High early pozzolanic reactivity of alumina-silica gel: A study of the hydration of composite cements with carbonated recycled concrete paste

Carbonated recycled concrete paste (cRCP) is a unique supplementary cementitious material with pozzolanic properties stemming from the alumina-silica gel. This study reveals that the high early strength of composite cements with cRCP is attributed to its rapid pozzolanic reactivity. ²⁷Al and ²⁹Si NMR investigations show that the alumina-silica gel is consumed after only 7 days of hydration. The rapid gel reaction results in the formation of a C-(A)-S-H phase with a low Ca/Si ratio. After 7 days, belite and remains of alite react further and increases the Ca/Si ratio of the C-(A)-S-H phase to a level typical for blended cements. As a result, a lower porosity forms already at early age, resulting in high strength. The reactivity of cRCP is influenced by its origin and carbonation conditions. Carbonation in sulfate-rich solutions enhances the gel surface area and accelerates the reaction. Slag remnants in cRCP react aiding to microstructure and strength development.