Abstract
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.
Highlights
Alkali activated materials (AAMs) have shown to be a promising alternative to Portland cement
This study investigates the mechanism of how the Al(OH)4− influences in-situ synthetic layered double hydroxides (LDHs) and gels formation in sodium aluminate activated slag (SAAS), and the consequent chloride absorption perfor mance
According to Paudel et al [34], the pH value of pore solution in the AAMs decreases with the curing ages, which is attributed to the process of reaction that consumed much of OH− in the pore solution
Summary
Alkali activated materials (AAMs) have shown to be a promising alternative to Portland cement. The mechanism of how the sodium aluminate affects the formation of cal cium (sodium) aluminosilicate hydrates (C(N)-A-S-H) phases still re mains poorly understood To discuss it further, the other physical aspects of chloride resistance, e.g. the lower porosity and more complex tortuosity of the microstruc ture in the AAMs matrix, in general, contribute to decreasing the chlo ride penetration. Whereas the mechanism of how the Al(OH)4− in sodium aluminate activator promotes the formation of LDHs and gels remains unknown in terms of alkali activated slag, and the insuffi cient understanding about the ions changes of pore solution explaining. This study investigates the mechanism of how the Al(OH)4− influences in-situ synthetic LDH and gels formation in sodium aluminate activated slag (SAAS), and the consequent chloride absorption perfor mance. A reaction mechanism was proposed to give an insight into the promotion of in-situ formed LDH and gels by sodium aluminate activator
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