Page 485 - New Trends in Eco efficient and Recycled Concrete

Microstructural studies on recycled aggregate concrete 435

Figure 14.6 SE image of concrete showing CH crystals with (A) hexagonal morphology
(w/c 5 0.53, 12 h); (B) massive agglomerates (w/c 5 0.53, 28 days); and (C) columnar
morphology. SE, Secondary electron.

Figure 14.7 TEM images of CH phase showing nanocrystalline CH domains in a hardened
paste (w/c 5 0.4; 19 days) (Richardson, 2004). TEM, Transmission electron microscopy.

Additional information regarding CH morphological nanofeatures such as crys-
tallinity and distribution of crystalline domains (see Fig. 14.7) has been obtained by
TEM observation.
As the paste matures it becomes more massive, evolving from a loosely packed
and interdispersed network of C S H and CH thin crystals to the formless and
massive characteristics of old pastes resulting from pore filling by hydration pro-
ducts (Franus et al., 2015). However, filling of the cellular structure also makes the
distinction between phenograin and outer C S H by SEM more difficult, as the
hydration products tend to fuse into a nearly continuous mass (Diamond, 2004).
Observation of cement paste polished sections shows CH as irregular masses of var-
ious sizes (Fig. 14.4A) and although somewhat brighter in appearance than
C S H gel, distinction is sometimes difficult in BSE images (Diamond, 2004).
C S H and CH in the ground mass are more clearly differentiated with AFM,