Fresh and hardened properties of alkali-activated fly ash/slag pastes with superplasticizers

This paper presents results of an experimental study carried out to investigate fresh and hardened properties of alkali-activated fly ash/slag pastes with superplasticizers. The binders were prepared by dry mixing of slag and fly ash with 5 slag-to-binder ratios, 0, 0.3, 0.5, 0.7, and 1. An alkali activator was prepared from NaOH solution and sodium silicate solution. The polycarboxylate-based and naphthalene-based superplasticizers were added to the mixes at the levels of 0%, 1%, 2%, 3%, and 4% by the mass of binder. All specimens were prepared under room-temperature curing conditions. The results showed that the higher contents of slag increased the compressive strength of alkali-activated fly ash/slag pastes, but caused rapid setting and crack due to autogeneous shrinkage in the cases of 70% and 100% of slag to binder ratios. The polycarboxylate-based superplasticizer showed retarding effect on alkali-activated fly ash/slag pastes with negligible effect on the heat of hydration, and improved the workability more significantly than naphthalene-based superplasticizer. From the results of the SEM/EDS tests, it was observed that a higher content of slag led to the formation of denser matrix of hydration products. However, formation of hydration products was not noticeably affected by the addition of superplasticizers.