Page 235 - New Trends in Eco efficient and Recycled Concrete

Recycled mollusc shells 203

Figure 8.9 SEM observation of microstructure of mussel shell plain concrete (Martı ´nez-
Garcı ´a et al., 2017).

occurs predominantly in the inner part of the shells, the nacre layer. This is proba-
bly due to chitin, which has a higher presence in the nacre layer than other organic
polymers which decrease the pull force of inorganic compounds (Ge ´nio et al.,
2012; Martı ´nez-Garcı ´a et al., 2017).
Furthermore, complementary SEM analysis was carried out on some specimens
of mussel shell structural concrete (Fig. 8.9), where the difference in bonding of
each side of the mussel shell can be noted. The interfacial transition zone with the
outer part of shell (periostracum) shows some little cracks and a high presence of
pores. However, the interfacial transition zone with the inner part of shell (nacre
layer) presents an absolute lack of bond, showing high porosity in the form of cavi-
ties. The reduced aggregate-paste bond and the increased porosity of mussel con-
crete, undoubtedly, affect the concrete properties, and especially the mechanical
ones.

8.5 Concluding remarks

The types of shells that have been recycled as aggregates in concrete production are
periwinkle, oyster, cockle, scallop and mussel, mostly in substitution of coarse
aggregate although there are some experiences in their use as fine aggregate. The
different publications on microscopic and mineralogical composition of different
types of marine shells coincide largely. All the shells are composed mainly of cal-
cium carbonate (CaCO 3 ) in 95% 99% and they present a flaky shape.
Most of the literature does not mention having carried out any cleaning or
deworming treatment on the seashells. Some of them used a simple treatment of

washing and sundry, others have cleaned the shells at 110 C and, in some cases,