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We now specifically consider the microstructure-transport property relationships in cement- based materials. We begin with cement paste, as this is the matrix material for the concrete composite, and it is difficult to understand the behavior of a composite without first understanding the matrix phase. Later we will consider mortar and concrete, composites made from cement paste at higher length scales, and individual phases of cement paste like C-S-H, handled at lower length scales.
The starting point for cement paste =cement + water, cement powder, is obtained by grinding cement clinker. The cement clinker is manufactured by firing mixtures of limestone and clay, which contain aluminate and ferrite impurities. After extraction from the kiln, gypsum (calcium sulphate dihydrate) is added to moderate the hydration process. After grinding together the clinker and gypsum, the cement powder consists of multi-size, multi-phase, irregularly shaped particles generally ranging in size from less than 1 µm to about 100 µm, with an average diameter of about 15-20 µm .
Figure 2 shows cross-sectional images of four different portland cements. These images were obtained by first taking a backscattered electron scanning electron micrograph, which determined the ranking of the average atomic number of each phase via the gray scale
contrast. X-ray bitmapping was also carried out using the SEM in order to determine relative atomic abundance of important elements like calcium, silicon, aluminum, sulfur, and iron. By combining the x-ray and backscattered electron images, the principal chemical phases of the cement particles could then determined [10]. Figure 2 clearly shows that even before hydration takes place, the cement powder is itself a random composite, with even most particles themselves being multi-phase composites.
When the cement is mixed with water, hydration reactions occur which ultimately convert the
water-cement suspension into a rigid porous material, which serves as the matrix phase for mortar
and concrete. The nominal point of hydration at which this conversion to a solid framework
occurs is called the set point. The degree of hydration at any time is the volume fraction of the
cement that has reacted with water, and is often denoted by the
symbol α. The ratio of
water to cement in a given mixture is specified by the water to cement ratio (w/c), which is the
ratio of the mass of water used to the mass of cement used. An ordinary concrete used in
buildings uses cement paste with w/c 
0.5. Newer high performance concretes often
have w/c ratios of 0.3 or lower [11].
The various chemical and mineral phases within the cement powder hydrate at different rates, depending on their size and composition, and interact with one another to form various reaction products. Some products deposit on the remaining cement particle surfaces (surface products) while others form as crystals in the water-filled pore space between cement particles (pore products). For simplicity, and because it still correctly captures the main features of the pore structure, cement paste can be thought of as consisting of four phases: 1) unreacted cement, 2) surface products (like C-S-H), 3) pore products (like CH = calcium hydroxide), and 4) capillary pore space. Surface products grow outward from the unreacted cement particles and contain connected (percolated) gel pores, while pore products are generally polycrystalline and fully dense, with no connected pores. The capillary pores are the remaining water-filled space between solid phases, left over after a given degree of hydration takes place. Capillary pores generally range from about 0.01 to 0.1 micrometers in size, in a reasonably well-hydrated cement paste (α > 0.5), although in early hydration, they can range up to a few micrometers in size.
While images of both initial and hydrated cement microstructures can be experimentally obtained in two dimensions, acquiring quantitative three-dimensional information is much more difficult. It is for this reason that computer models of the 3-D microstructural development of cement paste have been developed.
The actual process of cement hydration, for the purposes of modelling the development of microstructure, can be broken down into three parts: 1) material dissolves from the original cement particle surfaces, 2) diffuses within the available pore space, and 3) ultimately reacts with water and other dissolved or solid species to form hydration products. Therefore, in order to simulate the microstructure development of hydrating cement, the physical processes of dissolution, diffusion, and reaction must be simulated. Each of these processes may be conveniently simulated using cellular automaton-type rules as has been previously described [12,13]. Figure 3 shows four steps of simulated hydration for a C3S cement paste in 2-D. The original particle shapes are taken from a backscattered SEM image. The images have been colorized according to the major phases of portland cement [12].
Figure 3: Showing four stages of hydration in a microstructural model of
C3S hydration. The
degrees of hydration are: top left--0, top right--20 %, bottom left--50%,
bottom right--87%. Red=unreacted cement, blue=CH, yellow=C-S-H, and black=
porosity.
This brief description of the chemical hydration process, which is the basis of the microstructural formation of cement paste, of course glosses over a number of chemical details, many of which are not clearly understood [12]. However, this simple description is sufficient to be able to go on and investigate the various important percolation thresholds that occur in cement paste.
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