Carbonation durability-driven optimization of low-carbon mortar using an OPC-slag-wollastonite ternary binder system

초록

Low-carbon mortar design has traditionally focused on reducing initial carbon emissions while satisfying mechanical performance requirements, whereas carbonation resistance has received less attention. In this study, a carbonation-durability-driven optimization framework for an OPC-slag-wollastonite ternary mortar system was developed. Seven representative mixtures were evaluated, and response-surface models were used to optimize initial carbon emissions and carbonation depth subject to strength and workability requirements. Carbonation depth was significantly negatively correlated with calcium hydroxide (CH) content (r =-0.8279), suggesting that mixtures with higher CH contents tended to exhibit lower carbonation depths under the present accelerated test conditions. However, carbonation behavior should be interpreted as the combined outcome of alkalinity reserve, hydration-product evolution, pore-structure characteristics, and COQ transport resistance. When initial carbon emissions were minimized as the sole objective, the optimal mixtures favored reduced OPC contents and increased slag substitution. In contrast, when the carbonation depth and initial carbon emissions were simultaneously minimized, the resulting mixtures contained lower slag contents and higher OPC and wollastonite contents, thereby maintaining a higher alkalinity reserve and lower carbonation depths under the investigated conditions. These findings demonstrate a trade-off between clinker substitution and accelerated carbonation sensitivity. Although consideration of carbonation durability moderately increased initial carbon emissions, it resulted in optimized mixtures with lower carbonation depths under exposure conditions of 5% COQ, 60% relative humidity, and 20 degrees C for four weeks. The proposed framework provides an efficient empirical approach for low-carbon mortar design within the investigated ternary mixture domain. It was established and experimentally assessed at the mortar/paste scale; however, the safety-adjusted mixtures require further experimental verification, and concrete-scale validation under broader carbonation exposure conditions is needed before service-life-oriented application.

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