CEE April-June 2012

Nanotechnology in cement and concrete research and development Portland Cement produced by conventional methods have particle sizes in 0.5- 80 micrometer range. The chemical composition of the raw feed and pyroprocessing reactions in rotary kiln are almost fully understood. Although the molecular structures of the raw materials (limestone, clay), gypsum and the mineral admixtures used are known, the hydration reactions of the cements are not yet completely understood. Hydration reactions and different hydration products obtained can be investigated in a more detailed manner by making use of nanotechnological methods. Thus, stronger, more durable, sustainable and more effective concrete structures, which are the ultimate goals of the cement and concrete industries, can be attained. Detailed investigation of cement and concrete structures by using Nano technological methods Prior to the investigations at nano scale the C-S– H (Calcium Silicate Hydrates) molecular matrix which forms upon hydration was tried to be understood in several different ways. One of the first model s is Powers and Brownyard Colloid Model. According to this model, gel like particles are held together by van der Waals forces and the space between them is called as gel pores. Only water molecules can penetrate into this space. By a more advanced model developed by Feldman and Sereda, the function of the water was described in more detailed manner by using more advanced experimental methods. Besides describing the layered C-S-H structure, mechanical properties which are related with the water content were also made more understandable. Advances in experimental methods led to the development of new models such jenning's Colloid Model dealing with 5 nm sized colloid particles [5]. Advances in the instruments and the processes used to determine properties of materials have provided establishing a bridge from nanoscopic to macroscopic scale. AFM-Atomic Force Microscopy (a special type of it is LFM-Lateral Force Microscopy) [6] and Nanoindentation Techniques [3, 5, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18] have created a revolution in the research of material properties at nano dimensions. The other modern methods are: small– angle neutron scattering (SANS), ultrasmall-angle X– ray scattering (SAXS), quasi-elastic neutron scattering (QENS), nudear magnetic resonance (NMR), spectroscopy and nuclear resonance reaction analysis (NRRA) [3]. AFM (Atomic Force Microscopy) & LFM (Lateral Force Microscopy) When compared with SEM (scanning electron microscopy), AFM gives three dimensional, high resolution, digital and morphological information about the specimen. It operates under ambient conditions and does not have the possible disadvantages of SEM which requires vacuum that may result in surfaces damages [6, 10, 17, 19]. However, the sample examined should be polished in a smooth way and must be free of moisture. In AFM images, the structure of Ca (OH)2 molecule is flat, smooth and without any grains whereas C-S-H molecules have rather granular structure. Moreover, upon scanning with LFM (a special type of AFM) it was observed that C-S-H molecules have a thin coating or some protruding shapes on it also [6]. Besides identifying the surface characteristics, the observations made on hydrated specimens showed that particles of similar properties aggregate to form bundles of about 60x30x5 nm dimensions [28]. In addition to this in the measurement of active forces on the surfaces or between the particles of CSH, electrostatic forces have been effective and it can not be explained by classical "Derjaguin– Landau-Verwey-Overbeek theory" (DLVO) [28] as mentioned earlier, AFM studies of the surface of cement paste in a moist atmosphere, proved a change in the surface structure of the paste from coarse-grained part of the cake towards the more fine between 3 per cent and 30 in terms of relative humidity [3]. Thus, the damage that would occur due to condensation problems may be better understood and another step was taken towards developing more durable structures. Figure Ia. AFM image of an indentation formed by a nano indenter [8] II