## 4 Conclusions  
Hydrocalumite-like Ca\({}_{2}\)Al-NO\({}_{3}\)- LDH was synthesized based on a co-precipitation technique combined with an aging steps method. XRD, FT-IR, DLS, and SEM were used to characterize the crystalline properties, structure composition, particle size distribution, and morphology of CAL samples. The influences of contact time, initial concentration of Cl-, pH of solution, and coexistence anions on the chloride uptake of CAL were also investigated. Based on the experimental results, some conclusions can be drawn as follows:  
1. CAL samples synthesized at 60-120 \({}^{\circ}\)C performed a typical double hydroxide structure and the crystallinity of CAL increased slightly with increasing the aging temperature, which can be observed from XRD patterns. Enhancement of aging temperature also results in a sharper edge and larger particle size of CAL. The average particle size of CAL samples synthesized at 60 \({}^{\circ}\)C, 70 \({}^{\circ}\)C, 80 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, and 120 \({}^{\circ}\)C was 6.148 \(\upmu\)m, 11.580 \(\upmu\)m, 11.829 \(\upmu\)m, 14.278 \(\upmu\)m and 22.932 \(\upmu\)m, respectively.
2. The kinetic process of chloride ion adsorption by CAL was more in line with the description of a Pseudo-second-order kinetic model and the Langmuir isotherm could fit the adsorption results of CAL samples better than the Freundlich isotherm. Theoretical maximum adsorption capacity of Cl- on CAL prepared at 60-120 \({}^{\circ}\)C was 211.324 mg/g, 182.117 mg/g, 174.903 mg/g, 160.914 mg/g and 155.960 mg/g, respectively. Chloride uptake of CAL could be improved by increasing aging temperature due to particle size reduction.
3. The main adsorption mechanism of chloride ion on CAL was an interlayer anion exchange reaction other than the dissolution-precipitate mode. With increasing the pH of solution (7-13), chloride uptake of CAL increased slightly and a high pH level (11-12) was kept after adsorption. The presence of CO32- and SO42- with high concentration reduced the adsorption capacity of CAL significantly as compared with OH- due to the destruction of layered structure and precipitates formed (CaCO3 or CaSO4). Interference sequence of three anions on the chloride uptake of CAL was SO42- \(>\) CO32- \(>\) OH-, and the order of interlayer anionic affinity was Cl- \(>\) OH- \(>\) NO3- (4) Although the results showed that the presence of CO32- and SO42- could affect the application of LDH in concrete, in actual conditions the concentration of CO32- and SO42- in concrete is low and the carbonation and sulfate attacks happen from the exterior surface of the concrete structure. So, it is believed incorporation of LDHs into concrete would still maintain its adsorption effect on Cl-.  
Figure 9: XRD patterns of solid phases after reaction by CAL-60 sample and 80 mM NaCl solution with different interference anions ((**a**) OH-; (**b**) SO42-; (**c**) CO32-; (**d**) OH- and SO42– and CO32-).  
**Author Contributions:** Methodology, H.C. and S.Z.; experimental design and testing, F.Y., W.H. and D.Z.; data curation, W.H. and D.Z.; writing--original draft preparation, S.Z. and F.Y.; language modification, W.T.; writing--review and editing, H.C., L.L. and N.H.; funding acquisition, H.C. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The work described in this paper was fully supported by grants from Natural Science Foundation of China, grant number 51678366.  
**Conflicts of Interest:** The authors declare no conflict of interest.