Mechanisms Governing Rapid Stiffening and Subsequent Structuring in Sodium Carbonate‐Activated Glass Cullet Powder

ABSTRACT This study investigates the mechanisms governing rapid stiffening and subsequent structuring in sodium carbonate (Na 2 CO 3 )‐activated glass cullet powder. Pastes containing 5, 10, 15, or 25 wt% Na 2 CO 3 were monitored from the first minutes after mixing to 28 days by ultrasonic wave propagation, in situ and ex situ x‐ray diffraction, isothermal calorimetry, thermogravimetric analysis, and compressive strength testing. High Na 2 CO 3 contents caused stiffening within minutes, together with a sharp increase in ultrasonic velocity, intense early heat release, and precipitation of hydrated Na 2 CO 3 phases. These results show that initial stiffening is primarily associated with carbonate dissolution, hydration, and precipitation, rather than with glass dissolution. At 10 and 15 wt% Na 2 CO 3 , a transient decrease in ultrasonic velocity followed the initial structuring stage and occurred over the same period as the progressive disappearance of thermonatrite, consistent with a temporary microstructural rearrangement. By contrast, the 5 and 25 wt% systems showed more continuous structural development. At later ages, strength gain was associated with progressive glass reaction and formation of gaylussite and poorly crystalline or amorphous binding phases. The results distinguish two coupled mechanisms: rapid carbonate‐phase evolution associated with early stiffening, and delayed glass reaction associated with subsequent hardening. This mechanistic separation clarifies the phase–structure–property relationships in Na 2 CO 3 ‐activated recycled glass binders.