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Recycled mollusc shells 199

20%
Falade A
10%
Falade B
0%
Falade C
–10% Adewuyi A
–20%
Slump variation (%) –30% Olufemi
Adewuyi B
–40%
–50%
–60%
Yang B
–70% Yang A
–80%
Yang Aa
–90%
Yang Ba
–100%
0 10 20 30 40 50 60 70 80 90 100 Sabri
% Aggregate substitution
Periwinkle Oyster Cockle

Figure 8.4 Variation of slump in concrete with marine shells as aggregates.

undergo most important variations for smaller percentages, in comparison to those
presented in the Adewuyi (Adewuyi and Adegoke, 2008) and Olufemi (Olufemi
and Manasseh, 2009) studies. Yang (Yang et al, 2010) compares identical concrete
compositions for four types of series, the first two without an additive and the other
two with an additive, the A and Aa use substitution rates exceeding the normative
limit of chlorides, and the B and Ba used percentages that do not exceed this limit.
The variations of the slump are very pronounced even though this only tests the
included substitution of 20% of fine aggregate. In the case of cockle shells, the vari-
ation data of the slump are also high, showing a decrease in the values by 85% for
replacement percentages of 30%.

8.4.3 Compressive strength
The effect of the mollusc shells in compressive resistance is similar for all species.
It can be seen that the greater the percentage of substituted aggregate, the greater
the decrease in compressive strength at both 7 and 28 days (Figs. 8.5 and 8.6). Both
graphs include studies with periwinkle, scallop, oyster and cockle shells, and in
addition at 28 days, Nguyen’s study (Nguyen et al, 2017) with crepidula shells. It
should be noted that the highest reduction at 7 days has been detected by the studies
with a substitution of gravel per periwinkle gravel (70%) (Adewuyi and Adegoke,
2008) and substitution of the cement per cockle filler (73%, 3%) (Hazurina et al,
2013). This coincides with the tendency that coarse aggregates influence more than
fine aggregate properties, and that shells filler is an inert material with no binding
capacity. These reductions are maintained at 28 days, although the most important