Interfacial Transition Zone

Gallagher, KJ. Feifknecht,W. Marmweiler, U. (1968) Mechanism of oxidation of magnetite to Y-Fe20j. Nature 217 1118-1121 Gallias, J.L. (1998) Microstructure of the interfacial transition zone around corroded reinforcements. In Katz, A. Benier, M. Alexander, M. Arliguie, G. (eds.) The interfadal transition zone in cementitions composites. E.F.N. Spon, London, 171-178 Galvez, N. Barron,V. Torrent, J. (1999) Preparation and properties of hematite with structural phosphorus. Clays Clay Miner. 47 375-385... 

Microstructural Analysis of Paste and Interfacial Transition Zone in Cement Mortars Modified with Water-soluble Polymers... 

Keywords water-soluble polymer, polymer modification, film formation, interfacial transition zone... 

Abstract. The presence of water-soluble polymers affects the microstructure of polymer-modified cement mortar. Such effects are studied by means of SEM investigation. Polyvinyl alcohol-acetate (PVAA), Methylcellulose (MC) and Hydroxyethylcellulose (HEC) are applied in a 1 % polymer-cement ratio. The polymers provide an improved dispersion of the cement particles in the mixing water. The tendency of certain water-soluble polymers to retard the flocculation of the cement particles minimizes the formation of a water-rich layer around the aggregate surfaces. They also provide a more uniform distribution of unhydrated cement particles in the matrix, without significant depletion near aggregate surfaces. Both effects enable to reduce the interfacial transition zone (ITZ). The polymers also provide a more cohesive microstructure, with a reduced amount of microcracks. 

General characteristics of the interfacial transition zone. Fig. 5 illustrates the microstrueture of an unmodified eement mortar on SEM image in the backseattered mode (BSE). The large dark areas represent the sand particles. Unhydrated cement particles are responsible for white areas. The epoxy-impregnated pores have a low backscatter intensity and appear as black spots. 

Concrete is a composite typically composed of aggregate and cement paste. It is the connection between these phases - the interfacial transition zone (ITZ) that is most important.. The ITZ is the weakest link in the composite system with lowest mechanical properties of the three [1-4], The ITZ can affect the overall elastic module and the stress distributions in a concrete material. The ITZ is comparatively more porous than that of bulk cement paste, and often less well bonded to the aggregate [3]. This region can have a low formation of calcium-silicate-hydrate (C-S-H gel), a product of Portland cement hydration responsible for the good mechanical properties and durability [5]. 

E.J. Garboczi DP. Bentz, The effect of the interfacial transition zone on concrete properties the dilute limit. Proc. of the 4th Materials Conference, November (1996), Washington, DC. 

K.L. Scrivener A.K. Crumbie P. Laugesen. The interfacial transition zone between cement paste and aggregate in concrete. Interface Science (2005). DOI 10.1023/B INTS.0000042339.92990.4c. 

Keywords High-Performance, Lightweight, PVB, PVA (polyvinylalcohol) Fiber, Interfacial Transition Zone, Density, Fracture Toughness, Impact... 

Concrete is a multiple-phase material and the overall mechanical properties not only depend on each phase, but also upon the interface between them. Prior study shows that the interfacial transition zone (ITZ) which is characterized by the prevalence of calcium hydroxide and higher porosity, is the weakest region in a cementitious material. Interactions that take place there control many important properties such as strength, permeability and durability [1]. Methods have been... 

A H. Asbridge, GA. Chadboum and C.L. Page Effects of Metakaolin and the Interfacial Transition Zone on the Diffusion of Chloride Ions through Cement Mortars, Cement and Concrete Research Vol. 31, No. 11 (2001), p. 1567-1572. 

There were attempts to relate the permeability of concrete to the properties of interfacial transition zone. However, the unambiguous results were not obtained. According to Roy [142], the constraction of interfacial transition zone surface does not play important role in concrete permeability, while Valenta [143] has quite opposite opinion. This problem will be discussed in Chap. 6 where the construction and properties of interfacial transition zone will be presented [144], In the light of the studies by Richet and Oliver [145] it is evident that the porosity of inteifacial transition zone in traditional concretes (w/c = 0.5 or more) has a significant influence on the permeability of concrete this permeability is a hundred times higher than in the case of cement paste and rises with the size of aggregate (Fig. 5.68). However, the effect of the transition zone on the diffusion of ions is not so evident, because the locally increased water content in this zone, decrease the w/c ratio in cement matrix outside it, which consequently limits the diffusion, thus a total effect can be negligible [145],... 

Zaitsev and Wittmann [15] proposed a model of fracture, presented in Fig. 6.6. The authors assume in this model that the cracks start in the interfacial transition zone. 

The XRD studies of the interfacial transition zone (material produced by abrasion of paste layers) [16], as well as the SEM observations with EDS analysis [16] revealed the presence of transition zone surrounding the aggregate grains, determined by Maso as an aureole [ 10]. This relates to the former water film around the aggregate. This area shows higher w/c ratio and subsequently cement components can readily dissolve, as well as the hydration products crystallize from the solution. Calcium hydroxide crystallizes in this interfacial transition zone and the crystals are oriented in such a way that their (001) axis is perpendicular to the surface of aggregate, as it was reported by Barnes et al. [17]. The C-S-H is then formed and the two products occur together as a duplex film about 1 pm thick (Fig. 6.7). 

Neither the duplex film, nor the oriented portlandite crystals in the interfacial zone were found by all the authors [19, 20]. However, there is a common opinion that this area is enriched with calcium hydroxide crystals and exhibits higher porosity. Consequently, it will have a great impact on the corrosion resistance of concrete. The transition zone of high porosity is the weakest micro-area where the corrosion of concrete will begin [21]. For this reason, the interfacial transition zone became a subject of numerous studies [16,17,22-28]. The constitution of this zone can be easily observed on the model proposed by Rooij et al. [28] (Fig. 6.9). 

The following features have been attributed by Diamond and Huang [34] to the interfacial transition zone, as compared to the bulk paste matrix ... 

Fig. 6.11 Unhydrated clinker content in the interfacial transition zone. (According to [35])...

Quite another conditions occur in the case of concrete with porous aggregate, particularly a dry one, which will significantly absorb water. However, the construction of interfacial transition zone has not been studied in such a concrete as yet [10]. One can even suppose, based upon the model studies, that the growth of calcium hydroxide content around the aggregate grains will not be formed. There are some premises indicating the emichment in ettringite [10]. Incase of diy, porous... 

Fig. 6.18 Aggregate grain and dense cement matrix with C-S-H in an interfacial transition zone. (Photo of B. Trybalska)...

Fig. 6.19 Structure of interfacial transition zone and microcracks propagation (schematically). (According to [50])...

The construction of interfacial transition zone aroiuid the reinforcement in concrete is very similar to the aggregate paste interface. This is presumably the consequence of locally occurring higher w/c ratio, promoting dissolution of cement components and crystallization of cement hydration products from the liquid phase. This interfacial transition zone reveals also higher porosity and lower strength than the bulk cement matrix. 

This interfacial transition zone is enriched with portlandite crystals of hexagonal shape, with the c axis perpendicular to the surface of aggregate [49]. This layer is not continuous and there are in the formed pockets the C-S-H particles, surrounding sometimes the portlandite crystals [50, 51], as well as the ettringite is present too [52]. In Fig. 6.9 the construction of paste—reinforcement interface is shown [50]. The three zones can be noticed ... 

The thickness of interfadal transition zone is determined by the range of oriented portlandite crystals and equals 50-100 pm [16]. However, only a 10-20 pm thick layer of this zone shows clearly different mechanical properties, as compared to the bulk cement matrix. The microstmcture of interfacial transition zone is variable and depends on the type and properties of cement and reinforcement, presence of admixtures, concrete maturing regime, as well as the other factors (Fig. 6.19). 

There are also the other reactive aggregates, namely gneiss and mica containing shales [41], In the interfacial transition zone, in the vicinity of aggregate surface— kaolinite and hydromicas, while from cement paste side—gel of sodium-calcium silicate hydrate, respectively are formed. However, in the case of serpentine concrete deterioration is due to the formation of brucite [75]. The clay minerals, such as chlorites, vermiculite, as well as micas and feldspars, are also included to reactive aggregate components. 

The studies of the phase composition of interfacial transition zone were also developed. A white layer of reaction products is formed on the reactive aggregate surface, sometimes surrounded by a black gel. The crystalline thaumasite is often formed this white layer [60,71], Regourd et al. [92] found the two types of gel the... 

The difference between the microstructure of cement paste and concrete should be taken into accoimt when the diffusion conditions are compared. The paste (cement matrix in concrete)— aggregate interfacial transition zone is of special importance. This transition zone is formed of the thin layer of cement paste, about 50 pm wide, which has different microstmcture than the bulk cement matrix in this concrete (see Sect. 6.2). The porosity of this interfacial transition zone is significantly higher the portlandite content is higher as well. In the case of high performance concrete with low w/c ratio the microstructural difference between the interfacial region and the matrix is negligible or none. 

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