Compressive strength development and shrinkage of alkali-activated fly ash-slag blends associated with efflorescence

This study reports the changes in the compressive strength and shrinkage property of alkali-activated fly ash and slag (AAFS) blends associated with efflorescence. The AAFS specimens cured in air for different periods were further aged under the following three conditions: bottom-contact with water, fully immersed in water and in air. The second condition, i.e., bottom in contact with water, was used to accelerate efflorescence of AAFS. The efflorescence of the AAFS binders was dependent on the reaction extent of the solid precursors. A higher reaction extent present in the binder by extending initial-curing age can reduce efflorescence rate. The incorporation of low percentage of slag (lower than 50 wt%) in the AAFS binders has little influence on the efflorescence potential, while with the increased contents of slag higher than 50 %, the binders exhibit more rapid efflorescence than the 100 % fly ash binders. The efflorescence led to the strength loss because of the formation of carbonate crystals in the binder and the damage of the microstructure. The linear shrinkage of the binders was also related to efflorescence and alkali leaching. The slag addition can reduce the negative effects of efflorescence and alkali leaching on the absolute compressive strength and shrinkage of the binders.