In-situ formation of layered double hydroxides (LDHs) in sodium aluminate activated slag: The role of Al-O tetrahedra

Alkali activated materials (AAMs) have gained great attention as a new low-carbon binder. However, their durability performance, e.g. chloride ingress resistance, still needs further improvement. This study attempts to enhance the chloride binding of AAMs by activating slag with sodium aluminate with the aim to promote the in-situ formation of layered double hydroxides (LDHs). The evolution of pH and ions in the pore solution, reaction products and microstructure were determined to investigate the dynamic activation process. Results show that the sodium aluminate stabilizes the pH environment at around 12.7 during the curing ages. The Mg-Al-LDH with higher Al-O tetrahedra (denoted as Al(OH)4−) contents is promoted, enhancing the chloride absorption capacity. A new reaction mechanism is proposed to describe the activation process. This study reveals that the extra Al(OH)4−in a relatively low pH environment prevents the competition between Mg2+ and Si-O tetrahedra to react with Al(OH)4−, promoting the formation of LDH and C(N)-A-S-H simultaneously.