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Recycled mollusc shells 197

main publications on this topic. It describes the type of species, the grain size by
which it replaces the conventional aggregate (gravel or sand), the grain size of the
shell aggregate used, the cleaning treatment that each author has used prior to the
use of recycled aggregate, the reference, the country and the properties studied by
each researcher.
The aggregates used are periwinkle, oyster, cockle, scallop, and mussel, mostly
in substitution of coarse aggregates; with up to 25 mm in maximum size. In addi-
tion, there are some experiences of use as fine aggregate, up to a size of 5 mm. No
study includes the joint replacement of conventional aggregates with sand and
gravel from seashells.

8.4 Effect of mollusc shell in concrete properties

In this section, a detailed analysis of the properties analysed by the authors listed in
Table 8.1 is presented. The effect evaluated is the variation that the different per-
centages of substitution of conventional aggregates by marine shells produce with
respect to their reference, a concrete without shell aggregate. The variables ana-
lysed are consistency, hardened density and the mechanical resistances at different
ages. In this analysis, only authors are included who carried out the tests starting
from a standard mix composition. Using this as a basis, they substituted the natural
aggregate by different marine species shells. The works that did not include a refer-
ence were not discussed.

8.4.1 Hardened density

Regarding the density (hardened state), it can be observed, as shown in Fig. 8.3,
that the greater the percentage of substitution used, the greater the reduction of the
density with respect to the reference, and this applies to all the authors. The Falade
(Falade, 1995), Adewuyi (Adewuyi and Adegoke, 2008), and Olufemi (Olufemi
and Manasseh, 2009) studies, which replace conventional gravel with periwinkle
gravel, have large drops, registering maximum values of 237.4% for 50% substitu-
tion, 238.2% for 100%, and 222.5% for 100%, respectively. Falade (Falade,
1995) compares three different types of mix proportioning A, B and C with differ-
ent initial densities, all of them, for a replacement of 20% of aggregate present
reductions between 12% and 16% are observed. The studies of Adewuyi (Adewuyi
and Adegoke, 2008) and Olufemi (Olufemi and Manasseh, 2009), although they
also substitute conventional gravel for periwinkle, achieve lower reduction than any
Falade (Falade, 1995) series, even for 100% substitution percentage. If we compare
the series that substitute 50% of the aggregate, there are very notable differences in
the variations: 35% Falade (Falade, 1995), 10% and 15% Adewuyi (Adewuyi and
Adegoke, 2008), 14% Olufemi (Olufemi and Manasseh, 2009). This may be due to
the maximum size of the periwinkle aggregate used. In the case of the substitution
for scallop and crepidula, the falls are milder than those shown by studies with