Page 228 - Materials Chemistry, Second Edition

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Recycling of Municipal Solid Waste Rejects
Figure 6.6 also indicated an increase in compressive strength values with
the increase in mixing temperature. This resulted from the better mix homo-
geneity associated with high mixing temperature. Fewer polymer segregates
were observed for mixes prepared at 240°C in comparison to the 185°C mix-
ing temperature as revealed by the optical micrographs. Polymer segregates
will split apart from the matrix upon load application forming microcracks at
the interface, hence causing premature failure.
Following the ASTM specification C 936-96 (1998k), the average com-
pressive strength for solid concrete interlocking paving units should not be
less than 55 MPa for all test samples with no individual unit less than 50 MPa.
However, it was shown during trafficking tests in South Africa that the behav-
ior of block pavements was unaffected by changes in compressive strength
ranging from 25 to 55 MPa (Shackel, 1990). Hence, mix 12 prepared at a tem-
perature of 240°C with the large sand particles (sieve 2) at 60% sand content
has the potential for use as interlocking paving blocks. Moreover, the same
mix also satisfies the ASTM C 902-95 (1998j) specification for pedestrian and
light traffic paving bricks entitling a minimum compressive strength value of
20.7 MPa.

Flexural properties
The flexural strength for all the mixes was determined by test method I of the
ASTM D 790M-93 (1998). The results of all the flexure tests are summarized
in Table 3.2. The average modulus of rupture of the prepared mixes ranged
from 10.4 to 21.2 MPa. The ASTM standard C 410-60 (1998i) for industrial
floor bricks specifies a minimum modulus of rupture of 13.8 MPa for type M
and L industrial floor bricks, and 6.9 MPa for type T and H. About half the
investigated mixes satisfied the requirement for type M and L, and all the
mixes satisfied the type T and H minimum value. In addition, the modulus of
rupture of the produced composite material was higher than the modulus of
rupture of clay bricks ranging from 3.5 to 10.5 MPa (Cordon, 1979). These
results show the potential use of the composite material in industrial flooring
applications, as well as structural applications.
The effects of sand content, sieve size, and mixing temperature on flex-
ural strength was investigated graphically in Figure 6.7. It is evident from
the figure that for low mixing temperatures of 185°C, the flexural strength
decreases with the increase in the sand content. On the other hand, for a high
mixing temperature of 240°C, the flexural strength values increased with the
increase in sand content from 20% to 40% with a maximum value for the
40% sand content of 21.2 MPa and 16.3 MPa for sieve size 1 and 2, respect-
ively. However, increasing the sand content beyond this level resulted in a
decrease in the flexural strength. This decrease in the flexural strength with
the increase in sand content is attributed to the stress concentrations induced
by the filler particles. This stress concentration will promote failure upon load
application.