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Sewage sludge ash 135

5.4 Reactivity of sewage sludge ashes in Portland
cement mixtures

The data referring to the reactivity of SSA have been compiled for this area, taking
into account the different methods used from thermogravimetric analysis (TGA) to
Frattini tests. In general, in all of the reported articles, the pozzolanic character of
SSA was demonstrated.
Cyr et al. (2007) studied the reactivity of SSA in pastes with lime. The authors
made pastes with a ratio of 4:1:1 (SSA, lime and water respectively). They
concluded that the principal phases formed during the pozzolanic reaction were
ettringite, hydrated calcium aluminates and hydrated calcium aluminosilicates.
After three weeks, all the calcium hydroxide had reacted and the presence of
hydrated silicate phase was not detected.
Donatello et al. (2010a; 2010b) studied the pozzolanic activity and published
two articles on their findings. In the first paper, they studied the pozzolanic activity
using the Frattini test. Metakaolin (MK), quartz sand and FA were used for compar-
ative purposes. SSA, sand and FA were milled and all materials were washed with
sulphuric acid in order to study the influence of this process in the pozzolanic activ-
ity. The Frattini test focused on the study of the solubilisation of Ca(OH) 2 in a mix-
ture of cement (80%) and pozzolan (20%) with water. The samples were left for 8
or 15 days in a bath at 40 C, after which the samples were filtered. The filtrate was

2 21
analysed to determine concentrations of OH and Ca ions. The results were
21 2
plotted on the solubility chart of Ca /OH system and the points below the line
indicate a positive pozzolanic activity. The results obtained are represented in
Fig. 5.5. As can be seen in the figure, the grinding process improved the pozzolanic
activity of SSA. The process of washing the samples with acid worsened the behav-
iour of all the samples.

20
18
16
Sand acid MK acid
[CaO] mmol 14 Sand unmil Sand mil
12
10
ISSA acid
ISSA unmil
6 8 ISSA mil PFA acid Lime solubility curve
PFA unmil
4 PFA mil
2
MK unmil
0
35 45 55 65 75 85
[OH] mmol

Figure 5.5 Frattini test results (8 days) for unmilled, milled and sulphuric acid washed
materials (sand, MK, PFA, ISSA) (Donatello et al., 2010a). MK, Metakaolin; PFA,
Pulverised fuel ash; ISSA, incinerated sewage sludge.