82 Vol.29 No.1 WANG Baomin et al: Porosity and Pore Size Distribution Measurement of...
Porosity and Pore Size Distribution Measurement of Cement/Carbon Nano� ber Composites by 1H
Low Field Nuclear Magnetic Resonance
WANG Baomin, ZHANG Yuan, MA Hainan (Laboratory of Building Materials, School of Civil Engineering, Dalian University of Technology, Dalian 116024, China)
Abstract: The dispersion effect of carbon nano� bers (CNFs) in aqueous solution and the mechanical properties, porosity, pore size distribution and microstructure of CNFs reinforced cement-based composites were investigated in this paper. To achieve effective dispersion of CNFs, a method utilizing ultrasonic processing and a commercially surfactant were employed. CNFs were incorporated to cementitious materials with the addition of 0.1 wt% and 0.2 wt% of cement with a water/cement ratio of 0.35. The mechanical properties of CNFs/cement composites were analyzed, the porosity and pore size distribution were characterized by 1H low � eld nuclear magnetic resonance (NMR), and the microstructure was observed by scanning electron microscopy (SEM). The results indicate that the optimum concentration ratio of MC to CNFs is 2:1 for dispersing in aqueous solution. Moreover, in the field of mechanical properties, CNFs can improve the flexural strength and compressive strength. The increased mechanical properties and the decreased porosity of the matrices correspond to the increasing CNFs content and CNFs act as bridges and networks across cracks and voids.
WANG Baomin( ): Ph D; Prof.; E-mail: wangbm @dlut.edu.cn
Funded by the the National Natural Science Foundation of China (No.51278086), the Program for New Century Excellent Talents in University by Ministry of Education of the People’s Republic of China (No.NCET-12-0084), Liaoning BaiQianWan Talents Program (No.2012921073), Dalian Plan Projects of Science and Technology (Nos.2012A13GX024 and 2013A16GX113) and the Construction Safety and Environment State Key Laboratory Open Fund (No.201202)
DOI 10.1007/s11595-014-0871-1
1 Introduction
Portland cement is the most widely used construction material in the world. However, cementitious materials are typically characterized of brittle materials with low tensile strength and low strain capacity, which gives rise to the formation of cracks and finally weakens the durability of the matrix. Previous research showed that the macrofiber significantly improved the mechanical properties of cementitious materials[1-5]. However, microfibers
delay the development of the formed microcracks, and they do not stop their initiation[6,7]. The nanoscale fibers could control the formation of nanocracks. Nowadays, the vast majority of research has focused on using carbon nanotubes (CNTs) to reinforce the composites. The incorporation of multi-walled carbon nanotubes (MWCNTs) has been shown to arrest the nanocracks[8-13].
Similar to the MWCNTs, carbon nanofibers (CNFs) possess excellent properties including high stiffness, tensile strength, electrical and thermal conductivity. CNFs have a complex nanostructure as they combine microscopic lengths (0.5-100 m) with nanoscopic diameters (50-200 nm), and their outer surface usually consists of conically shaped graphite planes canted with respect to the longitudinal fiber axis[14-17]. They have a tensile strength of 3.34 GPa and a Young’s modulus of 180 GPa, tested by Ozkan et al. [18] Because of their remarkable properties, CNFs have a wide range of promising applications in such areas as biomedical science, field emission and composites[19-24]. Recently, CNFs have been acted as the reinforcement component in cement matrix to improve
Journal of Wuhan University of Technology-Mater. Sci. Ed. Feb.2014 83
its mechanical properties[25-28].The porosity and pore size distribution of
cement paste has a significant influence on its durability and strength, and an appropriate method is a prerequisite to research the pore structure. Compared to the traditional measurement method, e g, mercury intrusion porosimetry (MIP) and gas adsorption for characterizing pore size distribution, nuclear magnetic resonance (NMR) technique is a novel testing method to study pore structure of cement paste, which reduces the testing cost and never destroy the structure of tested samples. Few applications of low � eld NMR on cement paste testing have been reported in the literature, and it has a wide range of promising applications to study the porosity, pore size distribution and hydration of cement[29-32].
In this paper, the � rst objective was to determine the dispersion effect of CNFs in aqueous surfactant solution and obtain the optimum concentration ratio for dispersing. The second objective was to determine the mechanical properties, porosity, pore size distribution and microstructure of the CNFs/cement composites obtained. CNFs were incorporated to cement with addition of 0.1% and 0.2% by mass fraction of cement. The porosity and pore size distribution were analyzed by 1H low � eld nuclear magnetic resonance (NMR) and the microstructure was studied by an environmental scanning electron microscope (E SEM).
2 Experimental
2.1 MaterialsThe cementitious material was P.O 42.5R cement
which was purchased from Dalian Onoda Cement Co., Ltd., China (as shown in Table 1 and Table 2). The major composition of the cement- tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium alumino ferrite (C4AF) were 39.3%, 32.1%, 9.0% and 8.8%, respectively, as calculated by Bogue Eq.(1)-(4).
C3S=4.07(CaO) 7.60(SiO2) 6.72(Al2O3)
1.43(Fe2O3) 2.85(SO3) (1)
C2S=2.87(SiO2) 0.75(C3S) (2)
C3A=2.65(Al2O3) 1.69(Fe2O3) (3)
C4AF=3.04(Fe2O3) (4)
The surfactant was supplied by SINOPHARM Chemical Reagent (Shenyang) Co., Ltd., Shenyang,
China; CNFs were provided by Top Vendor Science & Technology Co., Ltd., Beijing, China. Their physical properties are shown in Table 3 and the microstructure of CNFs is shown in Fig.1; the defoamer was liquid tributyl phosphate (TBP) (supplied by Tianjin Chemical Reagent Plant, China), which was added in 0.13% by mass fraction of cement; the superplasticizer was supplied by Dalian Mingyuanquan Group Co., Ltd., China; the distilled water used in experiments was prepared in laboratory.2.2 Sample preparation
In order to obtain effective dispersion of CNFs, a homogenous CNF suspension was prepared � rst. CNFs were uniformly distributed in an aqueous solution by applying an ultrasonic energy in combination with a surfactant (methylcellulose), which was always used to disperse carbon filaments[33-35]. The weighed CNFs were put into the aqueous surfactant solution and the beakers were sealed up with a cap, and the homogenous CNF suspensions were finally gained after ultrasonic processing (DS-3510DT, Shanghai Sonxi Ultrasonics Instrument Co., Ltd., China, operating frequency 40
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kHz, power 180 W) for 15 minutes combined with mechanical stirring.
After ultrasonication, the dispersed solution, defoamer and dry cement were mixed in a multispeed planetary mixer for 5 minutes. The water/cement ratio of these cement paste was 0.35. Meanwhile, superplasticizer was used to keep a good � uidity. After pouring the mixture in the shaped mold, an electric vibrator was used to ensure good compaction. The samples were demolded after 24 h in a water vapor saturated atmosphere, and then cured in 20 water for 27 days until tests. The letters “0”, “1” and “2” indicate the CNFs dosages of 0, 0.1 and 0.2 wt% reinforcing cementitious materials, respectively.2.3 Testing procedure2.3.1 Dispersion in aqueous solution
The dispersion effect of CNFs in aqueous solution was characterized by measuring ultraviolet absorbency (UV absorbency) of each homogenous CNF suspension, which was gained by a UV-vis spectrophotometer (UV-560, Jasco Co., Ltd., Japan) with the chosen wavelength of 260 nm. In order to remove the MC effect on the UV absorbency, the UV absorbency of each aqueous MC solution was subtracted from that of CNF suspensions.2.3.2 Mechanical property
The mechanical performance of CNFs/cement composites was first evaluated by three-points bend tests (span=120 mm) onto the cured samples (40×40×160 mm3) at a rate of 0.2 mm/min. The surface stress at failure equals to:
(5)
where, Fmax is the maximum force at the instant of failure, l is the span, b is the width of the cured samples and t is the height.
After the three-points bend tests, the compressive strength measurement of each separated fragments was performed on a 40×40 mm2 surface. The compressive strength was evaluated by the maximum force at the instant of failure at a rate of 2 400 N/s, according to Chinese Standard GB 17671-1999.2.3.3 Porosity and pore size distribution
1H low � eld nuclear magnetic resonance (NMR) was employed to measure the porosity and pore size distribution of each cured samples (MiniMR60, supplied by Shanghai Niumag Co., Ltd., China, resonance frequency 23.309 MHz, magnet strength 0.55 T, magnet temperature 32.00 , effective sample detection volume 60×60×60 mm3). First, we need to
prepare the plain standard samples. The copper sulfate solutions with different mass were weighed and the corresponding volume was recorded, as shown in Table 4. Then the cured cement samples were put into a vacuum water-saturation device to have a water saturated treatment for 12 hours.
The PQ001 analysis software and Carr-Purcell-Meiboom-Gill-like (CPMG) pulse sequence were used to collect the signal values and draw the T2 attenuation curve of the tested samples. Meanwhile, the gained T2 attenuation curve was inversed to achieve the T2 relaxation information, which showed the porosity and pore size distribution of the cured CNFs/cement composites samples.2.3.4 Microstructure of the CNFs/cement composites
CNFs/cement composites samples were dried in an oven at 100 for about 12 hours, and crushed to tested samples (about 5 mm length, 5 mm width and 1 mm thickness). The tested samples were attached to a stub with carbon tape and then coated with an Au � lm about 20 nm thick. An environmental scanning electron microscope (E SEM) (QUANTA 450, FEI Co., Ltd., USA) was used to observe the fracture surface of the samples.
3 Results
3.1 Dispersion of CNFs in aqueous solution3.1.1 UV absorbency analysis
According to the Lambert-Beer’s law:
(6)
where, A is UV absorbency, T is light transmittance, E is absorption coef� cient, c is solution concentration and l is wavelength. The UV absorbency of CNF suspension can characterize its concentration when the testing wave and the wavelength are given as constant[36]. As is shown in Fig.2, the UV absorbency of each homogenous CNFs suspension was measured. The calibration of the variation of CNF concentration with respect to UV absorbency was obtained, which shows a good linearity and equation of linear regression is A=14.595C-0.0209 00, where A is the UV absorbency
Journal of Wuhan University of Technology-Mater. Sci. Ed. Feb.2014 85
and C is the concentration of CNFs in aqueous solution.With the increase of MC concentration, the
dispersion effect of CNFs changes remarkably (Fig.3). When the concentration of MC is too low or too high, the dispersion of CNF suspension is relatively poor. The concentration of CNFs increases corresponding to the increasing MC concentration at the range of 0-0.4 g/L, and reaches the maximum value when the concentration of MC is about 0.4 g/L in aqueous solution, which means the optimum dispersion effect. Then the concentration of CNFs decreases gradually when the concentration of MC continues to grow. The result indicated that the optimum MC concentration for dispersing was 0.4g/L and the optimum concentration ratio of MC to CNFs was 2:1.
3.1.2 TEM analysisR e s u l t s f r o m T E M m i c r o g r a p h s o f t h e
microstructure of CNFs in aqueous solution are presented in Fig.4. Without using MC as a surfactant, CNFs poorly disperse and agglomerate together in aqueous solution (Fig.4 (a)). Fig.4 (b) shows that CNFs disperse well in aqueous MC solution when the MC to CNFs ratio is 2:1, and the single CNFs can be observed. MC weakens the Van der Waals forces, and enhances the dispersibility of CNFs in aqueous solution.3.2 Mechanical properties
Both the flexural strength and the compressive strength of the cured CNFs/cement composites are shown in Table 5 on 3rd and 28th day. As is shown in Table 5, the addition of CNFs shows a decrease in strength on the 3rd day, but the average flexural strength is higher than the reference sample on the 28th day. The greatest increase in flexural strength (i e, 28.3%) occurs on the 28th day by No.2 sample. Moreover, the compressive strength of CNFs/cement composites is a little higher than that of the reference sample.
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3.3 Porosity and pore size distributionA lot of hydrogen protons are stayed in water
and the tested signal sources of 1H low � eld NMR are mainly from hydrogen protons, which indicates that a sample has a greater moisture and porosity when the larger amount of hydrogen protons are measured. The moisture and porosity of the cured CNFs/cement samples can be obtained when tested by 1H low field NMR.
First, three plain standard samples are measured and a good linearity between the mass of water and the water peak area is gained, which is shown in Table 6 and Fig.5. The equation of linear regression is y=1 196.952 1x+91.641 5 and its correlation coef� cient is 0.999, where y represents the water peak area and x means the mass of water, respectively.
The water peak area of each samples was measured after the water saturated treatment and the porosity can be obtained by utilizing the obtained equation of linear regression between the mass of water and the water peak area. The porosity of each cured CNFs/cement composites is given in Table 7. The addition of CNFs reduces the porosity of CNFs/cement composites. Compared to the reference sample (the porosity is 11.274%), the porosities of CNFs/cement composites contained 0.1 wt% and 0.2 wt% CNFs reduction and they are 9.294% and 5.680%, respectively.
T2 relaxation information, including T2 relaxation time, can be obtained by inversing the gained T2 attenuation curve in a given relaxation model. T2
relaxation time reflects the condition of chemical environment in which the internal proton stays in the tested samples, which has a close relationship with the structure of the samples. A porous material has a longer T2 relaxation time when it has a greater pore size
distribution. The T2 relaxation time spectrum of each tested sample is given in Fig.6.
Under the condition of no existed gradient � eld, for a porous material, the relationship between the T2
relaxation time and pore size is given by:
(7)
where, T2 is the relaxation time, is the relaxation rate constant, V is volume of the pore and S is the surface
Journal of Wuhan University of Technology-Mater. Sci. Ed. Feb.2014 87
area of the pore. Generally, the relaxation rate of the cementitious materials is in a certain range of 0.5-3
m/ms, and we get the middle value of 1 m/ms. Pores in the samples are assumed to be ideal spheres, and then S/V in Eq.(7) equals to 3/rc, where rc is the pore radius. The pore size distribution can be derived by transforming the T2 relaxation time distribution of the tested samples, which is shown in Fig.7. Compared to the reference sample, the presence of CNFs dose not significantly influence the pore diameter of the tested
cement matrix, and the pore size of 0.1 wt% and 0.2 wt% mixes increases a little.3.4 SEM analysis
Fig.8 shows the typical SEM images of CNFs/cement composites after curing for 28 days. It is observed that the diameter of CNFs is about 100 nm, which agrees with the manufacturer’s report. SEM images demonstrate that CNFs act as bridges and networks across the voids and cracks, and the embedded CNFs from the hydration products can be observed. Moreover, a large amount of ettringite wrap on the surface of CNFs, which indicates that high bonding strength is achieved between the CNFs and the cement matrix (Fig. 8 (c)).
4 Discussion
Previous research showed that the addition of CNFs could significantly improve the mechanical p rope r t i e s o f cemen t i t i ous ma te r i a l s i n the literature[25-28]. This increase was contributed to the microstructure of CNFs/cement composites. As a kind of porous material, cementitious material has a lower strength for its larger porosity. Due to the addition of CNFs, the porosity decreased and the mechanical properties were improved � nally. As is shown in Fig.8 (c) and (d), CNFs act as the bridges and networks across the voids and cracks, which improved the load-transfer intension. Meanwhile, the interfacial bond between the CNFs and the cement matrix has increased because of the high aspect ratio of CNFs and the increased amount of ettringite covered on the surface
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of CNFs. Certainly, the mechanical properties are enhanced.
5 Conclusions
In this paper, the dispersion effect of CNFs in aqueous MC solution and the mechanical properties, porosity and microstructure of cement paste samples reinforced with 0.1 wt% and 0.2 wt% of CNFs are researched systematically. The CNFs are uniformly distributed in aqueous solution by utilizing ultrasonic energy in combination with the use of MC. Stable and homogenous CNF suspensions can be obtained in aqueous MC solution, and the optimum MC to CNFs ratio of 2:1 by concentration is required to achieve dispersions with the maximum achievable dispersion of CNFs.
The addition of CNFs improves the mechanical properties of the cementitious materials, and the flexural strength increases up to 28.32% higher than that of the reference samples. Moreover, the compressive strength is a little higher than that of the reference samples. 1H low � eld NMR analysis indicates that the addition of CNFs reduces the porosity, which decreases from 11.274% to 5.680% in the tested samples. SEM images show the crack bridging, pulling out and network filling effect of CNFs. Moreover, CNFs give an improved performance of their high aspect ratio and a good interfacial interaction between the CNFs and the cement matrix.
Further studies are needed to obtain better homogeneity of CNFs/cement composites, and the microstructure properties shall be researched. We believe that our results are helpful for the study of cement/nanoparticles composites.
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