Page 167 - New Trends in Eco efficient and Recycled Concrete

Sewage sludge ash 139

Tay (1987) studied the use of SSA as a filler in concrete. Dewatered sludge
samples were collected weekly for 12 months, and burnt in a furnace at above

550 C to remove organic matter. In order to study the feasibility of the use of ash
as a filler, it was pulverised and passed through a 150 μm sieve. Then 5% 20%
(by weight) of PC was replaced by SSA. The results showed a decrease of compres-
sive strength as the percentage of sludge increased. After 28 days of curing, a
reduction in strength of 5% and 11% for 5% and 10% replacements, respectively,
was observed. Additionally, the shrinkage strain was reduced by 10% with respect
to the control sample. The author concluded that SSA could be used as a partial
replacement of cement for concrete mixing.
Monzo ´ et al. (1996, 1999a) studied the mechanical behaviour of PC mortars add-
ing SSA and curing at 40 C. In the first paper (Monzo ´ et al., 1996), 15% (by

weight) of PC was replaced by SSA and three size fractions (coarse, medium and
fine) were obtained by sieving. In general, all mortars containing SSA exceeded the
control mortar, and an increase of compressive strength with fineness was observed
(Fig. 5.7). The pozzolanic behaviour for the 3 28 day curing period was then con-
firmed when the curing temperature was slightly raised. In their second paper
(Monzo ´ et al., 1999a), the assessment of the sulphate content of SSA was carried
out. This parameter was very high in SSA (12.4%) and could influence the volume
stability of the concrete. The behaviour of mortars containing SSA (15% and 30%
replacements) and several PCs with different tricalcium aluminate contents was pre-
sented. Results showed that this SSA was compatible with PC with high C 3 A con-
tent, and a decrease in the mechanical properties were not observed after the 28-day

curing time at 40 C. This meant that the sulphate in the SSA was not reactive, and
no expansive products (e.g., secondary ettringite) were formed during the curing
process. The expansion was measured (Garce ´s et al., 2008) for SSA-cement

Figure 5.7 Compressive strength development of mortar cured at 40 C for 3 28 days.
Control mortar (only cement) was compared with 15% replaced mortars: SSA (as-received),
SSAC (coarse fraction) and SSAM (medium fraction) (Monzo ´ et al., 1996). SSA, Sewage
sludge ash.