To produce materials with acceptable or improved properties, adequate characterization of their microstructure is critical. While the microstructure can be viewed in two dimensions at a variety of resolutions (e.g., optical microscopy, scanning electron microscopy, and transmission electron microscopy), it is usually the three-dimensional aspects of the microstructure that have the largest influence on material performance. Direct viewing of the three-dimensional microstructure is a difficult task for most materials. Confocal microscopy and atomic force microscopy can each provide three-dimensional information on surface topography, but not a complete representation of the bulk three-dimensional microstructure. For this task, the most relevant technique is that of three-dimensional (micro)tomography.
Three-dimensional microtomography has been actively applied to the characterization of materials for about fifteen years [1], with much interest by the oil industry in the characterization of porous rocks [2,3]. More recently, it has been applied to building materials with applications to cement-based mortars [4,5], building bricks [6], and concrete aggregates [7]. With microtomography, a complete three-dimensional image of the microstructure is obtained. As capabilities of the tomography systems have increased, it has now become possible to obtain such images with a resolution better than one micrometer per voxel (image element), at facilities such as the European Synchrotron Radiation Facility (ESRF) in Grenoble, France [8]. This resolution is of particular interest for cement-based materials, where the starting particles are typically micrometers and tens of micrometers in diameter and many hydration products are also formed at the scale of micrometers [9]. Imaging cement-based materials at a pixel size of 1 micrometer will allow direct comparison to current digital-image-based microstructural models which also operate at this scale [10,11]. A variety of computational tools exist to directly calculate the transport and elastic properties of a three-dimensional image-based microstructure, when the corresponding properties of the component phases are known [12].