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Sewage sludge ash 137

(CaHPO 4 2H 2 O), was observed, which was generated from the reaction between
iron phosphate from the SSA and the Portlandite generated in the cement hydration.
The ICC test demonstrated that the evolved heat of the pastes containing SSA or
FSSA were higher than the control mix. In this case, the authors said that the nucle-
ation effect was the reason for this phenomenon.

5.5 Effect of sewage sludge ash on the rheology of
concrete

The principal characteristics of rheology studied on the role of SSA in cement mix-
tures are focused on the workability and the setting time parameters. The results
obtained in several of these investigations often do not agree, and this makes sense,
since the origins and compositions of the ashes are very variable.
Tay (1987) used SSA passing through a 150 μm sieve and established that the
initial setting time was similar to the control system with PC, but the final setting
time was less. The workability of the mixtures was superior, or similar, to the con-
trol for the proportions with 15% and 20% of SSA substituted for cement.
These results obtained by Tay in 1987 differed, for example, from those obtained
by Monzo ´ et al. (1996). In the last research, the SSA was used in three different
fractions: SSA (ash as-received, original ash), SSAC (ash retained in the 80 μm
sieve) and SSAM (ash retained in the 40 μm sieve). The percentage of substitution
was 15%, and in this case, the workability was inferior to the control mortar for all
ashes. The authors concluded that the irregular morphology and the water absorp-
tion on the surface of the particles were responsible for this loss of workability.
In the other article, the same authors studied the influence of different percen-
tages of substitution, water/cement ratios and the use of superplasticiser (Monzo ´
et al., 2003). The increase in the substitution produced a decrease in workability,
but the use of a superplasticiser improved the fluidity of the mortars. The finest
SSA had the worst workability.
One alternative to improve the workability of mortar with SSA is the use of FA
as a second replacement material. Paya ´ et al. (2002) demonstrated the good perfor-
mance of FA in mortars with RHA, SSA, SF and fluid catalytic cracking residue.
Pan et al. (2003) used milled SSA and established that a 20% substitution
enhanced the workability of the mixture. They reported that the mechanical grind-
ing of particles promoted the morphology change in the particles from an irregular
to a spherical form. It is likely that the use of the original ash without milling was
the cause of the poor workability due to the morphology and texture of the parti-
cles. The initial setting time was larger, but the final setting time was less than the
control. The reason for this trend was attributed to the greater quantity of alumin-
ium in the SSA.
Garce ´s et al. (2008) studied the effect of substitution of cement by SSA in per-
centages between 0% and 30%. Only the samples that had more than a 10%