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The complexity of concrete microstructure

Concrete is a composite material whose microstructure contains random features over a wide range of length scales, from nanometers to millimeters, with each length scale presenting a new random composite with which to deal [3]. In its actual uses, at the length scale of meters, concrete (almost always reinforced by steel) is usually considered to be a uniform material, with bulk properties like compressive strength, creep, and others. This is the usual engineering length scale.

Figure 1 shows four microscopic views of concrete, displaying the length scales of the material itself.


Figure 1: Showing concrete at four different length scales: upper left is concrete, upper right is mortar, lower left is cement paste, lower right is C-S-H.

In the upper left of the figure, there is an optical micrograph of concrete. The bars at the bottom of this image are millimeter bars. At this length scale, the concrete can be considered to be a mortar-large aggregate random composite, where the large aggregates range from 3-30 millimeters . The top right image shows a magnified optical micrograph of a piece of the matrix phase from the previous image, where now the mortar is itself seen to be a cement paste-sand composite. The bars at the bottom of the image are again millimeters. The size of the aggregates in this image range from a millimeter or so down to a few tenths of a millimeter. Looking closely at the matrix of the mortar composite, dark specks can be seen, of the order of tens of micrometers. These are pieces of unhydrated cement.

Turning to the lower left image in Fig. 1, we see a backscattered electron scanning electron micrograph of the cement paste matrix. The large white clumps are the largest of the unreacted cement grains, with the largest grains in the image being about 50-80 micrometers. These were seen as small dark specks in the previous mortar image. The contrast in a backscattered electron scanning electron micrograph is provided by average atomic number, and so the unreacted cement grains are now the brightest phase. Cement paste is clearly a random composite material, made up of unreacted cement, capillary pores, and various other chemical phases that are a result of the hydration reactions between water and cement [4]. The main reaction product phase is an amorphous or at best poorly crystalline calcium silicate hydrate gel, produced via a hydration reaction and denoted C-S-H (in cement chemistry notation, C=CaO, H=H2O, S=SiO2, F=Fe2O3, A=Al2O3, S=SO3). This is really the glue that holds the cement paste together, just as cement paste is the gluethat holds mortar and concrete together. In the image, this is the gray that lines the cement grains and is clumped in between, showing where small cement grains used to be before being consumed by the hydration reaction. The randomness in the cement paste microstructure is on the order of micrometers, because the original cement grains have an average diameter of approximately 10-20 micrometers, and these set the scale for the microstructure.

Finally, the lower right image in Fig. 1 is a transmission electron micrograph of C-S-H [5], which is itself seen to be a complex material with a random porous nanostructure. The size of the image is about 300 nanometers on a side. This random nanostructure can also be detected by neutron scattering [6].

The range of the random microstructure of concrete, from nanometers (C-S-H) to micrometers (cement paste) to millimeters (mortar and concrete) to meters (final end use) covers nine orders of magnitude in size! It is then a large and difficult task to try to theoretically relate the microstructure and the properties of concrete. This has led to the use of computer simulations, coupled with modern experimental probes, which have provided some insight into the microstructure.

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