CEE April-June 2012

~ore advanced outer part can be made. To 1mp_rove concrete properties nanoclay can be used. Vane~ of methods can be developed to monitor hydration of cement made materials. Energy needs ~n be reduced. Developing energy-efficient and msulated structures, thus reducing the amount of energy needed to warm up especially in buildings can be an economical solution. Methods that may decrease the water requirement of concrete can be found. _ Improving hydrated cement paste, contraction and shrinkage in concrete can be controlled. To monitor water equilibrium in hydration colour sensitive nano pigments materials can be used. Recycling concrete can be produced . To monitor ongoing hydration inside concrete embedded nanosensors can be used. Self-curing concrete can be produced. Viscosity of fresh concrete and moisture movements in hardened concrete can be monitored by nanomonitors. Water movements in concrete can be monitored by nanomonitors. To understand the freeze-thaw effect, sulphate effect and alkali-silica reactions new nanotechniques can be developed. To reduce the permeability of concrete surface and thus to increase the durability of concrete, nano additives can be used. Developing photocatalytic surface pavements, with the help of infrared rays, self– cleaning and self-polishing concrete surfaces can be made. Besides these, dirt and dust preventing thin nanofilm surfaces on concrete can be developed. Among the items listed above, transparent concrete, self-cleaning concrete, unstainable concrete, etc. are already being used. However the rest are either in theory or in very preliminary stages, currently. Therefore, it can be stated that nano technology is not fully implemented in the cement and concrete production [7,9, 26, 27, 28, 29, 30]. Enhancement of the C-S-H Nano molecular structure by incorporating Nano components Improvement of the nano structure of cement which constitutes about 15 per cent of concrete which is the most commonly used construction material throughout the world would lead to important macroscale structural improvements. The compressive strength of concrete can be as high as 200 Mpa. However, its tensi le and flexural strengths are low and it is a brittle material. It requires extreme care especially at early ages. The difference between the compressive and flexural and tensile strengths is due to the internal pore structure of concrete. The density is lower at the aggregate-matrix interface and the bind between the aggregate and matrix is weak. These lead to microcracking and propagation of these cracks. Conventionally, fibers or reinforcements are used to reduce these undesirable effects [5]. Since the contribution of other components of concrete (aggregates and water) will remain almost the same, if the structure of the C-S-H gel which is the major hydration product of cement can be improved at nano scale, it may be possible to improve the macro properties of concrete, also [5] . _ Cement production can be considered basically m two ways, from engineering point of view: The first is the fineness and the second is the composition. The conventional cements have particle sizes ranging between 0.5 to 100 1-1m and with a specific surface area of 3000 to 5000 cm 2 /g. Its mechanical and chemical properties are set by the standards. Nanotechnolgy can be applied to the cement fineness. In other words, when Si0 2 , Al 20 3 , Fe 2 0 3 , which are three of the basic oxides of cement, are used at nano sizes and hydrate, significant changes in the hydration products may result Besides that, without changing the fineness of the cement nano admixtures may be incorporated to result in important changes in the macro properties. In an experimental study [31] 2.5 per cent (by weight of cement) nano silica with an average particle size of 9 nm and specific surface area of 300 m 2 /g was used and rheological properties of the cement paste and mortar were investigated . The presence of nano Si0 2 decreases the amount of lubricating water available in the mortar mixture. The yield stress increases considerably when nano Si0 2 is incorporated in the paste, being the most affected rheological parameter. On the other hand, changes on plastic viscosity are less intense. The addition of nano Si0 2 also reduces the spread diameter due to the gain in cohesiveness of the paste. The relationship between spread and yield stress values better describes the effects of nano Si02 addition . For example, 2.5 wt per cent of nano Si0 2 causes a spread diameter reduction of 19.6 per cent, while the yield stress progresses 157 per cent. At the same time, the plastic viscosity increases only 3.6 per cent [31]. By adding nano Si02, the beginning of setting is anticipated and the dormant period is reduced. When 2.5 wt per cent of nano Si0 2 was added into the mixture, the setting time and the moment to reach the maximum hydration temperature decreased 60 per cent and 51.3 per cent, respectively. The XRD showed the presence of CH, already after 9 hours, in samples with nano Si~ addition. Therefore, the nano Si~ addition contributes to an increased production of CH at early age compared with sample without nano Si~. The nano Si0 2 addition decreases the apparent density and increases the air content in these mortars [31]. 14