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Sequestration of carbon dioxide by RCAs 491

Figure 16.11 Vickers microhardness developments at new ITZs around carbonated and non-
carbonated NRCAs.

NAs was noticeably higher than that of the cement pastes. The location of the new
ITZ between the old mortar in the NRCAs and the new mortar was in the area of
0 120 μm away from the old mortar, regardless of whether the NRCAs was the
non-carbonated or not. The microhardness value in the 0 40 μm area was lower
than that in the 40 160 μm area.
It was also observed that the measured microhardness of the cement pastes in
the C-NRCAs (97 6 9), was higher than that of the non-carbonated NRCAs
(87 6 17). After carbonation, the strength of NRCAs was improved. Moreover, the
microhardness on the new ITZ around the C-NRCAs was higher than that around
the non-carbonated NRCAs. This thus resulted in the improvement of the new ITZ.
This should be attributed to the lower water absorption value of the C-NRCAs than
that of the non-carbonated NRCAs, and then it would result in a relatively lower
water-to-cement ratio around the C-NRCAs to help the improvement of the new
ITZ. At 160 μm away from the old mortar of the NRCAs, the microhardness values
of the two types of concrete showed little difference.
The above findings imply that the micromechanical properties of the
C-NRCAs were improved as a result of the carbonation of the cement paste in
the adhered mortar. This was consistent with the results presented in Fig. 16.4.
Moreover, the mechanical properties of the new ITZ in RAC incorporating the
C-NRCAs were also enhanced. And due to these two aspects, the mechanical
properties of RAC prepared with the C-NRCAs were improved as shown in
Figs. 16.7 and 16.9.

16.3.6 Durability
The durability properties of RAC incorporated with non-carbonated NRCAs and C-
NRCAs, in terms of water absorption and permeability (bulk electrical conductivity,
air and chloride permeability), are presented in this section.