1. Introduction
Concrete is used as a structural material due to its low cost, wide availability and ideal mechanical properties. However, its most important ingredient, hydrated cement paste, is a quasi-brittle material and has disadvantages such as low tensile strength, low ductility and susceptibility to cracking due to early-age shrinkage, which would reduce the mechanical performance of concrete and the service life of concrete structures.
Adding fibers to cement-based materials to develop crack-free concretes with improved ductility is a strategy that has gained considerable attention worldwide in recent years. Among different fiber materials, carbon nanotubes (CNTs) are a promising candidate due to their excellent mechanical, thermal and electrical properties. Theoretically, for an individual CNT, the Young’s modulus and tensile strength could reach 1 TPa and 100 GPa, respectively, with an ultimate strain of up to 15% and specific surface area of up to 1000 m
2/g [
1,
2,
3]. To date, many investigations have shown the excellent mechanical and electrical properties of CNTs when they are added to cement-based composites [
4,
5,
6,
7,
8,
9,
10].
However, the tendency that CNTs have to agglomerate is still a key issue when they are dispersed in a cement matrix due to their high specific surface area and van der Waals’ force. This poor dispersion confines CNTs as reinforcements in the hydrated cement paste. Various physical and chemical techniques have been introduced to assist with the dispersion of CNTs, such as ultra-sonication, ball-milling, surface functionalization, ionic surfactant and non-ionic surfactant methods [
11,
12,
13,
14,
15,
16,
17]. The mechanisms for these dispersion methods could be explained as follows: ultra-sonication releases high levels of energy, which generate microscopic bubbles in suspension, and the bubbles separate individual CNTs from bundles [
18]; ball-milling process would charge the surface of CNTs and enable the individual CNTs insert into the composite [
19]; using functional groups introduced by surface functionalization and ionic surfactants could form micelles around the CNTs and inhibit its aggregation [
20]; the non-ionic surfactants could be absorbed into the surface of CNTs, which contributes to exfoliating individual CNTs from bundles through a steric stabilization effect [
14]. Aside from these mentioned methods, another optional method is dispersing CNTs in water with non-covalent functionalization surfactant, which has the characteristic of not changing the inherent electrical and mechanical properties of CNTs [
15].
The performance of CNTs/cement composite is not only influenced by the dispersion of CNTs in cement paste, but also related to their interaction. Computational simulation analyses demonstrated that CNTs will provide good reinforcement when the matrix is under loading, when a good connection between the CNTs and matrix is formed [
21]. Furthermore, Al–Rub et al. [
22] used the finite element simulation method to investigate the key parameters that control the pull-out behavior of a single CNTs from cement matrix and found that the interfacial adhesion governs the pull-out strength. However, due to its smooth surface at an atomic scale [
23], the CNTs shows little potential for forming a strong interface with a cement matrix. Few researchers have concentrated on the bonding strength between CNTs and the cement matrix in order to obtain ideal reinforcement of CNTs. The interfacial adhesion between CNTs and cement matrix may be improved either by functional treatment on the CNT surface or by adding additional materials in the composite. Previous efforts have been made to introduce hydroxyl, carboxyl and silica functional groups to the CNT surface for the purposes of higher interfacial strength between CNTs and cement matrix and greater mechanical enhancement of cement composites [
24,
25,
26,
27]. Also, due to small particle size and pozzolanic potential, silica fume is used to densify the interface between CNTs and cement matrix and enhance the capacity of load transfer and the overall strength of the cement composite [
28]. However, it is reported that silica functional groups actually retard the nucleation effects of CNTs during cement hydration [
27]. Although existing studies have shown the potential role of the silica-functional group in the interfacial bond between CNTs and the cement matrix, the probable role of NS in enhancement of interfacial bonds between CNTs and the cement matrix has not been widely studied in the current literature. As stated by Makar and Chan [
29], the nucleation of hydration products, calcium silicate hydrate (C–S–H), during the acceleration period by the CNT bundles is responsible for the reinforcements to the matrix. Makar et al. [
30] also prove that development of the strength of the matrix is due to the increase in the nucleation effect to the point where C–S–H is held together by the CNTs, producing a dense C–S–H structure against the surfaces of CNTs and leading to crack bridging. The influence of NS on the interaction between CNTs and the cement matrix are not well known, and efficient means for improving damping properties of CNTs/cement composite are urgently needed. Our study will close the knowledge gap concerning the improvement of interfacial interaction between CNTs and cement matrix and provide useful information to alleviate the dynamic vibration response and damage or fatigue in order to meet people’s safety and comfort needs.
In this work, the effects of NS on CNT performance, including nucleation and energy dissipation in the cement matrix, are studied systematically. Dispersion of CNTs in suspension is characterized with a UV-visible spectrophotometer. The influence of nano-silica (NS) on the microstructure and hydration promotion of CNT-reinforced cement pastes are investigated by means of scanning electron microscopy (SEM) and calorimetry tests, respectively. Dynamic mechanical analysis (DMA) is utilized to study the effects of NS on the energy dissipation behavior of CNTs in the cement matrix. The experimental program, analyses and discussions on the results are introduced in detail in this paper as well.
3. Discussions
This study investigates the effects of NS on interfacial adhesion between MWCNTs and cement matrix using various methods. Prior to the tests, UV-Visible spectroscopy showed that addition of NS does not affect the dispersion of MWCNTs in suspension. A calorimetry test indicated that NS promotes the nucleation of C–S–H around MWCNTs in cement hydration. Hence, it is proposed in this study that NS may adsorb on and activate the surface of MWCNTs, which offered more active nuclei for formation of C–S–H on MWCNTs and condensed the interface between MWCNTs and cement matrix. Further investigations are carried out to prove this hypothesis. The morphology observation also proves that NS absorbs on the surface of MWCNTs and accelerates the formation of hydration products, leading to a rough MWCNT surface and compact interface between MWCNTs and cement matrix. Based on the previous findings [
33], the damping mechanism in cement composite could be attributed to different sources of energy dissipation, including multiform interfaces of cement matrix, frictional damping due to slip-in the unbound regions and damping due to damaged cement matrix. For CNT-reinforced cement composite, addition of CNTs to the cement matrix would introduce diversified dislocation of cement matrix and additional friction damping in the interface. In this study, CNTs are dispersed in the same state and the samples are subjected to non-destructive periodic load, so both multiform interfaces and damping from damage would not contribute greatly to the difference of damping properties. During the small deformation, MWCNTs is able to form friction force with the cement matrix through marginal slippage and hence dissipate the vibration energy. Therefore, in this work, the frictional damping mechanism is highly expected to be the dominant factor affecting the damping properties of cement composite. Compared with the smooth surface of pure CNTs, the NS is able to densify the interface between CNTs and cement matrix through a pozzolanic reaction and enhance the capacity for load transfer [
28]. The higher densification between MWCNTs and cement matrix produces better reinforcement of MWCNTs through higher interface frictional strength, which is responsible for the significant energy dissipation of cement composite [
31], leading to higher loss modulus and better energy dissipation in DMA tests.
Figure 6 shows the schematic representation of the damping properties tested in DMA test. The interface between the NG hydration product and MWCNTs was physically connected, which is supposed to be a new form of friction regime introduced into the composite and should contribute to greater energy dissipation during the DMA analysis. In conclusion, addition of NS can improve the interfacial adhesion between MWCNTs and cement matrix. Compared with the negative nucleation effects of silica-functional CNTs during the cement hydration [
28], it is found that NS produces greater improvement in CNT nucleation, which may accelerate the early growth of cement composite strength. Due to the excellent damping ability, it is expected that NS-CNT-reinforced cement composite will be used for important civil infrastructure to alleviate the dynamic vibration response and damage or fatigue, in order to meet the safety and comfort needs of the people.