## 1 Introduction  
Coarse aggregates account for approximately 30% to 40% of the total mass of concrete [1]. Previous studies reported that construction wastes can be applied as recycled concrete aggregates (RCA) to replace natural aggregate in concrete [2; 3; 4]. However, due to the fact that concrete casted with RCA will increase its porosity, the resistance of concrete to chloride ions is reduced and their wide application in construction is restricted [5]. In the past, various technologies and methods were developed in an attempt to address the issues of RCA. Pre-treatment of RCA such as mechanical polishing [6] and chemical surface modification [7] have been found to be effective to remove the cement paste attached to the natural aggregate and improve the quality of recycled aggregate concrete. However, the implementation of these methods may also come with new problems such as high CO\({}_{2}\) emission, and environmental and safety concerns. Applying nanomaterials as filler to improve the compactness of concrete is another way to enhance the chloride resistance of recycled aggregate concrete. Nanoparticles such as nanoSiO\({}_{2}\)[8; 9], nano-modified fly ash [10], nano-Al\({}_{2}\)O\({}_{3}\)[11], graphene oxide [12] are effective in strengthening the mechanical properties and chloride binding capacity of blended cement pastes. A previous study shows that the porosity reduced by 11.8% when 2% nano-SiO\({}_{2}\) was applied and the chloride resistance also showed an obvious improvement [13]. Layered double hydroxides (LDHs) are a novel layered dimensional nanomaterial, which has a sandwich-like multilayer structure. One of the significant advantages of LDHs is that the nanoparticles can reduce the porosity while improving the chloride binding capacity of recycled aggregate concrete [14; 15]. Based on the brucite-like structure of LDH ([M\({}_{1-x}\)\({}^{2+}\)M\({}_{x}\)\({}^{3+}\)(OH)\({}_{2}\)]\({}^{+}\)), some trivalent metal cations (M\({}^{3+}\)) can be isomorphous substituted by divalent metal cations (M\({}^{2+}\)), which will form positively charged LDH layers compensated by some interlayer anions (A\({}^{n-}\)). These interlayer anions bind to the main layer structure through weak hydrogen bonds or electrostatic interaction. Hence, some guest anions in the surrounding environment will easily exchange the initial interlayer anion and thus intercalate into the layered structure. This anion exchangeability enables LDHs to be used as a vital adsorbent in chemical engineering. Hydrocalumite (Ca\({}_{2}\)Al(OH)\({}_{6}\)Cl\({}_{2}\)H\({}_{2}\)O, CAL) is one of typical natural LDHs, which has been applied to improve the durability of cementitious materials due to its excellent anion exchange or adsorption property. [PERSON] et al. reported that the CAL precursor prepared by the optimized synthesis method could effectively immobilize chloride from sodium chloride solution and it is a potential adsorbent to protect the reinforced concrete from chlorine corrosion [16]. [PERSON] et al. indicated that the free chloride ion in the pore solution could be effectively immobilized by CAL through an anion exchange mechanism [17]. [PERSON] et al. found that the compressive strength of cement mortar samples will not be significantly affected when the amount of CAL replacement for cement does not exceed 10% [18]. More recently, nano CAL with different particle sizes were prepared by varying pH value [19]. It was found that the particle size of CAL plays significant roles in the microstructure and permeability improvement of cement paste. CAL with a smaller particle size has better chloride binding and physical barrier effect than the larger ones. So, the mortars containing smaller CAL can reduce the chloride migration coefficient better. In this study, nano CALs with different microstructures were prepared by accurately controlling the aging temperature during the preparation.