INTRODUCTION

Portland cement-based materials such as concrete and mortar are widely used in building materials in today’s world and are mostly applied in the construction industry [1]. Nearly 4 billion tons of these materials were produced in 2011 [2]. The cement industry is currently generating about 8% of global greenhouse gas emissions, such as CO₂. While global cement production is expected to increase it by more than 4 billion tons due to an increase in demand for housing and infrastructure projects as shown in Fig. 1 [3-5]. Along with global policies to achieve sustainable development, many efforts have been made to reduce emissions of pollutants such as the use of replacing materials that reduce cement consumption [6]. Although some researchers have reported that increased cement consumption in cement-based products will improve mechanical properties, it is scientifically unacceptable. What is confirmed by the construction industry researchers is achieving an optimal percentage? On the other hand, for sustainable development and environmental protection, it is necessary to reduce cement consumption or at least not to increase it [7-10]. So, the use of nano-scale materials due to their unique characteristics and their performance differences over larger materials can be a good alternative to cement replacement, resulting in a significant reduction in its use [11-17]. Many researchers have made fundamental changes in science in recent years, utilizing nanotechnology and the use of various nanomaterials [18-22]. Among the most widely used nano-materials, there are carbon nanotubes (CNT), nano-titanium oxide or titanium (TiO₂) and nano-montmorillonite (NM). The preferred mechanical properties of carbon nanotubes have introduced this nanomaterial as a candidate for improving the durability of cement-based materials [23-28]. Carbon nanotubes have high flexibility, in addition to strength [29]. Titanium oxide nanoparticles also have opened a new chapter in nanotechnology research in recent years, due to their self-cleaning properties, excellent hydrophilicity, non-toxicity, abundance, and availability [30-32]. Nano-montmorillonite is one of the main nano-clay phases, and several structural properties of the clays have better reactivity and the ability to return to the initial dimensions in the vicinity of heat and flame retardation. This nanomaterial, as a modified natural substance, has achieved nanotechnology applications in nanocomposites and because of its high nature value, it can enter the industry at a lower cost than other nanomaterials [33-37]. In the above study, carbon nanotubes and titanium oxide nanoparticles were first synthesized and the dielectric properties of the powdery nanocomposites made from these materials and the montmorillonite nanoparticles and Portland cement were evaluated. Then, by design of experiments (DOE) by response surface methodology (RSM) concept, nanomaterials replaced cement in the cement paste by weight percentages. mechanical properties of the specimens were evaluated according to ASTM standards and through analyzing data, figures and spectra obtained from X-ray diffraction (XRD) and X-powder techniques, the effect of using nanomaterial in cement and its role in modifying the nanostructured properties of cement matrix were investigated.

EXPERIMENTAL PROGRAM AND SAMPLE PREPARATION

The design and method of this experimental study were approved by the nanophysics Research Center of the Faculty of Basic Sciences at the Mazandaran University.

Materials

Type II Portland cement produced in Mazandaran Cement Company was employed in the present study complying with ASTM C150, 2004 [38]. Chemical properties of the cement used in quality controlling of the cement company’s laboratory were studied with XRF analysis and are presented in Table 1. and Sodium dodecyl benzene sulfonate (SDBS) as a surfactant has also been used as a nanomaterial disperser.

Synthesis of MWCNT

To synthesize carbon nanotube by CVD technique, argon gas (for 15 minutes) was injected with a flow of 300 sccm into a quartz chamber containing 100 mg of catalyst substrate. Then, the flow of argon gas was reduced to 100 sccm and acetylene gas was released with a specific flux inside the quartz tube. In this way, the prepared substrate is placed adjacent to acetylene and argon gases. It takes about 15 minutes to complete the process of growing carbon nanotubes, dissolve the acetylene gas flow and let the material cool to room temperature in the vicinity of the argon gas (to prevent oxidation of the carbon nanotube).

Functionalization of MWCNT

For the production of operating carbon nanotubes, 5 g of synthesized carbon nanotubes were added to 200 ml of HNO₃ nitric acid solution and were diluted for 140 hours at 140° C. The material was washed with distilled water. Then solid separation from the liquid was performed by the use of centrifuge (3000 rpm). The solid products were completely dried at 60 ℃ to produce the resulting material as a MWCNT-COOH.

TEM analysis of nanomaterials

TEM analysis was used to describe the morphology and distribution of the size of carbon nanotubes and nanoparticles. The TEM image of the functionalized carbon nanotubes is shown in Fig. 2. The average outside diameter of the nanotubes is 15 to 35 nm. Montmorillonite nanoparticles were produced in Southern Clay Products, Inc. The TEM image of the nano-montmorillonite is shown in Fig. 3. The average length of the montmorillonite nanoparticles is 6 to 16 nm. Silicon carbide (β, 99%) was purchased from UD Research Nanomaterials, Inc. Titanium Oxide Nanoparticle (TiO₂, anatase) was purchased from Nanosony Corporation, Inc. The TEM image of the nano-TiO₂ is shown in Fig. 4. The average length of the TiO₂ nanoparticles is 18 to 45 nm. Physical properties of MWCNT-COOHs synthesized in the mentioned project can be observed in Table 2.

Preparation of cement paste samples

In the present work, five series of mixed cement paste including control samples as samples without nanomaterials and other samples containing CNT, TiO₂ and NM nanoparticles with the weight percentage of Portland cement were used according to Table 3.

The cement paste samples were prepared according to ASTM C109, 2002 [39] standard with the water to binder i.e. cement + MWCNT + NM or cement+ MWCNT + TiO₂ ratio of 0.425. For better dispersion of nanomaterials in the cement matrix, MWCNT was dispersed in deionized water using a magnetic stirrer before adding cement to make a suspension. After that, cement was added and the mixture was dispersed in water in an ultrasonic bath (BANDELIN, Germany) for 30 minutes at 25 ^oC for more uniform dispersion. Then, the mixture was blended by the use of a mechanical blender (TESTING Bluhm, Germany). The applied mixer for this experiment was in accordance with ASTM C511, 2013 [40] with a planetary motion of 285 rpm for paste. During the test, the laboratory temperature was 202 ^oC. The samples were demolded 24 hours after they were cast and then cured in a water basin at 23 ± 2 ° C for 27 days.

Test methods

Mechanical tests

Flexural and compressive strengths of the samples were studied in 28 days. Compressive strength was done on samples based on ASTM C109, 2002 [39] standard. For this purpose, 5 cubic specimens (50×50×50 mm3) were prepared. Flexural strength tests were done on samples based on the ASTM C348, 2002 [41] standard. In this regard, 5 prismatic samples (160×40×40 mm3) were prepared. The mechanical tests were done by a cement testing machine (DIGICON, Swiss) with a capacity of 200KN and a loading speed of 2.4 KN/s for compressive strength and with a capacity of 10KN and a loading speed of 0.5 KN/s for flexural strength.

Results of mechanical tests

Table 4 shows the results of the flexural and compressive strengths of the cement paste samples at different ages with different weight ratio nanomaterials. Results of variations in compressive and flexural strengths of pure cement composites (CS) and nanocomposites containing different nanomaterials with different weight percentages shown in Fig. 5 and Fig. 6 demonstrated that the compressive and flexural strength of samples containing nanomaterials is significantly improved compared to the control sample. The highest increase in compressive strength is related to the SNCS2 sample and the highest increase in flexural strength is related to the SNCS1 sample.

The compressive strength of the SNCS2 sample increased by about 30%, and the flexural strength of the SNCS1 sample increased by 39% compared to the control sample. It is clear that montmorillonite nanoparticles play the greatest role in improving the mechanical properties of cement composites, which can be attributed to specific surfaces and high reactivity of this nanoparticle. Another reason to improve the mechanical properties of cement-based nanocomposites is to increase the reactivity of cement products with the presence of montmorillonite and titanium dioxide nanoparticles. It means that with high surfaces and fast reactivity, the nanomaterials react with Ca(OH)₂ and produce a secondary gel which is the main factor in increasing the strength of cement-based nanocomposites. Another reason for the improvement of the mechanical properties of nanomaterial samples in comparison with the control samples is the presence of functionalized carbon nanotubes with the COOH group in the hydration process. The COOH function acts as a bridge between the polar hydrophilic cement matrix and non-polar carbon nanotubes, and it makes nanotubes better dispersed in the cement matrix. Furthermore, functionalized carbon nanotubes penetrate well inside cement products and by bridging, they help to reinforce interfaces. As a result, they improve the flexural strength of cementitious nanocomposite. Also, the presence of carbon nanotubes in the hydration process causes a delay in its duration, and more C-S-H is produced. Improvement of the mechanical properties of nanocomposites can also be due to the filling property of nanoparticles. Due to their very small dimensions, nanoparticles penetrate into cementitious pores as filler and fill the empty spaces and cause more cementitious cavitation and thus improve mechanical properties. In order to prevent agglomeration and produce better dispersion in the cementitious matrix, the SDBS surfactant and also the ultrasonic bath is used in the process of making nanocomposites.

MICROSTRUCTURE ANALYSIS

To confirm the mechanical test results in the previous section, the microstructure analysis of samples containing nanomaterials was compared with the control sample using the following methods.

XRD analysis and X-Powder technique

The XRD method can be used to determine phase identification, crystal size measurement, and crystallinity. The phases of the powdery samples, without and with nanomaterials were qualitatively and quantitatively analyzed by XRPD [W: 1200, 35: KV, 30: mA] with a tube FK 60-04, cobalt anode, and linear focus, and the data were collected as follows: Time per step: 1 sec; angular range: 5-80°; total acquisition time: 01:23 h; degrees/min: ½.

Fig. 7 shows the XRD patterns of pure cement paste (CS) and cement nanocomposites containing CNTs, TiO₂ nanoparticles (STCS1 and STCS2) and CNTs, NM nanoparticles (SNCS1 and SNCS2). According to the figure, the peaks located at 47.1°, 34.2°, 18.2°, and 8.50° correspond to Ca(OH)₂, where the intensity of SNCS2 and STCS1 decreased compared to CS, whereas the peak intensity at 29.5° , related to C-S-H, increased in samples containing nanomaterials, especially in the SNCS2 sample. The peak at 34° is related to the C-H bond, and its intensity has decreased in nanocomposite samples compared to CS. Also, peaks of 41.3° and 32.5° are related to the presence of C₂S and C₃S, which is reduced in samples containing nanomaterials, particularly SNCS2, compared to CS. All of these findings indicate improved nanostructured properties of cement-based nanocomposites that are consistent with mechanical test results. In addition, by reducing the structure of amorphous in cement composites, the electrical resistance of cement-based nanocomposites decreases.

The data were quantitatively analyzed by the Scherrer method using the X-Powder software. In order to measure the approximate sizes of the crystal phases, the width of the peak was measured at half the maximum intensity (FWHM) and the highest-intensity peak was chosen. Based on Scherrer’s model according to Eq. 1 [42]:

 (1)

D is the crystal size,  is the peak width at half the maximum intensity,  is the Bragg angle related to the peak, and  is the X-ray wavelength. Based on the equation, as the peak width increased, the crystal size decreased as a result of the addition of nanomaterials in cement paste. As shown in Fig. 8, the size of the nano-crystallites in the CS is 61 nm and 75 nm, in the SNCS1 sample, it is 29 nm and 37 nm, in the SNCS2 sample, 22 nm and 27 nm, in the STCS1 sample, 23 nm and 38 nm, and in STCS2 sample, it is 18nm, and 21 nm, which was obtained using both Sherer relation and the X-Powder technique.

FTIR properties

To determine the quantitative and qualitative properties of cement paste containing nanomaterials, type of the functional group and the bonds in their molecules can be determined using the FTIR spectrophotometric method. Fig. 9 illustrates the chemical bond between the samples containing nanomaterials and CS in a range of 400-4000 cm^-1. Fig. 9 shows the FTIR spectra of the control sample and cement-based nanocomposites containing different nanomaterials. The peak in the range of wave number 450 to 463 corresponds to the SiO(υ₄) functional group [43,44]. This peak in the sample without nanomaterials is in 462 Cm^-1 and in the samples containing carbon nanotubes, montmorillonite nanoparticles and, titanium oxide nanoparticles (SNCS1, SNCS2, STCS1 and STCS2) in the wave number 459 Cm^-1 shows more intensity. Due to the inverse relationship between the wave number and the displacement energy toward the lower wave number, it can be attributed to the stronger bond energy. The peak in the range of the wave number 874 to 878 Cm^-1 corresponds to the C(υ₄) bond [43] and the peak of samples containing nanomaterials is smaller than the control sample. In the range of 968 to 971Cm^-1, it is related to stretch vibration for SiO(υ₃) functional group [45-47] which is no significant change between samples containing nanomaterials and the control sample.

Considering the fact that the existing peak in the range of wavenumber 1426 to 1430 Cm^-1 is related to the functional group C(υ₃)[48,49], peak in the wavenumber 1426 Cm^-1 for the control sample and with changes fully reduced in the wavenumber 1420 Cm^-1, it is observed that the bonding related to C(υ₃) is a little weaker. In the wavenumber range of 1640 to 1650 Cm^-1, the peak related to the functional group is H₂O(υ₂) [47-50] where the target peak for the CS, the SNCS1 and SNCS2 is observed in the wavenumber 1641 Cm^-1 and for the STCS1 and STCS2, it is observed in the wavenumber 1652 Cm^-1. Also in the samples containing nanomaterials, the peaks are observed as intensified. The peaks in the range of wavenumber 3399 to 3444 Cm^-1 corresponded to the functional group O-H(υ₃) [51] which is observed in wavenumber 3438 Cm^-1 and the changes related to the samples containing nanomaterials compared to the CS are not observable. In the range of wavenumber 3641 to 3644 Cm^-1 the peak is corresponded to calcium hydroxide Ca(OH)₂ [50]. This peak for every five samples is observed in the wavenumber 3642 Cm^-1 and this peak is more intensive in the samples containing nanomaterials.

QUALITY AND EMISSION FACTORS OF THE SAMPLES

As stated before, the cement industry produces large amounts of CO₂. It is under pressure to reduce both greenhouse gas emissions and energy. For this purpose, the quality factor (Q_F) and dissipation factor (D_F) is studied where five capacitors were constructed. The capacity of the samples was measured by the use of GPS 132 A. Then dielectric constant (k) is calculated using the equation c=kε₀A/d. In the constructed tables: r=1.2 cm, d=2mm and ε₀.

Results in Tables 5 and 6 show that capacity (C), dielectric constant (k), quality factor (Q_F), and dissipation factor (D_F) change after adding different powdery materials to CNTs. The maximum amounts of dielectric constant at the frequency of 120 Hz (38.12) and 1 kHz (17.02) are related to SNCS2 sample. The maximum value of Q_F represents the minimum energy loss rate regarding the energy stored in the capacitor, which is obtained through the Eq. (2):

Q_F (2)

The D_F value is obtained from Eq. 3 which expresses the energy loss rate of the capacitor and varies with different dielectric materials, as presented in Tables 5 and 6.

 (3)

The estimated fraction emission factor, f in Eq. 4 and according to XRD patterns in Figs. 7 and 8 for samples in Tables 5 and 6 at 120 Hz for CS, SNCS1, STCS1, SNCS2 and STCS2 is about 0.875, 0.684,0.746, 0.612, and 0.743 respectively.

 (4)

Where E_cement the emission factor, f is is the fraction of CaO in cement,  and  are the molecular weight of CO₂ (44.01) and the molecular weight of CaO (56.08) respectively [55]. It is clear that E_SNCS2 has lower emission factor than the other present samples.

DESIGN OF EXPERIMENT BY RSM CONCEPT

In this study, the design of experiments was used to save time and money, and the results of the mechanical test section are the best results of this design. Design of experiment (DOE) is a systematic process to analyze the test and to optimize input variables levels to get optimum value for output response. Response surface methodology (RSM) is an important issue in the statistical design of experiment which is a complex of useful arithmetic and statistical techniques for molding and is a response variable that determines the relationship between one or more response variables and several independent inputs. The aim is to optimize this response.

One of the objectives of using RSM is to understand the topography of the response surface (local maximum, local minimum, ridge lines) and finding the regions where the optimal response occurs. RSM is an iterative process by which experiments are conducted along the pathway of steepest ascent (i.e. the direction of greatest improvement in the response variable) in order to identify the factor settings which optimize the response variable. The response changes in a given direction by adjusting the design variables. Using two-dimensional plots from the experimental design software, optimal concentrations of nanomaterials replaced in cement paste were predicted to achieve the highest results of mechanical tests as responses. The plots are helpful to see the shape of a response surface. The function  can be plotted versus the levels of (), () and (). In this graph, each value of  and  generates a y-value.

The contour plots show contour lines of (,), (,) and (,) pairs that have the same response value y. In this study, the contour plots illustrate contour lines of (NM, CNTs), (NM, SDBS), (CNTs, SDBS), (TiO₂, CNTs), (TiO₂, SDBS) and (CNTs, SDBS) pairs that have the compressive and flexural strengths as responses in Figs. 10 and 11. Using two-dimensional contour plots, we estimated the optimal concentrations of carbon nanotubes, titanium oxide, montmorillonite nanoparticles and SDBS in pairs which resulted in the highest compressive and flexural strengths.

CONCLUSION

In the present theoretical and empirical study, carbon nanotubes were synthesized and functionalized, and after the design of experiment, some nanoparticles (TiO₂, MWCNT, and NM) replaced cement in the cement matrix. The best results of mechanical tests have been reported. Then, the nano-structural factor, as well as quality and emission factors, were studied. The obtained results indicated that sample SNCS2 with higher dielectric constant (38.12), higher quality factor (33.24) and lower emission factor (0.612) is a suitable sample in the cement-based industry.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.