Aqueous carbonation of aged blended Portland cement pastes: Impact of the Al/Si ratio on the structure of the alumina-silica gel

Carbonation of recycled cement paste receives growing attention since it can carbonate end-of-life concrete at ambient conditions and chemically bind CO2 on a geological timescale. This work investigates aqueous carbonation of blended cement pastes, cured for 12–14 years and incorporating silica fume, fly ash, slags, and natural pozzolan as SCMs, using solid-state NMR, XRD, TGA and in-situ pH measurements. The carbonation products are calcite and an amorphous alumina-silica gel for all blends, where the initial calcium content governs the amount of calcite formed and, consequently, the CO2-uptake capacity. Aluminum is only present in tetrahedral coordination in the alumina-silica gel for all blends, where a higher aluminum content results in a reduced polymerization of the aluminosilicate network. A structural model for the alumina-silica gel is proposed, based on deconvolutions of the 29Si NMR spectra, which accounts for the Al/Si ratio and the reduced polymerization of silicate tetrahedra by the incorporated AlO4 units for the gels produced by carbonation of different blended cements.