Using silica fume for improvement of fly ash/slag based geopolymer activated with calcium carbide residue and gypsum

This study attempts to explore the feasibility of using calcium carbide residue (CCR) as a solid alkaline activator to prepare geopolymer binders. A mixture of fly ash and ground granulated blast furnace slag was activated with CCR to prepare one-part geopolymer. A conventional two-part geopolymer activated with sodium hydroxide (NaOH) solution was also made for comparison. The CCR-activated geopolymer with the highest compressive strength was selected to incorporate silica fume to improve its mechanical properties. Effects of the activator type/amount, the ratio of silica fume replacement and the curing age on the properties of geopolymer binders were evaluated. Mechanical behavior was mainly assessed by compressive strength tests, and hydration products were analyzed by FTIR, XRD and SEM-EDX. Also, the content of Ca(OH)2 in the system was calculated using TGA technique to investigate the effects of silica fume on the pozzolanic reaction. The results revealed that the CCR had high potential as an activator for the preparation of geopolymer binders, and geopolymer activated with 12 wt% CCR had the highest compressive strength. Increasing the CCR dosage could improve the compressive strength, while the addition of excess CCR led to a risk of strength loss. When the activator dosage was less than 8 wt%, the mechanical strength of CCR-activated geopolymer was found to be comparable with that of the NaOH-activated geopolymer. Moreover, the addition of silica fume was proved to enhance the strength of CCR-activated geopolymer. TGA results and SEM images analysis illustrated that adding silica fume to CCR-activated binders could promote the consumption of excessive CCR, facilitate the formation of hydration products and modify the pore structure.