Effect of MgO, Mg-Al-NO3 LDH and calcined LDH-CO3 on chloride resistance of alkali activated fly ash and slag blends

The mechanism of layered double hydroxides (LDHs) formation with the addition of magnesium oxide (MgO), the LDHs reconstruction of calcined layered double hydroxides (CLDHs) and the effect of externally added synthetic LDHs in alkali-activated materials (AAMs) system are the important factors influencing the chloride resistance of AAMs. The objective of this research is to investigate the different effects of MgO, nitrate intercalated LDH (Mg-Al-NO ₃ LDH) and calcined natural LDH-CO ₃ on the alkali activated fly ash and slag blends (AAFS) in terms of chloride resistance. Mg-Al-NO ₃LDH was successfully synthesized with the co-precipitation method. CLDH was also obtained by thermal treating natural Mg-Al-CO ₃ LDH. The reaction heat flow and reaction products of the AAFS were tested by isothermal calorimetry and X-ray diffraction (XRD), respectively. The microstructure of the AAFS was characterized by the nitrogen sorption tests (BET) and mercury intrusion porosimetry (MIP). The compressive strength of AAFS was also tested. The non-steady-state migration (NSSM) was applied to determine the chloride resistance of AAFS. The Ordinary Portland Cement (OPC) samples were also studied for comparative purpose. The results of the calorimeter test showed that the MgO, Mg-Al-NO ₃ LDH, and CLDH delayed the time to reach the reaction peak (TRRP) at 3.9%, 11.2%, and 9.2%, respectively. The MIP results illustrated that mesopores and micropores of AAFS were refined by the three admixtures. The NSSM results indicated that CLDH can significantly improve the chloride resistance of AAFS binder comparing to the samples containing MgO and Mg-Al-NO ₃ LDH. The different influences of MgO, Mg-Al-NO ₃ LDH and CLDH were compared in this study.