Pore structures in hardened concrete


In the previous section it was rather cavalierly remarked that at setting, `the mix solution is seamlessly carried over to become the pore solution’ of the hardened cement paste in concrete. The development and characteristics of the pores into which the pore solution is `carried over’ is an important and complicated feature of the internal structure of concrete binders. A completely realistic under- standing of the developing pore structures in concretes is not available, despite many years of active research and literally thousands of research publications. The usual division of pores in the binder of Portland concrete given in textbooks and even in most current research papers is overly simplistic. Pores are generally considered to be either capillary pores or gel pores after the classic proposal by Powers and Brownyard.26 As will be seen subsequently, such a division seems to be far from an adequate classification. A more detailed and a perhaps more realistic picture of various types of pores to be found in arche- typical concretes is presented in this section, along with some limited insight into how these pore structures evolve with time. In this treatment the pore structure of aggregates in concrete is generally ignored, as it usually is. However, most aggregates have some pores, some aggregates have substantial contents of pores, and lightweight aggregates in particular have very extensive pore structures. Thus in at least some cases aggregate pores may play a significant role in fluid or ion transport, and in such cases these effects need to be considered. It is also necessary to consider the possible contribution to permeation capacity of air voids, especially in heavily air-entrained concretes. Air voids occupy an ambiguous status in the literature; sometimes they are recognized as pores in the context of pastes in concretes; sometimes they are considered, but only as an afterthought; in many treatments of pore structures in concrete their existence is ignored entirely. Air voids are always present in archetypical concretes (and also in laboratory-mixed cement pastes), whether or not they have been deliberately air entrained. The only exception might be for concretes or cement pastes purposely mixed under vacuum.

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