Cements as Porous Materials
Handbook of Porous Solids
Introduction Concrete, found in roads, bridges, skyscrapers, and a myriad of other structures, is easily one of the most ubiquitous materials in modern civilization. Quite surprisingly, however, concrete is also a relatively difficult material to classify. It can easily be mistaken for a rock, possibly some form of artificial rock, or perhaps thought of as a composite. Adopting any one of these categories would not reveal the fundamental nature of concrete, namely that it is porous. In fact,
... porous. In fact, the binding matrix phase of concrete, called hardened cement paste, possesses between 25 and 50% porosity. This relatively large porosity is striking in itself; however, the variation in pore size is equally impressive, ranging from nanometer-sized gel pores, to micrometer-sized capillary pores, to millimeter-sized air voids. The complexity of concrete is compounded by its interactions with water. Hardened cement paste, formed from the reaction of the various minerals in concrete with water, will continue to react with its aqueous phase for several years, leading to progressive refinement and maturation of its porous structure. Much of this porous structure resides in the calcium silicate hydrate gel, causing the hydrate to have a surface area of several hundreds of square meters per gram. These colloidal dimensions result in an intimate association between the solid gel phase and the aqueous phase that resides in the gel pores. This coupling has important repercussions on the properties of hardened cement paste, especially in phenomena such as drying shrinkage and creep. Rather than attempting to cover all the peculiarities of cement science, this review concentrates on the pore structure of hardened cement paste and concrete. In Section 6.11.2, the structure of hardened cement paste is described after a brief overview of the hydration of Portland cement. Section 6.11.3 outlines pertinent issues concerning the characterization of the porous structure of hardened cement paste. Section 6.11.4 presents interpretations of the pore structure of hardened cement paste and, particularly, that of the calcium silicate hydrate product. Section 6.11.5 describes selected macroscopic properties of concrete such as strength, volume stability, and permeability, with emphasis on the influences of porosity and water on these properties. The concluding Section 6.11.6 concerns the durability of concrete and briefly reviews the debilitating effects of several important interactions between concrete and its environment.