Composition design and characterization of alkali-activated Slag–Metakaolin materials

This study explores the effects of the interactions among Na₂O content, metakaolin content and activator modulus on the compressive strength and autogenous shrinkage of alkali-activated slag–metakaolin (AASM) materials. The Box–Behnken RSM design was used to create an experimental scheme, establish a model, and optimize the mix proportions. Fourier transform infrared spectroscopy, scanning electron microscopy, and Mercury intrusion experiments were used to analyze the compositions, microstructures, and pore structures of the hydration products of the AASM, respectively. Results showed that the interactions between the activator modulus and metakaolin content, as well as Na₂O content and metakaolin content, are the key factors affecting the compressive strength and autogenous shrinkage, respectively, of the AASM. Under the optimal conditions of Na₂O content of 6%, sodium silicate modulus of 1.5, and metakaolin/slag ratio of 1: 3, the relative errors in the model verification test for compressive strength and autogenous shrinkage are 0% and 0.18%, respectively. In the water glass modulus and metakaolin content interaction, Ca²⁺, A1³⁺, and Si⁴⁺ ions in the composite system react with several [SiO₄]⁴⁻ groups to form C-A-S-H and N-A-S-H gels, which fill each other to make the composite structure denser. When Na₂O interacts with metakaolin, the OH⁻ and Na⁺ in the solution react with A1³⁺ and Si⁴⁺ to generate additional N-A-S-H, thereby reducing the compressive strength of the composite system, mitigating autogenous shrinkage, and increasing volume stability.

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