New insights into the reaction mechanisms of calcium-based activators modified sodium carbonated activated slag: Precursor dissolution and carboaluminate generation

This study investigated the regulatory mechanism of calcium-based activators on the reaction kinetics of sodium carbonate-activated slag (SAS), with the aim of revisiting the conventional interpretation based on liquid-phase alkalinity enhancement. By incorporating carbide slag (CS) into SAS, this study systematically clarified the regulatory mechanism of carboaluminate phase formation on the reaction kinetics, phase evolution, pore solution chemistry and microstructure of calcium-based activators-modified SAS (CSAS). The results demonstrated that the introduction of CS remarkably shortened the setting times of CSAS pastes. The incorporation of CS into the CSAS system promoted a cation exchange reaction with Na2CO3 in the solution, which elevated the pH and consequently enhanced slag dissolution. In CSAS containing a low CS dosage, the initial alkalinity was insufficient to initiate rapid slag depolymerization, resulting in a 3-day compressive strength and reaction degree of only 4.9 MPa and 6.2 %, respectively. As the reaction progressed, however, [Al(OH)6]3- species gradually released from the slag reacted with CO32- and Ca2+ to form carboaluminate phases. This phase formation significantly accelerated the further dissolution of aluminosilicate species and promoted the generation of C-A-S-H gel. Despite the negligible early heat evolution, the system achieved a 28-day compressive strength of 43.2 MPa and a reaction degree of 31.1 %. When the CS dosage was increased to 4 %, the precipitation of carboaluminate phases occurred earlier, resulting in improved kinetics of slag dissolution and gel formation. Consequently, the CSAS exhibited the highest pore tortuosity (8.88) and gel pore fraction (46.8 %), along with a 28-day compressive strength of 55.8 MPa, comparable to that of NaOH-activated slag. Notably, the CS dosage in CSAS systems could be flexibly adjusted based on reaction kinetics characteristics to meet different engineering demands. For large-volume concrete requiring controlled heat evolution, a lower CS content was recommended to mitigate thermal risks. In contrast, for rapid-setting applications such as emergency repairs, an increased CS dosage could accelerate reaction rates and early strength development. This study established a novel theoretical framework for understanding the kinetic mechanisms governing CSAS systems and provided a promising approach for designing alkali-activated materials with tunable reactivity.