One-part slag-mullite geopolymer: role of solid activators on connected porosity and mechanical properties

Geopolymer cements with reliable mechanical and durability properties could be a promising environmentally-friendly alternative to Portland cement. Connected porosity (CP) and compressive strength (CS) are critical parameters governing the durability of slag–mullite geopolymers. This study examines the relationship between connected porosity and mechanical performance in one-part slag–mullite geopolymer mortars activated by various solid alkaline salts, including sodium- and potassium-based activators. The results reveal a clear inverse relationship between connected porosity and compressive strength. Geopolymer mortars activated with sodium silicate and potassium silicate at silica moduli of 0.6 and 0.8 exhibited the highest compressive strength, along with the lowest connected porosity and water absorption. These improvements are attributed to the enhanced alkalinity and high ionic solubility of the silicate activators, which promote a denser geopolymeric network. In contrast, mortars activated with calcium hydroxide and sodium sulfate showed significantly higher connected porosity and lower strength, likely due to calcium sulfate precipitation and its detrimental effects on microstructural continuity. The incorporation of mullite further improved the material performance by promoting the formation of more densified aluminosilicate network and refining the planar silicate framework, resulting in reduced porosity. Overall, connected porosity is demonstrated to be a robust and quantifiable indicator of durability, reflecting the resistance of one-part slag–mullite geopolymers to fluid ingress and degradation under aggressive environmental conditions.