Effect of Metakaolin Substitution for GGBS on the Hydration, Rheology and Chloride Penetration Resistance of Supersulfated Cement

To reduce the dependence of supersulfated cement (SSC) on high-quality ground granulated blast-furnace slag (GGBS), metakaolin (MK) was used to replace 0–40% of GGBS by mass. The effects of MK content on rheology, hydration, microstructure, mechanical properties, and chloride penetration resistance were investigated. Increasing MK replacement increased the dynamic yield stress but reduced the cumulative heat release at 72 h. Ettringite and C-(A)-S-H gel remained the main hydration products, whereas residual gypsum increased at higher MK contents. Among all mixtures, the sample containing 20% MK exhibited the highest chemically bound water content, a relatively dense pore structure, and the lowest electrical flux at 28 d. However, MK replacement of 30% or more coarsened the pore structure, reduced strength, and increased chloride penetration. Therefore, replacing 20% of GGBS with MK reduced slag consumption while maintaining a favorable balance among bound-water content, compressive strength, and electrical flux.