Page 69 - New Trends in Eco efficient and Recycled Concrete

44 New Trends in Eco-efficient and Recycled Concrete

Unlike bottom ash, BFA is widely studied for its versatility. Studies reveal BFA
are effective in the development of the strength of the concrete. The behaviour
depends mainly on the level of SiO 2 in the ashes.

2.3.4 Durability-related properties: water absorption (by
immersion and capillarity), carbonation resistance and
chloride penetration, sulphate and acid attack
Durability problems play an important role, in particular among which the ASR is
the most frequent. They denote reactions between reactive aggregates and alkalis
released by the hydration of cement or other sources, such as mineral mixtures or
aggregates. These reactions lead to the expansion of concrete, cracks and even
faults, which are very damaging to concrete structures.
Cracking of the ASR (Fig. 2.10) generally occurs in areas with a frequent supply
of moisture, such as retaining walls exposed to the entrance of groundwater, marine
structures in direct contact with salt water and other areas with capillary suction.
A reactive by-product, like some BA, forms an alkaline silica gel by absorbing
the water present in the concrete (central zone in Fig. 2.10). This property is impor-
tant because the C S H gel (hydrated calcium silicate, also called calcium silicate
hydrate (CSH) gel with formula CaO SiO 2 H 2 O) produces an expansive pres-
sure, activating the crushing process between the aggregate and the surrounding
cement paste (Lindga ˚rd et al., 2012).
When biomass ash is used in concrete or mortar, the durability also depends on
whether the ash is pozzolanic or not. It depends on the structure of the particles in
terms of crystallinity and is essential to determine their pozzolanic activity, obtain-
ing improvements in terms of the durability of those ashes with pozzolanic activity
(Rajamma et al., 2009).

Figure 2.10 Cracking of the alkali silica reaction.