Nature of aluminates in C-A-S-H: A cryogenic stability insight, an extension of DNA-code rule, and a general structural-chemical formula

Herein, we report a cryogenic attack study on C-A-S-H with bulk Ca/(Al + Si) = 0.84–1.75, Al/(Al + Si) = 0.1, 0.2. The C-A-S-H nanostructure variations are quantitively analyzed using thermogravimetric analysis, single-pulse 29Si, and 27Al MAS NMR. Observing the conversion of tetrahedral aluminates to octahedral ones in the absence of depolymerization, we confirm the presence of bridging octahedral aluminates in C-A-S-H chains. The consistency between the slight polymerization and the decrease of pentahedral aluminate fraction in low-Al C-A-S-H reveals the existence of interlayer pentahedral aluminate. The depolymerization in high-Al C-A-S-H is accompanied by conversion of pentahedral aluminate to octahedral aluminate, which confirms the presence of bridging pentahedral and interlayer octahedral aluminates. We further identify the complexes for each structural unit. Then, a DNA-code rule is proposed to describe C-A-S-H atomistic structure considering both bridging and interlayer aluminates. Finally, we propose a general structural-chemical formula for C-A-S-H.