- Email: [email protected]
Evaluation of microstructure and mechanical performance of CNT-reinforced cementitious composites at elevated temperatures
Accepted Manuscript Evaluation of microstructure and mechanical performance of CNT-reinforced cementitious composites at elevated temperatures L.W. Zhang, M.F. Kai, K.M. Liew PII: DOI: Reference:
S1359-835X(17)30051-9 http://dx.doi.org/10.1016/j.compositesa.2017.02.001 JCOMA 4562
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
9 November 2016 25 January 2017 3 February 2017
Please cite this article as: Zhang, L.W., Kai, M.F., Liew, K.M., Evaluation of microstructure and mechanical performance of CNT-reinforced cementitious composites at elevated temperatures, Composites: Part A (2017), doi: http://dx.doi.org/10.1016/j.compositesa.2017.02.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of microstructure and mechanical performance of CNT-reinforced cementitious composites at elevated temperatures
L.W. Zhang 1,*, M.F. Kai2,3, K.M. Liew 2,3,* 1
School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
2
Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong, China 3
City University of Hong Kong Shenzhen Research Institute, Shenzhen 518057, China
Abstract Experimental studies are carried out to examine the microstructure and mechanical performance of carbon nanotube (CNT) reinforced cementitious composites at elevated temperatures (25oC, 200oC, 400oC and 600oC). Two different contents (0.1% and 0.2% by weight of cement) of CNT are added to cement paste. X-ray powder diffraction (XRD) results show that CNT does not improve or impede the hydration of cement minerals at room temperature, while at higher temperatures the further hydration process caused by high-pressure steam is impeded by CNT. The morphology observed by scanning electron microscopy (SEM) indicates that the bridging effect of CNT can be maintained below 400oC, while CNT is mostly spalled with the matrix on the walls of pores and gaps at 600oC. The residual flexural and compressive strength are measured. The reinforcing effect of CNT is most obvious at 400oC, which is due to the potential of CNT as channels for releasing high-pressure steam.
Keywords: CNT-reinforced cementitious composites; microstructure; mechanical performance; temperature
*Corresponding authors. E-mail addresses: [email protected] (L.W. Zhang); [email protected] (K.M. Liew)
1. Introduction Due to its extraordinary mechanical, electrical and thermal performance, CNT has become a candidate for use in many applications [1–3]. The tensile strength of CNT can be up to 500 GPa, while the strain at tensile failure has been predicted to be as high as 30% [4, 5]. The elastic modulus of CNT is reported to be near 1 TPa [6]. The electrical conductivity of CNT is reported to be in the range of 102–10-4 s/cm and strongly related to the clarity of its structure [7, 8]. The electric current density has been predicted to be more than 1,000 times greater than those of metals such as copper in theory [9]. In addition, CNT has significantly greater thermal stability than graphene, while it shares the same thermal conductivity as graphene [4]. The thermal stability is in the range of 600–750oC [7, 10]. The thermal conductivity along the axis of CNT is almost nine times greater than copper [11]. The thermal conductivity across its axis is reported to be as high as soil [12]. Due to substantial van der Waals attractions, sonication and surfactants are commonly used to disperse CNT in aqueous solution at first for its application in cementitious composites [13]. In addition, the stabilization of the solutions can be controlled by surfactants that decorate the CNT surface permanently and change the wettability of the CNT surface [10, 14]. Till now, the dispersion of CNT has been well solved and it can be well mixed in cement paste [6]. The mechanical and electrical performance of CNT-reinforced cementitious composites has been widely investigated [6]. Chan and Andrawes [15] experimentally and theoretically proved that CNT can enhance the mechanical properties of cement paste, as demonstrated by many other researchers [7, 13]. Eftekhari and Mohammadi [16] employed molecular dynamics simulation and proposed that the CNT orientation plays a decisive role in the enhancement effect of CNT in cement paste. Cwirzen et al. [17] investigated the compressive and flexural strength of CNT-reinforced cementitious composites and no
enhancement effect was found to exist, as stated by some other studies [18, 19]. However, some researchers [20, 21] found that the incorporation of CNT in cement matrix can reduce the mechanical properties. The compressive strength and rupture modulus measured by Musso et al. [20] showed sharp decreases. The improvement mainly resulted from the efficient bonding for load-transfer that was caused by CNT bridging in the cementitious composites [16]. Liew et al. [22] affirmed that the bonding strength can be affected by the type of surfactants and a lower bonding strength can improve the damping property of the CNT/cement composites. In addition, much effort has been made to research the electrical properties of CNT/cement matrix. It is a common conclusion that the addition of CNT decreases the electrical resistivity of cementitious composites while it increases their electrical conductivity. Its improvement range depends strongly on the concentration, the surface condition, the type and the degree of dispersion of the CNT [23–27]. Based on these, researchers have made many efforts to study the sensing behavior of CNT/cement composites for use in self-sensing and smart materials [23–28]. Though the thermal conductivity of CNT-reinforced composites has been researched in recent years [29, 30], there are few investigations about the high-temperature resistance performance of the composites. Fire is a risk for cementitious composites considering their unstable structures under high temperature and transformation of the hydrated products, so it is meaningful to investigate the effect of the incorporation of CNT into cement paste on the mechanical properties and microstructure at
elevated
temperatures,
and
to
determine
the
application
potential
of
CNT
on
high-temperature-resistant cementitious material. In this study, the compressive and flexural strength of the CNT/cement material treated with high temperatures were determined. The changes of mechanical properties of CNT/cement composites at high temperatures are correlated with chemical and microstructural changes, so the chemical
components were analyzed by XRD and the microstructure was detected by SEM.
2. Experiment 2.1 Material Hydroxylated multi-walled CNT (MWCNT-OH) was provided by Chengdu Organic Chemicals Co., Ltd. in China and produced by the chemical vapor deposition (CVD) method. The properties of CNT are presented in Table 1. The surfactant employed in this work was a type of non-ionic surfactant named polyvinylpyrrolidone (PVP), provided by Shenzhen Benno Industrial Co., Ltd. The binder material used in the research was Ordinary Portland Cement (OPC) type 42.5R, conforming to the requirements of Chinese Standard GB175-2007. 2.2 Preparation of specimens The compositions of two mixtures are presented in Table 2. First, CNT and PVP were added into water for dispersal. 20% of PVP by weight of CNT was used to help segregate the CNT and the technique of sonication (195 W, 15 min) was employed to achieve a satisfactory dispersion. After sonication, the cement was mixed together with the prepared solutions for 7 min using a cement paste mixer to obtain fresh cementitious composites with a water–cement ratio of 0.4. After mixing, the mixture was then poured into 40×40×160 mm molds for flexural strength and compressive strength tests. The mixture was compacted on a vibrating table by vibrating 30 times. The specimens were covered with plastic sheets till demolded after 24 h. Then the demolded specimens were kept in a moist room for 27 days in an environment of 20oC and relative humidity of 95%, leading to a total curing time of 28 days. Plain cement specimens (C0) were cast as a reference. All specimens were dried for 7 days at room temperature (25oC) and a relative humidity of 60% after curing.
2.3 Heating procedure Each specimen was placed in a furnace subjected to continuous heating at a rate of 3oC/min from the ambient temperature (25oC) to the target temperature (200oC, 400oC, 600oC). The target temperature was kept constant for a period of 1 hour in order to homogenize the inner temperature. After this, the specimens were naturally cooled in the oven with the furnace door closed to ensure a slow cooling rate. After cooling down to room temperature (25oC), each sample was kept in a hermetic plastic bag for tests and characterization. 2.4 Tests and characterizations The tests of flexural and compressive strength were conducted according to ASTM C 348 and ASTM C 349, respectively. After the 3-point flexural strength test, the compressive strength was measured from the end parts of the specimens. To portray the surface morphologies of the hydrated CNT-reinforced cementitious composites, scanning electron microscopy (SEM) (Hitachi, SU-70, Japan) was performed. The SEM samples were prepared in pellet form. The analysis of the crystalline phases was carried out using X-ray powder diffraction (XRD) (Bruker, AXS D8, German) using CuKα radiation (k = 1.540598Å) in the scattering range (2θ) of 5–70o with a scan rate of 0.1o/sec and slit width of 0.02o. The powder for XRD analysis, obtained by grinding the fragments of broken specimens, was sieved with a sieve having apertures of 40 μm. The theory of XRD is based on the diffraction properties of crystals. The Bragg equation (2dsinθ=nλ; d=interplanar spacing, θ=diffraction angle, λ=the wavelength of X-ray and n=arbitrary integer) was used to identify the crystals in a material. 3. Results and discussion 3.1 Phase composition The deterioration of the crystalline phase structure of plain cement and CNT/cement composites
subjected to various elevated temperatures was assessed by XRD analysis. Fig. 1 shows the deterioration of the cement hydrates, where the main peaks have been identified. The samples for XRD analysis were taken from the core of the 40 mm×40 mm×160 mm specimens. For each kind of specimens, three same samples were prepared to get the representative peak values. Considering that the peak at 18.007o is totally decided by calcium hydroxide crystals without the interference of other constituents on the images [31], it can be used to identify the hydration process. Fig. 1(a) and (b) show almost coincident images representing the crystalline phases of cement pastes with and without CNT, indicating that the CNT did not improve or impede the hydration process, which can also be concluded from the peak value of calcium hydroxide at 18.007o. The images showed that hydroxylated CNT did not combine with cement hydrates in a chemical way, which differs from the way that hydrates with carboxylated CNT are chemically combined, creating new peaks in the XRD images [32]. The peak values at 18.007o of plain cement at 25oC, 200oC, 400oC and 600oC are 1530, 1599, 1648 and 226. The difference of the peak values is obvious and can be well reflected by XRD test. The peak values at 18.007o of CNT/cement material at 25oC, 200oC, 400oC and 600oC are 1523, 1578, 1594 and 235. The difference of the peak values is also obvious and can be well reflected by XRD test. At the range of 25oC–200oC, the liberation of free water [33] and the decomposition of ettringite [31] occurred. It can be concluded from Fig. 1(c) and (d) (which represent the compositions of plain cement and CNT/cement materials respectively) that ettringite crystal almost totally deteriorated. It has been mentioned in several articles [34, 35] that the internal autoclaving conditions that form in cement paste under high temperature due to water vapor create high pressure (especially at the range of 100oC–300oC [34, 36]). The autoclaving conditions contribute to the additional hydration of
unhydrated cement particles. This explains why the peak value at 18.007o of calcium hydroxide under 200oC and 400oC increased slightly (Fig. 1(c), (d), (e) and (f)). This can also be found from the intensity of the superimposed reflections of the OPC minerals (C2S and C3S) [d=2.776–2.785Å] [37], which decreased due to further hydration. At 200oC, the peak value at 18.007o of CNT/cement composites is lower than that of the peak value of plain cement, which can be stated that 0.2% CNT can slightly decrease the further hydration process of cement matrix under a high temperature (200oC). From 200oC to 400oC, the physically bound water [35, 36] and chemically bound water of gel-like hydration products (C-S-H) [34, 38] were released from the specimens. However, the dehydration process of C-S-H gel did not enhance the intensity of the superimposed reflections of the OPC mineral crystal structure (Fig. 1(e) and (f)), which implied that the dehydration process was not severe and that the gel structure of C-S-H was not transferred into C2S or C3S crystal structure, though part of the chemically bound water was taken up. Comparing Fig. 1(c) with Fig. 1(d) or comparing Fig. 1(e) with Fig. 1(f), it can be clearly seen that the peak value of calcium hydroxide of plain cement at 200oC and 400oC was higher than that of 0.2% CNT/cement, indicating that the hydration process of pastes without CNT was better accelerated. At 400oC, the peak value at 18.007o of CNT/cement composites is lower than that of the peak value of plain cement, which can be stated that the further hydration process was decreased more obviously due to 0.2% CNT. At higher temperatures of 600oC, the change of the matrix with and without CNT was primarily caused by the further dehydration of C-S-H gel and calcium hydroxide [39, 40]. It is clear that broad superimposed reflections (Fig. 1(g) and (h)) of OPC minerals appeared, as observed in [34], indicating a huge decomposition of C-S-H gel and the formation of large quantities of C2S and C3S. Calcium hydroxide greatly deteriorated at 600oC as shown in Fig. 1(g) and (h). In general, the XRD images of
plain cement at 600oC were again basically coincident with those of paste with CNT. This implies that CNT did not influence the decomposition process of the hydration products at this temperature. 3.2 Microstructure observation In order to investigate the morphology of the cement matrix microstructure and the behavior of CNT subjected to various elevated temperatures, small sheet samples were taken from the cores of the 40 mm×40 mm×160 mm specimens with 0.2% CNT content for SEM observations. Fig. 2(a), (c), (e) and (g) show the general view of the surface morphology of the microstructure of the matrix before and after thermal treatment. Fig. 2(a) shows the unheated specimens that primarily comprise ill-crystallized and fibrous particles of calcium-silicate-hydrate (C-S-H) gel, amorphous and well-crystallized calcium hydroxide and invisible CNT [35]. It can be seen by comparing Fig. 2(a) with Fig. 2(c) that the matrix shows obvious integrality at 200oC, indicating that the crystal structure was not lost and the components of the main hydrates products did not decompose. When the temperature reached 400oC, there was a non-negligible change in the surface morphology, with part ill-crystallized or amorphous structures (Fig. 2(e)) resulting from the loss of bound water from the decomposition of C-S-H at the range from 200oC and 400oC [33]. The images at 400oC also show that the dehydration of part chemically bound water from C-S-H gel did not transfer from the gel structure into the crystal structure. Up to 600oC, most hydration products appear as ill-crystallized or amorphous structures with cracks on the surface (Fig. 2(g)), which is mainly due to the damage caused by the high temperature and the further decomposition of C-S-H gel and calcium hydroxide [31]. A more subtle level of images was performed on all the samples in order to study the state of CNT in cement paste after treatment at elevated temperature (Fig. 2(b), (d), (f), (h) and (i)). It can be concluded from Fig. 2(b) that the CNT is well mixed in the cement paste and it can be seen that the
cement hydrates grow on some tubes, as observed also by other researchers [41]. As a tube-shaped material, the shape of CNT is changed as the hydrates do not wrap the tube uniformly. There mainly two reasons why there are cement hydrates on CNT structure. First, the functional group (-OH) and hydrophilic group of surfactant PVP on CNT structure can absorb water which provides the potential condition for the cement particles to hydrate on the surface of CNT. Besides, as a kind of nano material with a huge specific surface area, CNT has very high surface energy which can promote the cement hydrates to cover on the surface of CNT. Individual CNTs can be distinctly observed, partly bridging the pores or gaps, and partly incorporated into the walls of the pores, gaps or the matrix surface. Comparing Fig. 2(b), (d) and (f), it can be seen that the bridging phenomenon of CNT on the pores or gaps was not affected by increasing the temperature from room temperature to 400oC, which also show that no spalling phenomenon occurred at the microstructure at 400oC, because spalling of cement hydrates on the walls of pores or gaps implies spalling of the bridging CNT too. When the temperature reaches 600oC, CNT cannot be easily found in the SEM image (Fig. 2(h)). It is a common phenomenon that there are no individual CNTs appearing in the pores and gaps, as shown in Fig. 2(h). This phenomenon implies that the CNT was spalled together with hydrates on the walls of the pores and gaps in the matrix. Fig 2(i) shows an individual CNT laid on the surface of the matrix, which demonstrates the existence of the spalling phenomenon at the micro level. In addition, in the process of taking images of specimens with 600oC heat treatment (Fig. 2(i)), it was found that the dislocated grains on the matrix surface easily fluctuated, which was attributed to the scanning electrons, and which was not seen in the process of taking images representing the surface treated at lower temperatures. This indicates that distinct spalling was caused when the temperature was raised to 600oC, while no apparent dislocation occurred at 400oC, as proved
by the continued existence of bridging CNT. 3.3 Residual mechanical properties To predict the mechanical behavior of CNT/cement material after exposure to high temperatures, the flexural and compressive strength were measured when the specimens were cooled to room temperature (25oC). The results of the mechanical strength of CNT-reinforced cementitious composites exposed to various high temperatures are shown in Fig. 3 and 4. In general, the failure section was happen at the middle of the specimens when the temperature was below 400 oC. However, when the temperature reached 600oC, the growth of micro-cracks caused the failure section was not occurred at the middle of the specimens, therefore, it is not meaningful to calculate the flexural strength of the composites according to ASTM C 348 at 600oC. As shown in Fig. 3, the flexural strength at 25oC was improved by 9.1% and 12.8% with the addition of 0.1% and 0.2% CNT content, respectively. At 200oC, a sharp decrease of the flexural strength of the cementitious composites was measured. The flexural strength of the reference cement paste decreased by 61.0%, while the flexural strength of 0.1% CNT-reinforced cement paste decreased by 58.7%, with 59.5% for 0.2% CNT addition, which showed that the CNT/cement composites had almost the same degradation in terms of their flexural strength when the CNT content ranges from 0 to 0.2%. This agrees with most other researches [42, 43] that the cementitious composites decrease dramatically at elevated temperature ranging from room temperature to 200oC. At 400oC, the flexural strength decreased by 76.0%, 73.9% and 70.2% with the addition of 0.0% CNT, 0.10% CNT and 0.20% CNT, respectively. At the range of 200–400oC, the trend of the degradation of CNT/cement composites is slower than at the range of 25–200oC. The addition of 0.1% CNT into the cement paste improved the flexural strength by 15.6% and 18.3% at 200oC and 400oC, respectively. The addition of 0.2% CNT
into the cement paste enhanced the flexural strength by 17.2% and 39.6% at 200oC and 400oC, respectively. In general, the degree of enhancement by CNT of the mechanical properties increased with increase of the temperature from 25–400oC. As shown in Fig. 4, there were improvements of 8.1% and 11.9% in the compressive strength of CNT-reinforced composites with the addition of 0.1% and 0.2% CNT content, respectively. At 200oC, the degradation in terms of compressive strength for the three batches of specimens with increased CNT content is 21.5%, 23.5% and 22.3% respectively, indicating the same degree of degradation existing in the range of 25–200oC. Up to 400oC, the reference cement paste showed a further obvious decrease in the compressive strength, while the paste containing CNT showed almost no decreasing trend. The strength of the reference decreased by 14.7% compared with that at 200oC, while the 0.1% CNT/cement paste only decreased by 0.4% and the 0.1% CNT/cement paste only decreased by 2.9%. Sahmaran et al., who incorporated PVA fiber into mortar, also found that the PVA fiber/mortar showed the same phenomenon, that the compressive strength showed no decrease as the heating temperature rose from 200oC to 400oC, while the mortar strength decreased obviously [35]. At 600oC, the CNT/cement composites have a sharper decrease than the reference specimen from 400–600oC. The compressive strength of the three batches of specimens with different CNT content decreased by 67.4% for the reference specimen, 71.5% for the specimen with 0.1% CNT content and 69.7% for the specimen with 0.2% CNT content. The addition of 0.1% CNT into the cement paste improved the compressive strength by 5.4%, 23.1% and -2.1% at 200oC, 400oC and 600oC, respectively. The addition of 0.2% CNT into the cement paste enhanced the flexural strength by 10.7%, 26.1% and 3.9% at 200 oC, 400oC and 600oC, respectively. In conclusion, the CNT enhanced the compressive strength well at 400oC, while the enhancement disappeared at 600oC.
3.4 Discussion The two main factors that affect the mechanical properties of cement matrix are the decomposition of hydration products (calcium silicate hydrates gel, calcium hydroxide and ettringite) [42] and the pore structure [35, 44]. By analyzing the XRD results for plain cement and CNT/cement paste, it can be found that the hydration process of cement particles continued at the range of 25–400oC due to the internal autoclaving conditions caused by high pressure water vapor. However, the high pressure could also cause the growth of the internal crack and pore size [35], which was the main reason for the degrading of the mechanical process at 200oC and 400oC, considering that the main components were not decomposed at 200oC and 400oC (XDR results). The XRD results show that the C-S-H gel and calcium hydroxide decomposed dramatically at 600oC. The SEM images indicate that the morphology of the CNT/cement material is greatly affected by the release of physically bound water and chemically bound water at 400oC. Obviously, the release of bound water from 200oC to 400oC did not cause a distinct loss of the mechanical properties of CNT-reinforced cementitious composites. The spalling of CNT happened only when the temperature reached 600oC. The C-S-H gel was partly decomposed into C2S and C3S crystal structure (according to the XRD images) and partly turned into ill-crystallized structure (according to the SEM images) at this temperature. Until reaching 600oC, the decomposition of the main components and further growth of cracks contributed to a sharp decrease in the strength of plain cement. The addition of CNT clearly did not enhance the mechanical properties to any great extent compared with the findings of other researchers [45, 46], indicating a moderate bonding between the CNT and cement hydrates due to the less strong friction properties of the carbon tube structure [47].
From 25oC to 200oC, the enhancement due to CNT of the mechanical properties resulted from the continued existence of bridging CNT in the gaps and pores. However, the degree of enhancement of the flexural strength was improved, while the degree of enhancement of the compressive strength was not. A possible reason is that the bending strength is more sensitive to inner cracks [42, 43], so the little limiting effect of CNT on the crack growth under 200oC could be reflected in the flexural strength measurement. From 200oC to 400oC, the flexural and compressive strength was improved most by CNT at 400oC. The compressive strength showed a negligible fall. At the range of 25oC to 400oC, the effect of CNT on the further hydration process of cement matrix is negative, as reflected by the peak value of calcium hydroxide. This indicated that the CNT had the potential to work as channels for the release of autoclaving steam, at the same time as a way to prevent the damage caused by the high-pressure steam. This explains why CNT can maintain the compressive strength and improve the mechanical properties most from 200oC to 400oC even though CNT is not good for the further hydration process of cement paste. Singh et al. [48] reported the process of water molecules passing through inner channels of CNT by simulations and experiments. Thomas et al. [49] observed in both experiments and simulation the extremely rapid water flow through inner channels of CNT. In addition, the production method and ultrasonication treatment can also introduce defects on the surface of the CNT, as a result of which the CNT provides more entrances and exits for water vapor [50]. Based on these findings, CNT has an obvious ability to deliver water vapor. From 400oC to 600oC, the CNT almost lost its bridging effect in the gaps and pores of the matrix. This was why the addition of CNT no longer enhanced or increased the properties. C-S-H gel and calcium hydroxide, the two main components of cement hydrates, were decomposed and obvious cracks appeared. These two reasons are attributed to the sharp decrease of mechanical properties.
From this study, CNT has the potential ability to provide channels for releasing harmful high-pressure steam by means of its hollow structure. Compared with other methods using microfibers (like polyvinyl alcohol (PVA) and polypropylene (PP)) to release the steam, these fibers have very low melting points and turn into channels after melting [35, 42, 51]. Sahmaran et al. [35] found that the compressive strength remained stable in the range of 200–400oC after adding PVA fiber into mortar. Some researchers [52, 53] have reported that melted PP fiber reduced the moisture content and improved the mechanical properties of mortar.
4. Conclusions The objective of this study was to investigate the effects of high temperature (25oC, 200oC, 400oC and 600oC) on the microstructure and residual strength of cementitious composites reinforced with CNT. CNT/cement mixtures were prepared with 0.1% and 0.2% CNT content. Some conclusions can be drawn from this study: 1. CNT has no effect on the hydration process and no potential to combine with cement hydrates in a chemical way according to the XRD results. 2. In the range of 25–400oC, the hydration process is slightly improved according to the peaks of calcium hydroxide at 18.007o and the OPC minerals (d=2.776–2.785Å). The addition of CNT impedes the further hydration process. Over 400–600oC, it is a decomposition process of the main cement hydrates (C-S-H gel and calcium hydroxide), which contributes to the sharp decrease of the mechanical performance of the cementitious matrix. 3. It can be concluded that when the temperature is over 200oC, the morphology of the CNT/cement matrix surface becomes more ill-crystallized with increasing temperature. From 400oC to
600oC, the decomposition leads to the spalling of cement hydrates and minerals from the walls of pores and gaps together with CNT, indicating the loss of its bridging effect. Multiple micro-cracks appear at 600oC. 4. Flexural strength drops faster than compressive strength because the bending test is more sensitive to inner cracks. The reinforcing effect of CNT on flexural strength was promoted with the increasing temperature, while the compressive strength remains stable between 200oC and 400oC. Both flexural and compressive strength exhibit the greatest enhancement effect at 400oC. The strength of CNT-reinforced cementitious composites at 600oC agrees with that of plain cement because of the spalling of CNT. 5. A possible reason why the further hydration process was impeded by the addition of CNT and the strength was improved greatly at 400oC is that CNT itself works as channels for the release of high-pressure steam to reduce the crack growth due to the steam.
Acknowledgements The work described in this paper was fully supported by grants from the National Natural Science Foundation of China (Grant Nos. 51378448 and 11402142) and the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 9042047, CityU 11208914).
References [1] Baughman RH, Zakhidov AA, de Heer WA. Carbon nanotubes--the route toward applications. Science. 2002;297(5582):787-92. [2] Valcarcel M, Simonet BM, Cardenas S, Suárez B. Present and future applications of carbon nanotubes to analytical science. Analytical and bioanalytical chemistry. 2005;382(8):1783-90. [3] Liew KM, Kai MF, Zhang LW. Carbon nanotube reinforced cementitious composites: An overview. Composites Part A: Applied Science and Manufacturing. 2016;91:301-23. [4] Xie X-L, Mai Y-W, Zhou X-P. Dispersion and alignment of carbon nanotubes in polymer matrix: a review. Materials Science and Engineering: R: Reports. 2005;49(4):89-112. [5] Yu M-F, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS. Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load. Science. 2000;287(5453):637-40. [6] Han B, Sun S, Ding S, Zhang L, Yu X, Ou J. Review of nanocarbon-engineered multifunctional cementitious composites. Composites Part A: Applied Science and Manufacturing. 2015;70:69-81. [7] Thostenson ET, Li C, Chou T-W. Nanocomposites in context. Composites Science and Technology. 2005;65(3):491-516. [8] Lu X, Chen Z. Curved pi-conjugation, aromaticity, and the related chemistry of small fullerenes and and single-walled carbon nanotubes. Chemical Reviews. 2005;105(10):3643-96. [9] Hong S, Myung S. A flexible approach to mobility. Nature Nanotech. 2007;2:207-8. [10] Al-Hamadani YA, Chu KH, Son A, Heo J, Her N, Jang M, et al. Stabilization and dispersion of carbon nanomaterials in aqueous solutions: A review. Separation and Purification Technology. 2015;156:861-74. [11] Pop E, Mann D, Wang Q, Goodson K, Dai H. Thermal conductance of an individual single-wall
carbon nanotube above room temperature. Nano letters. 2006;6(1):96-100. [12] Sinha S, Barjami S, Iannacchione G, Schwab A, Muench G. Off-axis thermal properties of carbon nanotube films. Journal of Nanoparticle Research. 2005;7(6):651-7. [13] Zou B, Chen SJ, Korayem AH, Collins F, Wang C, Duan WH. Effect of ultrasonication energy on engineering properties of carbon nanotube reinforced cement pastes. Carbon. 2015;85:212-20. [14] Ravindran S, Chaudhary S, Colburn B, Ozkan M, Ozkan CS. Covalent coupling of quantum dots to multiwalled carbon nanotubes for electronic device applications. Nano Letters. 2003;3(4):447-53. [15] Chan LY, Andrawes B. Finite element analysis of carbon nanotube/cement composite with degraded bond strength. Computational Materials Science. 2010;47(4):994-1004. [16] Eftekhari M, Mohammadi S. Molecular dynamics simulation of the nonlinear behavior of the CNT-reinforced calcium silicate hydrate (C–S–H) composite. Composites Part A: Applied Science and Manufacturing. 2016;82:78-87. [17] Cwirzen A, Habermehl-Cwirzen K, Nasibulin A, Kaupinen E, Mudimela P, Penttala V. SEM/AFM studies of cementitious binder modified by MWCNT and nano-sized Fe needles. Materials characterization. 2009;60(7):735-40. [18] del Carmen Camacho M, Galao O, Baeza FJ, Zornoza E, Garcés P. Mechanical properties and durability of CNT cement composites. Materials. 2014;7(3):1640-51. [19] Coppola L, Cadoni E, Forni D, Buoso A. Mechanical characterization of cement composites reinforced with fiberglass, carbon nanotubes or glass reinforced plastic (GRP) at high strain rates. Applied Mechanics and Materials: Trans Tech Publ; 2011. p. 190-5. [20] Musso S, Tulliani J-M, Ferro G, Tagliaferro A. Influence of carbon nanotubes structure on the mechanical
behavior
of
cement
composites.
Composites
Science
and
Technology.
2009;69(11):1985-90. [21] Makar J, Margeson J, Luh J. Carbon nanotube/cement composites-early results and potential applications.
Proceedings of the 3rd International Conference on Construction Materials:
Performance, Innovations and Structural Implications. Vancouver, B.C., Canada.2005. p. 1-10. [22] Liew KM, Kai MF, Zhang LW. Mechanical and damping properties of CNT-reinforced cementitious composites. Composite Structures. 2017;160:81-8. [23] Han B, Yu X, Ou J. Multifunctional and smart carbon nanotube reinforced cement-based materials. Nanotechnology in civil infrastructure: Springer; 2011. p. 1-47. [24] Han B, Zhang K, Yu X, Kwon E, Ou J. Fabrication of piezoresistive CNT/CNF cementitious composites with superplasticizer as dispersant. Journal of Materials in Civil Engineering. 2011;24(6):658-65. [25] Han B, Yu X, Kwon E, Ou J. Piezoresistive multi-walled carbon nanotubes filled cement-based composites. Sensor Letters. 2010;8(2):344-8. [26] Han B, Yu X, Ou J. Effect of water content on the piezoresistivity of MWNT/cement composites. Journal of materials science. 2010;45(14):3714-9. [27] Han B, Zhang K, Yu X, Kwon E, Ou J. Electrical characteristics and pressure-sensitive response measurements of carboxyl MWNT/cement composites. Cement and Concrete Composites. 2012;34(6):794-800. [28] Konsta-Gdoutos MS, Aza CA. Self sensing carbon nanotube (CNT) and nanofiber (CNF) cementitious composites for real time damage assessment in smart structures. Cement and Concrete Composites. 2014;53:162-9. [29] Veedu VP. Multifunctional cementitious nanocomposite material and methods of making the same.
Google Patents; 2011. [30] Yakovlev G, Kerienė J, Gailius A, Girnienė I. Cement based foam concrete reinforced by carbon nanotubes. Materials Science [Medžiagotyra]. 2006;12(2):147-51. [31] Alonso C, Fernandez L. Dehydration and rehydration processes of cement paste exposed to high temperature environments. Journal of Materials Science. 2004;39(9):3015-24. [32] Singh AP, Govind, Dhawan SK, Gupta BK, Mishra M, Chandra A, et al. Multiwalled carbon nanotube/cement composites with exceptional electromagnetic interference shielding properties. Carbon. 2013;56(5):86-96. [33] Alarcon-Ruiz L, Platret G, Massieu E, Ehrlacher A. The use of thermal analysis in assessing the effect of temperature on a cement paste. Cement & Concrete Research. 2005;35(35):609-13. [34] Piasta J, Sawicz Z, Rudzinski L. Changes in the structure of hardened cement paste due to high temperature. Matériaux et Construction. 1984;17(4):291-6. [35] Şahmaran M, Özbay E, Yücel HE, Lachemi M, Li VC. Effect of fly ash and PVA fiber on microstructural damage and residual properties of engineered cementitious composites exposed to high temperatures. Journal of Materials in Civil Engineering. 2011;23(12):1735-45. [36] Englert G, Wittmann F. Water in hardened cement paste. Materials & Structures. 1968;1(6):535-46. [37] Chem. A. JOINT COMMITTEE ON POWDER DIFFRACTION STANDARDS. Analytical Chemistry. 1970. [38] Janotka I, Nürnbergerová T. Effect of temperature on structural quality of the cement paste and high-strength concrete with silica fume. Nuclear Engineering & Design. 2005;235(17):2019-32. [39] Chan YN, Peng GF, Anson M. Residual strength and pore structure of high-strength concrete and
normal strength concrete after exposure to high temperatures. Cement & Concrete Composites. 1999;21(1):23-7. [40] Khoury GA, Sullivan PJE, Grainger BN. Transient thermal strain of concrete: Literature review, conditions within specimen and behaviour of individual constituents. Magazine of Concrete Research. 1985;37(132):131-44. [41] Makar JM, Chan GW. Growth of cement hydration products on single walled carbon nanotubes. Journal of the American Ceramic Society. 2009;92(6):1303-10. [42] Çavdar A. A study on the effects of high temperature on mechanical properties of fiber reinforced cementitious composites. Composites Part B: Engineering. 2012;43(5):2452-63. [43] Çavdar A. The effects of high temperature on mechanical properties of cementitious composites reinforced with polymeric fibers. Composites Part B Engineering. 2013;45(1):78–88. [44] Rostasy F, Weiβ R, Wiedemann G. Changes of pore structure of cement mortars due to temperature. Cement and Concrete Research. 1980;10(2):157-64. [45] Konsta-Gdoutos MS, Metaxa ZS, Shah SP. Highly dispersed carbon nanotube reinforced cement based materials. Cement & Concrete Research. 2010;40(7):1052-9. [46] Lai YC, Andrawes B. Finite element analysis of carbon nanotube/cement composite with degraded bond strength. Computational Materials Science. 2010;47(4):994-1004. [47] Tamimi A, Hassan NM, Fattah K, Talachi A. Performance of cementitious materials produced by incorporating surface treated multiwall carbon nanotubes and silica fume. Construction & Building Materials. 2016;114:934-45. [48] Chan WF, Chen H, Surapathi A, Taylor MG, Shao X, Marand E, et al. Zwitterion Functionalized Carbon Nanotube/Polyamide Nanocomposite Membranes for Water Desalination. Acs Nano.
2013;7(6):5308-19. [49] Thomas M, Corry B. Thermostat choice significantly influences water flow rates in molecular dynamics studies of carbon nanotubes. Microfluidics & Nanofluidics. 2015;18(1):41-7. [50] Zou B, Chen SJ, Korayem AH, Collins F, Wang CM, Duan WH. Effect of ultrasonication energy on engineering properties of carbon nanotube reinforced cement pastes. Carbon. 2015;85:212-20. [51] Nishida A, Yamazaki N, Inoue H. Study on the properties of high strength concrete with short polypropylene fibre for spalling resistance. Shimizu Technical Research Bulletin. 1995;14:1-6. [52] Sarvaranta L, Mikkola E. Fibre mortar composites under fire conditions: effects of ageing and moisture content of specimens. Materials & Structures. 1994;27(9):532-8. [53] Leena S, Esko M. Fibre mortar composites in fire conditions. Fire & Materials. 2004;18(1):45-50.
Intensity/counts
2000
o
(a) C0-25 C
B
1600
B 1200 800
C
400
B
A
B
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
2000
o
Intensity/counts
(b) C2-25 C B
1600
B 1200 800
C
400
B
B
A
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
Intensity/counts
2000
o
(c) C0-200 C
B
1600
B
1200 800
B
C
400
B
c
0 5
10
15
20
25
30
35
40
degree
45
50
55
60
65
70
2000
o
1600
Intensity/counts
(d) C2-200 C
B B
1200 800
B
C
400
B
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
Intensity/counts
2000
o
(e) C0-400 C
B
1600
B
1200 800
B B
C
400 0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
Intensity/counts
2000
o
(f) C2-400 C
B
1600
B 1200 800
B
C
400
B
0 5
10
15
20
25
30
35
40
degree
45
50
55
60
65
70
Intensity/counts
2000
o
(g) C0-600 C
1600 1200 800
C 400
B
B
B
B
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
2000
o
Intensity/counts
(h) C2-600 C 1600 1200 800
C
400
B
B
B
B
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
Fig. 1 The typical XRD images of plain cement and CNT-reinforced cementitious composites at elevated temperatures (A=ettringite; B=calcium hydroxide; C=OPC minerals): (a) C0-25oC (The average peak value at 18.007o is 1530); (b) C2-25oC (The average peak value at 18.007o is 1523); (c) C0-200oC (The average peak value at 18.007o is 1599); (d) C2-200oC (The average peak value at 18.007o is 1578); (e) C0-400oC (The average peak value at 18.007o is 1648); (f) C2-400oC (The average peak value at 18.007o is 1594); (g) C0-600oC (The average peak value at 18.007o is 226); (h) C2-600oC (The average peak value at 18.007o is 235).
(a)
(b) CNT
Hydrates on CNT
(c)
(d) CNT
(e)
(f)
CNT
(g)
crack
(h) gap
(i)
CNT
Fig. 2 SEM images of 0.20%CNT/cement matrix before and after thermal treatment: (a) and (b) control (unheated); (c) and (d) after 200°C heat treatment; (e) and (f) after 400°C heat treatment; (g), (h) and (i) after 600°C heat treatment.
10
C0 C1 C2
Flexural strength (MPa)
8
6
4
2
0 0
200
400
600
o
Temperature ( C)
Fig. 3 Degradation of CNT-reinforced cementitious composites in terms of flexural strength
60
C0 C1 C2
Compressive strength (MPa)
55 50 45 40 35 30 25 20 15 10
0
200
400
600
o
Temperature ( C)
Fig. 4 Degradation of CNT-reinforced cementitious composites in terms of compressive strength
Table 1 Property of CNT Type
Aspect ratio
Diameter
Length
OH content
Purity
Ash
Surface area
MWCNT-OH
<400
>50nm
20μm
0.76wt%
>90wt%
<6wt%
>40m2/g
Table 2 Mix design of CNT-reinforced cementitious composites Mix
Cement (g)
Water (g)
CNT (g)
PVP (g)
CNT/cement
C0
100
40
-
-
-
C1
100
40
0.1
0.02
0.10wt.%
C2
100
40
0.2
0.04
0.20wt.%
S1359-835X(17)30051-9 http://dx.doi.org/10.1016/j.compositesa.2017.02.001 JCOMA 4562
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
9 November 2016 25 January 2017 3 February 2017
Please cite this article as: Zhang, L.W., Kai, M.F., Liew, K.M., Evaluation of microstructure and mechanical performance of CNT-reinforced cementitious composites at elevated temperatures, Composites: Part A (2017), doi: http://dx.doi.org/10.1016/j.compositesa.2017.02.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of microstructure and mechanical performance of CNT-reinforced cementitious composites at elevated temperatures
L.W. Zhang 1,*, M.F. Kai2,3, K.M. Liew 2,3,* 1
School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
2
Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong, China 3
City University of Hong Kong Shenzhen Research Institute, Shenzhen 518057, China
Abstract Experimental studies are carried out to examine the microstructure and mechanical performance of carbon nanotube (CNT) reinforced cementitious composites at elevated temperatures (25oC, 200oC, 400oC and 600oC). Two different contents (0.1% and 0.2% by weight of cement) of CNT are added to cement paste. X-ray powder diffraction (XRD) results show that CNT does not improve or impede the hydration of cement minerals at room temperature, while at higher temperatures the further hydration process caused by high-pressure steam is impeded by CNT. The morphology observed by scanning electron microscopy (SEM) indicates that the bridging effect of CNT can be maintained below 400oC, while CNT is mostly spalled with the matrix on the walls of pores and gaps at 600oC. The residual flexural and compressive strength are measured. The reinforcing effect of CNT is most obvious at 400oC, which is due to the potential of CNT as channels for releasing high-pressure steam.
Keywords: CNT-reinforced cementitious composites; microstructure; mechanical performance; temperature
*Corresponding authors. E-mail addresses: [email protected] (L.W. Zhang); [email protected] (K.M. Liew)
1. Introduction Due to its extraordinary mechanical, electrical and thermal performance, CNT has become a candidate for use in many applications [1–3]. The tensile strength of CNT can be up to 500 GPa, while the strain at tensile failure has been predicted to be as high as 30% [4, 5]. The elastic modulus of CNT is reported to be near 1 TPa [6]. The electrical conductivity of CNT is reported to be in the range of 102–10-4 s/cm and strongly related to the clarity of its structure [7, 8]. The electric current density has been predicted to be more than 1,000 times greater than those of metals such as copper in theory [9]. In addition, CNT has significantly greater thermal stability than graphene, while it shares the same thermal conductivity as graphene [4]. The thermal stability is in the range of 600–750oC [7, 10]. The thermal conductivity along the axis of CNT is almost nine times greater than copper [11]. The thermal conductivity across its axis is reported to be as high as soil [12]. Due to substantial van der Waals attractions, sonication and surfactants are commonly used to disperse CNT in aqueous solution at first for its application in cementitious composites [13]. In addition, the stabilization of the solutions can be controlled by surfactants that decorate the CNT surface permanently and change the wettability of the CNT surface [10, 14]. Till now, the dispersion of CNT has been well solved and it can be well mixed in cement paste [6]. The mechanical and electrical performance of CNT-reinforced cementitious composites has been widely investigated [6]. Chan and Andrawes [15] experimentally and theoretically proved that CNT can enhance the mechanical properties of cement paste, as demonstrated by many other researchers [7, 13]. Eftekhari and Mohammadi [16] employed molecular dynamics simulation and proposed that the CNT orientation plays a decisive role in the enhancement effect of CNT in cement paste. Cwirzen et al. [17] investigated the compressive and flexural strength of CNT-reinforced cementitious composites and no
enhancement effect was found to exist, as stated by some other studies [18, 19]. However, some researchers [20, 21] found that the incorporation of CNT in cement matrix can reduce the mechanical properties. The compressive strength and rupture modulus measured by Musso et al. [20] showed sharp decreases. The improvement mainly resulted from the efficient bonding for load-transfer that was caused by CNT bridging in the cementitious composites [16]. Liew et al. [22] affirmed that the bonding strength can be affected by the type of surfactants and a lower bonding strength can improve the damping property of the CNT/cement composites. In addition, much effort has been made to research the electrical properties of CNT/cement matrix. It is a common conclusion that the addition of CNT decreases the electrical resistivity of cementitious composites while it increases their electrical conductivity. Its improvement range depends strongly on the concentration, the surface condition, the type and the degree of dispersion of the CNT [23–27]. Based on these, researchers have made many efforts to study the sensing behavior of CNT/cement composites for use in self-sensing and smart materials [23–28]. Though the thermal conductivity of CNT-reinforced composites has been researched in recent years [29, 30], there are few investigations about the high-temperature resistance performance of the composites. Fire is a risk for cementitious composites considering their unstable structures under high temperature and transformation of the hydrated products, so it is meaningful to investigate the effect of the incorporation of CNT into cement paste on the mechanical properties and microstructure at
elevated
temperatures,
and
to
determine
the
application
potential
of
CNT
on
high-temperature-resistant cementitious material. In this study, the compressive and flexural strength of the CNT/cement material treated with high temperatures were determined. The changes of mechanical properties of CNT/cement composites at high temperatures are correlated with chemical and microstructural changes, so the chemical
components were analyzed by XRD and the microstructure was detected by SEM.
2. Experiment 2.1 Material Hydroxylated multi-walled CNT (MWCNT-OH) was provided by Chengdu Organic Chemicals Co., Ltd. in China and produced by the chemical vapor deposition (CVD) method. The properties of CNT are presented in Table 1. The surfactant employed in this work was a type of non-ionic surfactant named polyvinylpyrrolidone (PVP), provided by Shenzhen Benno Industrial Co., Ltd. The binder material used in the research was Ordinary Portland Cement (OPC) type 42.5R, conforming to the requirements of Chinese Standard GB175-2007. 2.2 Preparation of specimens The compositions of two mixtures are presented in Table 2. First, CNT and PVP were added into water for dispersal. 20% of PVP by weight of CNT was used to help segregate the CNT and the technique of sonication (195 W, 15 min) was employed to achieve a satisfactory dispersion. After sonication, the cement was mixed together with the prepared solutions for 7 min using a cement paste mixer to obtain fresh cementitious composites with a water–cement ratio of 0.4. After mixing, the mixture was then poured into 40×40×160 mm molds for flexural strength and compressive strength tests. The mixture was compacted on a vibrating table by vibrating 30 times. The specimens were covered with plastic sheets till demolded after 24 h. Then the demolded specimens were kept in a moist room for 27 days in an environment of 20oC and relative humidity of 95%, leading to a total curing time of 28 days. Plain cement specimens (C0) were cast as a reference. All specimens were dried for 7 days at room temperature (25oC) and a relative humidity of 60% after curing.
2.3 Heating procedure Each specimen was placed in a furnace subjected to continuous heating at a rate of 3oC/min from the ambient temperature (25oC) to the target temperature (200oC, 400oC, 600oC). The target temperature was kept constant for a period of 1 hour in order to homogenize the inner temperature. After this, the specimens were naturally cooled in the oven with the furnace door closed to ensure a slow cooling rate. After cooling down to room temperature (25oC), each sample was kept in a hermetic plastic bag for tests and characterization. 2.4 Tests and characterizations The tests of flexural and compressive strength were conducted according to ASTM C 348 and ASTM C 349, respectively. After the 3-point flexural strength test, the compressive strength was measured from the end parts of the specimens. To portray the surface morphologies of the hydrated CNT-reinforced cementitious composites, scanning electron microscopy (SEM) (Hitachi, SU-70, Japan) was performed. The SEM samples were prepared in pellet form. The analysis of the crystalline phases was carried out using X-ray powder diffraction (XRD) (Bruker, AXS D8, German) using CuKα radiation (k = 1.540598Å) in the scattering range (2θ) of 5–70o with a scan rate of 0.1o/sec and slit width of 0.02o. The powder for XRD analysis, obtained by grinding the fragments of broken specimens, was sieved with a sieve having apertures of 40 μm. The theory of XRD is based on the diffraction properties of crystals. The Bragg equation (2dsinθ=nλ; d=interplanar spacing, θ=diffraction angle, λ=the wavelength of X-ray and n=arbitrary integer) was used to identify the crystals in a material. 3. Results and discussion 3.1 Phase composition The deterioration of the crystalline phase structure of plain cement and CNT/cement composites
subjected to various elevated temperatures was assessed by XRD analysis. Fig. 1 shows the deterioration of the cement hydrates, where the main peaks have been identified. The samples for XRD analysis were taken from the core of the 40 mm×40 mm×160 mm specimens. For each kind of specimens, three same samples were prepared to get the representative peak values. Considering that the peak at 18.007o is totally decided by calcium hydroxide crystals without the interference of other constituents on the images [31], it can be used to identify the hydration process. Fig. 1(a) and (b) show almost coincident images representing the crystalline phases of cement pastes with and without CNT, indicating that the CNT did not improve or impede the hydration process, which can also be concluded from the peak value of calcium hydroxide at 18.007o. The images showed that hydroxylated CNT did not combine with cement hydrates in a chemical way, which differs from the way that hydrates with carboxylated CNT are chemically combined, creating new peaks in the XRD images [32]. The peak values at 18.007o of plain cement at 25oC, 200oC, 400oC and 600oC are 1530, 1599, 1648 and 226. The difference of the peak values is obvious and can be well reflected by XRD test. The peak values at 18.007o of CNT/cement material at 25oC, 200oC, 400oC and 600oC are 1523, 1578, 1594 and 235. The difference of the peak values is also obvious and can be well reflected by XRD test. At the range of 25oC–200oC, the liberation of free water [33] and the decomposition of ettringite [31] occurred. It can be concluded from Fig. 1(c) and (d) (which represent the compositions of plain cement and CNT/cement materials respectively) that ettringite crystal almost totally deteriorated. It has been mentioned in several articles [34, 35] that the internal autoclaving conditions that form in cement paste under high temperature due to water vapor create high pressure (especially at the range of 100oC–300oC [34, 36]). The autoclaving conditions contribute to the additional hydration of
unhydrated cement particles. This explains why the peak value at 18.007o of calcium hydroxide under 200oC and 400oC increased slightly (Fig. 1(c), (d), (e) and (f)). This can also be found from the intensity of the superimposed reflections of the OPC minerals (C2S and C3S) [d=2.776–2.785Å] [37], which decreased due to further hydration. At 200oC, the peak value at 18.007o of CNT/cement composites is lower than that of the peak value of plain cement, which can be stated that 0.2% CNT can slightly decrease the further hydration process of cement matrix under a high temperature (200oC). From 200oC to 400oC, the physically bound water [35, 36] and chemically bound water of gel-like hydration products (C-S-H) [34, 38] were released from the specimens. However, the dehydration process of C-S-H gel did not enhance the intensity of the superimposed reflections of the OPC mineral crystal structure (Fig. 1(e) and (f)), which implied that the dehydration process was not severe and that the gel structure of C-S-H was not transferred into C2S or C3S crystal structure, though part of the chemically bound water was taken up. Comparing Fig. 1(c) with Fig. 1(d) or comparing Fig. 1(e) with Fig. 1(f), it can be clearly seen that the peak value of calcium hydroxide of plain cement at 200oC and 400oC was higher than that of 0.2% CNT/cement, indicating that the hydration process of pastes without CNT was better accelerated. At 400oC, the peak value at 18.007o of CNT/cement composites is lower than that of the peak value of plain cement, which can be stated that the further hydration process was decreased more obviously due to 0.2% CNT. At higher temperatures of 600oC, the change of the matrix with and without CNT was primarily caused by the further dehydration of C-S-H gel and calcium hydroxide [39, 40]. It is clear that broad superimposed reflections (Fig. 1(g) and (h)) of OPC minerals appeared, as observed in [34], indicating a huge decomposition of C-S-H gel and the formation of large quantities of C2S and C3S. Calcium hydroxide greatly deteriorated at 600oC as shown in Fig. 1(g) and (h). In general, the XRD images of
plain cement at 600oC were again basically coincident with those of paste with CNT. This implies that CNT did not influence the decomposition process of the hydration products at this temperature. 3.2 Microstructure observation In order to investigate the morphology of the cement matrix microstructure and the behavior of CNT subjected to various elevated temperatures, small sheet samples were taken from the cores of the 40 mm×40 mm×160 mm specimens with 0.2% CNT content for SEM observations. Fig. 2(a), (c), (e) and (g) show the general view of the surface morphology of the microstructure of the matrix before and after thermal treatment. Fig. 2(a) shows the unheated specimens that primarily comprise ill-crystallized and fibrous particles of calcium-silicate-hydrate (C-S-H) gel, amorphous and well-crystallized calcium hydroxide and invisible CNT [35]. It can be seen by comparing Fig. 2(a) with Fig. 2(c) that the matrix shows obvious integrality at 200oC, indicating that the crystal structure was not lost and the components of the main hydrates products did not decompose. When the temperature reached 400oC, there was a non-negligible change in the surface morphology, with part ill-crystallized or amorphous structures (Fig. 2(e)) resulting from the loss of bound water from the decomposition of C-S-H at the range from 200oC and 400oC [33]. The images at 400oC also show that the dehydration of part chemically bound water from C-S-H gel did not transfer from the gel structure into the crystal structure. Up to 600oC, most hydration products appear as ill-crystallized or amorphous structures with cracks on the surface (Fig. 2(g)), which is mainly due to the damage caused by the high temperature and the further decomposition of C-S-H gel and calcium hydroxide [31]. A more subtle level of images was performed on all the samples in order to study the state of CNT in cement paste after treatment at elevated temperature (Fig. 2(b), (d), (f), (h) and (i)). It can be concluded from Fig. 2(b) that the CNT is well mixed in the cement paste and it can be seen that the
cement hydrates grow on some tubes, as observed also by other researchers [41]. As a tube-shaped material, the shape of CNT is changed as the hydrates do not wrap the tube uniformly. There mainly two reasons why there are cement hydrates on CNT structure. First, the functional group (-OH) and hydrophilic group of surfactant PVP on CNT structure can absorb water which provides the potential condition for the cement particles to hydrate on the surface of CNT. Besides, as a kind of nano material with a huge specific surface area, CNT has very high surface energy which can promote the cement hydrates to cover on the surface of CNT. Individual CNTs can be distinctly observed, partly bridging the pores or gaps, and partly incorporated into the walls of the pores, gaps or the matrix surface. Comparing Fig. 2(b), (d) and (f), it can be seen that the bridging phenomenon of CNT on the pores or gaps was not affected by increasing the temperature from room temperature to 400oC, which also show that no spalling phenomenon occurred at the microstructure at 400oC, because spalling of cement hydrates on the walls of pores or gaps implies spalling of the bridging CNT too. When the temperature reaches 600oC, CNT cannot be easily found in the SEM image (Fig. 2(h)). It is a common phenomenon that there are no individual CNTs appearing in the pores and gaps, as shown in Fig. 2(h). This phenomenon implies that the CNT was spalled together with hydrates on the walls of the pores and gaps in the matrix. Fig 2(i) shows an individual CNT laid on the surface of the matrix, which demonstrates the existence of the spalling phenomenon at the micro level. In addition, in the process of taking images of specimens with 600oC heat treatment (Fig. 2(i)), it was found that the dislocated grains on the matrix surface easily fluctuated, which was attributed to the scanning electrons, and which was not seen in the process of taking images representing the surface treated at lower temperatures. This indicates that distinct spalling was caused when the temperature was raised to 600oC, while no apparent dislocation occurred at 400oC, as proved
by the continued existence of bridging CNT. 3.3 Residual mechanical properties To predict the mechanical behavior of CNT/cement material after exposure to high temperatures, the flexural and compressive strength were measured when the specimens were cooled to room temperature (25oC). The results of the mechanical strength of CNT-reinforced cementitious composites exposed to various high temperatures are shown in Fig. 3 and 4. In general, the failure section was happen at the middle of the specimens when the temperature was below 400 oC. However, when the temperature reached 600oC, the growth of micro-cracks caused the failure section was not occurred at the middle of the specimens, therefore, it is not meaningful to calculate the flexural strength of the composites according to ASTM C 348 at 600oC. As shown in Fig. 3, the flexural strength at 25oC was improved by 9.1% and 12.8% with the addition of 0.1% and 0.2% CNT content, respectively. At 200oC, a sharp decrease of the flexural strength of the cementitious composites was measured. The flexural strength of the reference cement paste decreased by 61.0%, while the flexural strength of 0.1% CNT-reinforced cement paste decreased by 58.7%, with 59.5% for 0.2% CNT addition, which showed that the CNT/cement composites had almost the same degradation in terms of their flexural strength when the CNT content ranges from 0 to 0.2%. This agrees with most other researches [42, 43] that the cementitious composites decrease dramatically at elevated temperature ranging from room temperature to 200oC. At 400oC, the flexural strength decreased by 76.0%, 73.9% and 70.2% with the addition of 0.0% CNT, 0.10% CNT and 0.20% CNT, respectively. At the range of 200–400oC, the trend of the degradation of CNT/cement composites is slower than at the range of 25–200oC. The addition of 0.1% CNT into the cement paste improved the flexural strength by 15.6% and 18.3% at 200oC and 400oC, respectively. The addition of 0.2% CNT
into the cement paste enhanced the flexural strength by 17.2% and 39.6% at 200oC and 400oC, respectively. In general, the degree of enhancement by CNT of the mechanical properties increased with increase of the temperature from 25–400oC. As shown in Fig. 4, there were improvements of 8.1% and 11.9% in the compressive strength of CNT-reinforced composites with the addition of 0.1% and 0.2% CNT content, respectively. At 200oC, the degradation in terms of compressive strength for the three batches of specimens with increased CNT content is 21.5%, 23.5% and 22.3% respectively, indicating the same degree of degradation existing in the range of 25–200oC. Up to 400oC, the reference cement paste showed a further obvious decrease in the compressive strength, while the paste containing CNT showed almost no decreasing trend. The strength of the reference decreased by 14.7% compared with that at 200oC, while the 0.1% CNT/cement paste only decreased by 0.4% and the 0.1% CNT/cement paste only decreased by 2.9%. Sahmaran et al., who incorporated PVA fiber into mortar, also found that the PVA fiber/mortar showed the same phenomenon, that the compressive strength showed no decrease as the heating temperature rose from 200oC to 400oC, while the mortar strength decreased obviously [35]. At 600oC, the CNT/cement composites have a sharper decrease than the reference specimen from 400–600oC. The compressive strength of the three batches of specimens with different CNT content decreased by 67.4% for the reference specimen, 71.5% for the specimen with 0.1% CNT content and 69.7% for the specimen with 0.2% CNT content. The addition of 0.1% CNT into the cement paste improved the compressive strength by 5.4%, 23.1% and -2.1% at 200oC, 400oC and 600oC, respectively. The addition of 0.2% CNT into the cement paste enhanced the flexural strength by 10.7%, 26.1% and 3.9% at 200 oC, 400oC and 600oC, respectively. In conclusion, the CNT enhanced the compressive strength well at 400oC, while the enhancement disappeared at 600oC.
3.4 Discussion The two main factors that affect the mechanical properties of cement matrix are the decomposition of hydration products (calcium silicate hydrates gel, calcium hydroxide and ettringite) [42] and the pore structure [35, 44]. By analyzing the XRD results for plain cement and CNT/cement paste, it can be found that the hydration process of cement particles continued at the range of 25–400oC due to the internal autoclaving conditions caused by high pressure water vapor. However, the high pressure could also cause the growth of the internal crack and pore size [35], which was the main reason for the degrading of the mechanical process at 200oC and 400oC, considering that the main components were not decomposed at 200oC and 400oC (XDR results). The XRD results show that the C-S-H gel and calcium hydroxide decomposed dramatically at 600oC. The SEM images indicate that the morphology of the CNT/cement material is greatly affected by the release of physically bound water and chemically bound water at 400oC. Obviously, the release of bound water from 200oC to 400oC did not cause a distinct loss of the mechanical properties of CNT-reinforced cementitious composites. The spalling of CNT happened only when the temperature reached 600oC. The C-S-H gel was partly decomposed into C2S and C3S crystal structure (according to the XRD images) and partly turned into ill-crystallized structure (according to the SEM images) at this temperature. Until reaching 600oC, the decomposition of the main components and further growth of cracks contributed to a sharp decrease in the strength of plain cement. The addition of CNT clearly did not enhance the mechanical properties to any great extent compared with the findings of other researchers [45, 46], indicating a moderate bonding between the CNT and cement hydrates due to the less strong friction properties of the carbon tube structure [47].
From 25oC to 200oC, the enhancement due to CNT of the mechanical properties resulted from the continued existence of bridging CNT in the gaps and pores. However, the degree of enhancement of the flexural strength was improved, while the degree of enhancement of the compressive strength was not. A possible reason is that the bending strength is more sensitive to inner cracks [42, 43], so the little limiting effect of CNT on the crack growth under 200oC could be reflected in the flexural strength measurement. From 200oC to 400oC, the flexural and compressive strength was improved most by CNT at 400oC. The compressive strength showed a negligible fall. At the range of 25oC to 400oC, the effect of CNT on the further hydration process of cement matrix is negative, as reflected by the peak value of calcium hydroxide. This indicated that the CNT had the potential to work as channels for the release of autoclaving steam, at the same time as a way to prevent the damage caused by the high-pressure steam. This explains why CNT can maintain the compressive strength and improve the mechanical properties most from 200oC to 400oC even though CNT is not good for the further hydration process of cement paste. Singh et al. [48] reported the process of water molecules passing through inner channels of CNT by simulations and experiments. Thomas et al. [49] observed in both experiments and simulation the extremely rapid water flow through inner channels of CNT. In addition, the production method and ultrasonication treatment can also introduce defects on the surface of the CNT, as a result of which the CNT provides more entrances and exits for water vapor [50]. Based on these findings, CNT has an obvious ability to deliver water vapor. From 400oC to 600oC, the CNT almost lost its bridging effect in the gaps and pores of the matrix. This was why the addition of CNT no longer enhanced or increased the properties. C-S-H gel and calcium hydroxide, the two main components of cement hydrates, were decomposed and obvious cracks appeared. These two reasons are attributed to the sharp decrease of mechanical properties.
From this study, CNT has the potential ability to provide channels for releasing harmful high-pressure steam by means of its hollow structure. Compared with other methods using microfibers (like polyvinyl alcohol (PVA) and polypropylene (PP)) to release the steam, these fibers have very low melting points and turn into channels after melting [35, 42, 51]. Sahmaran et al. [35] found that the compressive strength remained stable in the range of 200–400oC after adding PVA fiber into mortar. Some researchers [52, 53] have reported that melted PP fiber reduced the moisture content and improved the mechanical properties of mortar.
4. Conclusions The objective of this study was to investigate the effects of high temperature (25oC, 200oC, 400oC and 600oC) on the microstructure and residual strength of cementitious composites reinforced with CNT. CNT/cement mixtures were prepared with 0.1% and 0.2% CNT content. Some conclusions can be drawn from this study: 1. CNT has no effect on the hydration process and no potential to combine with cement hydrates in a chemical way according to the XRD results. 2. In the range of 25–400oC, the hydration process is slightly improved according to the peaks of calcium hydroxide at 18.007o and the OPC minerals (d=2.776–2.785Å). The addition of CNT impedes the further hydration process. Over 400–600oC, it is a decomposition process of the main cement hydrates (C-S-H gel and calcium hydroxide), which contributes to the sharp decrease of the mechanical performance of the cementitious matrix. 3. It can be concluded that when the temperature is over 200oC, the morphology of the CNT/cement matrix surface becomes more ill-crystallized with increasing temperature. From 400oC to
600oC, the decomposition leads to the spalling of cement hydrates and minerals from the walls of pores and gaps together with CNT, indicating the loss of its bridging effect. Multiple micro-cracks appear at 600oC. 4. Flexural strength drops faster than compressive strength because the bending test is more sensitive to inner cracks. The reinforcing effect of CNT on flexural strength was promoted with the increasing temperature, while the compressive strength remains stable between 200oC and 400oC. Both flexural and compressive strength exhibit the greatest enhancement effect at 400oC. The strength of CNT-reinforced cementitious composites at 600oC agrees with that of plain cement because of the spalling of CNT. 5. A possible reason why the further hydration process was impeded by the addition of CNT and the strength was improved greatly at 400oC is that CNT itself works as channels for the release of high-pressure steam to reduce the crack growth due to the steam.
Acknowledgements The work described in this paper was fully supported by grants from the National Natural Science Foundation of China (Grant Nos. 51378448 and 11402142) and the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 9042047, CityU 11208914).
References [1] Baughman RH, Zakhidov AA, de Heer WA. Carbon nanotubes--the route toward applications. Science. 2002;297(5582):787-92. [2] Valcarcel M, Simonet BM, Cardenas S, Suárez B. Present and future applications of carbon nanotubes to analytical science. Analytical and bioanalytical chemistry. 2005;382(8):1783-90. [3] Liew KM, Kai MF, Zhang LW. Carbon nanotube reinforced cementitious composites: An overview. Composites Part A: Applied Science and Manufacturing. 2016;91:301-23. [4] Xie X-L, Mai Y-W, Zhou X-P. Dispersion and alignment of carbon nanotubes in polymer matrix: a review. Materials Science and Engineering: R: Reports. 2005;49(4):89-112. [5] Yu M-F, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS. Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load. Science. 2000;287(5453):637-40. [6] Han B, Sun S, Ding S, Zhang L, Yu X, Ou J. Review of nanocarbon-engineered multifunctional cementitious composites. Composites Part A: Applied Science and Manufacturing. 2015;70:69-81. [7] Thostenson ET, Li C, Chou T-W. Nanocomposites in context. Composites Science and Technology. 2005;65(3):491-516. [8] Lu X, Chen Z. Curved pi-conjugation, aromaticity, and the related chemistry of small fullerenes and and single-walled carbon nanotubes. Chemical Reviews. 2005;105(10):3643-96. [9] Hong S, Myung S. A flexible approach to mobility. Nature Nanotech. 2007;2:207-8. [10] Al-Hamadani YA, Chu KH, Son A, Heo J, Her N, Jang M, et al. Stabilization and dispersion of carbon nanomaterials in aqueous solutions: A review. Separation and Purification Technology. 2015;156:861-74. [11] Pop E, Mann D, Wang Q, Goodson K, Dai H. Thermal conductance of an individual single-wall
carbon nanotube above room temperature. Nano letters. 2006;6(1):96-100. [12] Sinha S, Barjami S, Iannacchione G, Schwab A, Muench G. Off-axis thermal properties of carbon nanotube films. Journal of Nanoparticle Research. 2005;7(6):651-7. [13] Zou B, Chen SJ, Korayem AH, Collins F, Wang C, Duan WH. Effect of ultrasonication energy on engineering properties of carbon nanotube reinforced cement pastes. Carbon. 2015;85:212-20. [14] Ravindran S, Chaudhary S, Colburn B, Ozkan M, Ozkan CS. Covalent coupling of quantum dots to multiwalled carbon nanotubes for electronic device applications. Nano Letters. 2003;3(4):447-53. [15] Chan LY, Andrawes B. Finite element analysis of carbon nanotube/cement composite with degraded bond strength. Computational Materials Science. 2010;47(4):994-1004. [16] Eftekhari M, Mohammadi S. Molecular dynamics simulation of the nonlinear behavior of the CNT-reinforced calcium silicate hydrate (C–S–H) composite. Composites Part A: Applied Science and Manufacturing. 2016;82:78-87. [17] Cwirzen A, Habermehl-Cwirzen K, Nasibulin A, Kaupinen E, Mudimela P, Penttala V. SEM/AFM studies of cementitious binder modified by MWCNT and nano-sized Fe needles. Materials characterization. 2009;60(7):735-40. [18] del Carmen Camacho M, Galao O, Baeza FJ, Zornoza E, Garcés P. Mechanical properties and durability of CNT cement composites. Materials. 2014;7(3):1640-51. [19] Coppola L, Cadoni E, Forni D, Buoso A. Mechanical characterization of cement composites reinforced with fiberglass, carbon nanotubes or glass reinforced plastic (GRP) at high strain rates. Applied Mechanics and Materials: Trans Tech Publ; 2011. p. 190-5. [20] Musso S, Tulliani J-M, Ferro G, Tagliaferro A. Influence of carbon nanotubes structure on the mechanical
behavior
of
cement
composites.
Composites
Science
and
Technology.
2009;69(11):1985-90. [21] Makar J, Margeson J, Luh J. Carbon nanotube/cement composites-early results and potential applications.
Proceedings of the 3rd International Conference on Construction Materials:
Performance, Innovations and Structural Implications. Vancouver, B.C., Canada.2005. p. 1-10. [22] Liew KM, Kai MF, Zhang LW. Mechanical and damping properties of CNT-reinforced cementitious composites. Composite Structures. 2017;160:81-8. [23] Han B, Yu X, Ou J. Multifunctional and smart carbon nanotube reinforced cement-based materials. Nanotechnology in civil infrastructure: Springer; 2011. p. 1-47. [24] Han B, Zhang K, Yu X, Kwon E, Ou J. Fabrication of piezoresistive CNT/CNF cementitious composites with superplasticizer as dispersant. Journal of Materials in Civil Engineering. 2011;24(6):658-65. [25] Han B, Yu X, Kwon E, Ou J. Piezoresistive multi-walled carbon nanotubes filled cement-based composites. Sensor Letters. 2010;8(2):344-8. [26] Han B, Yu X, Ou J. Effect of water content on the piezoresistivity of MWNT/cement composites. Journal of materials science. 2010;45(14):3714-9. [27] Han B, Zhang K, Yu X, Kwon E, Ou J. Electrical characteristics and pressure-sensitive response measurements of carboxyl MWNT/cement composites. Cement and Concrete Composites. 2012;34(6):794-800. [28] Konsta-Gdoutos MS, Aza CA. Self sensing carbon nanotube (CNT) and nanofiber (CNF) cementitious composites for real time damage assessment in smart structures. Cement and Concrete Composites. 2014;53:162-9. [29] Veedu VP. Multifunctional cementitious nanocomposite material and methods of making the same.
Google Patents; 2011. [30] Yakovlev G, Kerienė J, Gailius A, Girnienė I. Cement based foam concrete reinforced by carbon nanotubes. Materials Science [Medžiagotyra]. 2006;12(2):147-51. [31] Alonso C, Fernandez L. Dehydration and rehydration processes of cement paste exposed to high temperature environments. Journal of Materials Science. 2004;39(9):3015-24. [32] Singh AP, Govind, Dhawan SK, Gupta BK, Mishra M, Chandra A, et al. Multiwalled carbon nanotube/cement composites with exceptional electromagnetic interference shielding properties. Carbon. 2013;56(5):86-96. [33] Alarcon-Ruiz L, Platret G, Massieu E, Ehrlacher A. The use of thermal analysis in assessing the effect of temperature on a cement paste. Cement & Concrete Research. 2005;35(35):609-13. [34] Piasta J, Sawicz Z, Rudzinski L. Changes in the structure of hardened cement paste due to high temperature. Matériaux et Construction. 1984;17(4):291-6. [35] Şahmaran M, Özbay E, Yücel HE, Lachemi M, Li VC. Effect of fly ash and PVA fiber on microstructural damage and residual properties of engineered cementitious composites exposed to high temperatures. Journal of Materials in Civil Engineering. 2011;23(12):1735-45. [36] Englert G, Wittmann F. Water in hardened cement paste. Materials & Structures. 1968;1(6):535-46. [37] Chem. A. JOINT COMMITTEE ON POWDER DIFFRACTION STANDARDS. Analytical Chemistry. 1970. [38] Janotka I, Nürnbergerová T. Effect of temperature on structural quality of the cement paste and high-strength concrete with silica fume. Nuclear Engineering & Design. 2005;235(17):2019-32. [39] Chan YN, Peng GF, Anson M. Residual strength and pore structure of high-strength concrete and
normal strength concrete after exposure to high temperatures. Cement & Concrete Composites. 1999;21(1):23-7. [40] Khoury GA, Sullivan PJE, Grainger BN. Transient thermal strain of concrete: Literature review, conditions within specimen and behaviour of individual constituents. Magazine of Concrete Research. 1985;37(132):131-44. [41] Makar JM, Chan GW. Growth of cement hydration products on single walled carbon nanotubes. Journal of the American Ceramic Society. 2009;92(6):1303-10. [42] Çavdar A. A study on the effects of high temperature on mechanical properties of fiber reinforced cementitious composites. Composites Part B: Engineering. 2012;43(5):2452-63. [43] Çavdar A. The effects of high temperature on mechanical properties of cementitious composites reinforced with polymeric fibers. Composites Part B Engineering. 2013;45(1):78–88. [44] Rostasy F, Weiβ R, Wiedemann G. Changes of pore structure of cement mortars due to temperature. Cement and Concrete Research. 1980;10(2):157-64. [45] Konsta-Gdoutos MS, Metaxa ZS, Shah SP. Highly dispersed carbon nanotube reinforced cement based materials. Cement & Concrete Research. 2010;40(7):1052-9. [46] Lai YC, Andrawes B. Finite element analysis of carbon nanotube/cement composite with degraded bond strength. Computational Materials Science. 2010;47(4):994-1004. [47] Tamimi A, Hassan NM, Fattah K, Talachi A. Performance of cementitious materials produced by incorporating surface treated multiwall carbon nanotubes and silica fume. Construction & Building Materials. 2016;114:934-45. [48] Chan WF, Chen H, Surapathi A, Taylor MG, Shao X, Marand E, et al. Zwitterion Functionalized Carbon Nanotube/Polyamide Nanocomposite Membranes for Water Desalination. Acs Nano.
2013;7(6):5308-19. [49] Thomas M, Corry B. Thermostat choice significantly influences water flow rates in molecular dynamics studies of carbon nanotubes. Microfluidics & Nanofluidics. 2015;18(1):41-7. [50] Zou B, Chen SJ, Korayem AH, Collins F, Wang CM, Duan WH. Effect of ultrasonication energy on engineering properties of carbon nanotube reinforced cement pastes. Carbon. 2015;85:212-20. [51] Nishida A, Yamazaki N, Inoue H. Study on the properties of high strength concrete with short polypropylene fibre for spalling resistance. Shimizu Technical Research Bulletin. 1995;14:1-6. [52] Sarvaranta L, Mikkola E. Fibre mortar composites under fire conditions: effects of ageing and moisture content of specimens. Materials & Structures. 1994;27(9):532-8. [53] Leena S, Esko M. Fibre mortar composites in fire conditions. Fire & Materials. 2004;18(1):45-50.
Intensity/counts
2000
o
(a) C0-25 C
B
1600
B 1200 800
C
400
B
A
B
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
2000
o
Intensity/counts
(b) C2-25 C B
1600
B 1200 800
C
400
B
B
A
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
Intensity/counts
2000
o
(c) C0-200 C
B
1600
B
1200 800
B
C
400
B
c
0 5
10
15
20
25
30
35
40
degree
45
50
55
60
65
70
2000
o
1600
Intensity/counts
(d) C2-200 C
B B
1200 800
B
C
400
B
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
Intensity/counts
2000
o
(e) C0-400 C
B
1600
B
1200 800
B B
C
400 0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
Intensity/counts
2000
o
(f) C2-400 C
B
1600
B 1200 800
B
C
400
B
0 5
10
15
20
25
30
35
40
degree
45
50
55
60
65
70
Intensity/counts
2000
o
(g) C0-600 C
1600 1200 800
C 400
B
B
B
B
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
2000
o
Intensity/counts
(h) C2-600 C 1600 1200 800
C
400
B
B
B
B
0 5
10
15
20
25
30
35
40
45
50
55
60
65
70
degree
Fig. 1 The typical XRD images of plain cement and CNT-reinforced cementitious composites at elevated temperatures (A=ettringite; B=calcium hydroxide; C=OPC minerals): (a) C0-25oC (The average peak value at 18.007o is 1530); (b) C2-25oC (The average peak value at 18.007o is 1523); (c) C0-200oC (The average peak value at 18.007o is 1599); (d) C2-200oC (The average peak value at 18.007o is 1578); (e) C0-400oC (The average peak value at 18.007o is 1648); (f) C2-400oC (The average peak value at 18.007o is 1594); (g) C0-600oC (The average peak value at 18.007o is 226); (h) C2-600oC (The average peak value at 18.007o is 235).
(a)
(b) CNT
Hydrates on CNT
(c)
(d) CNT
(e)
(f)
CNT
(g)
crack
(h) gap
(i)
CNT
Fig. 2 SEM images of 0.20%CNT/cement matrix before and after thermal treatment: (a) and (b) control (unheated); (c) and (d) after 200°C heat treatment; (e) and (f) after 400°C heat treatment; (g), (h) and (i) after 600°C heat treatment.
10
C0 C1 C2
Flexural strength (MPa)
8
6
4
2
0 0
200
400
600
o
Temperature ( C)
Fig. 3 Degradation of CNT-reinforced cementitious composites in terms of flexural strength
60
C0 C1 C2
Compressive strength (MPa)
55 50 45 40 35 30 25 20 15 10
0
200
400
600
o
Temperature ( C)
Fig. 4 Degradation of CNT-reinforced cementitious composites in terms of compressive strength
Table 1 Property of CNT Type
Aspect ratio
Diameter
Length
OH content
Purity
Ash
Surface area
MWCNT-OH
<400
>50nm
20μm
0.76wt%
>90wt%
<6wt%
>40m2/g
Table 2 Mix design of CNT-reinforced cementitious composites Mix
Cement (g)
Water (g)
CNT (g)
PVP (g)
CNT/cement
C0
100
40
-
-
-
C1
100
40
0.1
0.02
0.10wt.%
C2
100
40
0.2
0.04
0.20wt.%