## 1 Introduction  
A huge amount of construction and demolition waste is being produced globally, which brings in many environmental problems and induces a heavy burden to the limited landfill capacity especially in populated areas such as Hong Kong. In the past decades, much research has been conducted to utilize waste concrete as recycled coarse aggregate (RCA) as partial or whole replacement of natural coarse aggregate (NCA) to produce recycled aggregate concrete (RAC). It can not only reduce the amount of waste concrete, but also decrease the consumption of the natural resources to produce NCA. Nevertheless, because of the inferior physical properties of RCA when compared to NCA, such as higher water absorption, higher porosity, lower density and the presence of initial cracks [1, 2, 3, 4], the mechanical properties and durability of RAC are lower than that of natural aggregate concrete (NAC) [5, 6, 7, 8, 9]. As a result, the use of RAC is currently still limited, and most of the applications are in non-structural uses [10]. To broaden the application of RAC, many RCA treatment techniques have been proposed to enhance the performance of RAC in the past few decades. In general, there are three categories of RCA treatment techniques in terms of their enhancement mechanisms. The first type is to remove the old mortar that is attached on RCA by mechanical grinding [11, 12], heat grinding [13, 14], ultrasonic cleaning in water [15], soaking in acid stress-strain curve solutions [16], etc. However, these techniques have obvious disadvantages such as high energy consumption, high CO\({}_{2}\) emission, large amount of waste fines produced and the increased chloride and sulfate contents [3]. The second type is to enhance the old mortar of RCA and the old interface transition zone (ITZ) with methods such as accelerated carbonation [17; 18; 19; 20], microbial carbonate precipitation [21], pre-soaking in sodium silicate solution [22] or polyvinyl alcohol [23; 24], etc. The third type is to enhance the new ITZ between RCA and new mortar and many surface treatment techniques have been proposed to realize this purpose such as coating RCA with cement slurry [25], pozzolan slurries [26], nano materials [27; 28]. Among these RCA treatment techniques, accelerated carbonation has attracted much research interest. According to the review study by [PERSON] et.al [29], four types of acceleration carbonation methods have been utilized to pretreat RCA, which includes standard carbonation [30], pressurized carbonation [17; 18; 19], flow-through CO2 curing (or flow-through carbonation) [31], and water-CO\({}_{2}\) cooperative curing (or wet carbonation) [32]. By using acceleration carbonation, CO\({}_{2}\) reacts with the hydration product of cement such as Ca(OH)\({}_{2}\) and C-S-H and un-hydrated cement clinkers to produce calcium carbonate and silica gel, which densifies the microstructures of RCA [29; 33]. As a result, the physical properties of RCA and the performance of RAC could be improved. However, different carbonation treatment methods vary a lot in terms of efficiency and operation easiness. It is necessary to compare the efficiencies of different carbonation treatment methods. The use of nano silica (NS) to pretreat RCA has also attracted many researchers' interests. Currently, the commonly used technique is by pro-soaking RCA in colloidal NS [27; 28; 34]. This technique not only improves the physical properties of RCA, but also enhances the new ITZ between RCA and new mortar. However, this technique may have low economic feasibility because it consumes a large amount of NS due to high water absorption of RCA. Recently, a new NS treatment method, namely pre-spraying NS on the surface of RCA, was proposed by the authors [35]. It consumes much less NS than using the NS pre-soaking method. Moreover, it causes better improvement in the performance of RAC than using the pre-soaking method.