The effect of calcined clay reactivity on the mechanical properties and chloride diffusion resistance of alkali-activated calcined clay-GGBFS concrete

The increasing demand for sustainable and durable construction materials, particularly in aggressive environments such as those exposed to chloride ingress, necessitates the exploration of alternative binders that offer enhanced performance and lower environmental impact compared to traditional Portland cement. This study investigates the chloride resistance of calcined clay/ground granulated blast furnace slag (GGBFS) geopolymer concretes, focusing on variations in precursor proportions, alkali activator concentrations, and the amorphous content of two calcined clays. Durability tests, including standard bulk chloride diffusion, modified-rapid chloride permeability test (RCPT-10V), and NT Build 492, were conducted to evaluate the concrete performance. The compressive strength achieved ranged from 31.5 to 63.5 MPa. Notably, high-grade calcined clay with a 10 % Na₂O achieved the highest compressive strength of 63.5 MPa. Electrical resistivity tests indicated that free ions in the pore solution, especially unbound alkalis, contributed to increase the conductivity. A correlation between RCPT results and chloride diffusion coefficients categorized most concretes as low performers in chloride environments, with the exception of the 50HG50S-10 sample, which was categorized as medium performance. Notably, increasing the Na 2 O concentration improved the chloride resistance of low-grade calcined clay, and this was further enhanced with high-grade calcined clay. Finally, exposure to NaCl resulted in no new phases except for halite in low-grade calcined clay. No Friedel's salt was detected in any sample. • Compressive strength of calcined clay-GGBFS concretes ranges from 31.5 to 63.5 MPa, suitable for structural use. • Higher Na₂O levels reduced water absorption and voids, enhancing microstructure density in high-grade calcined clay mixes. • The 50HC50S-10 mix had the lowest chloride diffusion coefficient, suitable for marine environments. • Low-grade calcined clay with 10 % Na₂O improved chloride resistance performance significantly. • RCPT results correlate well with chloride resistance, with the 50HG50S-10 mix matching OPC concretes' durability.