Impact of Ca/Si and Al/Si ratios on the composition and structure of the alumina-silica gel formed by wet carbonation of synthesized C-S-H in blends with portlandite and ettringite

It is well-established that aqueous carbonation of recycled concrete paste produces calcium carbonate and an amorphous alumina-silica gel as the main products. However, sparse information is available about the structure and composition of the gel. This work focuses on these properties by carbonation of synthesized C-S-H (Ca/Si = 1.75) alone and blended with portlandite and ettringite. Wet carbonation of pure ettringite results in an alumina gel with the approximate composition, Al₂O₃·7.4H₂O, and including two distinct octahedral Al sites. For carbonated blends of C-S-H with ettringite, this phase is observed along with the alumina-silica gel. Increasing ettringite contents lead to significant changes in the structure of the alumina-silica gel and to increasing Al/Si ratios approaching the maximum of Al/Si = 1.0. For the blended samples, portlandite and ettringite carbonate independently and faster than the C-S-H phase, and both additional Ca and Al slow down the carbonation rate for the C-S-H phase.