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Effect of silicon carbide nanowhiskers on hydration and mechanical properties of a Portland cement paste
Construction and Building Materials 169 (2018) 388–395
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of silicon carbide nanowhiskers on hydration and mechanical properties of a Portland cement paste Nagilla Huerb de Azevedo ⇑, Philippe J.P. Gleize Laboratory of Application of Nanotechnology in Civil Construction - LabNANOTEC, Civil Engineering Department, Federal University of Santa Catarina, CxP 476, 88040-900 Florianópolis, SC, Brazil
h i g h l i g h t s SiC nanowhiskers (SIC NWS) were mixed with Portland cement pastes at 0.25, 0.50, 1.00 and 1.50 wt%. The Portland cement pastes showed that SIC NWS act as nucleation sites and accelerate cement hydration. The mechanical properties of Portland cement pastes can be modified with small additions of SIC NWS.
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Article history: Received 24 November 2017 Received in revised form 19 January 2018 Accepted 24 February 2018
Keywords: SiC nanowhiskers Portland cement paste Reinforcement Cement hydration Mechanical strength
a b s t r a c t This paper investigates the effect of silicon carbide nanowhiskers (SiC NWS) on cement pastes. SiC NWS were used at 0.25–1.50 wt%, significant increases in the compressive and flexural strength were observed (up to 25% and 75%, respectively). However, an optimum effect plateau seems to be reached at content of 0.25–1.00 wt% for compressive and flexural strength, respectively. The microstructural observations also show the ability of the SiC NWS to act as bridges across the matrix cracks. It was concluded that the enhancements observed are mainly due to the SiC NWS (wt%) and the dispersion has a lesser effect. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The use of nanomaterials has been the subject of investigation in vast array of diverse areas, mainly to enhance the properties of traditional materials, for example, to make them lighter and more resistant [1–9]. Silicon carbide nanowhiskers (SiC NWS) have been attracting considerable attention as a novel type of reinforcement due to their excellent properties, including high thermal conductivity (50–120 W/mK), and stability, low density (3.13 g/cm3) and excellent oxidation resistance [4]. Values of 610–660 GPa and 53.4 GPa for the elastic modulus and ultimate bending strengths, respectively, of SiC nanorods with several tens of nanometers of thickness have been reported [5]. In recent years the number of investigations on the use of SiC nanomaterials for the reinforcement of monolithic matrices of metals [6,7], polymers [8,9] and even Portland cement [10] has ⇑ Corresponding author. E-mail address: [email protected] (N.H.d. Azevedo). https://doi.org/10.1016/j.conbuildmat.2018.02.185 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
been increasing, because this nanomaterial promotes a considerable improvement in the mechanical properties of the matrices to which it is added. In a study by Bahari, Berenjian and SadeghiNik [10], SiC nanoparticles with different weight ratios (1.25, 2.00 and 3.30 wt%) were added to cement mortars. The authors reported that with 2 wt% SiC nanoparticles the values for the 28 day flexural and compressive strength of cement mortars increased by 11.3% and 6%, respectively. In general, cementitious materials are brittle and characterized by low tensile strength and strain capacity. The use of discrete fibers results in a more uniform distribution of their remarkable mechanical, chemical and thermal properties in reinforcing cementitious materials. Microfibers can delay the growth of cracks at the microscale, whereas nano-reinforcements will delay the growth of cracks at the nanoscale and halt their propagation to the microlevel. In addition, nanofibers can act as stress transfer bridges during the application of load to cementitious materials, increasing their strength and durability over time. In recent years, carbon nanotubes and carbon nanofibers have emerged as
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promising candidates for the next generation of high-performance additives for cement-based materials [2,3,11]. Despite the excellent results that nano-SiC can provide as a nano-reinforcement for the mechanical reinforcement of matrices [6–10], it is difficult to obtain a uniform distribution in the matrix, and the adequate dispersion of nanomaterials is crucial for the success of applications. Due to its high specific area and surface energy as well as strong van der Waals and electrostatic forces between the nanoparticles, inhibiting their agglomeration remains a challenge. Many researches have addressed this issue, for instance, through studies on physical methods, where the nanomaterials can be appropriately dispersed via external mechanical forces by applying ultrasonication [2,3,11,12] or milling [13]. Another alternative is to apply chemical methods, using various dispersants [14–16]. This paper investigates the effect of SiC NWS on some of the properties of Portland cement pastes, including the cement hydration, Young modulus, compressive and flexural strengths and water absorption.
Table 2 Properties of SiC nanowhiskers (SiC NWS). Free Carbon Diameter: Length: Crystal Type: Decomposition Temperature: Density (15 °C): Hardness (Mohs):
<0.05% 0.1–2.5 mm 2.0–50.0 mm Beta 2700 °C 3.216 g/cm3 9.5
2. Materials and methods 2.1. Materials 2.1.1. Cement The cementitious material used in this study was ordinary Portland cement, CPII-F-32, conforming to Brazilian standard NBR 11578 with the addition of 6–10% calcareous filler [17]. Its chemical composition and physical properties are shown in Table 1.
Fig. 1. SEM micrograph of b-SiC nanowhiskers.
2.1.2. SiC nanowhiskers The b-SiC nanowhiskers used in this study are a commercial product purchased from Nanostructured & Amorphous Materials, Inc., with the properties given in Table 2. The SiC nanowhiskers were used as received and the size and morphology were examined by scanning electron microscope (SEM, JEOL JSM-6701F). Fig. 1 shows that the material had a mean diameter of 1 mm and length of 25 mm. X-ray diffraction (XRD, Philips X-Pert using CuKa I = 15418 Å) was carried out to characterize the nanomaterial (Fig. 2) and indicated that the powder was mainly b-SiC (3C-polytype). The chemical composition of the NWS was determined by energy dispersive X-ray detection (EDS, JEOL JSM-6390LV) as follows: Si 46.48 wt%; C 50.99 wt% and O 2.53 wt%. 2.1.3. Superplasticizer A sodium polycarboxylate superplasticizer, with a relative density of 1.10, was incorporated into all mixes. The content was adjusted for each mix to ensure that all of the pastes had the same workability as the reference paste (without SiC NWS). 2.2. Dispersion of SiC NWS in aqueous medium The SiC NWS were added to 95% of the deionized water to be used in the mixture and the remaining 5% of the water was added to the sodium polycarboxylate superplasticizer (SP). The suspension was sonicated for 6 min. using a Vibra-Cell 750 Watts sonicator (Sonics & Materials, Inc.) with an amplitude of 50% and intervals of 20 s pulse and 20 s standing. To prevent temperature
Fig. 2. XRD pattern for as-synthesized b-SiC nanowhiskers.
rising, the suspensions were kept in a water–ice bath during sonication. After this process the solution was allowed to stabilize and was maintained at 23 °C however, it was verified that the SiC NWS started to decant soon after the end of the sonication process. 2.3. Production of the cement pastes The water-cement ratio (w/c) was 0.4 for all of the mixtures and the mix proportions for the materials are shown in Table 3. For the mixtures with SiC NWS, the appropriate amount of sodium
Table 1 Chemical and physical properties of Portland cement. SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Specific surface Blaine, m2 kg1
Compressive strength, 7-day, MPa
Compressive strength, 28-day, MPa
18.34
4.40
2.93
60.94
4.96
2.70
3.333
35.90
42.40
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Table 3 Mix proportion and consistency of the cement pastes. Cement Pastes
Cement (g)
Water(g)
SiC NWS (g)
SP (wt% cement)
Diameter – Mini Slump (mm)
REF NWS NWS NWS NWS
975 975 975 975 975
390 390 390 390 390
– 2.44 4.87 9.75 14.62
– 0.010 0.015 0.025 0.035
77.33 79.10 75.76 76.73 78.93
0.25% 0.50% 1.00% 1.50%
polycarboxylate superplasticizer (SP) was added in order to maintain constant the mini-slump consistency (diameter nearly 77 mm, similar to the reference), as it was considered that a part of the superplasticizer is consumed by the SiC NWS and not available for the dispersion of cement particles. Therefore, this will allow more objective comparisons between the results obtained for the reference and the nanocomposite samples, as noted by Tyson et al. [2] and Mohsen et al. [3], who studied carbon nanotubes in a cement matrix. The cement pastes were mixed in a mechanical mixer as follows:
Three-point bending flexural strength tests were carried out on 100 20 20 mm3 rectangular specimens (three samples) following procedures described in the Brazilian standard NBR 13279 [20]:
Sf ¼
1; 5 Fc L bh
2
where, Sf = flexural strength (MPa); Fc = load at the fracture point (N); b = sample width; h = sample thickness; L = length of the support span (60 mm). 2.7. Water absorption by immersion
1. The sonicated solution (95% water + SiC NWS) was mixed with the cement for 1 min at 440 rpm. 2. The superplasticizer with the remaining 5% water was added to the cement paste during mixing. 3. The mixture was homogenized for a further 2 min at the same speed.
The determination of the water absorption by immersion followed the test method specified in the Brazilian standard NBR 9778 [21]. The test was carried out on the prismatic specimens (50 20 20 mm3) used in the flexural strength tests and consisted of measuring the mass of the specimens in saturated and oven-dried states (three samples).
Approximately 10 g of the freshly prepared cement mixture was used for the isothermal calorimetry and the rest was poured into nylon molds for use in other tests. Special care was taken to release any residual air bubbles. After demolding (24 h), the specimens were cured for 7 and 28 days in saturated limewater at 23 ± 2 °C. The results were subjected to statistical analysis through analysis of variance (ANOVA) and the Tukey test using Past Software, version 2.17c.
2.8. Microstructural analysis (SEM) The SEM microstructural analysis of the broken specimens was performed to qualitatively examine the effect of the weight fraction of the NWS on their dispersion in the cement hydration products. This was carried out by observing the fracture surfaces of the specimens. The cement hydration process was stopped using the freeze drying technique at 28 days old. 3. Results and discussion
2.4. Isothermal calorimetry 3.1 Isothermal calorimetry A TAM Air isothermal calorimeter was used to assess the evolution of the heat flow of the cement pastes. Immediately after mixing, around 10 g of cement paste was placed into standard glass containers and the paste was then loaded into the isothermal calorimeter. The tests were performed on the pastes described previously (SiC NWS + SP) and on pastes with only the addition of the (SP) in order to separate the effects of the SiC NWS and the SP on the cement hydration. 2.5. Dynamic Young’s modulus The Young’s modulus was determined by the impulse excitation technique with SonelasticÒ equipment (ATCP – Physical Engineering) [18]. This technique consists of promoting a slight impact on the prismatic specimen with dimensions of 100 20 20 mm3. The acoustic response is captured and this is processed using software that calculates the Young’s modulus and were tested 3 samples in each age. 2.6. Compressive and flexural strengths The compressive strength measurements were performed on 20 40 mm2 cylindrical specimens (eight samples) following the recommendations of the Brazilian standard NBR 7215 [19].
The cement hydration kinetics is influenced by particle surface reactions and the surface area or size of the particles [14]. It is well known that the addition of nanomaterials to cement stimulates the nucleation process that occurs during the early cement hydration and the faster the nuclei are formed the earlier they can grow to larger crystals, accelerating the cement hydration. Due to their small size, nanomaterials provide very large surface areas that are highly reactive and can act as nucleation sites during cement hydration [22,23]. Table 4 and Figs. 3 and 4 provide the cumulative heat evolution and rates of heat generation (until 40 h), respectively, during the hydration for Portland cement pastes, according to the different additives used. The first peak is typically associated with the wetting of the cement powder, the dissolution of the silicate and gypsite and the formation of ettringite. After the induction period, the second peak is mainly due to the dissolution and hydration of alite, which induces the formation of C-S-H and CH. The second peak also corresponds to the acceleration zone which is associated with the formation of sulfate (type AFm) [23]. The superplasticizer does not strongly affect cement hydration and only a slight increase in the dormant period was observed as its content was increased. The retarder effect of superplasticizers on cement hydration is well known, and may be due to the formation of an insoluble layer or a ‘‘barrier” of calcium compounds surrounding the hydrating grains
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N.H.d. Azevedo, P.J.P. Gleize / Construction and Building Materials 169 (2018) 388–395 Table 4 Calorimetry results for the cement pastes. Cement Pastes
Dormant period (h:min)
I (%)
Cumulative heat evolution at 40 h (J/g)
I (%)
Acceleration process (a)
REF SP 0.010% SP 0.015% SP 0.025% SP 0.035% NWS 0.25% NWS 0.50% NWS 1.00% NWS 1.50%
2:25 2:19 2:31 2:39 2:51 2:05 2:09 2:19 2:48
– 4.14 4.14 9.65 17.93 13.79 11.03 4.14 15.86
162.90 165.81 165.93 166.92 167.85 167.25 160.61 164.49 160.80
– 1.79 1.86 2.47 3.04 2.67 1.40 0.98 1.29
0.288 0.292 0.285 0.288 0.290 0.347 0.302 0.318 0.309
+ + + +
SP SP SP SP
0.010% 0.015% 0.025% 0.035%
Fig. 3. Heat flow calorimetry results for the cement pastes with only SP.
Fig. 4. Heat flow calorimetry results for the cement pastes with NWS and SP.
[24]. Also, a slight increase in the cumulative heat evolution was observed with the increase in the amount of superplasticizer (Table 4), which can indicate better cement hydration due to greater dispersion of the cement particles. In most cases the dormant period was reduced with the addition of the SiC NWS + SP, indicating that due to the high surface area, the SiC NWS particles act as nucleation sites and accelerate the cement hydration [25]. It should be noted that there is ‘‘competition” between the retarder effect of the superplasticizer and the accelerating effect of the NWS.
Compared with the reference sample, the slopes of the curves for the pastes with NWS are clearly greater during the acceleration period (Fig. 4), as evidenced by the a value of the straight line during the acceleration period of the cement hydration of the pastes (Table 4). 3.2 Young’s modulus The effect of the NWS on the Young’s modulus (E) of the cement pastes, measured by impulse excitation vibration was also
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investigated. The results are reported in Fig. 5 and Table 5 (mean values and standard deviations). After 7 days, there is a slight decrease in the modulus with the addition of the NWS, which can be attributed to a delay in the formation of high stiffness CS-H in the early ages, as reported by Tyson et al. [2], who studied carbon nanotube cement composites. In contrast, at 28 days old, all of the pastes with NWS showed a slight modulus increase. However, statistically this enhancement is not significant.
1.00 wt% NWS (up to 107% and 75% at 7 and 28 days, respectively) and, statistically, the flexural strength after this content is not significant. In general, the literature reports that for cement pastes with a ratio of nanomaterials higher than 0.50% the improvement in the mechanical properties is not significant due to agglomeration [2,3,27].
3.3. Flexural strength
Fig. 7 and Table 6 show that the addition of NWS increases the compressive strength of pastes at 7 and 28 days. However, this improvement does not reach that observed for the flexural strength and the optimum effect seems to plateau at 0.25 wt% of NWS content.
3.4. Compressive strength
The flexural strength results are shown in Fig. 6 and Table 5 (mean values and standard deviations). It can be observed that there were increases in the flexural strength with the addition of
Fig. 5. Young modulus values for cement pastes with NWS at the ages of 7 and 28 days.
Table 5 Tukey’s scores for all possible pairs of different Young modulus and flexural strength values for the cement pastes. Cement Pastes REF NWS NWS NWS NWS
0.25% 0.50% 1.00% 1.50%
+ + + +
SP SP SP SP
0.010% 0.015% 0.025% 0.035%
E(GPa) 7 days
I (%)
E(GPa) 28 days
I (%)
Flexural (MPa) 7 days
I (%)
Flexural (MPa) 28 days
I (%)
19.49 ± 0.26 19.26 ± 0.37 18.47 ± 0.13 19.00 ± 0.27 18.79 ± 0.10
– 1.18 5.23 2.51 3.59
19.84 ± 0.27 21.01 ± 0.30 20.70 ± 0.13 21.10 ± 0.16 20.48 ± 0.40
– 5.90 4.33 6.35 3.22
4.40 ± 0.23 7.37 ± 0.47 7.14 ± 0.40 9.12 ± 0.40 7.88 ± 0.75
– 67.50 62.27 107.27 79.09
5.39 ± 0.16 7.95 ± 0.15 9.31 ± 0.54 9.43 ± 0.22 9.42 ± 0.36
– 47.49 72.73 74.95 74.77
Fig. 6. Flexural strength values for cement pastes at the ages of 7 and 28 days.
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Fig. 7. Compressive strength values for cement pastes at the ages of 7 and 28 days.
Table 6 Tukey’s scores for all possible pairs of different compressive strength values for the cement pastes. Cement Pastes
Compressive (MPa) 7 days
I (%)
Compressive (MPa) 28 days
I (%)
REF NWS NWS NWS NWS
37.17 ± 0.83 49.22 ± 1.09 45.51 ± 0.63 42.72 ± 1.44 42.30 ± 1.08
– 32.42 22.44 14.93 13.80
51.42 ± 1.22 64.21 ± 2.02 58.42 ± 1.87 57.56 ± 1.93 57.81 ± 0.72
– 24.87 13.61 11.94 12.43
0.25% 0.50% 1.00% 1.50%
3.5. Water absorption The water absorption test results for the pastes after 28 days of curing are shown in Fig. 8. All pastes showed a reduction in the open porosity when compared to the reference, mainly those with low contents of SiC NWS (0.25 wt% and 0.50 wt%). When, higher percentages of SiC NWS were added, the reduction in the porosity was not significant and, as mentioned in another research, and this may be due to some structural defects such as the increase of pores or voids in the cement paste and at the interface between nanowhiskers and cement paste [28].
This slight reduction in the porosity cannot explain the notable improvement in the mechanical properties with the addition of NWS to the cement paste. Thus, it can be concluded that these improvements are due essentially to the addition of the NWS. 3.6. SEM microstructural analysis The microstructural analysis shows the ability of the NWS to bind the particles of the cement hydration products and also to bridge cracks (Fig. 9). In previous studies it has been shown that the appropriate quantity of carbon nanotubes promotes an increase in the mechanical properties of cementitious materials.
Fig. 8. Water absorption results for the cement pastes at the age of 28 days.
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Fig. 9. SEM micrographs showing crack bridging and binding of cement hydration products by SiC NWS (a) 0.25% NWS and (b) 0.50% NWS.
Fig. 10. Low-content-SiC NWS samples showing good dispersion with (a) 0.25% SiC NWS (b) and 0.50% SiC NWS.
These nanomaterials can also act as bridges across pores and cracks, thus indicating that high bonding strength between them and the cement matrix is achieved [26,27,29]. With regard to the SiC NWS weight fraction, samples with low NWS contents (0.25 and 0.50 wt%) showed a relatively homogeneous dispersion in the cement matrix (Fig. 10). Almost no agglomeration was observed with 0.25 wt% NWS and few agglomeration zones were found with 0.50 wt% NWS. In contrast, for the pastes
with higher NWS contents of 1.00 and 1.50 wt% there were many zones with NWS covered by cement hydration products (Fig. 11), as previously reported for carbon nanotubes [2,3]. On associating these microstructural observations with the mechanical performance results, it can be noted that, in spite of the occurrence of NWS agglomeration bundles within the cement matrix for 0.50 and 1.00 NWS wt%, the flexural and compressive strengths exhibit a significant improvement compared with the
Fig. 11. High-content-SiC NWS samples covered by cement hydration products (a) 1.00% SiC NWS and (b) 1.50% SiC NWS.
N.H.d. Azevedo, P.J.P. Gleize / Construction and Building Materials 169 (2018) 388–395
reference. It can be concluded that the SiC NWS content (by weight) seems to have a greater effect on the overall strength improvement that the dispersion quality of the nanofilaments. However, with the addition of 1.50% NWS the improvement in the flexural and compressive strength is not as great compared with the other pastes. 4. Conclusions In this experimental study, Portland cement pastes were prepared with the addition of different amounts of SiC NWS (0, 0.25, 0.50, 1.00 and 1.50 wt%). As expected, SiC NWS act as nucleation sites and accelerate the cement hydration despite the retarder effect of the superplasticizer. No significant increase in the Young’s modulus was observed. In contrast, there was an increase in the flexural strength for 1.00 wt% of SiC NWS (up to 107% and 75% at 7 and 28 days, respectively). The compressive strength also increased with the addition of SiC NWS however, this improvement was not as great as that observed for the flexural strength. The enhancement in the compressive strength and especially in the flexural strength is essentially due to the presence of SiC NWS. Microstructural observations show the ability of the SiC NWS to act form bridges across the cement paste pores and cracks, indicating that there is a high bonding strength between them. As expected, it was also observed that as the SiC NWS content increased the occurrence of zones with SiC NWS agglomerated in bundles increased however, these zones were mostly, at least partially, covered by cement hydration products. This finding could explain the plateau reached by the mechanical strength, since it was observed that, in spite of the occurrence of these bundles within the cement matrix with 1.00 wt% NWS content, the flexural and compressive strengths exhibited a significant improvement as the SiC NWS content increased. Further studies are necessary to fully understand where and how the SiC nanowhiskers are acting on cement hydration. Acknowledgments The authors gratefully acknowledge Laboratório Central de Microscopia Eletrônica (LCME) at Federal University of Santa Catarina and the Brazilian governmental research agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo a Pesquisa do Estado de Santa Catarina (FAPESC) for providing the financial support for this research. The Coordenação de Aperfeiçoamento de Pessoal do Nível Superior – CAPES provided a scholarship to the first author. References [1] R.B. Hamada, H. Chacham, Nanotubos e a nova era do carbono, Ciência Hoje 198 (2003). [2] B.M. Tyson, R.K.A. Al-Rub, A. Yazdanbakhsh, Z. Grasley, Carbon nanotubes and carbono nanofibers for enhancing the mechanical properties of nanocimposite cementitious materials, J. Mater. Civ. Eng. 23 (2011) 1028–1035. [3] M.O. Mohsen, R. Taha, A.A. Taqa, A. Shaat, Optimum carbono nanotubes’ contente for improving flexural and compressive strength of cement paste, Constr. Build. Mater. 150 (2017) 395–403.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of silicon carbide nanowhiskers on hydration and mechanical properties of a Portland cement paste Nagilla Huerb de Azevedo ⇑, Philippe J.P. Gleize Laboratory of Application of Nanotechnology in Civil Construction - LabNANOTEC, Civil Engineering Department, Federal University of Santa Catarina, CxP 476, 88040-900 Florianópolis, SC, Brazil
h i g h l i g h t s SiC nanowhiskers (SIC NWS) were mixed with Portland cement pastes at 0.25, 0.50, 1.00 and 1.50 wt%. The Portland cement pastes showed that SIC NWS act as nucleation sites and accelerate cement hydration. The mechanical properties of Portland cement pastes can be modified with small additions of SIC NWS.
a r t i c l e
i n f o
Article history: Received 24 November 2017 Received in revised form 19 January 2018 Accepted 24 February 2018
Keywords: SiC nanowhiskers Portland cement paste Reinforcement Cement hydration Mechanical strength
a b s t r a c t This paper investigates the effect of silicon carbide nanowhiskers (SiC NWS) on cement pastes. SiC NWS were used at 0.25–1.50 wt%, significant increases in the compressive and flexural strength were observed (up to 25% and 75%, respectively). However, an optimum effect plateau seems to be reached at content of 0.25–1.00 wt% for compressive and flexural strength, respectively. The microstructural observations also show the ability of the SiC NWS to act as bridges across the matrix cracks. It was concluded that the enhancements observed are mainly due to the SiC NWS (wt%) and the dispersion has a lesser effect. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The use of nanomaterials has been the subject of investigation in vast array of diverse areas, mainly to enhance the properties of traditional materials, for example, to make them lighter and more resistant [1–9]. Silicon carbide nanowhiskers (SiC NWS) have been attracting considerable attention as a novel type of reinforcement due to their excellent properties, including high thermal conductivity (50–120 W/mK), and stability, low density (3.13 g/cm3) and excellent oxidation resistance [4]. Values of 610–660 GPa and 53.4 GPa for the elastic modulus and ultimate bending strengths, respectively, of SiC nanorods with several tens of nanometers of thickness have been reported [5]. In recent years the number of investigations on the use of SiC nanomaterials for the reinforcement of monolithic matrices of metals [6,7], polymers [8,9] and even Portland cement [10] has ⇑ Corresponding author. E-mail address: [email protected] (N.H.d. Azevedo). https://doi.org/10.1016/j.conbuildmat.2018.02.185 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
been increasing, because this nanomaterial promotes a considerable improvement in the mechanical properties of the matrices to which it is added. In a study by Bahari, Berenjian and SadeghiNik [10], SiC nanoparticles with different weight ratios (1.25, 2.00 and 3.30 wt%) were added to cement mortars. The authors reported that with 2 wt% SiC nanoparticles the values for the 28 day flexural and compressive strength of cement mortars increased by 11.3% and 6%, respectively. In general, cementitious materials are brittle and characterized by low tensile strength and strain capacity. The use of discrete fibers results in a more uniform distribution of their remarkable mechanical, chemical and thermal properties in reinforcing cementitious materials. Microfibers can delay the growth of cracks at the microscale, whereas nano-reinforcements will delay the growth of cracks at the nanoscale and halt their propagation to the microlevel. In addition, nanofibers can act as stress transfer bridges during the application of load to cementitious materials, increasing their strength and durability over time. In recent years, carbon nanotubes and carbon nanofibers have emerged as
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promising candidates for the next generation of high-performance additives for cement-based materials [2,3,11]. Despite the excellent results that nano-SiC can provide as a nano-reinforcement for the mechanical reinforcement of matrices [6–10], it is difficult to obtain a uniform distribution in the matrix, and the adequate dispersion of nanomaterials is crucial for the success of applications. Due to its high specific area and surface energy as well as strong van der Waals and electrostatic forces between the nanoparticles, inhibiting their agglomeration remains a challenge. Many researches have addressed this issue, for instance, through studies on physical methods, where the nanomaterials can be appropriately dispersed via external mechanical forces by applying ultrasonication [2,3,11,12] or milling [13]. Another alternative is to apply chemical methods, using various dispersants [14–16]. This paper investigates the effect of SiC NWS on some of the properties of Portland cement pastes, including the cement hydration, Young modulus, compressive and flexural strengths and water absorption.
Table 2 Properties of SiC nanowhiskers (SiC NWS). Free Carbon Diameter: Length: Crystal Type: Decomposition Temperature: Density (15 °C): Hardness (Mohs):
<0.05% 0.1–2.5 mm 2.0–50.0 mm Beta 2700 °C 3.216 g/cm3 9.5
2. Materials and methods 2.1. Materials 2.1.1. Cement The cementitious material used in this study was ordinary Portland cement, CPII-F-32, conforming to Brazilian standard NBR 11578 with the addition of 6–10% calcareous filler [17]. Its chemical composition and physical properties are shown in Table 1.
Fig. 1. SEM micrograph of b-SiC nanowhiskers.
2.1.2. SiC nanowhiskers The b-SiC nanowhiskers used in this study are a commercial product purchased from Nanostructured & Amorphous Materials, Inc., with the properties given in Table 2. The SiC nanowhiskers were used as received and the size and morphology were examined by scanning electron microscope (SEM, JEOL JSM-6701F). Fig. 1 shows that the material had a mean diameter of 1 mm and length of 25 mm. X-ray diffraction (XRD, Philips X-Pert using CuKa I = 15418 Å) was carried out to characterize the nanomaterial (Fig. 2) and indicated that the powder was mainly b-SiC (3C-polytype). The chemical composition of the NWS was determined by energy dispersive X-ray detection (EDS, JEOL JSM-6390LV) as follows: Si 46.48 wt%; C 50.99 wt% and O 2.53 wt%. 2.1.3. Superplasticizer A sodium polycarboxylate superplasticizer, with a relative density of 1.10, was incorporated into all mixes. The content was adjusted for each mix to ensure that all of the pastes had the same workability as the reference paste (without SiC NWS). 2.2. Dispersion of SiC NWS in aqueous medium The SiC NWS were added to 95% of the deionized water to be used in the mixture and the remaining 5% of the water was added to the sodium polycarboxylate superplasticizer (SP). The suspension was sonicated for 6 min. using a Vibra-Cell 750 Watts sonicator (Sonics & Materials, Inc.) with an amplitude of 50% and intervals of 20 s pulse and 20 s standing. To prevent temperature
Fig. 2. XRD pattern for as-synthesized b-SiC nanowhiskers.
rising, the suspensions were kept in a water–ice bath during sonication. After this process the solution was allowed to stabilize and was maintained at 23 °C however, it was verified that the SiC NWS started to decant soon after the end of the sonication process. 2.3. Production of the cement pastes The water-cement ratio (w/c) was 0.4 for all of the mixtures and the mix proportions for the materials are shown in Table 3. For the mixtures with SiC NWS, the appropriate amount of sodium
Table 1 Chemical and physical properties of Portland cement. SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Specific surface Blaine, m2 kg1
Compressive strength, 7-day, MPa
Compressive strength, 28-day, MPa
18.34
4.40
2.93
60.94
4.96
2.70
3.333
35.90
42.40
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Table 3 Mix proportion and consistency of the cement pastes. Cement Pastes
Cement (g)
Water(g)
SiC NWS (g)
SP (wt% cement)
Diameter – Mini Slump (mm)
REF NWS NWS NWS NWS
975 975 975 975 975
390 390 390 390 390
– 2.44 4.87 9.75 14.62
– 0.010 0.015 0.025 0.035
77.33 79.10 75.76 76.73 78.93
0.25% 0.50% 1.00% 1.50%
polycarboxylate superplasticizer (SP) was added in order to maintain constant the mini-slump consistency (diameter nearly 77 mm, similar to the reference), as it was considered that a part of the superplasticizer is consumed by the SiC NWS and not available for the dispersion of cement particles. Therefore, this will allow more objective comparisons between the results obtained for the reference and the nanocomposite samples, as noted by Tyson et al. [2] and Mohsen et al. [3], who studied carbon nanotubes in a cement matrix. The cement pastes were mixed in a mechanical mixer as follows:
Three-point bending flexural strength tests were carried out on 100 20 20 mm3 rectangular specimens (three samples) following procedures described in the Brazilian standard NBR 13279 [20]:
Sf ¼
1; 5 Fc L bh
2
where, Sf = flexural strength (MPa); Fc = load at the fracture point (N); b = sample width; h = sample thickness; L = length of the support span (60 mm). 2.7. Water absorption by immersion
1. The sonicated solution (95% water + SiC NWS) was mixed with the cement for 1 min at 440 rpm. 2. The superplasticizer with the remaining 5% water was added to the cement paste during mixing. 3. The mixture was homogenized for a further 2 min at the same speed.
The determination of the water absorption by immersion followed the test method specified in the Brazilian standard NBR 9778 [21]. The test was carried out on the prismatic specimens (50 20 20 mm3) used in the flexural strength tests and consisted of measuring the mass of the specimens in saturated and oven-dried states (three samples).
Approximately 10 g of the freshly prepared cement mixture was used for the isothermal calorimetry and the rest was poured into nylon molds for use in other tests. Special care was taken to release any residual air bubbles. After demolding (24 h), the specimens were cured for 7 and 28 days in saturated limewater at 23 ± 2 °C. The results were subjected to statistical analysis through analysis of variance (ANOVA) and the Tukey test using Past Software, version 2.17c.
2.8. Microstructural analysis (SEM) The SEM microstructural analysis of the broken specimens was performed to qualitatively examine the effect of the weight fraction of the NWS on their dispersion in the cement hydration products. This was carried out by observing the fracture surfaces of the specimens. The cement hydration process was stopped using the freeze drying technique at 28 days old. 3. Results and discussion
2.4. Isothermal calorimetry 3.1 Isothermal calorimetry A TAM Air isothermal calorimeter was used to assess the evolution of the heat flow of the cement pastes. Immediately after mixing, around 10 g of cement paste was placed into standard glass containers and the paste was then loaded into the isothermal calorimeter. The tests were performed on the pastes described previously (SiC NWS + SP) and on pastes with only the addition of the (SP) in order to separate the effects of the SiC NWS and the SP on the cement hydration. 2.5. Dynamic Young’s modulus The Young’s modulus was determined by the impulse excitation technique with SonelasticÒ equipment (ATCP – Physical Engineering) [18]. This technique consists of promoting a slight impact on the prismatic specimen with dimensions of 100 20 20 mm3. The acoustic response is captured and this is processed using software that calculates the Young’s modulus and were tested 3 samples in each age. 2.6. Compressive and flexural strengths The compressive strength measurements were performed on 20 40 mm2 cylindrical specimens (eight samples) following the recommendations of the Brazilian standard NBR 7215 [19].
The cement hydration kinetics is influenced by particle surface reactions and the surface area or size of the particles [14]. It is well known that the addition of nanomaterials to cement stimulates the nucleation process that occurs during the early cement hydration and the faster the nuclei are formed the earlier they can grow to larger crystals, accelerating the cement hydration. Due to their small size, nanomaterials provide very large surface areas that are highly reactive and can act as nucleation sites during cement hydration [22,23]. Table 4 and Figs. 3 and 4 provide the cumulative heat evolution and rates of heat generation (until 40 h), respectively, during the hydration for Portland cement pastes, according to the different additives used. The first peak is typically associated with the wetting of the cement powder, the dissolution of the silicate and gypsite and the formation of ettringite. After the induction period, the second peak is mainly due to the dissolution and hydration of alite, which induces the formation of C-S-H and CH. The second peak also corresponds to the acceleration zone which is associated with the formation of sulfate (type AFm) [23]. The superplasticizer does not strongly affect cement hydration and only a slight increase in the dormant period was observed as its content was increased. The retarder effect of superplasticizers on cement hydration is well known, and may be due to the formation of an insoluble layer or a ‘‘barrier” of calcium compounds surrounding the hydrating grains
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Dormant period (h:min)
I (%)
Cumulative heat evolution at 40 h (J/g)
I (%)
Acceleration process (a)
REF SP 0.010% SP 0.015% SP 0.025% SP 0.035% NWS 0.25% NWS 0.50% NWS 1.00% NWS 1.50%
2:25 2:19 2:31 2:39 2:51 2:05 2:09 2:19 2:48
– 4.14 4.14 9.65 17.93 13.79 11.03 4.14 15.86
162.90 165.81 165.93 166.92 167.85 167.25 160.61 164.49 160.80
– 1.79 1.86 2.47 3.04 2.67 1.40 0.98 1.29
0.288 0.292 0.285 0.288 0.290 0.347 0.302 0.318 0.309
+ + + +
SP SP SP SP
0.010% 0.015% 0.025% 0.035%
Fig. 3. Heat flow calorimetry results for the cement pastes with only SP.
Fig. 4. Heat flow calorimetry results for the cement pastes with NWS and SP.
[24]. Also, a slight increase in the cumulative heat evolution was observed with the increase in the amount of superplasticizer (Table 4), which can indicate better cement hydration due to greater dispersion of the cement particles. In most cases the dormant period was reduced with the addition of the SiC NWS + SP, indicating that due to the high surface area, the SiC NWS particles act as nucleation sites and accelerate the cement hydration [25]. It should be noted that there is ‘‘competition” between the retarder effect of the superplasticizer and the accelerating effect of the NWS.
Compared with the reference sample, the slopes of the curves for the pastes with NWS are clearly greater during the acceleration period (Fig. 4), as evidenced by the a value of the straight line during the acceleration period of the cement hydration of the pastes (Table 4). 3.2 Young’s modulus The effect of the NWS on the Young’s modulus (E) of the cement pastes, measured by impulse excitation vibration was also
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investigated. The results are reported in Fig. 5 and Table 5 (mean values and standard deviations). After 7 days, there is a slight decrease in the modulus with the addition of the NWS, which can be attributed to a delay in the formation of high stiffness CS-H in the early ages, as reported by Tyson et al. [2], who studied carbon nanotube cement composites. In contrast, at 28 days old, all of the pastes with NWS showed a slight modulus increase. However, statistically this enhancement is not significant.
1.00 wt% NWS (up to 107% and 75% at 7 and 28 days, respectively) and, statistically, the flexural strength after this content is not significant. In general, the literature reports that for cement pastes with a ratio of nanomaterials higher than 0.50% the improvement in the mechanical properties is not significant due to agglomeration [2,3,27].
3.3. Flexural strength
Fig. 7 and Table 6 show that the addition of NWS increases the compressive strength of pastes at 7 and 28 days. However, this improvement does not reach that observed for the flexural strength and the optimum effect seems to plateau at 0.25 wt% of NWS content.
3.4. Compressive strength
The flexural strength results are shown in Fig. 6 and Table 5 (mean values and standard deviations). It can be observed that there were increases in the flexural strength with the addition of
Fig. 5. Young modulus values for cement pastes with NWS at the ages of 7 and 28 days.
Table 5 Tukey’s scores for all possible pairs of different Young modulus and flexural strength values for the cement pastes. Cement Pastes REF NWS NWS NWS NWS
0.25% 0.50% 1.00% 1.50%
+ + + +
SP SP SP SP
0.010% 0.015% 0.025% 0.035%
E(GPa) 7 days
I (%)
E(GPa) 28 days
I (%)
Flexural (MPa) 7 days
I (%)
Flexural (MPa) 28 days
I (%)
19.49 ± 0.26 19.26 ± 0.37 18.47 ± 0.13 19.00 ± 0.27 18.79 ± 0.10
– 1.18 5.23 2.51 3.59
19.84 ± 0.27 21.01 ± 0.30 20.70 ± 0.13 21.10 ± 0.16 20.48 ± 0.40
– 5.90 4.33 6.35 3.22
4.40 ± 0.23 7.37 ± 0.47 7.14 ± 0.40 9.12 ± 0.40 7.88 ± 0.75
– 67.50 62.27 107.27 79.09
5.39 ± 0.16 7.95 ± 0.15 9.31 ± 0.54 9.43 ± 0.22 9.42 ± 0.36
– 47.49 72.73 74.95 74.77
Fig. 6. Flexural strength values for cement pastes at the ages of 7 and 28 days.
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Fig. 7. Compressive strength values for cement pastes at the ages of 7 and 28 days.
Table 6 Tukey’s scores for all possible pairs of different compressive strength values for the cement pastes. Cement Pastes
Compressive (MPa) 7 days
I (%)
Compressive (MPa) 28 days
I (%)
REF NWS NWS NWS NWS
37.17 ± 0.83 49.22 ± 1.09 45.51 ± 0.63 42.72 ± 1.44 42.30 ± 1.08
– 32.42 22.44 14.93 13.80
51.42 ± 1.22 64.21 ± 2.02 58.42 ± 1.87 57.56 ± 1.93 57.81 ± 0.72
– 24.87 13.61 11.94 12.43
0.25% 0.50% 1.00% 1.50%
3.5. Water absorption The water absorption test results for the pastes after 28 days of curing are shown in Fig. 8. All pastes showed a reduction in the open porosity when compared to the reference, mainly those with low contents of SiC NWS (0.25 wt% and 0.50 wt%). When, higher percentages of SiC NWS were added, the reduction in the porosity was not significant and, as mentioned in another research, and this may be due to some structural defects such as the increase of pores or voids in the cement paste and at the interface between nanowhiskers and cement paste [28].
This slight reduction in the porosity cannot explain the notable improvement in the mechanical properties with the addition of NWS to the cement paste. Thus, it can be concluded that these improvements are due essentially to the addition of the NWS. 3.6. SEM microstructural analysis The microstructural analysis shows the ability of the NWS to bind the particles of the cement hydration products and also to bridge cracks (Fig. 9). In previous studies it has been shown that the appropriate quantity of carbon nanotubes promotes an increase in the mechanical properties of cementitious materials.
Fig. 8. Water absorption results for the cement pastes at the age of 28 days.
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Fig. 9. SEM micrographs showing crack bridging and binding of cement hydration products by SiC NWS (a) 0.25% NWS and (b) 0.50% NWS.
Fig. 10. Low-content-SiC NWS samples showing good dispersion with (a) 0.25% SiC NWS (b) and 0.50% SiC NWS.
These nanomaterials can also act as bridges across pores and cracks, thus indicating that high bonding strength between them and the cement matrix is achieved [26,27,29]. With regard to the SiC NWS weight fraction, samples with low NWS contents (0.25 and 0.50 wt%) showed a relatively homogeneous dispersion in the cement matrix (Fig. 10). Almost no agglomeration was observed with 0.25 wt% NWS and few agglomeration zones were found with 0.50 wt% NWS. In contrast, for the pastes
with higher NWS contents of 1.00 and 1.50 wt% there were many zones with NWS covered by cement hydration products (Fig. 11), as previously reported for carbon nanotubes [2,3]. On associating these microstructural observations with the mechanical performance results, it can be noted that, in spite of the occurrence of NWS agglomeration bundles within the cement matrix for 0.50 and 1.00 NWS wt%, the flexural and compressive strengths exhibit a significant improvement compared with the
Fig. 11. High-content-SiC NWS samples covered by cement hydration products (a) 1.00% SiC NWS and (b) 1.50% SiC NWS.
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reference. It can be concluded that the SiC NWS content (by weight) seems to have a greater effect on the overall strength improvement that the dispersion quality of the nanofilaments. However, with the addition of 1.50% NWS the improvement in the flexural and compressive strength is not as great compared with the other pastes. 4. Conclusions In this experimental study, Portland cement pastes were prepared with the addition of different amounts of SiC NWS (0, 0.25, 0.50, 1.00 and 1.50 wt%). As expected, SiC NWS act as nucleation sites and accelerate the cement hydration despite the retarder effect of the superplasticizer. No significant increase in the Young’s modulus was observed. In contrast, there was an increase in the flexural strength for 1.00 wt% of SiC NWS (up to 107% and 75% at 7 and 28 days, respectively). The compressive strength also increased with the addition of SiC NWS however, this improvement was not as great as that observed for the flexural strength. The enhancement in the compressive strength and especially in the flexural strength is essentially due to the presence of SiC NWS. Microstructural observations show the ability of the SiC NWS to act form bridges across the cement paste pores and cracks, indicating that there is a high bonding strength between them. As expected, it was also observed that as the SiC NWS content increased the occurrence of zones with SiC NWS agglomerated in bundles increased however, these zones were mostly, at least partially, covered by cement hydration products. This finding could explain the plateau reached by the mechanical strength, since it was observed that, in spite of the occurrence of these bundles within the cement matrix with 1.00 wt% NWS content, the flexural and compressive strengths exhibited a significant improvement as the SiC NWS content increased. Further studies are necessary to fully understand where and how the SiC nanowhiskers are acting on cement hydration. Acknowledgments The authors gratefully acknowledge Laboratório Central de Microscopia Eletrônica (LCME) at Federal University of Santa Catarina and the Brazilian governmental research agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo a Pesquisa do Estado de Santa Catarina (FAPESC) for providing the financial support for this research. The Coordenação de Aperfeiçoamento de Pessoal do Nível Superior – CAPES provided a scholarship to the first author. References [1] R.B. Hamada, H. Chacham, Nanotubos e a nova era do carbono, Ciência Hoje 198 (2003). [2] B.M. Tyson, R.K.A. Al-Rub, A. Yazdanbakhsh, Z. Grasley, Carbon nanotubes and carbono nanofibers for enhancing the mechanical properties of nanocimposite cementitious materials, J. Mater. Civ. Eng. 23 (2011) 1028–1035. [3] M.O. Mohsen, R. Taha, A.A. Taqa, A. Shaat, Optimum carbono nanotubes’ contente for improving flexural and compressive strength of cement paste, Constr. Build. Mater. 150 (2017) 395–403.
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