5.4.2     Calcium-Silicate-Hydrate (C-S-H) gel

    The C-S-H gel is not only the most abundant reaction product, occupying about 50% of the paste volume, but it is also responsible for most of the engineering properties of cement paste. This is not because it is an intrinsically strong or stable phase (it isn't!) but because it forms a continuous layer that binds together the original cement particles into a cohesive whole. All the other hydration products form as discrete crystals that are intrinsically strong but do not form strong connections to the solid phases they are in contact with and so cannot contribute much to the overall strength. The ability of the C-S-H gel to act as a binding phase arises from its nanometer-level structure. Because of its importance and complexity, an entire chapter (Chapter 8) is devoted to the structure and properties of C-S-H. Here we will discuss two of its most important general features: the internal pore system and the two morphologies.
    As C-S-H gel grows outward from the cement particles, it does not take the form of a monolithic solid phase but instead develops an internal system of tiny pores, called gel pores, which are hundreds or thousands of times smaller than the original capillary pores. Although the liquid water in the gel pores is not part of the solid C-S-H phase in a chemical sense, it is physically isolated and thus cannot undergo further chemical reaction with the cement minerals. This is the main reason for the range of water contents of C-S-H gel (the variable x in eqs. 5.1 and 5.2). The C-S-H gel, including its internal gel pores, occupies significantly more volume than the original C3S and C2S mineral that it replaces. This causes the layers of C-S-H gel to expand outward and interconnect into a continuous phase, causing the cement paste to first set and then harden into a strong solid. Because the overall volume of the cement paste does not change significantly after mixing, the increase in the volume of solid phases causes the capillary pore system to decrease in volume and, if the w/c is reasonably low, to become discontinuous. This greatly decreases the permeability of the cement paste, meaning that it is more difficult for liquid water and dissolved ions to move through the pore system.
    When a hydrated cement paste is viewed in a microscope at moderate magnifications, two apparently d;ifferent types or morphologies of C-S-H gel can be seen. One of these is less dense (more porous) and appears to occupy space that was originally water-filled, while the other appears more dense and is found primarily in areas originally occupied by the cement particles. The less dense morphology forms rapidly during the early hydration period surrounding setting (Stage 3), while the denser morphology fills in more slowly over days and weeks during Stage 4 (see Figure 5-4). Based on these characteristics, the two types have been given a variety of distinguishing labels, including "early" and "late" and "outer" and "inner". We prefer the terms "low-density" and "high-density" C-S-H, because the other terms are only approximately correct (i.e., some low-density C-S-H can form at later times and some high-density C-S-H is found outside of the original particle boundaries). Since the most important characteristic of C-S-H gel is its tendency to grow outward into the porosity, the low-density C-S-H is much more important than the high-density C-S-H.



Figure 5-4: Left: Rate of hydration vs. time, showing when the low-density and high-density morphologies form. Right: Backscattered SEM image of a 28 d old cement paste (courtesy of Paul Stutzmann, Concrete Microscopy Library). The white area is unreacted cement. The surrounding smooth grey area is high-density C-S-H. The multiphase areas are a mixture of capillary pores (black), low-density C-S-H, and other solid hydration products. The image is approximately 100 £gm wide.

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