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Sewage sludge ash 141

strength of the mortars increased with the SSA fineness. Donatello et al. (2010a)
also demonstrated that the milling is an appropriate treatment for activating the
reactivity of ashes, and a significant increase in the SAI was achieved. For example,
0.8 was the SAI value for 20% SSA replaced mortars after 28 days, while the SAI
value was raised to 0.96 for mortar with milled SSA. They also reported that acid
washing (H 2 SO 4 ) had a negative effect in the pozzolanic behaviour of SSA in terms
of compressive strength development. Kappel et al. (2017) found a similar enhance-
ment in strength with the milling of the SSA when a 20% replacement in mortar was
tested. The higher SSA fineness led to a reduction in the ASR process with respect to
the control mortar (Chen and Poon, 2017b). After 14 days of storing the control mor-
tar in 1 M NaOH solution, it developed an expansion close to 0.25%. However, when
ground SSA was used for 10% replacement, the expansion was reduced to 0.06%.
The replacement of 20% of cement by SSA (as-received or ground) led to a decrease
in the ASR expansion after 14 days (less than 0.01%). On the other hand, due to
water absorption ability of SSA particles, the shrinkage rate for SSA containing mor-
tar was significantly greater than that observed for the control mortar.
The reactivity of SSA also was studied with regards to calcination temperature.
Naamane et al. (2016) compared the development of strength for mortars containing
SSA in the 0% 30% range, and submitted the SSA to different calcination tem-

peratures (300 800 C). Fig. 5.9 shows the developed strengths after 90 days of

curing. Calcination temperatures in the 300 500 C range did not improve the

strength development, whereas the 700 800 C range of treatment temperatures
yielded the best results. Yang et al. (2014) also found that the calcination of SS at
800 C for 2.5 h was the best option in terms in compressive strength.

Also, a complementary study in terms of lime addition in the obtaining of SSA
was carried out (Vouk et al., 2017b). Different SSA samples were obtained from:
(1) calcination of SS; (2) calcination of SS previously stabilised with lime; and

Figure 5.9 Influence of SSA calcination temperature. Compressive strength values for
mortars with 0% 30% SSA content after 90 days of curing (Naamane et al., 2016). SSA,
Sewage sludge ash.