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

• Higher contents of slag increased the compressive strength, but caused rapid setting.
• Polycarboxylate-based superplasticizer retarded the setting with negligible effect on the heat of hydration.
• The workability was significantly improved with the addition of the polycarboxylate-based superplasticizer.
• Formation of hydration products was not noticeably affected by the addition of 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.