²⁹ Si and ²⁷ Al MAS‐NMR of NaOH‐Activated Blast‐Furnace Slag

Abstract

b 29 Si and 27 Al MAS‐NMR were performed on NaOH‐activated blast‐furnace slag to better characterize the amorphous and poorly crystalline phases which occur in this system. The unreacted glass has a mainly dimeric silicate structure represented by a broad 29 Si peak (FWHM = 15 ppm) centered at –74.5 ppm [ Q 1 ], with aluminum present exclusively in tetrahedral coordination. Upon reaction with 5M NaOH ( w/s = 0.4), three new 29 Si peaks with widths of ca. 2 ppm are formed at ‐78.5 Q 1 , –81.4 [ Q 2 (1Al)J, and ‐84.3 [ Q 2 ]. Relative peak areas indicate a mostly dimeric silicate structure for the tobermorite‐like C─S─H layers, with roughly a third of the bridging sites occupied by aluminum, and less than 10% by silicon. In addition to the tetrahedrally coordinated aluminum substituted in the C─S─H structure, 27 Al MAS‐NMR reveals the presence of aluminum in octahedral sites, which is attributed to the aluminate phase (C,M) 4 AH 13 . 29 Si results indicate rapid initial consumption of the glass, with roughly a third of the glass reacting within the first day and another third consumed over the following 27 days.