Does carbonation affect the microstructural inhomogeneity of recycled aggregate concrete?

In this study, two types of modelled recycled aggregate concrete were prepared using the recycled aggregates in the lab. The evolutions of three kinds of interfacial transition zones (ITZs) and two kinds of mortars in these concretes before and after accelerated carbonation were separately studied using phenolphthalein spray test, CO2 diffusion coefficient modelling, pore structure test, backscattered electron test and nanoindentation test. The results showed that carbonation increased the inhomogeneity of recycled aggregate concrete by dividing the concrete into an irregular carbonation zone and a noncarbonation zone. Additionally, the average ITZ width showed the highest difference before and after carbonation, compared with the average elasticity modulus, average unhydrated cement content and average porosity. Carbonation also changed the differences in the microstructures between different kinds of ITZs and between different types of mortars. The inhomogeneity changes with time in the whole service life and further affects the long-term mechanical properties and durability of recycled aggregate concrete.