Question

What are the problems that occur with concrete that can be solved by nanotechnology?

Answer 1

Concrete is the single most commonly used construction material in the world. More than 2.6 billion tons of OPC was produced worldwide in 2007 , corresponding to more than 17 billion tons of concrete. This production of OPC was used in a wide range of products, ranging from basement foundations and house walls through road pavement and pre-cast lamp poles to bridges, dams and high rise towers. Concrete products are generally intended to have long life-spans and be resistant to local environmental conditions. However, in most cases they will eventually be demolished and possibly recycled when they reach the end of their service lives.

In addition, the nature of the construction industry is such that it is easier to implement process innovations, rather than disruptive product innovations. Construction integrates products from a wide range of suppliers and skills from a wide range of contractors, subcontractors and trades into a single finished structure. A change in the way a structure is built can be determined by the construction company itself, but a significantly new product needs to be created by a supplier, understood and approved by architects, engineers and the client and implemented by on-site workers who may need to be specifically trained in its use.

All of these factors must be taken into consideration in developing nanotechnology for use in concrete. First, concrete and related products are bulk commodities. Even high value concrete structures require low materials costs and the ability to handle large quantities of material in a safe and environmentally acceptable manner. Second, innovations need to be thoroughly developed and field tested in order to build the knowledge and confidence in the construction community. Finally, concrete structures can be difficult to demolish, often requiring explosive or other high-energy approaches as an initial step to break up the major components of the structure. Nanotechnology used in concrete must therefore be compatible to these traditional practices.

Given these constraints, the initial nanotechnology applications in construction are those that provide a clear benefit in terms of added functionality with relatively small amounts of nanomaterials that can be delivered in using standard construction practices and will not affect other aspects of the performance of the material. Novel products that improve the delivery of existing materials, such as the control released admixture work described here, are likely to be next to market. Other innovations, such as nanocalcium carbonate accelerators, will become more common as the price of the nanomaterial falls to the point where it can be used in bulk. Carbon nanotube/cement composites, in contrast, will likely take the longest time to implement as they will require further fundamental research, reductions in CNT prices, the development of specialized delivery techniques and equipment, greater understanding of the environmental impact of CNTs and specialized demolition methods before wide spread adoption can occur.

The emergence of nanotechnology applications in the area of concrete construction is relatively recent and many developments are still in the commercialization process. However, the research described here has economic implications ranging from the indirect impact of improved understanding of the performance of cement and concrete to new products that are in the process of becoming market ready. In terms of basic science, improvements in the understanding of the characteristics of the hydration products of Portland cement (particularly C-S-H) at the nanoscale should facilitate the more efficient manipulation of the nature of cement-based materials. For example, current research has demonstrated that the incorporation of nano-sized C-S-H particles as seeds into cement systems can be used to tailor the overall composition of the C-S-H and monitored using newly developed quantitative 29Si MAS NMR techniques [85]. This makes possible the development of a new generation of concretes that can address specific durability and environmental concerns. Commercial products comprised of C-S-H seeding material are currently available. The economic implications are significant in terms of improvements in life-cycle performance and maintenance costs.

Calcium Silicate Hydrate (C-S-H)

*Formation and properties

The main product of the hydration of Portland cement is a nearly amorphous material − Calcium Silicate Hydrate (C-S-H) − that forms up to about 60% by volume of the paste. In cement chemistry, CaO, SiO2, and H2O are represented by C, S, and H respectively. The hyphens in C-S-H indicate indefinite stoichiometry and the hydrate is sometimes referred to as “C-S-H gel”. C-S-H is produced along with calcium hydroxide in the chemical reaction of the silicate phases (i.e., β-C2S and C3S) with water. C-S-H is the principal binding agent in the cement paste and is responsible for its important properties such as strength and shrinkage. Resolving the structure of this material at the nano scale is an essential part of understanding and predicting its behavior. It is also important in the context of modification and development of novel C-S-H systems discussed in the next section.

The molar ratio of CaO to SiO2 (C/S ratio) in C-S-H is one of the main parameters in defining and controlling the properties of a calcium silicate hydrate system. This value varies from 1.2 to 2.1 in hydrated silicate phases and has an average of about 1.75 .The C-S-H systems may be divided into low and high lime content categories partitioned by the C/S ratio of about 1.1 where chemical and physical properties change noticeably . The state of water in a C-S-H system is also vaguely defined. Water can be present within the interlayer structure of C-S-H (either in the form of H2O or OH-). Water molecules can also be physically adsorbed on the surface of solid phases. Finally, the capillary pores (10–50 nm in diameter in well hydrated pastes and as large as 3–5 micrometeres at early ages) between C-S-H clusters can contain free water. Distinction of water states is not simple as the energy by which the water molecules are held in C-S-H varies over a wide range and may overlap for different locations .

There are several more ordered calcium silicate hydrates that are structurally related to the C-S-H. Tobermorite and jennite (with approximate stoichiometry of C5S6H5 and C9S6H11 respectively), for example, have a defined crystal structure and have been studied for many years as possible analogues to C-S-H. The reaction between lime and silica in excess water results in the formation of tobermorite-like and jennite-like systems most commonly known as C-S-H(I) and C-S-H(II). These hydrates can also be prepared through mixing sodium silicate and calcium salt in aqueous solution, although they are less crystalline. These phase pure materials are relatively easy to produce and are convenient for systematic research work on C-S-H.

* Nanostructural models of C-S-H

Study of the structure of C-S-H in Portland cement systems using X-ray diffraction is limited due to its poorly crystalline nature. Early research investigations were conducted using mainly surface area and density measurements, and, weight and length change isotherms in order to characterize this material . In the last few decades, many new aspects of the C-S-H have been revealed with the advancements in the analytical techniques and application of new methods such as nuclear magnetic resonance (NMR) spectroscopy.

The nanostructure of C-S-H has been the subject of much research, yet is still not clearly understood with suggested models ranging from colloidal to “layer-like”. One of the first physical models was proposed by Powers and Brownyard.It describes C-S-H as a colloidal material. In this model the gel particles are held together mainly by van der Waals’ forces and the space between them is called “gel porosity” which is accessible only by water molecules. A more comprehensive model was developed later by Feldman and Sereda based on extensive experimental studies of hydrated cement systems .The role of water in this model is explained in more detail and the changes in the mechanical properties of C-S-H related to water content can be easily described. The main feature of their model is concerned with the layered nature of the C-S-H. Structural roles that are assigned to the interlayer water of the C-S-H, exhibit irreversible behavior during the adsorption and desorption processes.

Simplified physical model for hydrated Portland cement.

Advancement in experimental techniques, has led to the development of new models. Jennings’ colloid model features globules of about 5 nm in diameter for C-S-H and proposes the existence of intraglobular pores (IGP) and small gel pores .The viability of using a layered model for the C-S-H in cement paste seems, however, also plausible according to recent work , which used helium inflow technique as a nanostructural probe along with XRD to follow the changes at the nano-level in the properties of C-S-H(I), a layered semi-crystalline material. The helium inflow results are analogous to those for C-S-H in hydrated Portland cement. They can be best explained by a layered model for C-S-H in cement paste. The Jennings colloidal model is essentially a hybrid where the ‘globules’are comprised of C-S-H layers. The layered model is incompatible with the colloid model in its explanation of physico-chemical and engineering behavior. The colloid model neglects the structural role of interlayer water in cement paste as evidenced by the corresponding behavior of synthetic C-S-H (I) and the more amorphous C-S-H present in the paste. A primary difference is rooted in the inability of the colloid model to separate the ‘reversible’ and ‘irreversible’ thermodynamic aspects of sorption phenomena. It is unable, for example, to rigorously explain elastic and viscoelastic behavior and their dependence on relative humidity. A schematic of the calcium silicate structure of a tobermorite layer is shown in Figure 2. It is suggested that the omission of bridging tetrahedra and further defects in the silicate chain in addition to the presence of calcium ions in the interlayer region can accommodate a variety of compositional changes for C-S-H systems .

The schematic molecular structure of a single sheet of tobermorite. Circles: calcium atoms located at the center of Ca-O octahedra; Triangles show silicate tetrahedra ; OH- attachments are not shown. Various tobermorite systems exist that vary in the ...

*. Mechanical properties

The mechanical properties of phase pure C-S-H systems have rarely been studied. The intrinsic modulus of elasticity of C-S-H appears to be independent of its C/S ratio and degree of polymerization . Nanoindentation methods have been employed in order to study the elastic character of C-S-H nanoparticles . Two types of C-S-H are suggested − low and high density. The low and high density C-S-H phases, with a volume fraction of 30 and 70% in hydrated OPC, have a mean stiffness of about 21.7 and 29.4 GPa, respectively. Work by the authors using dynamic mechanical analysis methods has shown that the complex stages of changes in the storage modulus (E’) and internal friction (tan δ) of phase pure C-S-H(I) on the removal of adsorbed and interlayer water is consistent with that for the C-S-H in OPC . It has also been shown that the decrease in C/S ratio of C-S-H increases its stiffness.

Dynamic molecular modeling and free energy minimization techniques have also been used to estimate the elastic properties of C-S-H. It has been reported that the average Young’s modulus (E) increases with the increase in the C/S ratio of the C-S-H . Another study suggests that the C/S ratio is not the only governing parameter in determining E although it exhibits a slight overall decrease as C/S ratio increases . A recent study, however, demonstrated that the modulus value of the C-S-H increases with the increase in the mean silicate chain length . The bulk modulus of tobermorite was computed in two separate studies to be about 70 GPa .

* Engineering C-S-H-based Nanocomposites

*. Background

Environmental, socio-economic and modern engineering advances are contributing factors for sustainable development in the construction industry. Innovation has included global efforts to enhance the durability and performance of concrete structures. Strategies for achieving this goal include the fabrication of organic/inorganic nanocomposites where the continuous inorganic phase is calcium silicate hydrate-the principal binding component of cement-based products. Key goals include obtaining enhanced engineering properties (e.g., modulus of elasticity and strength) and improvement of durability. Organic moieties have been shown to be useful instruments for the nanostructural modification of C-S-H. There appears to be a number of different interaction mechanisms, which are summarized below.

*. Surface adsorption and grafting of polymers at defect sites

One possible mechanism is grafting at sites of missing silica tetrahedra on the silicate chain comprising C-S-H .Analysis of the 29Si MAS NMR spectra indicate an increase in the Q2/Q1 ratio following the interaction of several organic molecules (e.g., hexadecylmethylammonium (HDTMA); poly(ethylene glycol) (PEG); poly(vinyl alcohol) (PVA); poly(acrylic acid) (PAA) and methylene blue (MB) . This increase suggests that the chemical shift of silicon in the vicinity of the polymer can mimic that obtained with a Si-O-Si bond resulting in an apparent increase in the number of Q2 sites. This is illustrated by the schematic (Figure 3), which depicts the polymer modified C-S-H nanostructure.

Schematic of polymer-modified C-S-H nanostructure. a: the nanostructure of pristine C-S-H. b: The nanostructure of modified C-S-H after the interaction with polymer molecules.

The effectiveness of the grafting process (involving ionic or van der Waals forces) is dependent on the C/S ratio of the C-S-H as the number of defect sites generally increases with C/S ratio >1.0. It is possible that differences in reported results may occur due to differences in the preparation procedures of the C-S-H nanocomposites. The NMR evidence for a C-S-H-polymer interaction was supported by results from other analytical techniques.