Carbon Nanotubes and Carbon Nanofibers in Concrete—Advantages and Potential Risks

By Andrzej Cwirzen

This book focuses on the application of carbon nanotubes and carbon nanofibers in traditional concretes based on Portland cement. Fundamental information is given related to the production technologies of carbon nanotubes and carbon nanofibers, as well as concretes and methods of incorporation. It also contains a section focusing on the possible negative effects of carbon nanotubes and carbon nanofibers on animals and humans.

The book indicates benefits and possible problems related to the application of carbon nanotubes and carbon nanofibers in concrete. It is designed to be easy to access and digest for the reader, aiming to reach an audience, not only from academia, but also from the construction industry, materials producers, and contractors who might work with nanomaterials.

• Outlines the major properties and synthesis methods for carbon nanomaterials in concrete engineering;
• Explains the role of carbon nanotubes and nanofibers in creating high-performance concrete;
• Assesses the major challenges of integrating carbon nanomaterials into concrete manufacture on an industrial scale.

• Language: English
• Publisher: Elsevier Science
• Release date: Jul 10, 2021
• ISBN: 9780323855723

Andrzej Cwirzen

Prof. Andrzej Cwirzen has obtained his Master’s degree in Civil Engineering from the Silesian University of Technology and his PhD in Concrete Technology from the Helsinki University of Technology. As an associate professor, he led the Concrete Technology Research Group at Aalto University. Later, he was appointed as a full professor at the Luleå?University of Technology and chair of the Structural Engineering Research Group. Since 2019, he is a chaired professor of the Building Materials Research Group at the same university. In parallel to his academic career, Prof. Cwirzen has been working as a consultant and R&D project manager for construction companies and concrete producers. In his 20 years of work with concrete technology, he has focused on nanotechnology, nanomaterials especially including carbon nanotubes and carbon nanofibers, alternative ecological cementitious binders, high-strength concretes, ultrahigh-performance concretes, durability, and production technology of concrete structures. He is a RILEM senior member, member of several committees at the Transportation Research Board (TRB) in the United States and at the American Nano Society. Prof. Cwirzen holds two patents related to ecological cements. He is an editor and reviewer for several top international journals related to building materials. He has been a visiting professor at several universities in Europe, Canada, and Australia. https://www.researchgate.net/profile/Andrzej_Cwirzen/info

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1

Introduction to concrete and nanomaterials in concrete applications

Abstract

The majority of new structures are built at least partially of concrete, a material that was developed centuries ago but only became widely used with the invention of Portland cement in the early 1800s. Sadly, concrete is often perceived as a boring gray material that is not environmentally friendly or durable. However, intensive research and developmental work in recent decades, performed both in academia and industry, have given rise to a material that is modern, smart, and eco-friendly. A material that can compete even with steel, timber, and polymers. New cementitious binders, combined with optimized mix designs and novel casting technologies, have reduced cement’s carbon footprint, and improved its strength. Engineers are now able to design lighter and more sophisticated structures with the durability and longevity to last one hundred or more years maintenance free, even in the most harsh environmental conditions. Furthermore, the combination of nanotechnology with modern electronics and information technology systems has led to the creation of smart concrete, which has self-monitoring capabilities, and can even harvest its own energy through thermoelectric or piezoelectric effects. The following chapter will introduce the reader to concrete technology with a special emphasis on cementitious binders, durability, sustainability, and fibers. It also contains a short review of the current and future applications of nanomaterials.

Keywords

Concrete; cementitious binders; durability; fibers; nanomaterials; mix design

1.1 Introduction

Concrete is the second most used substance on Earth, following water. While it is often described as an artificial rock, concrete can be produced at normal temperatures (unlike natural rocks, which would have to be liquefied and exposed to thousands of degrees prior to being cooled and formed). Furthermore, it is made of materials widely available on our planet and can be very durable if produced properly.

The history of concrete dates all the way back to the Roman Empire, but it was the invention of Portland cement that kick started its rapid development in the 19th century. Concrete quickly grew into a high performance material that could compete even with steel, and could be used in complex engineering structures, such as bridges and skyscrapers. Sadly, the very same Portland cement, which over one hundred years ago initiated the concrete revolution and the beginning of the concrete age, is now the main reason why modern concrete is not considered a sustainable and environmentally-friendly construction material. Compared with other modern materials, concrete is seen as having a relatively short life span. It is often pictured with cracks and marked with rusty stains around steel reinforcements, indicating extensive corrosion and alerting consumers to potential danger or at least expensive repairs in the near future. Environmental concerns often put concrete into the undesirable material category, prompting consumers to explore true green alternatives. In recent times, timber has been heavily promoted and marketed as the only sustainable alternative, especially in European countries, (Watts, 2019). It is priced for its sustainability and supposedly unlimited availability, even though the most of its availability is confined to only a few regions of the world, including Russia, Canada, and Scandinavia. Carbon dioxide (CO2) emissions are a result of the processing and transportation of timber, putting into question the true sustainability of this material. It is also important to consider the costs of planting and maintaining the growth of new trees, and the lower consumption of CO2 which results in the successive extinction of the Earth’s forests and jungles. There are strong arguments in support of which material is greener on both sides, and this debate is still ongoing today. It is the opinion of the author that the solution to these complex problems lies somewhere in the middle. Each type of construction material has its own unique set of properties and availability, and should be used in combination with others to see optimum results. Some structures should be built predominantly from timber; some from concrete, while others from steel glass and polymers.

Concrete material certainly has changed and improved tremendously, especially over the last few decades. Many sustainability issues have been addressed in several ways, including the development of new cementitious binders, optimization of mix designs, and using novel casting and printing technologies (Fig. 1.1).

Figure 1.1 Massive concrete dam.

Engineers are now able to design lighter and more sophisticated structures that better meet the needs of a modern society where an increasing part of the world’s population moves into big metropolises. The durability has been improved and an increasing number of structures are designed to withstand one hundred years or longer without the need for significant maintenance, even when exposed to very harsh conditions.

Nanotechnology and nanomaterials, including carbon nanotubes (CNTs) and carbon nanofibers (CNFs), combined with modern electronics and information technology systems, enable the creation of smart concrete structures. Structures are able to monitor themselves or harvest energy through thermoelectric or piezoelectric effects (Fig. 1.2).

Figure 1.2 Possible smart functions achievable by modification of Portland cement-based binder matrix by incorporation of various types of nanomaterials.

The following chapter will introduce the reader to concrete technology with a special emphasis on cementitious binders, durability, sustainability, and fibers. It also contains a short review of the current and future applications of nanomaterials.

1.2 Concrete material and concrete technology

Concrete is one of the few construction materials that can be designed and manufactured directly on the construction site. Most other materials, including steel, glass, ceramics, polymers, and rubber are designed and produced off-site and according to certain specifications by external suppliers. The then ready elements are delivered to the building site and mounted in specified locations within a structure. On the positive side, the quality of externally produced materials is guaranteed by producers, which might limit the legal responsibility of a contractor. Unfortunately, these externally made components are often produced far away from the actual construction site, which adds to their cost and related CO2 emissions. The transportation of these materials might also be impossible or extremely difficult when the element size is considerable. On the contrary, concrete is made predominantly from locally available materials and often directly on the construction site and can be cast in large volumes. Typical examples include dams, bridges, marine structures, or foundations for skyscrapers. Here there is simply no other alternative than concrete, which has to be used even despite its very high carbon footprint and other known deficiencies.

Concrete is a composite, multiphase material. It is often compared to an artificial rock, which is formed without the application of high temperatures to liquefy the minerals. Instead, concrete consists of a binder matrix that cements together various types of inclusions. Initially it takes the form of a fluid, plastic, or moist substance depending on the mix composition. Later, after a certain period of time, when cementitious material is dissolved in the mixing water, the solidification process of the binder matrix begins. The process depends on the types of cementing materials used. When only a pure Portland cement is used, the hydration process dominates. While addition of SCMs, including for example, blast furnace slag, metakaolin, or fly ash, the solidification is controlled by a combination of hydration and alkali activation processes, (Taylor, 1997). Systems without Portland cement can be dominated only by alkali activation and geopolymerization processes. The inclusions that are cemented together by the binder matrix include a wide range of materials; the most significant being fine and coarse aggregates, mineral fillers, nanomaterials, nanofibers, microfibers, synthetic fibers, etc. Properties of concrete in its fresh and hardened states depend on the ingredients used, mix proportions, the curing procedure, and also on exposure conditions. The effects of various types of binders, water-to-cement ratio, SCMs and aggregates will be described in more detail in the following sections. A typical cross section of a normal strength concrete based on Portland cement with incorporated CNTs is shown in Fig. 1.3. The solidified binder matrix itself has the strongest effect on most concrete properties, including compressive and flexural strength, modulus of elasticity, microstructure, and chemical compositions, durability, which are crucial elements from an engineering perspective.

Figure 1.3 Scanning electron microscopy image of polished cross section of concrete based on Portland cement and incorporating carbon nanotubes.

1.3 Concrete ingredients

The following section describes the most important ingredients used to produce concrete including cementitious binders, secondary cementitious binders, aggregates, and commonly used chemical admixtures.

1.3.1 Binders and secondary cementitious binders

The Romans developed the first cementitious binders as early as the years 100 BCE to the year 400. In this process, limestone was heat-treated in a kiln, which removed carbon and some of the oxygen in the form of carbon dioxide, leaving behind a highly reactive product known as quicklime or calcium oxide. Placing the calcium oxide in water created a white colloidal paste called hydrated lime, or slaked lime, which was then stored in large clay jars known as amphorae until it was needed. The hydrated lime could then be mixed with a clean river sand to produce mortar that was used to join bricks and stones in aqueducts, bridge columns, piers, and other harbor structures. Between 118–128 jKr, the Romans developed the next generation of cementitious binder by combining fine volcanic ash with lime. The resulting material showed a greater strength and excellent durability, which enabled its survival until the present times, (Lothenbach et al., 2011).

1.3.1.1 Portland cement-based binders

Ordinary Portland cement (OPC), which is the most commonly used cementitious binder today, was developed and patented in 1824 by Joseph Aspdin. The production process included the intergrinding of limestone with clay, followed by clinkering at very high temperatures. After cooling down, the produced clinker was grinded to a very fine powder. Later, in 1844, Isaac Charles Johnson discovered the importance of intermixing chalk, clay, and water for the clinkering process. He also modified the kiln by tapering the chimney to strengthen the draught thus increasing the clinkering temperature, which reached 1400°C. The invention of Portland cement initiated a rapid development of concrete, which successfully replaced bricks, timber, stones, and even steel, as the main construction material. Contrary to the production of Roman cement, which often relied on a limited supply of volcanic ash, Portland cement could be manufactured in a number of locations around the world where limestone and only a few other minerals were available. The main elements needed to produce Portland cement belong to the eight most common elements present in the earth’s crust, (Scrivener and Nonat, 2011), including, silicon (25.8%), aluminum (7.57%), iron (4.7%), and calcium (3.39%). Some indicate that due to the large amount of cementitious binder required for the production of concrete, the availability of needed elements is a key parameter. Critics believe that most of the alternative cements developed for application in concrete are not sustainable due to a limited amount of resources. For example, alkali activated binders based on fly ash or blast furnace slag will always remain a niche product, as the amount of the produced coal ash is decreasing.

A known chemical process called hydration drives the solidification of Portland cement. The main components of that cement, including tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite, react with the mixing water. The process is exothermal with four distinctive phases: the initial reaction, period of slow reaction (or dormant period), acceleration, and deceleration, see Fig. 1.4. The type and amount of hydration products formed depends on the time after mixing, cement composition, curing temperature, moisture availability, and the presence of SCMs, along with other factors.

Figure 1.4 Hydration rate of tricalcium silicate measured by isothermal calorimetry. Source: From Bullard, J.W. et al., 2011. Mechanisms of cement hydration, Cement and Concrete Research. https://doi.org/10.1016/j.cemconres.2010.09.011.

The dominant component of the resulting hydrated binder matrix is calcium silicate hydrate (C-S-H) which ultimately plays a key role in determining the physical properties, strength, and durability of the matrix once it is solidified. The C-S-H gel will eventually occupy between 50% and 60% of the volume of the binder matrix and the calcium to silica ratio varies between 1.2 and 2.0, (Hewlett, 2003). The morphology of the matrix depends on various factors including curing temperature, curing procedure, admixtures used, the water-to-cement ratio and the time after mixing with water, (Zhang et al., 2018). At the beginning of the hydration process, C-S-H appears as a foil-like, flakey material that goes on to transform into a gelatinous amorphous layer. Other observed morphologies include small disks or spheres, and tapered, needle-like, fibers (Fig. 1.5) (Jennings et al., 1981; Ménétrier, 1979).

Figure 1.5 Example SEM images of C-S-H phase. Source: Adapted with permission from Zhang, Z., Scherer, G.W., Bauer, A., 2018. Morphology of cementitious material during early hydration, Cement and Concrete Research. https://doi.org/10.1016/j.cemconres.2018.02.004.

Additionally, the reaction of di- and tri- calcium silicates produces calcium hydroxide (Portlandite) (Fig. 1.6), which can be directly observed by the formation of hexagonal-like plates. This hydration product is characterized by low strength and high chemical reactivity. Consequently, the presence of Portlandite lowers the strength of the solidified matrix and also causes a number of durability problems, as described later. On a positive note, it provides a high pH, which is crucial for preventing corrosion of steel reinforcements, (Hewlett, 2003; Taylor, 1997; MacLaren and White, 2003). Tricalcalcium aluminate and tetra calcium aluminoferrite react rapidly causing flash setting. To combat this issue, gypsum can be added to the cement. It reacts rapidly with these phases to form Ettringite, which prevents flash set and provides a dormant period when hydration processes are relatively slow (Fig. 1.7) (Corstanje et al., 1973). The dormant period is necessary in order to be able to transport the concrete to the construction site, place it and initiate the curing process and other potential after-treatments, (Kosmatka et al., 2002). Several theories have been formulated to explain the existence of the dormant period. One of the early theories proposed the formation of a metastable barrier on cement particles (Stein and Stevels, 2007; Jennings and Pratt, 1979). According to this theory, the formed barrier is sufficiently dense to restrict the access of water to any remaining unhydrated cement particles. Acceleration of hydration at the end of the dormant period has been related to rupture of that barrier. Unfortunately, SEM or Atomic Force Microscopy (AFM) have not been able to replicate or confirm its existence. Other alternative theories assumed the formation of a superficially hydroxylated layer characterized by a slower dissociation of ions (Bullard, 2011). After the solution reached the supersaturation needed for C-S-H, its nucleation began rapidly, marking the acceleration period (Garrault et al., 2005). Yet another theory related the slow initial dissolution to a formation of local pits on the surface of cement particles. SEM studies showed formation of C-S-H only in areas containing crystallographic defects (Odler and Dörr, 1979). The nucleation of Portlandite particles was also indicated in this study as possibly controlling the C-S-H growth rate (Gartner et al., 2002). The acceleration period has ultimately been assumed to be controlled by nucleation and growth mechanisms (Thomas et al., 2009; Thomas, 2007). The agglomeration of C-S-H nanoparticles has been indicated as another alterative where C-S-H grows only on a certain particle size (Thomas et al., 2009). During the deceleration period, the hydration rate is controlled by the diffusion process, decreasing the amount of free available space, lowering the amount of water, and increasing the size of unreacted cement particles (Bishnoi and Scrivener, 2009; Constantinides and Ulm, 2007).

Figure 1.6 Formation of calcium silicate hydrate and Portlandite during hydration of di- and tri- calcium silicate.

Figure 1.7 Formation of ettringite (AFt phase) through a reaction of tricalcium aluminate with gypsum and water followed by its reaction with tricalcium aluminate and formation of calcium monosulphoaluminate (AFm phase).

Mixtures that are exposed to high temperatures during hydration may suffer several negative effects, including delayed ettringite formation or thermal microcracking of the binder matrix. Both scenarios could lead to a shorter service life of the affected structure (Taylor et al., 2001; Collepardi, 2003; Pavoine et al., 2012).

In summary, the hydration of Portland cement is not yet fully understood. Unfortunately, most studies were performed on separated calcium silicate and aluminum silicate-based phases, and the quantitative description of the basic hydration mechanism is still lacking.

Climate change and increasing global temperatures have forced most companies and states to actively seek ways to reduce CO2 emissions. Portland cement, one of the major mass produced materials, is significantly contributing to total global CO2. Additionally, the produced amounts of Portland cement are predicted to continue increasing in the coming decades. All these factors have forced the construction industry to search for feasible alternatives. The easiest and most efficient short-term solution to reduce concrete’s carbon footprint is to partially replace Portland cement with SCMs. The most common SCMs are silica fume, and natural puzzolans, including metakaolin, coal fly ash, and blast furnace slag. Each SCM is characterized by different chemical and mineralogical compositions, type and content of crystalline phases, content of amorphous phases, particle size distribution, particle shape, particle surface morphology, etc. The closest substance to Portland cement, in terms of chemical composition, is blast furnace slag (fly ash and natural puzzolans contain less CaO and Al2O3) (Fig. 1.8). The most radical SCM is silica fume, which contains only SiO2.

Figure 1.8 Ternary diagram of secondary cementitious materials in comparison with Portland cement. Source: From Scrivener, K.L., Nonat, A., 2011. Hydration of cementitious materials, present and future, Cement and Concrete Research. https://doi.org/10.1016/j.cemconres.2011.03.026.

The ultimate interaction between SCMs and Portland cement is based on the so-called filler effect and chemical reactions. The exact contribution of each interaction depends on the type of SCM added. In general, most SCMs affect the microstructure of the solidified binder matrix and thus its mechanical properties and durability. Both properties can be either enhanced or worsened (Lothenbach et al., 2011). Optimum and maximum replacement levels depend on the type of SCM used. For example, silica fume can usually replace 5%–30w% of Portland cement, while the blast furnace slag can replace up to 95% weight of cement. Natural puzzolans and limestone seldom exceed a 50% weight replacement level.

At an early age, the hydration of Portland cement is affected by the addition of SCMs, mostly through the so-called filler effect. The SCM particles do not dissolve in the pore water, due to its low pH, and only act as nonreactive fillers (Gutteridge and Dalziel, 1990). The alkalinity of the pore solutions present in Portland cement-based systems builds up slowly over the first few days, yielding a very limited initial reaction from most SCMs. As a support of this mechanism, Lothenbach et al. (2011) indicated that both quartz filler, which is an inert material, and silica fume, had very similar effects on the strength development at an early age. The Nuclear Magnetic Resonanse (NMR) study indicated no change of the ²⁹Si, thus confirming a negligible reaction of silica (Hjorth et al., 1988). The nucleation effect appeared to be stronger for alumina phases at later stages of the hydration process. The developed hydration heat was significantly higher after 10 hours when fine inner fillers were present in the mix. At that time, the transformation of the AFt/AFm phase occurred.

The chemical interaction between hydrating Portland cement and SCMs occurs later when the pH of the pore solution is sufficiently high. Two major effects can be distinguished: alterations in the C-S-H phase and consumption of Portlandite through the puzzolanic reaction (Eq. 1.1).

(1.1)

In comparing the chemical reactions of silica fume, blast furnace slag and fly ash, the intensity is certainly the highest in the case of silica fume. It was estimated that between 20%–80% of silica fume reacts after the first two days (García Calvo, 2010; Hjorth et al., 1988). The puzzolanic reactivity of silica fume consumes Portlandite and results in the formation of more C-S-H. In this process, the pH of the pore solution decreases and slows down the reaction. The type of C-S-H formed depends on the amount of available SiO2. Thermodynamic modeling, as well as experimental data, have indicated that at lower amounts, (<20% weight) a jennite-like high Ca/Si C-S-H is mostly formed. At higher amounts, a tobermorite-like low Ca/Si C-S-H begins to form. Others indicated that the low Ca/Si C-S-H has the ability to incorporate more aluminum (Pardal et al., 2009). High amounts of silica fume also elongated hydration and caused consumption of all Portlandite (Cheng-yi and Feldman, 1985; Scrivener and Nonat, 2011). The presence of fly ash, which also contains Al2O3 and CaO, contrary to silica fume, induced puzzolanic reactions, but the amount of remaining Portlandite was higher in comparison with silica fume (Lam et al., 2000). This was related to its generally lower level of reactivity and supply of additional CaO. Fly ash tended to decrease the amount of Ettringite and to increase the amount of AFm phases. The filler effect was also shown to be stronger at earlier stages in the hydration phase. The chemical reactivity of blast furnace slag is somewhere between the reactivity levels of silica fume and fly ash. Its level of reactivity was found to decrease at lower levels of pH, which has also been observed with silica fume. This leads to a slower strength development at early stages. The amount of calcium hydroxide during the first days of hydration is similar to systems containing only Portland cement, but tends to be lower at later stages indicating an occurrence of puzzolanic reaction (Codina, 2008; Escalante, 2001; Pane and Hansen, 2005). Chemically, the blast furnace slag contains higher amounts of CaO and lower amount of Al2O3, compared to fly ash. The phase composition of hydrated binders containing Portland cement and blast furnace slag is very similar to when only Portland cement is used. Experimental studies have shown a formation of C-S-H, ettringite, and AFm phases, but also hydrotalcite-like phases (Chindaprasirt et al., 2005; Pane and Hansen, 2005).

1.3.1.2 Non-Portland cement-based cementitious binders

Several non-Portland cement-based systems had already been developed between the 1950s and 1970s. The most commonly used examples included alkali-activated systems based on aluminosilicates, which are also often called geopolymers, reactive magnesia, calcium aluminate cements, and calcium sulphoaluminate cements.

Geopolymers

A more detailed description of alkali-activated concretes is given in Chapter 10, Effects of carbon nanotubes and carbon nanofibers on properties of alkali-activated concretes. The first reported research on alkali-activated binders dates all the way back to the 1940s (Purdon, 1940; Davidovits, 2002; Roy, 1999; Provis and Van Deventer, 2009). The solidification mechanism of these systems is controlled by a combination of hydration and geopolymerization processes. The ultimate contribution of each process depends on the type of alkali activator and precursors used. The geopolymerization process includes dissolution of the solid aluminosilicate precursor, polymerization of the alumina/silica-hydroxyl species (monomers and oligomers), and a polycondensation and stabilization phase during which oligomers and small gel units are transformed into extensive cross-linked networks via through-solution reorganization (Provis and van Deventer, 2007). Typical examples of materials used as precursors for geopolymers are fly ash, granulated blast furnace slag, and metakaolin (White et al., 2010a; White et al., 2010b) whose mechanical properties and durability are comparable to ordinary Portland cement (OPC). Precursors can be used in various combinations. For example, concretes containing a mix of blast furnace slag, fly ash, and 5wt.% of Portland cement and activated with sodium silicate achieved 28-day compressive strengths of over 55 MPa (Fig. 1.9). No strength regression was observed at later ages (Cwirzen et al., 2014a,b).

Figure 1.9 Compressive strength development of alkali activated mixes based on binders composed of 44wt.% of ground granulated blast furnace slag, 44wt.% of fly ash and 12wt.% of OPC. All mixes were activated with sodium silicate. Source: Data adapted from Cwirzen, A. et al., 2014a. Effects of curing: comparison of optimised alkali-activated PC-FA-BFS and PC concretes, Magazine of Concrete Research. Available from: https://doi.org/10.1680/macr.13.00231; Cwirzen, A., et al., 2014b. The effect of limestone on sodium hydroxide-activated metakaolin-based geopolymers. Construction and Building Materials. Available from: