ABSTRACT
Abstract The notion that a definite interfacial transition zone (ITZ) some 30 to 50 u.m thick is formed between aggregate and paste is challenged. Microstructural studies show that the ITZ is neither continuous nor uniform. Furthermore, the appearance of the paste near the aggregate as compared to the bulk of the paste discloses marginal microstructural differences. Nonetheless, it appears that the near interfacial zone (< 10-15 jam) bears some attributes that distinguishes it from the surrounding by the occurrence of a higher amount of calcium hydroxide and larger pores. The association of calcium hydroxide, large pores, and nucleation sites makes the near interfacial zone vulnerable to any kind of chemical attack. Despite the non-uniformity, the role of the near interfacial zone is best manifested by secondary mineralization. There is a consistent pattern indicating that the near interfacial zone offers little resistance to solution transport and precipitation. Under corrosive conditions, calcium hydroxide is the first to react out to form deposits along aggregate boundaries. Common thickening of secondary mineralization exerts stresses that facilitate degradation. Keywords: cement microstructure, deposition site, interfacial transition zone, secondary mineralization
1 Introduction
To date a large body of literature deals with the various effects of the interfacial transition zone (ITZ) developed between the paste and aggregate. The notion that concrete is a three-phase composite consisting of cement paste, aggregate, and ITZ has become so popular that attempts to assign measurable properties to it have been made[l,2,3]. Mindess[l] has reviewed the literature and pointed out to the inconsistencies in the ITZ properties. At large this has been attributed to differences in specimen geometries and test procedures. In turn, the difficulties in determining the ITZ properties might also be related to assumption that the ITZ of a given concrete has a specific microstructure that lends certain properties. The question arises to what extent
such an assumption is justifiable? Otherwise, what are the attributes that characterizes this zone?
