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4.2.2 Short description

With a production volume of more than 14 billion tons per year concrete is the most widely used material on earth [^I]. Concrete offers a fairly large compressive strength of up to 50 MPa. However, due to its poor tensile strength a reinforcement is needed in most applications. The traditional approach is based on steel reinforcements, the so called rebars, but other materials such as textiles and other fibres have also been investigated intensively. The comparatively high strength of modern concrete is based on crystal structures in the nanometer range. The most important physical properties of concrete and other cement based materials are strongly dependant on the complex chemical reactions during the setting and hardening process. Therefore it is not astounding that nanotechnological approaches are mainly based on these reactions, either by changing the chemical composition or by manipulating the hydration process of the clinker. During the hydration process calcium silicate hydrate (CSH) is formed. The nanocrystalline structures of these so called CSH-phases are mainly affecting the resulting strength of the material. For this reason the formation of CSH-phases is a major focus of research. But in addition to the compressive and tensile strength, also the elastic modulus, the drying shrinkage, the resistance to carbonation and freezethaw processes are of great importance.

The innovation strategy for modern cement based materials is mainly based on admixtures or chemical additives. Several admixtures have been investigated to improve the physical properties of concrete. Silica fume, fly ash and blast furnace slag are residues or byproducts from industry which offer a "cementitious reaction" and can be used as admixtures to increase the density and strength or to replace a part of the Portland cement clinker. Such materials are known since ancient times as Pozzolans and the chemical reaction is referred to as pozzolanic reaction. The pozzolanic reaction is also producing CSH-crystals but as a secondary reaction which is much slower than the initial hydration process as described above. Besides CSH-phases also Ettringite and Portlandite crystals are formed during the complex hydration process of cement and water. Ettringite crystals strongly influence the setting of the concrete mixture and are responsible for the hardening time. The unwanted formation of Portlandite crystals (Ca(OH)2) leads to a deterioration of strength. This limitation can be addressed by suitable "pozzolanic" admixtures inducing a secondary cementitious reaction.

Primary reaction: Cement + Water ---> CSH + Ca(OH)₂

Secondary reaction: Ca(OH)₂ + SiO₂ + Water ---> CSH ("Pozzolanic Reaction")

Today fly ash or silica fume are often used as admixture reducing the number of Portlandite crystals and increasing the strength of the concrete. Natural pozzolanes have already been used by the Romans. They used volcanic ashes from a site near the village Pozzuoli as hydraulic binder in ancient concrete.

However, the grain size and the size distribution of such admixtures are a critical issue which affect the reaction kinetics and the density of the resulting concrete. A smaller grain size is advantageous for concretes with superior strength. This is where nanotechnological innovations come into play. The specific surface area of such admixtures can be increased by grinding techniques such as ball milling or high energy ball milling whereby the latter technique is able to reduce the grain size down to the nanometer regime [^II]. The result is a high performance concrete (HPC) with increased strength (Fig.1). A nanotechnological cement additive named Gaia nanosilica has been reported [^III]. The use of Gaia nanosilica has led to a compressive strength of around 92 Mpa. However, due to the comparatively slow speed of the secondary "cementitious reaction" the maximum compression strength occurred after 28 days [^IV].

Fig. 1: Traditional vs. high performance concrete. Adapted from RWTH Aachen. http://www.imb.rwth-aachen.de/publikationen/umdrucke/mb5.html

Another important issue for the strength and durability of concrete is the water/cement ratio (w/c). Usually the water content is higher as needed for the chemical reaction from pure stoichiometric considerations. The reason is the workability of the cement mixture. Typical w/c ratios are 0.35-0.5. A lower w/c ratio will positively affect strength and durability but also leads to a higher viscosity. This again results in a poorer workability or placement ability of the concrete mixture. For this reason so called plasticizers and superplasticizers are in use as additives which are able to influence the rheological properties of the concrete mixture. Today, highly effective superplasticizers are in use especially for the so called self compacting concrete (SCC) [^V], the high performance concrete (HPC) and for ultra-high performance concrete (UHPC). Self compacting concrete offers a superior filling ability which does not require vibration techniques for placing and compaction [^VI]. A nanotechnological superplasticizers based on polycarboxylate ether has been introduced for readymix purposes in 2003 [^VII].

A superior ultra-high performance concrete (UHPC) has been introduced in recent years. Offering a w/cratio as low as 0.2 and compressive strength of more than 200 MPa [^VIII] UHPC could be the cement based material of the future. UHPC offers also a large flexural strength of up to 50 MPa. Therefore UHPC is also a ductile material offering a fairly large flexibility depending on the reinforcement [^IX]. Due to the extremely low water content UHPC has a lower porosity and higher density compared to traditional concrete and high performance concrete. The low porosity makes UHPC more resistant against freezethaw cycles and also against aggressive chemicals. However, a major disadvantage of UHPC is its relatively high cement content which has not been fully hydrated and does not account for the stability of the resulting concrete. This limitation may be addressed by using a nanotechnological improved admixture instead of traditional silica fume [^X]. UHPC has already been implemented in several building parts or passenger bridges in many different countries but only for testing purposes. UHPC is sometimes also referred to as ultra-high strength concrete (UHSC), which has been defined through compressive strengths above 150MPa. High performance and ultra-high performance concrete is sometimes also referred to as "reactive powder concrete" (RPC).

Traditional cement based materials have also several limitations. Especially their low resistance to freezethaw cycles and aggressive chemicals are of major concern. Polymer modified mortars and concrete (Polymer Cement Concrete, PCC) have the potential to overcome these drawbacks. However, their comparatively high costs have prevented an extensive use in today's construction industry so far.

In order to overcome the limitation in tensile strength different reinforcing materials have been introduced. Besides the traditional steel rebars also steel fibres and textile fibres are in use. Ultra-high performance fibre reinforced concrete is sometimes referred to as UHPFRC.

The use of carbon nanotubes (CNT) as reinforcing material has also been reported [^XI]. A major advantage of CNT reinforced concrete will be the reduction in concrete volume needed to build a certain structure. New structural designs and concepts are also predicted [^XII].