Internal carbonation curing for autogenous shrinkage mitigation

Amino acid salts (AASs) reversibly react with CO₂ to form carbamate, making them promising for CO₂ capture. Herein, we propose a concept of internal carbonation curing by absorbing carbamate solution with superabsorbent polymer (SAP) and incorporating the resulting hydrogel into cement to mitigate autogenous shrinkage and maintain mechanical performance. We discovered that the incorporation of only 0.1% SAP‑serine‑based carbamate hydrogel (S-SAP) almost completely eliminates autogenous shrinkage at 7 days and simultaneously preserves 28‑day compressive strength, thereby overcoming the traditional dilemma between autogenous shrinkage and strength. This performance is attributed to the combined effect of internal curing and internal carbonation, in which the released carbamate decomposes to form CaCO₃ that fills voids and promotes hydration. In contrast, 0.1% SAP‑arginine‑based carbamate hydrogel (A-SAP) exhibited stronger retardation due to arginine adsorption and Ca²⁺ complexation, leading to shrinkage reduction at the cost of strength. This distinct performance originates from their molecular structures, where serine contains a neutral hydroxymethyl group that facilitates moisture transport and nucleation, while arginine features a positively charged guanidinium group that adsorbs onto cement surfaces and coordinates Ca²⁺, thereby delaying hydration. These findings suggest that the efficiency of internal carbonation curing can be enhanced through the selection of different amino acid molecules and the regulatory effects of their characteristic side chains.