As with all materials, the microstructures of cement pastes, mortars, and concretes provide the bridges between materials processing and engineering properties. Unlike many other materials, however, cement-based materials exhibit a highly dynamic (micro)structure that is extremely sensitive to initial conditions, processing, and environmental exposure. There are few materials where water plays such a critical role in processing, microstructure development, performance, and durability. In cement-based materials, water is the liquid that provides flowability to the raw materials, serves as a vehicle for and participant in the numerous and complex cement hydration reactions, exerts forces on the solid components of the porous microstructure during self-desiccation, drying, freezing, alkali-silica gel formation, and exposure to fire, and provides the pathways for the ingress of deleterious ions. From a geometrical/structural viewpoint, the characteristics of both the porosity where the water resides and of the particles initially present in the mixing water are critical influences on hydration rates, microstructure, and performance properties. In analogy to large scale construction, cement hydration can be viewed as the process of building bridges to connect cement particles and dams to disconnect the water-filled capillary pore space.
Because many of the cement hydration products form around the initial cement clinker particles, as shown in Figure 1, the initial configuration of these particles is crucial in providing a "scaffold" on which a network of (porous) solid bridges will form. Thus, the initial water-to-cement ratio (w/c) [1], particle size distribution [2-4], and dispersion/flocculation state [5] of the particles all exert major influences not only on the developed microstructure (through and beyond setting) but also on the hydration kinetics. The bridging process can be conveniently explored via a coordinated experimental and computer modeling approach, as will be demonstrated in this paper.
Figure 1: Color-coded three-dimensional microstructures (100 µm x 100 µm x 100 µm) of real (left) and model (right) hydrating cement pastes of Cement and Concrete Reference Laboratory cement 133 with a w/c≈0.47 and a degree of hydration of about 0.62. Red represents unhydrated cement particles, yellow hydration products, and blue capillary porosity. The real microstructure was captured by x-ray microtomography at beam line ID 19 of the European Synchrotron Research Facility in Grenoble, France, in September 2000 [6, 7].
While the microstructural bridges are critical for strength development and mechanical properties, the microstructural dams are more important for limiting transport and improving the durability of cement-based materials. As cement hydration connects the original cement particles together, it may also disconnect the water-filled capillary porosity, at least at the micrometer scale. Because there are nanometer-sized pores present in the calcium silicate hydrate gel (C-S-H) hydration product, the porosity always remains percolated at some scale. Although the capillary pores present in both the real and model microstructures in Figure 1 appear depercolated in two dimensions, in three dimensions, they are still highly connected. But, as first noted by Powers many years ago [8], for lower w/c pastes, sufficient hydration will result in depercolation of the initial water-filled capillary pores. Here, this depercolation, along with the role of curing conditions and cement alkali content, will be examined experimentally using low temperature calorimetry (LTC).