Page 176 - New Trends in Eco efficient and Recycled Concrete

148 New Trends in Eco-efficient and Recycled Concrete

5.9 Conclusions

Chemical, physical and mineralogical properties of SSAs are highly variable
because of the different sources of wastewater, the nature of the soil and the type of
treatment in the WWTP, among other factors. In general, silicon dioxide is usually
the major component, although depending on the SS, calcium carbonate, iron oxi-
des, phosphorous and sulphur-derived compounds are also present. This variability
must be taken into account, because it could result in significant differences in the
behaviour of cement-based SSA containing mortars and concrete.
SSA has interesting pozzolanic reactivity, which has been tested by several tech-
niques: thermogravimetry, Frattini test and compressive strength development,
among others. In some cases, SSA delays the hydration rate of PC. However, this
behaviour apparently does not affect the final strength obtained. It is important to
highlight that this pozzolanic reactivity, because it opens up the possibility of using
SSA containing mortars and concrete. Another important fact that must be taken
into account in the use of SSA in cement mixtures is the effect on the rheology of
the systems. Usually, a significant decrease in workability was observed in fresh
concrete or mortar, due to the irregular morphology, porosity and roughness of SSA
particles. Several treatments, such as milling or high temperature calcination, have
been proposed in order to minimise this negative effect.
The pozzolanic role of SSA in cement mixtures has been widely explored. The
reactivity of the ash in terms of strength contribution has been considered as a mod-
erate rate. In some cases, increasing reactivity has been achieved by controlling the
calcination temperature of the SS (usually 800 C was the best condition), or by

grinding. Durability aspects were less assessed, and research was focused on para-
meters such as porosity, absorption, ASR expansion and drying shrinkage.
In the last decade, there has been an interest in the use of SSA in geopolymer
binders. Reported results demonstrated that in most cases, blending SSA with other
reactive mineral additions (MK, BFS) led to the production of stable systems with
sufficient mechanical properties. SSA was also successfully used in the fabrication
of clinker and ceramic bricks.
The global view of the treatment of wastewater can be seen as a good example
of the zero-waste concept and circular economy. The removal of contaminants from
the wastewater provides an opportunity to reuse the water and/or to avoid damage
to the environment, mainly rivers, lakes and the ocean. The by-product from the
water treatment, namely the SS, can be partially reused, mainly in the agriculture
sector. Finally, the discarded SS can be transformed into ash by calcination, in
many cases taking advantage of its organic matter content. The resulting ash,
namely SSA, can be reused in many fields of the construction sector: ceramics,
clinker and cement/mortar/concrete. Certainly, the reuse of this ash in the prepara-
tion of mortars and concrete is a very promising option. Properties of the manufac-
tured elements based on the cement ash system showed that, in many cases, the
quality of the final product was appropriate. In this way, the circle is closed and no
material is lost through the different processes.