Setting time, compressive strength and sulfuric acid resistance of a high calcium fly ash geopolymer containing borax

Abstract

The objective of this research was to study the influence of borax on setting time and other properties viz., workability, compressive strength and sulfuric acid resistance of a high calcium fly ash (HCFA) geopolymer. Four borax types viz., deca-hydrate borax, deca-hydrate borax heat-treated for 5-minutes and 10-minutes and anhydrous borax were used to replace fly ash. A liquid to binder ratio (L/B) of 0.60, a 10M NaOH solution, and Na2SiO3/NaOH at a ratio of 1.0 were used in the formulation of the geopolymer. The results indicated that the replacement of fly ash with borax accelerated the initial setting time but retarded the final setting time of the geopolymer paste. The flow of mortar with deca-hydrate borax remained unaffected by the increase in the amount of borax. For other borax types, the mortar flow was reduced and this was related to the water molecules in the borax. The compressive strength of the mortar tended to decrease with increasing borax content. The resistance of the mortar to 3% sulfuric acid was reduced with increased borax content. The use of borax should be around 2.5% to control the setting of a high calcium fly ash geopolymer and to obtain good strength and resistance to acid.

Keywords

• Geopolymer
• High calcium fly ash
• Borax
• Workability
• Compressive strength
• Sulfuric acid resistance