The Effect of Multi-Walled Carbon Nanotubes on the Compressive Strength of Cement Mortars

Abstract

In this work, multi-walled carbon nanotubes (MWCNTs) have been synthesized using a modified method of solid-phase pyrolysis. The MWCNTs are effectively dispersed using a simple and facile method such as ultrasonic energy without and with surfactant for two different sonication times (15 min and 40 min). In the present study, the effect of MWCNT concentration (0.001, 0.01, 0.05, 0.1 wt.%) on the compressive strengths of cement mortars has been investigated. Compressive tests were carried out on an automatic pressure machine (C089) with a loading rate of 0.5 kN/s at the age of 7 days and 28 days. It is shown that the optimal value of the nanotubes’ concentration does not exist in the case of 15 min of sonication time, whereas the optimal value for 40 min of sonication time without and with surfactant is 0.01%. Moreover, in the absence of surfactants, the strength of the specimen over 7 days of hardening increased by 13%, and by 19.5% in the presence of surfactants. The compressive strength for a curing period of 28 days increased by 6.3% and 13.8%, respectively.

1. Introduction

Due to the increasing demand for high-performance cement composites, the properties of cement composites, such as architectural versatility, excellent mechanical properties, and durability, have been undergoing continuous improvements [1].

To this end, in the literature, various agricultural wastes [2,3], industrial wastes [4,5], natural minerals [6], and synthetic materials [7] have been successfully incorporated. These binders exhibited enhancement in the fresh, mechanical, durability, shrinkage, and microstructural properties of construction products. One such material sparingly used in the literature is carbon nanotubes (CNTs).

CNTs can be considered as seamless cylinders formed of one or more graphene roll sheets and can be classified based on the number of graphene layers [8]. Based on the number of graphene layers, there are two types: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) [9]. MWCNTs comprise manifold-wrapped graphene sheets arranged in concentric hollow tubes with outside diameters ranging from 2 nm to 100 nm [10]. High modulus of elasticity (≥1 TPa), outstanding tensile strength (65–93 GPa), excellent thermal conductivity (two times more than that of a diamond), high aspect ratio (100–2500), and excellent electrical conductivity are the physical and mechanical characteristics of MWCNTs [11].

The use of CNTs in Portland cement matrices applied in civil engineering has a significant potential to enhance the mechanical properties of composites [12,13,14]. MWCNTs are used more frequently than SWCNTs because of their lower manufacturing costs and improved reinforcement. According to Lai and Basem [15], adding 0.25 percent MWCNTs to cement mass increased the flexural and tensile strengths of cement-based mortars by 25%. Using small amounts of MWCNTs at 0.08%, the authors reported enhanced flexural strength of 25% [16]. According to Jeevanagoudar et.al [17], MWCNT-reinforced mortars exhibit improved engineering properties compared to ordinary mortars. In particular, it has been discovered that 0.4 percent is the optimum concentration of MWCNT concentration in order to get the highest compressive strength.

CNTs’ optimal dispersion is one of the important factors for the preparation of enhanced cement-based composite materials. There are two main methods for dispersing nanotubes: mechanical and chemical [18,19]. In this work, the dispersion of nanotubes by mechanical, in particular ultrasonication, method has been carried out. It is worth noting that the ultrasonication efficiency for MWCNT dispersion depends on many factors, including duration, sonicator type, energy, temperature, and the properties of MWCNTs [20,21,22,23]. Due to their high surface energy, MWCNTs have a tendency towards agglomeration, which may lead to the creation of week zones in the final product. By using surfactants, the efficiency of sonication increases; as a result, a more uniform dispersion of MWCNTs in cementitious matrices can be seen [24].

The effect of the concentration of MWCNTs synthesized by different methods, the duration of sonication, and the use of surfactants on the mechanical properties of cement-based materials, particularly cement mortar, still requires further investigation.

In the present work, the effects of concentration of MWCNTs, duration of sonication, and the use of surfactants on the compressive strength of cement-based mortar were investigated.

2. Experiment

2.1. Materials

Ordinary Portland cement 52.5 (GOST 31108-2020, which is available in Araratcement Factory, Yerevan, Armenia) has been used as a binder in the mortars within the framework of this study. The short MWCNTs have been synthesized (Figure 1) using a modified method of solid-phase pyrolysis of cobalt phthalocyanine. The pyrolysis was carried out in a closed quartz ampoule at a temperature of 900 °C and a duration of 30 min using pyrolysis analogs of Ni and Fe phthalocyanines [25,26]. The physical properties and the chemical composition of the used cement [27] (GOST EN 196-1-2002, 196-2-2002, 196-3-2002) are shown in

Table 1. Physical properties and chemical composition of cement.

Characteristics			Days			Results Obtained
Standard consistency (%)			-			31
Specific gravity (g/sm3)			-			3.1
Blain’s fineness (m2/kg)			-			354.8
Compressive strength (MPa) (EN 196-1)			3 days			23
			7 days			38
			28 days			52
Setting time (min)			Initial			60
			Final			330
Chemical composition of cement (wt.%)
Al2O3	SiO2	Fe2O3	CaO	MgO	SO3	Loss of Ignition	Insol. Resid.	Free CaO
3.21	23.2	1.25	57.5	5.1	2.9	3.7	2.1	1.13

, while the physical properties of the used sand are shown in

Table 2. Physical properties of sand and MWCNTs.

Sand	Fineness Modulus 2.43	Specific Gravity 2.17	Zone II	Bulk Density in Compact State (kg/m3) 1829	Bulk Density in Loose State (kg/m3) 1609
MWCNTS	Outer diameter 40–50 nm	Length < 1 μm	Purity > 90%

.

2.2. Dispersion of MWCNTs

Many researchers have reported MWCNT dispersion using various techniques. In this work, MWCNTs, in the required amount, were mixed with water and stirred continuously to ensure proper mixing. Two different sonication times were considered (15 min and 40 min). The sonication process is conducted at room temperature using the ultrasonic device UP400S. In the present investigation, DISPERBYK 199 was also used in order to increase the efficiency of the MWCNT dispersion process in the water. A similar procedure was followed for solutions containing different wt.% of MWCNT content (0.001, 0.01, 0.05, 0.1).

2.3. Mixing and Sample Preparation

The w/c ratio used in the present work was 0.47, and the cement to sand proportion used was 1:4. First, cement and sand were mixed (E095 Mortar mixer, Matest, Treviolo, Italy) for 2.0 min, then the MWCNTS/water mixture was added and mixed for 5 min. The size of the molds were 40 mm × 40 mm × 160 mm. The mortar was compacted through a vibration machine (C278 Vibrating table, Matest, Treviolo, Italy) for 30 s. Similarly, a series of mortars and the one without different MWCNT contents (0.001%, 0.01%, 0.05%, and 0.1% by weight of cement) were cast. The specimens were also prepared by adding in the MWCNTS/water surfactant DISPERBYK-199 (produced by company BYK, Wesel, Germany). The weights of the surfactants were 4.4, 44, 220, and 440 mg, respectively. After 24 h, the specimens were de-molded, and the mortar sample was immersed in water at 20 ± 0.2 °C temperature (Figure 2).

2.4. Compressive Strength Testing

Three cubes of each specimen were incidentally selected from each batch to test their average compressive strength according to the Concrete Compression Machine (Matest, Treviolo, Italy) 2000 kN automatic, Servo-Plus Progress (following the standard EN 196-1, and specimen sizes were 40 mm × 40 mm). Compressive tests were carried out on an automatic pressure machine (C089) (Matest, Treviolo, Italy) with a loading rate of 0.5 kN/s at the age of 7 days and 28 days.

3. Results and Discussions

First of all, the results obtained with 15 min sonication time and without surfactant are presented. Figure 3a,b show the compressive strength of the mortar with different wt.% of MWCNTs for 7 and 28 days, respectively. The results indicate that, for both cases of curing days of the cement mortar, the optimal value of MWCNT concentration does not exist. Furthermore, the compressive strength of the cement mortar in the presence of MWCNTs is lower than the compressive strength of the reference specimen. This can be explained due to the low-efficiency dispersion of the MWCNTs in the water, which will reduce the hydration degree. The results indicate that the compressive strength increases with increased curing period, which can be attributed to increased hydration with time.

Figure 4a,b show the compressive strength of the mortar with different wt.% of MWCNTs for 7 and 28 days, respectively. C0, C1, C2, C3, and C4 correspond to the 0%, 0.001%, 0.01%, 0.05%, and 0.1% of MWCNTs, respectively. The results indicate that the compressive strength of each specimen increases as the curing period increases. This is associated with increased hydration over time. The addition of nanotubes with an appropriate concentration of MWCNTs leads to an increase in compressive strength. The compressive strength reaches the maximum value, then starts to decrease. In other words, the optimal value of the nanotube concentration at which the compressive strength for both cases of curing days reaches its maximum value was obtained. The indicated optimal value of nanotubes is 0.01 wt.%. This is due to the chosen composition of the mortar, as well as the physical and mechanical properties (structure and size) of nanotubes. It is known that in order to make the effect of nanotubes on the physical and mechanical properties of cement-based mortars or concretes effective, it is first necessary to ensure a homogeneous distribution of nanoparticles throughout the volume. For each case, with the increase of nanotube concentration, under the same conditions, the degree of homogeneous distribution decreases, as a result of which the compressive strength decreases. Elsewhere [17], the authors studied rather large values of nanotube concentration, and at 0.4 wt.% of MWCNTs the maximum compressive strength was found only for 28 days of curing (in this work for 7 curing days the optimal values of MWCNTs were not obtained). It can be seen from the figures that the strength of specimens with 7 curing days increased by 13%, and in the case of 28 curing days it increased by 6.3%.

Figure 5a,b show the compressive strength of the mortar with different wt.% of MWCNTs for 7 and 28 days, respectively, when the surfactant in the MWCNT/water was added.

For both cases of curing days, the optimal values of the MWCNT concentration have been obtained at 0.01 wt.%. The improvement in the efficiency of the dispersion process of MWCNTs in the water brought an increase in the maximum value of the compressive strength. In particular, the strength of a sample with 7 curing days increased by 19.5%, and by 13.8% in the case of 28 curing days.

4. Conclusions

In the present research, the mechanical properties, such as compressive strength, of cement mortar with different concentrations of MWCNTs have been investigated. The sonication technique with and without surfactant, in particular DISPERBYK-199, was employed. For 15 min sonication time, the optimal values of MWCNT concentration to obtain maximum compressive strength do not exist, while in the case of 40 min of sonication it was found to be 0.01%, which is much less than the optimal value obtained in known works. In particular, in the absence of surfactants, the strength of the specimen over 7 days of hardening increased by 13%, and by 19.5% in the presence of surfactants. The compressive strength for a curing period of 28 days increased by 6.3% and 13.8%, respectively. Thus, the optimal mass of surfactants was found, and it was shown that when added to MWCNTs/water, the maximum value of compressive strength can increase by up to 7.5%.