Page 530 - New Trends in Eco efficient and Recycled Concrete

478 New Trends in Eco-efficient and Recycled Concrete

these previous studies, there are four main techniques available to improve the
properties of RCAs and the corresponding RAC, as summarised next:

Removal of old adhered mortar of RCAs. The adhered mortar content in RCAs has an
obvious influence on mechanical properties and durability of RAC. After removing the
adhered mortar by a microwave heating and mechanical rubbing process (Akbarnezhad
et al., 2011), the strength of RAC would approach over 90% of normal concrete.
Moreover, when the mortar content is decreased approximately from 60% to 20%, the
total charge passed in Coulombs (as per ASTM C1202) of RAC can be obviously
decreased (Duan and Poon, 2014).
Enhancement of RCAs by physical and chemical methods without removal of residual
mortar of RCAs. In general, encapsulation and impregnation by a foreign chemical is used
to block the open porosity of RCAs. After this treatment, the strength of RAC can be
enhanced and its durability is comparable to normal concrete (Kou and Poon, 2010).
Improvement of new cement matrix of RAC by adding pozzolanic materials or other che-
micals (e.g. water repellent). This is a general technique to improve the properties of con-
crete as the cement matrix or mortar in concrete is the most important constituent
controlling the strength and permeability (Kou and Poon, 2013).
Modification of mixing method of RAC. Two-stage or three-stage mixing methods with the
use of pozzolanic materials have been verified to improve the performance of RAC (Kong
et al., 2010; Tam and Tam, 2007). When a two-stage mixing approach was adopted, it can
lower the chloride permeability of RAC (with 100% RCAs replacement) by about 30%
and improve its strength by about 17% (Tam and Tam, 2007). The main reason is attrib-
uted to the improvement of the interfacial transition zone (ITZ) between RCAs and the
new cement mortar.
Adopting the accelerated carbonation technique to improve the quality of RCAs
has been proposed by some researchers recently (Thiery et al., 2013; Zhan et al.,
2014; Xuan et al., 2017; Xuan et al., 2016a). It is one of ways to enhance RCAs by
chemical reactions without removal of the residual mortar. The chemical reactions
are between an external CO 2 source and the hydration products/anhydrous compo-
nents of cement in concrete: 3CaO SiO 2 ,2CaO SiO 2 ,Ca(OH) 2 , xCaO SiO 2 yH 2 O
gel, etc. (Morandeau et al., 2014; Kashef-Haghighi et al., 2015), as expressed by
Eqs. (16.1 16.4). The theoretical calculation indicates that the full carbonation of
concrete could take up 50% of CO 2 by cement mass. The reaction product, CaCO 3 ,
precipitates in the pore space of the system and densifies the microstructure of RCAs.
The carbonated RCAs, therefore, have increased densities, aggregate strengths and
decreased water absorption values.

ð
3CaO SiO 2 1 32xð ÞCO 2 1 yH 2 O ! xCaO SiO 2 yH 2 O 1 3 2 xÞCaCO 3
(16.1)

ð
2CaO SiO 2 1 22xð ÞCO 2 1 yH 2 O ! xCaO SiO 2 yH 2 O 1 2 2 xÞCaCO 3
(16.2)
ð
Ca OHÞ 1 CO 2 ! H 2 O 1 CaCO 3 (16.3)
2