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17 process to occur. Although most experts agree that secondary C-S-H, referred to as gel pores or interlayer hydration space, ettringite formation will not generate sufficient expansive make up the smallest individual elements of the total cement pressures to cause concrete fracture, its presence in the air- paste porosity. Their characteristics cannot be altered by void structure may limit the ability of the paste to resist changing mix design parameters. freeze-thaw deterioration (Ouyang and Lane 1999). In contrast, capillary porosity can be significantly modi- DEF, on the other hand, can lead to destructive expansion fied by altering mixture properties, especially the w/c ratio. within the paste, resulting in microcracking and separation of The capillary pore system is the space between anhydrous the paste from aggregate particles. DEF is most often associ- cement grains that is filled with water and not hydration prod- ated with steam curing the concrete because primary ettringite ucts. This system is typically irregular in both shape and spa- will not properly form at elevated temperatures (Thaulow tial distribution, with the pore sizes and connectivity depen- et al. 1996, Klemm and Miller 1997). After the concrete has dent on the size of the initial water-filled space (a direct cured and temperatures are reduced to ambient conditions, function of the w/c ratio) and on the degree of hydration. In sulfates and aluminate phases in the paste may react to form well-hydrated, loww/c-ratio systems common in EOT con- expansive ettringite, disrupting the concrete matrix. It is still crete mixtures, capillary pores will be much smaller than in speculative, however, whether cast-in-place concrete can highw/c-ratio systems or systems at early stages of hydra- experience DEF. But there is little doubt that under certain tion (Mehta and Monteiro 1993). The use of admixtures that conditions (e.g., thick slab, high cement content, and high disperse cement grains, such as water reducers, typically ambient temperature), EOT concrete may experience temper- result in smaller, more uniformly distributed capillary pores. atures in excess of that required for DEF during curing, espe- The largest elements of the pore structure are the air voids. cially if curing blankets are used during summer placements. The air voids are generally classified into two groups, those The manifestation of internal sulfate attack is character- that are intentionally entrained and those that are unintention- ized by a series of closely spaced, tight map cracks, with ally entrapped. Entrained air voids are essentially spherical wide cracks appearing at regular intervals. DEF can only be and tend to be randomly distributed throughout the cement identified through petrographic microscopic analysis in accor- paste. They are created through the addition of admixtures dance with ASTM C 856, "Standard Practice for Petrographic specifically designed to produce large quantities of micro- Examination of Hardened Concrete." scopic air bubbles when mixed into fresh concrete. While it is virtually impossible to clearly distinguish between entrained and entrapped air voids, quite often voids larger than 1 mm 3.4 MICROSTRUCTURE (0.04 in.) in diameter and/or irregular in shape are labeled as entrapped (ASTM C 125). These larger air voids contribute For the most part, concrete mechanical properties and dura- significantly to the total air content of concrete, but not to the bility are controlled by the paste microstructure. Detailed dis- frost resistance of concrete. cussions of concrete microstructure can be found in Mindess The interfacial transition zone (ITZ) between hydrated et al. (2003) and Mehta and Monteiro (1993). The hydrated cement paste and the coarse aggregate particles is also an cement paste microstructure that binds the aggregates together important element of the paste microstructure. The ITZ usu- consists of solid phases and a pore system. The solid phases, ally has a different microstructure than the rest of the paste which consist of both unhydrated cement grains and the system, with a higher proportion of CH and a greater poros- related hydration products, can be characterized by type, size, ity than the bulk paste. With time, the highly porous transi- and relative percentages. The number of unhydrated cement tion zone may become filled with additional hydration prod- grains increases markedly in highcement-content, loww/c- ucts resulting from chemical reactions between the cement ratio EOT concrete mixtures. While a variety of hydration paste phases and the aggregates (Mehta and Monteiro 1993). products exist in cement paste, the primary phases of interest Because these reactions reduce the porosity of the interfacial in determining the behavior of concrete are calcium-silicate- zone and consume calcium hydroxide, they tend to increase hydrate (C-S-H), calcium hydroxide (CH), and calcium sul- the paste strength in this zone. foaluminates (ettringite [AFt] and monosulfate [AFm]). The Microcracking of hydrated cement paste may occur rela- nature of the solid phases in a cement paste changes with tively early in the hydration process (before the paste had time. At time zero, when the anhydrous cement grains first gained significant strength) when internal stress exceeds the come into contact with the mix water, the microstructure con- strength of the paste. Shrinkage and/or thermal strain could sists of the unhydrated cement particles surrounded by water. produce such stress when restrained by the aggregates. Both As hydration proceeds, space that was initially water-filled is autogenous shrinkage and rapid changes in temperature of progressively occupied by hydration products. EOT concrete mixtures could lead to excessive microcrack- The cement paste pore structure can generally be classified ing of the paste. into three distinct groups: cement gel pores, capillary pores, A number of techniques are commonly used to character- and air voids (Neville 1996). The paste/aggregate interfacial ize the microstructure of concrete, including staining and transition zone and microcracking represent additional ele- various microscopy techniques (Van Dam et al. 2002b). Of ments of the concrete pore structure. The pores within the all the available methods, the stereo optical microscope is the