1. Introduction
Over the past decades, investigations of construction material properties have only been possible on a macro scale. However, new materials have been developed and knowledge of the nano scale behavior is imperative. For example, research of the cement matrix and its interaction with other concrete design constituents is a powerful approach to the development of concretes with better properties and more controlled degradation.
The construction industry is a branch of engineering that is of great importance to society, ranked among the top forty industrial sectors likely to be soon affected by nanotechnology. Concrete and its related products are considered essential inputs for this industry [
1,
2].
On the other hand, since 1991, carbon nanotubes have been used by several industries (electronic, automobile, and astronautic) in the research of nano-size particles. The carbon nanotubes (CNTs) have outstanding mechanical, thermal and electrical properties including an elastic modulus of 1 TPa, tensile strength of 63 GPa, thermal conductivity of 6600 W m
−1 K
−1, electrical resistivity in the order of 10
−4 Ω-cm, and a current density capacity up to 106 A cm
2 [
3]. Carbon nanotubes can be found in two major forms: single-walled carbon nanotubes and multi-walled carbon nanotubes [
1]. Another carbon structure is the nanotube in which carbon atoms are connected in the form of hollow tubes, such as fullerenes, with diameters of one to several tens of nanometers. Carbon atoms can also be combined in nanosheets and nanofites, which have membrane-like structures a few nanometers thick [
4].
Understanding, at the nano scale, the performance of the cement matrix and its interaction with other components can be a powerful step in the development of superior concrete with enhanced properties and a more effectively controlled deterioration process. Concrete requires excellent filling capacity and adequate resistance to segregation. In general, conventional research on concrete considers the effects of the addition of different substances, such as ash, ground granulated blast-furnace slag, and limestone, on the performance of the mechanical properties [
5].
Experiments with nanomaterials have already allowed for the development of lower cost high-performance cement compounds used extensively in civil engineering [
2,
6]. The properties of concrete in its initial state, such as its fluidity and workability, are governed by the distribution and size of the particles, while the properties of concrete in its hardened state, such as strength and durability, are affected both by the size of the particles and by the arrangement or grouping of the aggregate [
7].
One of the most desirable properties of the nanomaterials used in the construction industry is their ability to confer mechanical reinforcement to the concrete structure. Due to the remarkable properties of carbon nanotubes, this can be achieve by the incorporation of this material into cement compounds, resulting in a new class of cement products valuable to the construction industry [
8,
9]
The nanotubes modify the molecular structure of the cementitious materials, leading to improved physical and mechanical properties. These nanotubes directly influence the mechanical performance, volumetric stability, durability and sustainability of the concrete structure [
10,
11].
Nowadays, carbon nanotubes are also being used to reinforce many types of materials including metals [
12]. Studies have shown that pressure above 500 MPa can be transferred across the interface between a polymer and carbon nanotube, this strength is 10 times higher than that between polymer and carbon fibers [
13].
Makar et al. [
14] presented hardness measurements, evidence that carbon nanotubes can affect cement hydration in their initial state and that a strong bond is possible between cement paste and carbon nanotubes. They reported that nanotubes have a special role in controlling the cracking of the composite as, in terms of the size and proportion incorporated into the matrix, they are dispersed better than the fibers used as reinforcement. Agullo et al. [
15] observed an increase in early age compressive strengths by adding a low concentration of multi-walled carbon nanotubes (MWCNTs) to the cement composites.
Balaguru and Chong [
16] believed that the development of nanoscience for concrete was needed since properties such as low shrinkage, resistance to high and low temperatures, compatibility with different types of fibers, and responsiveness to nanomaterials such as carbon nanotubes can be used to create new products with far better performance. Carbon nanotubes and carbon nanofibers appear to be some of the most promising nanomaterials to improve the mechanical properties of cementitious material due to their resistance to crack propagation and other abnormalities [
17].
Hunashyal et al. [
18] noted higher tensile strength and a better stress–strain relationship, in direct tension, for cement samples containing carbon nanotubes. Han et al. [
19] showed that the addition of multi-walled carbon nanotubes (MWCNTs) could decrease the water absorption and permeability coefficient of reinforced cement-based materials. Manzur et al. [
20] found that, when used in the mortar, MWCNTs with an OD (outside diameter) of 20 nm or less obtained higher compressive strengths, compared to larger OD MWCNTs, while the maximum compressive strength was achieved by the smallest size MWCNTs considered in the study.
Lelusz [
21] found that the compressive strength of cement composites decreased with the increase of the CNT dosage. This research used different dosage rates of MWCNTs, ranging from 0.00% to 0.12% by weight of cement. A suitable mix proportion was proposed by Manzur et al. [
22] in a study aiming to produce surface-treated, MWCNT-reinforced cement composites based on their flexural and compressive strengths. It is, therefore, evident that carbon nanotube reinforcement could result in robust and improved cementitious composites.
Campillo et al. [
23] experimented on a cementitious compound containing carbon nanotubes under compression and found that SWCNTs ( single walled nanotubes) and MWCNTs increased the compression resistance in relation to that of pure cement when at 6% and 30%, respectively. Lai and Bassem [
24] conducted experiments to investigate the effectiveness of the carbon nanotubes, uniformly dispersed and randomly oriented, added for the reinforcement of cementitious composites.
Results of the work by References [
8,
25,
26] proved that the nanomodification of cementitious material could lead to significant improvement of its mechanical properties, compactness, and durability. Several researchers have been investigating the effects of different concentrations of CNTs on the strength of cement-based materials but no consensus has yet been reached [
26].
This research focused on the investigation of the optimal incorporation of carbon nanotubes in cementitious composites. The objective was to optimize the cementitious composite design to evaluate the behavior of cement mortar produced by the addition of multi-walled carbon nanotubes (MWCNTs) in different concentrations by comparing their physical and mechanical properties with the corresponding properties of the nanotube-free material. It is important to highlight that the majority of published studies evaluating the ideal content of MWCNTs [
20,
21,
22,
26] randomly selected the number of mixtures used in the experimental procedures. Alternatively, this work uses the design of experiments (DoE) methodology to determine the number of mixtures. The aim was to obtain more accurate results based on an experimental design including destructive and non-destructive tests and SEM observations.
2. Experimental Design
The experimental design was developed in the laboratories of the Universitat Politecnica de Catalunya (UPC), Barcelona, Spain, using destructive and non-destructive testing with mortar samples with and without carbon nanotubes.
The capacity to bear compression and tension forces were important indicators for the evaluation of the mechanical properties of the cementitious materials. Although the performance of these materials depended on several other factors, it provided a reliable indication of product quality. Low strength indicated that the mortar or concrete had problems in its structure that may have derived from the use of unsuitable material and may lead to the development of internal defects stemming from the lack of compaction or absence of proper hydration (cure procedure) [
27].
2.1. Material Characteristics
This study was primarily oriented to obtain comparative results between cement mortar mixtures with and without carbon. Therefore, the following commercial materials typical of the construction industry were used in this study:
Portland cement CEM I 52.5 R UNC-EM 197-1 manufactured by Cimento Molins Industrial S.A., Barcelona, Spain;
Sand from Arids Catalunya S.A., Barcelona, Spain;
Potable water from the Water and Sanitation Company of Barcelona, Barcelona, Spain;
Superplasticiser (SP) polycarboxylate ADVA Flow 401 produced by GRACE Construction Products according to European Standard EN 934-2.28 [
28];
Carbon nanotubes produced and supplied by the Nanomaterials Laboratory of Physics, Department of Universidade Federal de Minas Gerais (UFMG), Brazil. The nanotubes were produced by chemical deposition in vapor phase and identified by MWCNT HP2627. They have the following characteristics: type—multi-walled carbon nanotubes (MWCNTs); weight—60 g, purity >93%; other carbon structures <2%; contaminants <5% of catalyst powder type MgOCo-Fe; and external diameter dimensions, 99% of the CNTs, between 5 and 60 nm and an estimated length between 5 and 30 µm.
2.2. Cement Mortar Compositions
To analyze the benefits of CNT incorporation, cement mortars were produced with a
w/
c of 0.5 with the carbon nanotube addition of 0.00%, 0.20%, 0.40% and 0.60%, by cement weight, and a constant sand/cement ratio of 3:1, according to BS EN 196-1 [
29].
Table 1 presents the mixture compositions used to evaluate the behavior of cement mortars with different concentrations of CNTs.
2.3. Cement Mortar Production and Specimen Preparation
The cylindrical (44 mm × 80 mm) and prismatic (40 mm × 40 mm × 160 mm) specimens were casted according to European standard EN 196-1 [
29], which recommend the standard composition for mortar production to be as follows: 1:3 cement/sand ratio and 1:0.5 cement/water ratio. The amount of superplasticizer (SP) and carbon nanotubes obtained was with respect to the cement mass.
The specimens were produced in batches of six due to the small capacity of the mixer and according to the following procedure:
- (1)
Cement and sand were weighed on a precision balance (Gram brand, model ST-4000, with a maximum capacity of 4000 g and accuracy of 0.1 g);
- (2)
Sand and cement were mixed manually until acquiring a homogeneous appearance;
- (3)
Water and SP were weighed on the same balance. SP and water were then mixed manually in a plastic container for about 5 min;
- (4)
Nanotubes were weighed on the same balance. CNTs were then added to the water and SP mixture [Sikora 2018] and mixed by hand with a glass rod for 5 min;
- (5)
Dispersion of the nanotubes and homogenization of the CNTs occurred and they were submitted to a physical and chemical procedure involving sonication, to deagglomerate the CNT bundles, for 60 min (see
Figure 1). For this purpose, we used “ultrasonic P2000 clinging qteck Gmbh (Gemarny)” equipment;
- (6)
Cement and sand (previously mixed) were poured into the mixer containing the mixture of the water, SP, and CNT after 15 min of mixing.
After completely mixing all the components, the cement mortar compaction was done in two layers using a manual vibrating platform. The specimens were kept for 24 h in a chamber at a temperature of 21.4 °C and 99% relative humidity. After 24 h, the specimens were demolded and returned to the humid chamber until the test date at 3, 7 or 28 days.
2.4. Mechanical Properties
The compressive strength and splitting tensile strength were performed at 3, 7, or 28 days, according to standards of UNE-EN 196-1 [
29]. These were adopted for this test and a hydraulic-digital machine, INCOTECNIC model PA/MPC-2 (Seville, Spain), with a 20 ton capacity was used, as shown in
Figure 2. For each mixture and age, three samples (replication factor) were used, resulting in a total of 72 samples. More tests were conducted for the mixtures CN0 and CN4 to obtain the flexural strength (three-point bending test). This test was performed in prismatic specimens with the dimensions 40 mm × 40 mm × 160 mm at 28 days.
2.5. Physico-Chemical Performance and Non-Destructive Tests
To obtain the physicochemical properties, porosity tests were performed (see
Figure 3). The pore structures of cement mortars incorporating variable nanotube concentrations were analyzed by vacuum porosimetry. Additionally, microscopic observations (SEM) were made to evaluate the microstructure characteristics, the composition, crystallographic and morphology. A Hitachi model S-4100 scanning electron microscope (Tokyo, Japan) was used. Samples for this test were obtained at 28 days. Furthermore, non-destructive tests such as ultrasonic pulse velocity (55 kHz) were done according to EN 14579 [
30], to obtain the dynamic elastic modulus and the density of the cement mortar mixtures.