Page 531 - New Trends in Eco efficient and Recycled Concrete

Sequestration of carbon dioxide by RCAs 479

xCaO SiO 2 yH 2 O 1 CO 2 ! CaCO 3 1 ðx 2 1ÞCaO SiO 2 yH 2 O (16.4)

Therefore, it is noteworthy that the potential CO 2 uptake by concrete during its
service life and demolition phases would partly offset the CO 2 emission from the
production of cement. When taking into account the CO 2 sequestration potential
and the quality enhancement of RCAs, the carbonation pre-treatment method for
enhancing the properties of RCAs would be of significant contribution to the sus-
tainability of concrete. This chapter presents the potential CO 2 capture ability of
RCAs estimated by a developed empirical model as well as the performance of
RAC incorporating the carbonated RCAs.

16.2 Estimation of CO sequestration by RCAs
2

In theory, the maximum CO 2 uptake of cement by carbonation can be calculated by
the chemical compositions of its constituents (Xuan et al., 2018; Huntzinger et al.,
2009), as expressed by:

%Theory
3
CO 2 ðcementÞ 5 0:785 X CaO 2 0:56X CaCO 2 0:70X SO 3
1 1:09X MgO 1 0:71Na 2 O 1 0:47X K 2 O (16.5)
where X is the mass ratio of individual subscripted oxides to cement.
However, in practice, the carbonation rate and CO 2 uptake of concrete or RCAs
would be greatly influenced by external factors (humidity, temperature, CO 2 partial
pressure and concentration of CO 2 ) and internal factors (water/binder ratio, binder
content, particle size etc.) (Thiery et al., 2013; Kashef-Haghighi et al., 2015;
Papadakis et al., 1991; Jae-Dong et al., 1990). Although these variables have been
identified to influence the carbonation of concrete, studies on integrally quantifying
these effects were limited. The following section will present an empirical model
developed to estimate the CO 2 uptake of RCAs with different material characteris-
tics in relation to various accelerated carbonation conditions.

16.2.1 Materials and experimental
16.2.1.1 RCAs used

The RCAs, coded new recycled concrete aggregates (NRCAs), were obtained from
crushing a batch of new concrete produced by a ready-mixed concrete supplier after
6 months of outdoor curing. The crushed NRCAs were further sieved into three size
fractions: 10 20, 5 10, 2.36 5 and ,5 mm. The basic characteristics of NRCAs
were tested in accordance with BS EN 1097 and are listed in Table 16.1. The
cement content was determined in accordance with the ASTM-C1084 standard.