NEW MATERIALS FOR CONCRETE TECHNOLOGY: NANO POWDERS

Remzi Sahin*, Atatürk University, Erzurum, Turkey Meral Oltulu, Atatürk University, Erzurum, Turkey

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33rd Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 – 27 August 2008, Singapore

NEW MATERIALS FOR CONCRETE TECHNOLOGY: NANO POWDERS

Remzi Şahin*, Atatürk University, Erzurum, Turkey Meral Oltulu, Atatürk University, Erzurum, Turkey

Abstract

In recent years, the world of science has started to produce new materials and to research their properties with nanotechnology. The use of the technology, which is also called molecular production, has become widespread in all branches of science. Nano materials such as nanoparticles, carbon nanotubes and nanofibres offer a potential for developing much stronger, tougher and more durable structural materials for construction sector. Concrete, which is an indispensable building material, can be accepted as a nano technological material due to, in particular, having nano-sized binder phase. So, the studies in nano scale should be increased for concrete technology. In this study, after giving information about nanotechnology and nanotechnological products includings nanopowders, the use of nano powders in cement and concrete world have been summarized and evaluated in terms of the future of concrete.

Keywords: concrete, nanotechnology, nano powder.

1. NANOTECHNOLOGY

For the first time in science history, R. Feynmann who won Nobel Prize pointed out the

importance of the structure with small size in 1960, and N. Taniguchi used the term “nanotechnology” in his article in 1974 [1]. Nanotechnology is a manufacturing technology that allows thorough and inexpensive control of the structure of matter by working with atoms [2].

The use of this technology, which is also called “molecular production”, is getting increase in

every field of science. Nanotechnology helps us in producing materials with new properties, combining interdisciplinary sciences (physics, chemistry, bio sciences, engineering etc.) [1], bridging between live and alive world, and between quantum and conventional world. Because, to produce the new material starting from its atoms, and to control the events by using the substances in the atomic and moleculer size are possible by this technology [2,3]. Scientists have been studying on processing of each molecule and combining them by their scientific backgrounds skillfully [4], and they can improve the smart materials which develops properties such as durability, mechanical performance, thermal, electrical and magnetical conductivity/isolation, etc. [5]. Nanotechnology provides the facility better, cheaper, faster and smarter formation of the structures [1]. It can be said that the greatest power will stem from the smallest matter in the future.

There are two different approaches in nanoscale studies. The first one is to form nanoscale structure by reducing the size of the structure (top-down approach). The other one is to form the desired material by starting from atom and molecule (bottom-up approach) [1]. For the development of nano structures, it should be able ; i) to understand the interactions in nanoscale, ii) to see the structures in nanoscale, and iii) to form, process, and combine the nano structure [4].

Interdisciplinary researches and the cooperation of theoretical knowledges and experimental

works have become a necessity in the development of this technology. In recent years, it has been observed that the number of the studies related with nanotechnology, which almost started to determine the strategies of military, science and technology of the countries [6] have increased significantly. For example, in this field, the number of the articles in SCI increased to 33060 in 2004, while it was 4552 in 1991 [7].

2. APPLICATION FIELDS OF NANOTECHNOLOGY and NANOPOWDERS

Nanotechnology which determines the power of the countries in the future will have a wide range application, such as communication, defence industry, space and aircraft technology, molecular biology, genetic engineering, and material science etc. Furthermore, it is estimated that this technology will be indispensable part of human life in the future. Especially, with this technology having unbelievable development in medical field, nano surgery has been started to be used in brain and eye operations. In the coming years, it is expected that the micro robots designed with nanotechnology will be introduced into the circulatory system and will remove obstructions in the system [8].

In electronics, the products such as nano tubes, flat screen televisions, computers with advanced storage systems, electronic papers, soft and flexible screens can be manufactured. In textile industry, it has been tried to produce bullet resistant textiles from nano tubes, and textiles which can be dried faster and do not need ironing and free of dirts and bacteria.

In material science, it is possible to produce and to control the smart materials which can perform

some functions by reacting the change in the material itself and its surrounding [6]. By the nanotechnology, it is possible to develop materials such as dyes, which can clean itself, materials which are resistant to corrosion and erosion, coatings which provide thermal isolation for glasses, composites which are sensitive to small stresses, new nano engineering polymers, high strength fibers, nanoparticles which increase workability and strength [9].

Figure 1 shows an estimation of the market area influenced by nanotechnology at the present

time. On the other hand, a chart, which was prepared by Bartos [10] and covers basic existing construction materials, is given in Figure 2. In this chart, it is seen that incremental improvements achieved by the exploitation of nanotechnology will lead to big commercial and environmental/social gains because of the extremely high ‘multiplying factors’ attached to such materials.

Figure 1. Nano powder-world market divided in application fields [11]

Figure 2. Roadmap for traditional bulk construction materials [10]

By the use of nanotechnology in concrete industry it is expected to make possible the production

of materials with high durability. The concrete technologists have been carried out some researchs such as use of nano powders and nanosensors in concrete, and the production of nano composites. In addition to these studies, it has been also conducted some projects related to nano cement [12]. To emphasize the effect of nanotechnology on reinforced concrete, Ratner [13] expressed that it will be possible to produce composite structures with great fracture strength if nanotubes are used instead of reinforcement and if plastics are used instead of concrete. Moreover, with some planned projects, the studies have been carried out such as (i) the modification of rheology and improvement of the mechanical and durability characteristics of fresh and hardened concrete by using ultra fine material which prevents flocculation of granular material, (ii) the development of sensing and observing the behaviour of underground structures by special nano materials (e.g., nanotubes), and (iii) the development of new generation additives in nanoscale [14].

Nanopowders which have grain sizes in the order of 1-100 nm in at least one coordinate and

nomally in three have high surface area. As the substances with high surface areas have enhanced chemical, optical and mechanical properties, the powders contribute a variety of structural and non-structural applications [2]. For instance, new and high quality composites can be produced by the use of nano powders. It is expected that addition of nano powders into composites will increase strentgh, reduce voids, and improve self control and cleaning.

Several methods are available for the production of nanomaterials other than direct atom

manipulation [2]: plasma arcing, chemical vapour deposition, electrodeposition, sol-gel synthesis, ball milling, and use of natural nanoparticles.

3. NEEDS FOR NEW ADDITIVES FOR CONCRETE

Cement is one of the main constituents of concrete. However, cement spreads greenhouse gas

entering the atmosphere during the production. Mann [15] indicated that “global cement production is currently around 1.6bn tonnes/yr, and through the calcination of limestone to produce calcium oxide

and carbon dioxide, approximately 0.97 tonnes of CO2 is produced for each tonne of clinker produced. Around 900kg of clinker is used in each 1000kg of cement produced so the global cement industry produces around 1.4 tonnes of CO2 each year. This represents about 6% of the total worldwide man-made CO2 production”. Therefore, the usage of the cement will be questioned in concrete technology in the next future.

On the other hand, due to economical and technical reasons, some industrial waste products,

which are called mineral additives, such as fly ash, silica fume, blast furnace slag etc. have been used in concrete industry. It can be said that the most used ones among the additives are silica fume and fly ash. But, the amount, and thus their sufficiency of the additives must be investigated.

Reserve of fly ash has a good position according to the other additives. Malhotra [16] argued that

fly ash will be available in huge quantities until by 2050. Besides, it has to be taken into account that fly ash obtained from thermal power plants which use coal as a fuel can not response the need when the type of fuel is changed due to environmental causes. Silica fume, in contrast to flay ash, will not be able to fulfill this need when considering the increasing need of the high strength (or high performance) concrete due to limited production in the future. So, new additives such as nanopowders should be investigated in addition or alternative to conventional mineral additives. The new additives can also have extra properties. For instance, the puzzolanic activity of nano-silica is higher than silica fume [17].

4. CONCRETE and NANOTECHNOLOGY

Since all materials are composed of grains, which in turn comprise many atoms [2], it can be said

that concrete which is an indispensable building material is also composed of structures in nano scale. The scaling from moleculer size to experimental sample size is shown in Fig.3.

Figure 3. Scales of various constituents of concrete [12,18] As the properties of the concrete which is a multiscale and interdisciplinary technological product [19] are affected by its nano and micro structure, it is important to make the works concentrate on these scales together with nanotechnology. Hydration products of cement having a complex and nanosized structure (see Fig.4), which has not yet understood are also controlled and regulated with nanotechnology [20,21]. For example, C-S-H gels which are nano sized [1, 22, 23] and responsible for the mechanical and physical properties of cement pastes including shrinkage, creep, porosity, permeability and elasticity [15] make difficult to understand the relation of structure-properties of concrete [24], since its quantitative analysis is very difficult [22]. Therefore, many researchers [e.g., 25, 26] have started to investigate the structure and composition of C-S-H gels in nano scale.

Figure 4. Reaction products of cement at nano level [20] The discovery of nano-sized hydration products became possible by the development of microscopes which are called nanoscopes. And with these microscopes invented in 1980, the specifications of a material can be investigated starting from the smallest size [27], and the new concretes which can be called as “nano-engineered concrete” have been developed (Fig. 5).

Figure 5. The improvement of concrete subject to particle size and surface area of constituents [28]

In this kind of concretes it has been attempted to improve the properties of concrete by using nano-sized particles. When looking at the cross section in Fig. 6, it can be said that since nano powders have smaller size, thus larger surface area, than the mineral additives they fill the voids out of cement constituents and, therefore, the concretes with more compact, less penetrable, higher strength, and more durable can be produced [17, 28-30].

Unhydrated

cement

Mineral additive Nano-Silica

Portland cement components

Figure 6. The image showing mineral additive, cement constituent and nano-SiO2 [28]

5. THE USE OF NANOPOWDERS IN CONCRETE TECHNOLOGY The use of the powders in concrete technology affects the cement kinetics and accelerates

hydration significantly. It was expressed that larger surface area and stronger electrostatic forces of the powders lead to these changes [26, 28-31]. The nanopowders, additionally, are not only as filler, but also as an activator to promote hydration and to improve the microstructure of the concrete when they are uniformly dispersed [29, 30, 33, 34]. Furthermore, it was seen that the mortars produced with the powders have very high compacity, and compressive and bending strength, are more uniform, and more durable for water penetration [28, 32].

It was observed, by Scanning Electron Microscopy (SEM) and Enviromental Scanning Electron

Microscopy (ESEM) which give an insight into the morphologic variety of the microstructure on nm and µm level, that nano-SiO2 participates in the pozzolanic reactions resulting in the consumption of Ca(OH)2 and formation of extra C-S-H. Nanopowders improve the structure of the interfacial transation zone (ITZ), resulting in a better bond between aggregates and cement paste [17, 28, 35].

It was also found that the mortars with nano-Al2O3 additives increase elasticity modulus [35].

Besides, it was expressed that mortars including nano-Fe2O3 increase the determining effect of the stresses inside themselves with increasing nano-Fe2O3 content, and so, it was said that the materials containing nano-Fe2O3 can be smart materials which are sensitive to stresses [32]. .

6. CONCLUSIONS

Under the light of above mentioned information, it can be said that;

1- It is necessary to reduce the size of concrete researches which have been concentrated on meso- and macro-structure at the present time, and to control the hydration reactions by nanotechnology. 2- The properties of concrete are improved by the use of nanopowders, and the behaviour of concrete is determined by the observation of microstructure with the help of some nanomaterials which can be defined as nanosensors.

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