions

A thorough study of the mechanical and fracture properties of cementitious nanocomposites reinforced with well dispersed multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) was performed. Flexural strength, Young’s modulus, flexural toughness, effective crack length and critical fracture toughness were determined and compared with similarly processed reference cement based mixes without the nano-reinforcement. The excellent reinforcing capability of MWCNTs and CNFs is demonstrated by a significant improvement in flexural strength (87% for MWCNTs and 106% for CNFs reinforcement), Young’s modulus (95%), and fracture toughness (86% for MWCNTs and 119% for CNFs reinforcement).

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