The improvement of concrete properties through its interaction with admixtures has been the focus of attention since its emergence as a construction material. Apart from steel reinforcing bars, different embedded admixtures have been added to cement composites to primarily improve their mechanical performance [1
]. In more recent times, nanoadmixtures have also been attracting the widespread interest of researchers due to their capability not only to further improve several mechanical properties of cement-based materials, but also to provide new properties that may lead to a wide range of potential applications. These materials include concrete, mortar and cement paste, which are used in structural elements, pavements, and finishing and repairing products [2
In the last two decades, research on the study and manipulation of matter at the nanoscale has been expanding exponentially, supported by the advances achieved in visualising technologies, such as the atomic force microscope, scanning tunnelling microscope and focused ion beam lithography [4
]. This expansion is also corroborated by the acceleration in the proliferation of scientific literature published all around the world [5
], as a result of a research race between the world powers [6
]. However, investments seem to partially neglect the construction sector to date, since few nanotech applications are currently in implementation. The situation is even more critical in the fields of sustainable construction [7
] and environmental applications [9
], despite the demonstrated benefits in water treatment [10
], soil and water remediation [11
], self-cleaning concrete and glass surfaces, photovoltaic coatings [13
], or electrochromic windows—which may potentially provide heating, cooling and lighting savings [14
As far as the construction industry is concerned, it is obvious that this sector faces certain obstacles to the penetration of new materials and technologies. Within this highly fragmented industry, new knowledge is still based on empirical approaches, as construction works are long-term processes which involve high investments. In such conditions, construction companies usually avoid risks that are inherent to research and tend to be reluctant to use materials that are not specifically listed in official construction Codes [7
]. Consequently, investment efforts involving nanotechnology are mostly focused on higher profit areas, such as electronics, IT (information technology) and health [15
Despite the modest resources allocated to construction research, some recent findings regarding cement matrix reinforced with nanoinclusions point to a noticeable improvement in the mechanical performance and durability of the hardened cement-matrix composite. Additionally, it can be provided with smart features by virtue of its physical characteristics, either through its dispersion into the cement matrix or by being applied as coatings on the cement matrix surface.
This paper collects the key findings regarding all the aforementioned functionalities derived from the combination of the most studied nanomaterials with the cement matrix. Besides, it gathers the most updated information with regard to the results concerning the strengthening of the cement matrix. In order to illustrate the evolution of this field, a bibliometric study has been conducted (see Figure 1
). Although relevant studies were published at the beginning of the 21st century, Figure 1
reveals that the scientific literature has significantly increased in the last decade. Therefore, the authors have mainly focused on the findings reported in this period, paying special attention to the last two years.
At the beginning of the research expansion of nano-modified cement composites, Sobolev et al. published a pioneering two-part review [17
] on this field. In their work, they paid special attention to nanoparticles and carbon nanotubes (CNTs), as Balaguru et al. also did in their review published in the same year [19
]. At that early stage of research, other papers that investigated cement composites containing specific inclusions attracted the attention of the scientific community worldwide; for instance, those focused on nanosilica compared with micro-sized silica fume [20
], nanosilica [21
], nanosilica and nano-Fe2
], nanoalumina [24
], nanoclay [25
], and carbon nanotubes [26
More recently, a general description of the applications of nanotechnology to cement was made by Sanchez et al. [29
] in 2010. Their work covered the analysis of nanostructures present in the cement matrix, the technological advances that have allowed the characterization of nanomaterials, the effect of carbon nanotubes on the matrix, and the most studied nanoparticles, including references which date primarily from the period 2004–2009. Pacheco-Torgal et al. [30
] made a review on the application of nanotechnology to building materials—with a focus on the photocatalytic effect of nanotitania particles—based on different papers mainly published in the period 2003–2010. They also included the currently active topic of the toxicity of nanoparticles. In 2014, the review by Chuah et al. [31
], covering mainly the period 2008–2013, incorporated the topic of the fabrication of nano-modified cement composites, focusing on carbon nanotubes and graphene oxide. They analysed their effect on the properties of the fresh cement matrix, the hardened matrix, and the kinetics of hydration. As for the cementitious composites using specific types of nanoinclusions, the following reviews can be highlighted: nanoparticles [32
], carbon-based nanomaterials [34
], and CNTs [36
]. Although more extensive studies based on the collection of previous research have recently been made [38
], some relevant topics have not been covered yet, such as the use of graphene oxide as a reinforcement, the hardening of the surface through the electrochemical migration of nanoparticles, the crack filling carried out by bacterial activity, or the health risks that nanomaterials involve.
In this paper, the authors provide a general overview of the technological peculiarities of nanotechnology when applied to cement-matrix composites and, also, an outline of the state-of-the-art research on mechanical reinforcement from a quantitative approach. In addition, the most remarkable and novelty features achieved are also highlighted. These physical synergies allow cement-based products to become either highly specialised—usually in terms of mechanical performance—or multi-functional—with regard to their electrical, auto-sensing, and thermal behaviour [41
]. Moreover, nanomaterials have the potential of leading to considerable savings in terms of materials and resources at each stage in the life cycle of cement products [42
Therefore, this article is structured as follows. Section 2
introduces nanoscience in the context of cementitious products and, in particular, Section 3
focuses on the techniques required to maximise the interaction between nanomaterials and the cement matrix. The most studied materials, or those with the most promising features because of their synergies with the cement matrix, are described in Section 4
. Section 5
collects the main risks that the contact with nanomaterials poses to human health. In Section 6
the findings from the most recent studies are discussed, and, finally, the conclusions are presented in Section 7
2. Concepts of Nanoscience and Nanotechnology Applied to Cement-Based Composites
Given their small size, nanomaterials require advanced technology to be studied, produced, and applied. In order to better understand the properties of nano-modified cement-based materials at the macroscale, it is essential to know the nanostructure of the cement matrix. This knowledge helps to understand the interaction between the matrix and the nanoinclusions, as well as the properties that emerge from their synergy.
Nanoscience is a modern discipline concerned with the novel properties of materials that emerge at the nanometric scale—which is somehow equivalent to the molecular scale. Given the small size of nanoentities, their high specific surface area, the self-assembly characteristic of molecules and the quantum effect, the properties of materials at this scale can vary substantially when compared to higher scaled bulk materials. Thus, for instance, the melting temperature can decrease, and substances can become soluble, transparent, flammable, more reactive, more electrically conductive or catalytic, among other aspects [43
]. Therefore, as we move downwards from this scale, the behaviour of the matter becomes far more complex, thus involving the need of an interdisciplinary approach and the confluence of different fields for its study, such as chemistry, physics, engineering, medicine and computing [44
Nanotechnology studies the manipulation techniques of materials at the molecular level to create large structures. The aim is to exploit the novel and significantly improved properties by gaining control of the structures, while maintaining them stable, in order to integrate these nanostructures at higher scales. Therefore, the obtained composites can be multifunctional, i.e., showing two or more properties, such as greater mechanical performance, different electrical resistance, or self-sensing, self-cleaning and self-healing features [45
]. In the scientific literature consulted for this paper, the numeric value adopted for this scale is 1–100 nm, as proposed by the National Nanotechnology Initiative, a programme released by the US Government [46
In the field of cement matrix, nanoscience studies the structural variables at the micro- and nanoscales in cement-based composites and, by using characterization techniques and molecular modelling, it also analyses their influence on the properties of the composite at the macro-scale. Nanotechnology focuses on the manipulation techniques of these materials with the main goal of improving their performance in a certain way [46
]. This enhancement can be applied to three different technical aspects: the behaviour of the fresh cement matrix; the chemical, thermal and mechanical progress during the curing process; and the properties of the hardened composite [47
]. To this end, several nanoinclusions can be added to the cement. They basically consist of nano-sized particles, fibres or sheets that are either embedded into the matrix to control the behaviour of the bulk material or grafted onto cement matrix molecules, aggregates or additives in order to modify the interaction between interfaces [29
]. They can even be applied as a coating when a small quantity is required or when the desired feature consists in a superficial interaction.
Before covering the benefits provided by nanoinclusions, it must be remarked that the cement matrix is a nano-structured material itself. Since the properties of each scale derive from the structure of the next smaller scale [29
], nanoscience has been aimed at revealing the complex relationships between nanostructures and the properties of the bulk material of cement matrices. A micrograph of plain concrete is shown in Figure 2
. At the nanoscale, the cement-based composite is a complex structured material, composed of an amorphous phase, nano- to micro-sized crystals, and bound water. To this end, the attention is focused on the binding phase, named calcium–silicate–hydrate (C–S–H) gel, since it is responsible for both the intrinsic cohesion of the cement paste and the adherence of the cement paste to fine and coarse aggregates. Therefore, it also provides the mechanical strength to the composite [48
]. In fact, the C–S–H gel has been the subject of intensive research, given its complexity, the increasing range and complexity of admixtures and blending materials, and the wide range of experimental and computational tools that have been applied in recent decades to cementitious materials. Some of these tools include: X-ray diffraction (XRD) [49
]; Scanning and Transmission Electron Microscopies (SEM and TEM) [50
]; nuclear magnetic resonance and small angle neutron scattering [51
]; atomic force microscopy [45
]; and nanoindentation [53
Papatzani et al. [56
] reviewed the models proposed in the period 2000–2014 which describe the relation between the C–S–H nanostructure and the mechanical properties at the macro-scale of the hardened composite. They concluded that modern models were, essentially, an extension of the colloidal or layered models suggested in the 1960s, rather than providing a ground-breaking new approach in relation to nanotechnology and computational advances. Regardless of this aspect, these technologies have facilitated the shift from descriptive to predictive models, have contributed to save research resources, and have paved the way to the production of nano-modified cement matrices with minimum Portland cement content. As for the results of nanotechnology when applied to the study of the C–S–H gel, Raki et al. [57
] reviewed the pieces of research that focused on the relation between the C–S–H structure and the mechanical properties of the cement matrix, as well as on the techniques to modify C–S–H composites at the nanoscale.
3. Dispersion of Nanoinclusions in the Cement Matrix
When nanoelements are mixed with an aqueous compound, they tend to form agglomerates because of the attractive Van der Waals forces—especially with 1D and 2D morphologies with high aspect ratios. Thus, one of the main obstacles to prepare a cement-matrix composite lies in the difficulty to obtain a mix with a uniformly dispersed inclusion [58
]. Techniques to achieve the homogeneous dispersion of nanoadmixtures are often required, and these are classified in four main groups: chemical surface modification, physical surface modification (through surfactants or polymer wrappings), and mechanical methods of ultrasonication and stirring. Other used mechanical methods include ball milling, shear mixing, calendaring and extrusion. Ultrasonication has been commonly used to attain uniform dispersion in the cement matrix: an ultrasonic probe imparts excitation energy to break up the nanotube clusters at the expense of achieving decreased aspect ratios [31
Parveen et al. [59
] and Chuah et al. [31
] presented their respective reviews on the dispersion methods of CNTs in cement matrices, a nanomaterial with great potential but with challenging dispersion problems. They also included a review on the mechanical improvements obtained by researchers, which resulted to be highly variable. Hassan et al. [60
] analysed how the protocol followed in the preparation of CNT-concrete affects compressive strength, thus confirming such high variability. In addition, they also highlighted the need of providing the exact details involved in the dispersion procedure and of standardising the optimal methods.
In the specialised literature consulted, the concept of “functionalization” is often used as a synonym for “chemical modification”. However, in some cases, “chemical modification” refers to the introduction of weak, non-covalent interactions with relatively unreactive molecules. Although “functionalization” is also used in that context, it usually makes reference to the covalent bonding of reactive functional groups with the nanostructure of the matrix [61
]. In spite of the fact that non-covalent modification can be an excellent solution for certain applications, the focus is put on the design of new and more efficient covalent linking techniques. Therefore, functionalization is the most common approach to achieve the satisfying dispersion of CNTs; more specifically by applying acid mixtures, which oxidise CNTs and add carboxylic (–COOH) or hydroxyl groups (–OH), thus increasing the solubility of CNTs in the aqueous matrix. Through this method, CNTs contribute to the rigidisation of the hardened matrix, since the attraction created by the covalent bonds of oxide groups makes them become tightly wrapped by the C–S–H phase. Graphene oxide has properties that are substantially different than those of graphene, given the important changes that the functionalization of graphene sheets implies [59
Apart from solubility, functionalization can provide graphene and carbon nanotubes with additional properties which are suitable for those applications related to electromechanical behaviour, electrochemical sensing, catalysis, or biocompatibility. For this reason, they have become a subject of intensive research for the fabrication of novel hybrid composites [62
5. Health impact of Nanomaterials
The on-going growing industry of nanoproducts has led to an increase in the studies about the effects that nanomaterials have on the environment and human health [203
]. More specifically, there has been an increase in the number of papers that review the health and safety considerations related to the use of nanomaterials in the construction industry during their whole life cycle [205
]. The main reasons for their toxicity lie in their reduced size and high reactivity.
Many studies have proven the harmful effects of airborne particulates on the respiratory and cardiovascular systems, including a greater incidence of atherosclerosis and a higher rate of asthma [207
]. Well known examples of these particulates—which have a diameter of less than 100 nm—are usually generated by high temperature processes, such as welding and smelting, combustion, and industrial processes [208
]. Now, nanomaterials are being manufactured on a large scale, and, therefore, the risks for workers and users should be assessed.
Nanosilica particles with a 42 nm diameter were demonstrated to penetrate into the human skin [209
]. Hirai et al. [210
] reported that nanosilica with a 70 nm diameter, applied for three days on mice skin, could penetrate and be transported throughout the body via the lymphatic system [211
The second most used nano-sized metal oxide worldwide is TiO2
]. Chang et al. [213
] analysed 62 papers which focused on the study of the health consequences from the exposure to nano-TiO2
. In these pieces of research, some clues towards the hypothesis that this nanomaterial could have an impact on human health were found. Nano-TiO2
was detained in several organs and possible cell damage was reported. However, further research is still needed to demonstrate its toxicity for the human body, especially epidemiological studies, as they can show the relation between the occupational exposure and the development of health problems more directly.
Ong et al. [214
] reviewed the studies on SWCNTs, from their absorption into a body to the accumulation and induction of organ-specific toxicity. Although earlier studies had reported that the harmful effects of SWCNTs were similar to those of other conventional fibres such as asbestos, recent pieces of research have suggested that the nanometric nature of SWCNTs can have further consequences on human health. For example, Ong et al. indicated more toxic effects on numerous cell types, when compared to the same nanoparticulate mass for carbon and quartz—which are commonly adopted as yardsticks for harmful particles. Both SWCNTs and MWCNTs pose risks to the respiratory system [215
] and exhibit antibacterial properties [205
]. Singh [216
] reviewed the toxicological studies on the graphene family nanomaterials in the context of their applications.
A common conclusion of the medical studies regarding health-related issues of nanomaterials is the need of further research to confirm these risks to human health. To this end, many researchers have remarked the convenience of standardising the nanomaterial itself [214
], as well as the protocols of both the experiments and long-term studies [218
6. Discussion of Results
The studies included in this paper demonstrate that nanotechnology applied to the cement matrix is still in an intense phase of research: searching for more efficient and environmentally sustainable synthesis methods, exploring attractive applications, and designing the first prototypes of nano-technological buildings. Table 4
and Table 5
gather the findings and tested applications included in the analysed studies.
Many of these pieces of research share a common characteristic: a significant variability in the strength parameters when reinforcing cement matrices. The reasons for this variation lie in all the stages of the research process. Firstly, there is not a common and widely accepted synthesis method for nanomaterials and research groups have to use expensive devices to identify the nanostructure of the material used. Nanoparticles are in a favourable situation compared to carbon-based nanomaterials, as their manufacture is easier and has been evolving for a longer time. However, in cement products with nanoparticles, the proportions of the mixture and the components are far from being commonly adopted in the short term. Secondly, the characteristics and preparation of specimens follow different standards, as it happens to testing devices. The properties of the specimens will depend on the instruments and materials available where the experiment takes place. Thirdly, there is no coincidence in the mechanical parameters measured in each study. Compressive strength is usually provided, but flexural, toughness, and tensile tests are carried out less frequently.
Nanoparticles have been proven to provide valuable improvements in the mechanical properties of the cement matrix, as well as new advanced properties—such as strain and temperature auto-sensing and self-cleaning capability. Nevertheless, it is a fact that laboratory work at the molecular level is a costly activity and the companies from the cement and construction sectors usually work with limited budgets. Expensive microscopes are needed to examine nanostructures, high technology is required to synthetise high purity graphene and CNTs [220
], and an extra effort is also necessary to disperse these nanoinclusions uniformly within the cement matrix. Consequently, both the commercial nanoinclusions currently available and the products made of them are still limited [221
]. Mahdavinejad et al. [222
] highlighted the lack of coherence between the academic field and the industrial requirements.
Experimentation with GO- and CNT-cement is a very active line of study. The obstacles derived from their high cost and poor binding properties are expected to be gradually overcome in the future. At present, further work regarding building Codes needs to be done in order to achieve a widespread application of these nano-modified composites and, even of the well-known macroscopic fibre concretes [54
]. In cement composites, the research on CNTs prevails over the studies on CNFs. However, CNFs, due to their lower cost, are still an interesting object of study with regard to the reinforcing and electrical functions. For the same reason, CB is used as a nanomaterial in order to enhance the conductivity of the cement matrix.
As previously exposed, additional physical properties—beyond the fundamental mechanical requirements for the cement matrix—have already been successfully tested. The study of smart structures has been highlighted through the examples of the strain self-sensing and auto-healing capacities. However, there is still a lack of on site experiments and prototypes that include such features.
Medical studies on the health risks posed by nanomaterials have been warning about absorption, detaining or cell damage in animals. These findings constitute a strong indicator of the possible risks to human health. Further research is strongly recommended to confirm this hypothesis.
Given the limited international commitment towards the introduction of multi-functional concrete in architecture, influential building certificates—such as BREEAM and LEED—can play a key role to foster innovation within construction. In this regard, a remarkable example would be the last guide on the selection of materials published by The Concrete Centre and developed within the REEAM framework [223
], as it opens the door to provide new features to concrete with regard to the reduction of gas emissions, the comfort of the user, the use of recycled materials, etc.
Although most of the activity around nanoinclusions is still in a research stage, some findings have already proven that there is great room for improvement in the mechanical performance of the cement matrix, either with the addition of nanoparticles or with the use of carbon-based nanomaterials. Moreover, nanoelements can usually be combined with micro- and macro-admixtures and reinforcements in order to further enhance the strength of the hardened cement composite. Nevertheless, the variability in the strength parameters of cement-matrix composites reflects the need of standardising the activities related to nanotechnology.
As previously mentioned, it is important to channel efforts in order to provide efficient and, therefore, standardised synthesis methods for nanomaterials to improve their production on a large scale. If research groups had access to similar products, the comparison of results would be more reliable. The next step would be to make standards based on the most efficient processes for mixing the cement-based composite blend.
Standardisation is a key line of action, as high quality standards are needed to facilitate the transferability of the results from the research field to the global market. Nanoscience is a new promising field that, in order to develop further, needs to establish an internationally agreed terminology, as well as commonly adopted methods of measurement and characterisation. Construction would particularly benefit from the standardisation of technical requirements, since its activity is regulated by mandatory codes that evolve, in the long term, on the basis of solid empirical experience.
Nanotechnology also helps to decrease the environmental impact derived from the construction activity. Firstly, the strengthening provided by nanoproducts can lead to a reduction in the carbon footprint of the cement matrix by cutting down the consumption of cement. Secondly, recycled elements have been successfully tested as nanoadmixtures in the cement matrix, or, at least, the reinforcing effect of nanomaterials provides room to include recycled elements—which would otherwise imply an expensive disposal process. Influential building certificates, such as LEED and BREEAM, will probably play a key role in fostering the application of innovative technologies within the construction industry.
Several novelty properties of nanomaterials, which are primarily based on the electrical characteristics of nanoelements, have been successfully tested in lab work. A common research topic is the piezoresistive characteristic of CNTs, which could lead to the design of a load-sensing structure or a thermal-sensing coating. These advanced features can be used by entrepreneurs as a way to differentiate their products from those that have traditionally been in the market.
Research groups and private initiatives should carefully choose the line of research they are eager to follow, given the high variability of results found in this field. Regardless of this aspect, it cannot be denied that, taking into account the huge consumption of Portland cement worldwide, any interesting improvement in the cement matrix that reaches an acceptable cost-effectiveness ratio will have great economic and positive environmental impacts at an international level.