Multi-scale Performance and Durability of
Carbon Nanofiber/Cement Composites
F. Sanchez, L. Zhang, and C. Ince1
Abstract. This paper reports on recent work that is directed at understanding the
fundamental controlling mechanisms of multi-scale, environmental weathering of
nano-structured cement-based materials through an integrated experimental and
computational program. The effect of surface treatment and admixture addition on
the incorporation of carbon nanofibers (CNFs) in cement composites was studied.
Silica fume and surface treatment with nitric acid facilitated CNF dispersion. The
CNFs were found as individual fibers anchored in the hydration products throughout the cement pastes and as entangled networks in cavities. The presence of the
CNFs did not modify the compressive or tensile strength of the composite but did
provide it with a fair level of mechanical integrity post testing. Preliminary results
on durability indicated a residual effect of the CNFs after decalcification of the
composites as manifested by a slow load dissipation after peak load under compression. Molecular dynamics modeling of the reinforcing structure-cement phase
interface demonstrated that manipulation of the interface characteristics may provide a method to control the composite properties.
1 Introduction
Nano-level modifications of the structure of cement-based materials have the potential of greatly enhancing the material mechanical properties and durability and
of opening the door for new applications in civil engineering infrastructure. A
promise of nanotechnology is the use of carbon nanofibers and nanotubes as nanoreinforcement, or nano-rebar, to replace the steel rebar, a main cause of concrete
degradation. High specific strength, good chemical resistance, and electrical and
thermal conductivity are several properties that make carbon nanofibers/nanotubes
interesting as cement reinforcement [1, 2]. However, understanding the evolution
and performance of the nano-reinforcement interface is of critical importance.
F. Sanchez, L. Zhang, and C. Ince
Vanderbilt University
e-mail: [email protected]
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F. Sanchez et al.
Vapor grown carbon nanofibers (CNFs) are multiwall, highly graphitic structures with diameters ranging from 70 to 200 nm and lengths up to a few hundred
microns. CNFs present numerous exposed edge planes along their surface,
which in turn constitute potential sites for advantageous chemical or physical interaction. In addition, these fibers are well characterized, offer similar benefits
as carbon nanotubes at a lower cost, and are already produced in ton per year
quantities [3].
There is a complex, time-dependent and multi-scale interaction that occurs between an aging material and its surrounding environment. Exposure to weathering
forces moves components into and out of the material causing internal chemical
changes and stresses that affect the reinforcing fiber-cement interface. The properties of nanofiber reinforced, cement-based materials exist in, and the degradation
mechanisms occur across multiple length scales (nano to macro). The nano-scale
ultimately affects the properties and performance of the bulk material.
This paper reports on recent work [4-6] that is directed at developing
CNF/cement composites that have long-term performance and durability. The objective is to understand the fundamental controlling mechanisms of multi-scale,
environmental weathering of nano-structured cement-based materials through an
integrated experimental and computational program focusing on how molecular
level, chemical phenomena at internal interfaces influence long-term, bulk material performance. The performance of CNF/cement composites is discussed in
terms of microstructural, physical, and mechanical properties.
2 Experimental Approach
Commercially available vapor grown CNFs (Pyrograf®-III PR-19-LHT, Applied
Sciences, Inc., Cedarville, OH, USA) were used for the study. The CNFs were
used “as received” and after surface treatment with 70% nitric acid. The CNFs
were added to Portland cement (PC) pastes and PC pastes with 10 wt% silica fume
(SF cement). The following materials were prepared: (i) plain reference PC paste,
(ii) PC paste containing 0.5 wt% of “as received” CNFs, (iii) PC paste containing
0.5 wt% of surface treated CNFs with nitric acid, (iv) reference SF cement paste,
and (v) SF cement paste containing 0.5 wt% of “as received” CNFs. A water to
cementitious material (cement + SF) ratio of 0.33 was used for all mixes. After a
minimum curing time of 28 days, some specimens were conditioned for 95 days
under a concentrated solution of ammonium nitrate (590 g/L NH4NO3) to accelerate decalcification. A variety of tests were conducted on the non-degraded and degraded composites, including compression and splitting tensile tests, scanning
electron microscopy (SEM) observation of the fracture surface, x-ray diffraction,
BET analyses, and thermal analyses. A summary of the main findings is provided
below. Details of the experimental techniques can be found in [4, 5].
Multi-scale Performance and Durability of Carbon Nanofiber/Cement Composites
347
3 Results and Discussion
3.1 Microstructure of CNF/Cement Composites
For all composites examined, SEM observations of the fractured surface revealed
entangled networks of CNFs filling cavities created in the cement paste. Van der
Waals interactions between “as received” CNFs presented a significant barrier to
fiber dispersion. The current challenge to improving the composite properties is
the break-up of the initial clumps of fibers. In general, a certain level of break-up
was observed to occur with the addition of SF and after surface treatment of the
CNFs with nitric acid [5, 6]. For these two cases, the CNFs were found as individual fibers well anchored inside the hydration products throughout the cement
pastes (Fig. 1 ) in addition to the entangled networks (clumps of intertwined
CNFs) in cavities. These results clearly demonstrated the potential for CNFs to intimately interact with the cement phases.
Fig. 1 SEM of the fracture surface of
CNF/cement composites with nitric
acid surface treated CNFs, showing
individual CNFs anchored in the paste
3.2 Macroscopic Properties of CNF/Cement Composites
For all mixes tested, the splitting tensile strength of the CNF/cement composites
was comparable to the reference cement pastes (Fig. 2 ). Subjected to compressive
loads, though no significant change in the strength was observed, the CNF/cement
composites retained a certain mechanical integrity post testing (Fig. 3 ). The
propagation of cracks may have been limited by (i) the entangled clumps of CNFs
inside the cavities, (ii) the well anchored fibers at cavity edges bridging the paste
and the CNF networks, and/or (iii) the individually dispersed fibers (SF and surface treated CNF composites only). While static compression and tensile tests are
an incomplete measure of the mechanical properties, these results are encouraging
because no attempt to optimize the dispersion was made. Performance enhancements may be expected from on-going work using chemical functionalization of
the surface, optimum physical blending, and/or the use of surfactants. Aspects of
this work are being guided by the use of molecular dynamics modeling.
Splitting tensile strength (MPa)
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F. Sanchez et al.
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
max
median
min
PC pastes
Ref.
* As received
†
SF pastes
0.5wt%
CNF*
0.5wt%
CNF †
Ref.
0.5wt%
CNF*
Surface treated with nitric acid
Fig. 2 Splitting tensile strength of CNF/cement composites
a)
b)
Fig. 3 CNF/ cement composites post compression testing. a) Reference PC paste and b) PC
paste with 0.5 wt% nitric acid surface treated CNFs
3.3 Durability of CNF/Cement Composites
Many types of concrete degradation are closely associated with decalcification of
the cement paste. It has been shown that calcium can be used as a good indicator
of the chemical deterioration of concrete [7]. The load-displacement curves of the
PC pastes with 0 wt% and 0.5 wt% CNF loading obtained before and after exposure to accelerated decalcification using ammonium nitrate solution for 95 days
are presented in Fig. 4 . These initial results showed no evident difference in compressive strength after exposure to ammonium nitrate between the reference PC
paste and the corresponding paste with CNFs. Decalcification resulted in ca. 50%
reduction in compressive strength. With decalcification, the compressive strength
behavior evolved from a more brittle to a more ductile behavior with a slow load
dissipation after failure. This was more pronounced for the PC paste with CNFs
than for the reference PC paste, indicating a residual effect of the CNFs.
Multi-scale Performance and Durability of Carbon Nanofiber/Cement Composites
60
Compressive load (MPa)
Fig. 4 Comparison of compressive load displacement
curves of CNF/cement composites for PC pastes before
and after decalcification for 95
days (AN95d)
349
PC pastes
50
Reference
0.5 wt% CNF
40
30
0.5 wt% CNF - AN95d
20
10
Reference - AN95d
0
0
1
2
3
4
5
Displ. (mm)
3.4 Fiber-Cement Interaction
Molecular dynamics simulations were performed to investigate the interactions
between PC pastes and surface treated carbon fibers [8]. A model derived from a
model for the 9 Å tobermorite structure was used to represent the C-S-H phase of
cement. Standard models were used for graphite surfaces with several different attached, reactive moities and a plain surface with no attached moities. In the development of CNF/cement composites, they offer insight into the local interactions
among individual atoms, groups of atoms, and phases. The results indicated that
significant improvement in interfacial interaction is possible through appropriate
surface functionalization of the graphite surface. H-bonds and calcium counter
ions played a significant role in bridging the structure across the interface. Careful
control of the type and amount of functionalization is necessary to optimize the
strength of the H-bond network and other ionic interactions.
4 Conclusions
Silica fume and surface treatment with nitric acid facilitated CNF dispersion and
improved the interfacial interaction between the CNFs and the cement phases.
Though the ultimate load failure during static compression and tensile testing were
unchanged, improvements were observed post failure with a fair level of mechanical integrity observed for composites containing CNFs. Additionally, preliminary
results on durability indicated that after decalcification the CNF composite was
more ductile, retaining some residual strength post peak load. Molecular dynamics
modeling was found to be a useful and promising technique for understanding the
interfacial interaction between the cement phases and the reinforcing structure.
Acknowledgments. Funding from the National Science Foundation under NSF CAREER
CMMI 0547024 is gratefully acknowledged.
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