The role of alumina on performance of alkali-activated slag paste exposed to 50 °C

• Strength of alkali-activated slag (AAS) pastes after exposure to 50 °C is studied.
• AAS pastes with high alumina content lose strength after the exposure.
• C4AH13 and CAH10 form in these AAS pastes.
• Conversion of these calcium alumina hydrates is associated with the strength loss.
• AAS pastes with low alumina content maintain its strength after the exposure.

The strength and microstructural evolution of two alkali-activated slags, with distinct alumina content, exposed to 50 °C have been investigated. These two slags are ground-granulated blast furnace slag (containing 13% (wt.) alumina) and phosphorous slag (containing 3% (wt.) alumina). They were hydrated in the presence of a combination of sodium hydroxide and sodium silicate solution at different ratios. The microstructure of the resultant slag pastes was assessed by X-ray diffraction, differential thermogravimetric analysis, and scanning electron microscopy. The results obtained from these techniques reveal the presence of hexagonal hydrates: CAH10 and C4AH13 in all alkali-activated ground-granulated blast-furnace slag pastes (AAGBS). These hydrates are not observed in pastes formed by alkali-activated ground phosphorous slag (AAGPS). Upon exposure to 50 °C, the aforementioned hydration products of AAGBS pastes convert to C3AH6, leading to a rapid deterioration in the strength of the paste. In contrast, no strength loss was detected in AAGPS pastes following exposure to 50 °C.