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Microstructural studies on recycled aggregate concrete            431


           increment the volume of mercury intruded into the pores is recorded and the total
           intruded volume is determined (Cnudde et al., 2009). This information is then used
           to quantify several features related to the pore structure of the material, including
           pore size distribution, total pore volume, bulk and absolute density of the material
           and presence of cracks (Abell et al., 1999). MIP is widely used to study pore dia-
           meters ranging from 360 μm to less than 100 nm (Abell et al., 1999; Kumar and
           Bhattacharjee, 2003; Diamond, 2004; Cnudde et al., 2009), but cannot provide
           information regarding gel pores (that remain non-intruded and are not quantified),
           closed pores or pore connectivity (Kumar and Bhattacharjee, 2003). The relation-
           ship between MIP results and the actual pore distribution and connectivity can be
           better understood combined with BSE image analysis (Abell et al., 1999) since the
           lower limit of conventional SEM-detectable pore sizes is usually larger than the
           upper limit of pore sizes reported in MIP studies of hydrated cement paste
           (Diamond, 2004).




           14.3    Overview of microstructural features of natural
                   aggregate concrete

           14.3.1 The microstructure of hydrated cement paste

           The microstructure resulting from cement hydration comprises two structural units:
           phenograins and groundmass. Phenograins include unhydrated clinker grains, partially
           hydrated grains with core composed of unhydrated clinker surrounded by a dense
           hydration rim, completely hydrated grains and hollow shell hydrated grains (Bonen,
           2006). The groundmass is composed of small particles with undefined shape embed-
           ded in pore spaces, composing a network with characteristic cellular structure that
           confines most of the system’s porosity (Bonen, 2006). Overall, cementitious products
           include non-crystalline hydrated phases, hydrated crystalline phases and unhydrated
           crystalline phases, with different chemistry, microstructure and size-scale. A wealth
           of information on composition, microstructure and spatial distribution of cement
           hydration products has been gathered through the years by optical and SEM. As a
           result, abundant characterisation of cementitious structure at the micro and submicron
           scale is available. And although detailed characterisation at the nanoscale is still a
           challenge, a relevant database regarding the nanostructure of cementitious products
           has also been built in recent years, mainly by TEM and AFM.
              Unhydrated cement particles. Individual anhydrous cement particles in calcium
           silicate clinkers (Fig. 14.2A) are typically constituted by an intimate mixture of
           fragments of several crystalline phases, including C3S, C2S, C3A, C4AF and minor
           components (Stutzman et al., 2015). Such unhydrated particles are retained in
           nearly all cement pastes (Fig. 14.2B), since only the finer cement grains are able to
           quickly hydrate completely (Scrivener, 2004). The unhydrated cores of individual
           cement particles in a cement paste are readily identifiable at the bright, white end
           of the grey scale when observed in BSE mode (Fig. 14.3).
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