Performance Analysis of a Recycled Concrete Interfacial Transition Zone in a Rapid Carbonization Environment

Based on the characteristics of recycled concrete interface structures, a multi-interface reconstruction model was established. To study the microstructure evolution of the interfacial transition zone (ITZ) during the carbonization process of recycled concrete, the microstructure characteristics of the ITZ of C30, C40, and C50 grade recycled concrete and the mortar matrix before and after carbonization were studied through the microhardness tester and SEM./e results show that the microhardness values of the ITZ and the mortar matrix are obviously increased and that the width of the ITZ decreases, while the ITZ performance of the C50 grade recycled concrete is not significantly changed. /e ITZ exhibits a large amount of granular CaCO3 after carbonization, the pores are refined, and microcracks are generated. Overall, there are significant differences in the microstructures between the ITZ and the mortar matrix before and after carbonization.


Introduction
With the acceleration of industrialization and urbanization, accompanied by increasing construction waste, China's annual output of construction waste has reached 3.5 billion ton, including 1.5 billion ton in the past year, which causes a significant threat to the environment in China. e resulting economic and social problems are becoming increasingly prominent [1,2]. Recycled concrete recycles not only conserve many natural aggregate resources but also reduce construction waste pollution in the environment, in line with the requirements for the sustainable development of the construction industry. At present, scholars in various countries have conducted extensive research on the performance of recycled concrete and have found that its workability, mechanical properties, and durability are lower than those of ordinary concrete [3,4]. Durability is the basis of engineering applications, and concrete carbonation is not only an important factor affecting concrete durability but also the main factor in the corrosion of steel [5,6]. e carbonation reaction reduces the pH values of concrete and destroys the passivation film on the surface of steel bars, resulting in the corrosion of steel in a concrete structure.
Domestic scholars have performed systematic researches on the macroscopic properties of recycled concrete after carbonization but have conducted little researches from a microscopic viewpoint [7,8]. Because recycled concrete is a type of multiphase, multi-interface, and heterogeneous complex alkaline material, the microstructure is more complicated than that of ordinary concrete, and the main reason for the low performance of recycled concrete is the existence of multiple weak interfacial structures [9][10][11][12]. erefore, it is necessary to systematically study the microstructure of recycled concrete and find out the corresponding methods for improvement.
In this paper, the Vickers hardness values of the interfacial transition zone (ITZ) of recycled concrete are measured by a microhardness tester, and variations in the ITZ width and microhardness of old aggregate-old paste (LG-LJ), old slurry-new slurry (LJ-XJ), and old aggregatenew slurry (LG-XJ) interfaces are analysed, respectively. e microstructure evolution of the ITZ of the recycled concrete before and after carbonation was analysed by SEM and BES, and the mechanism of the carbonation reaction of recycled concrete was revealed.

Raw Materials and Preparation of Test Samples. Ordinary
Portland cement (P.O. 42.5) was used in this project. Table 1 lists the basic properties of cement, which were measured according to the Chinese standard [13]. Coarse aggregates were in 5 to 25 mm continuous graded limestone gravel, and the technical indicators are shown in Table 2. Fine aggregates were natural sand, and the fineness modulus is 2.5. A polycarboxylic high-performance water-reducing agent was used with a water reduction rate of 30% at an amount of 1.2%. e water used is tap water.
According to the reconstructed model of the recycled concrete multi-interface structure, core-like concrete samples (C30, C40, and C50 grades) with different strength grades were prepared, respectively. e mixing ratio of the mortar is determined by reducing the watercement ratio by 0.02 after removing the coarse aggregate. Test concrete samples were produced with the mix ratios shown in Table 3.
Waste concrete was drilled to obtain Φ75 mm × 100 mm cylindrical core samples. e surface of the core-like concrete vertical to the size of the 100 mm cube test centre was saturated, and then different grades of cement mortar were poured ( Figure 1). After the standard block was cured for 26 days, it was cut into 20 mm slices ( Figure 2). Sections were divided into two groups: one group was utilized for the un-carbonated specimen and was immersed in anhydrous ethanol to terminate the hydration, and the other group underwent a rapid carbonation test based on the "Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete" (GB/T 50082-2009) [14].

Vickers Hardness and Data Processing.
e surface to be tested is prepolished as per the microhardness test requirements. As the LG-XJ and LJ-XJ interfaces exist between the interface between the core-like concrete and the new slurry, the contact surface is correspondingly similar to a straight line under a 100-fold microscope; here, in order to make it easy to identify, the beat method as the matrix point is employed. Nine indentation regions are selected, where each region is a 4 × 5 lattice, as shown in e LG-LJ interface only exists within the core-like concrete, and the appearance of aggregates causes the LG-LJ interface to appear as an irregular curve, vertical to the RBI direction. e height difference between the two adjacent sides is 10 μm, as shown in Figure 3(c).
Due to the influence of surface roughness, the fine aggregate distribution, and the unhydrated cement clinker     New mortar

LJ-XJ interface
Old mortar LG-XJ interface LG-LJ interface Old aggregate Figure 2: Section of a waste concrete specimen with a core sample.
Advances in Materials Science and Engineering

Scanning Electron Microscopy.
One noncarbonized sample and one sample that was carbonized for 28 days were cut into cubes of 10 mm × 10 mm × 10 mm, dried to a constant weight, and subjected to a top gold-plating treatment. e microstructure of the ITZ was observed using a scanning electron microscope (SEM, EVO18 type, Germany). Figures 6 and 7 show SEM images of the ITZ before and after carbonization. Based on a comparison between Figures 6(a) and 6(b), it is clear that the LG-LJ and LG-XJ interfaces are similar before carbonization. After standard curing for 28 days, the LG-XJ interface exhibits microcracks and high porosity. e interface structure is loose and filled with a large amount of CH product before carbonization. e reaction of CO 2 with CH in the weak transition zone produces CaCO 3 , which fills some of the pores, improving the ITZ microstructure and reducing the porosity after carbonization, as depicted in Figure 7. It is noting that the H 2 O produced during the reaction leaves a small hole after evaporation and causes the cement stone to shrink, resulting in a slight crack in the ITZ. As the hydration duration of the LG-LJ interface is long and the hydration products are rich, the interface transition area becomes denser, with less CH. Figures 7(a) and 7(b) show that carbonation effect of the latter is weaker than that of the former. As the elastic modulus and thermal expansion coefficient of the old and new mortar are similar, the micropores on the surface of the old mortar absorb the cement particles of the new mortar; thus, the new and old mortar are firmly engaged, and the ITZ is denser, which lessens the carbonization effect [15,16].

Effect of Carbonation on Microhardness of Aggregate-Slurry Interfaces in Recycled Aggregate
Concrete. In Figure 8, it can be seen that the microhardness of the LG-XJ interface increases and that the width of the ITZ decreases after 28 days carbonization; thus, the interface of low-strength-grade recycled concrete is affected by carbonization. Taking the C40 LG-XJ interface for instance, before carbonization, the ITZ microhardness is 125-171 MPa, and the ITZ width is 70-80 μm. After carbonization, the ITZ microhardness is 144-182 MPa, and the ITZ width is reduced by approximately 20 μm. Due to the sidewall effect in the LG-XJ ITZ, the porosity near the surface of the old aggregate is higher than that in the mortar matrix, which facilitates the migration of Ca 2+ , OH − , Al 3+ , and SO 4 2− ions during the cement hydration process. is phenomenon leads to the enrichment of CH in the ITZ [17], and the impact of carbonation is more obvious. Figure 8(d) shows that the difference in microhardness between the LG-LJ ITZ and the mortar matrix is relatively small, and the microhardness and ITZ width of the transition zone before and after carbonization are lower than those of the LG-XJ interface. As the C40 grade recycled concrete, the hydration duration of the LG-LJ interface is long, the cement hydration degree is high, and the zone is compact [18]; thus, the effect of carbonation on its hardness value is relatively small. e coarse aggregate itself will not be affected by the carbonation, and the microhardness value is stable.

Effect of Carbonation on the Microhardness of the LJ-XJ
Interface in Recycled Concrete. Figure 9 shows that the microhardness value of the new slurry and the old slurry of the low-strength-grade recycled concrete is obviously improved. With increasing strength grade in the recycled concrete, the microhardness of the LJ-XJ interface increases, and the ITZ width decreases slightly. Taking the LJ-XJ interface of C40 recycled concrete as an example, the microhardness values of the LJ-XJ ITZ are 170-180 MPa, and the ITZ width is approximately 40-50 μm before carbonization. After carbonization, the hardness of ITZ is 171-182 MPa, and the ITZ width is reduced by approximately 10 μm.
Compared with the LG-XJ interface, the effect of carbonation on the ITZ and mortar matrix of LJ-XJ is not obvious. As shown in the SEM image, the LJ-XJ interface is more compact, giving the ITZ a lower porosity. e moisture absorbed by the surface of the old mortar not only ensures the hydration of the cement particles around the interface but also produces a large water-filling space [19,20]. CH cannot be fully grown in this space [21], and the microstructure of LJ-XJ ITZ can be improved, which is also an important benefit of carbonization.

Conclusions
(1) e carbonation reaction produces a large amount of granular CaCO 3 , which improves the microstructure of the ITZ and reduces the porosity. Notwithstanding, the generated water vaporization can cause cracks due to cement stone shrinkage, resulting in improved pore connectivity in some sense.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.