Microstructure Evaluation of Fly Ash Geopolymers Alkali-Activated by Binary Composite Activators

Abstract

An efficient fabrication of fly-ash-based geopolymer is urgent and necessary to develop solid waste recycling techniques. Herein, an attempt to investigate the effect of binary composite activators on the microstructure of fly-ash-based geopolymers is conducted through the comparison of 24 experiments, which consisted of Na2SiO3·9H2O, Na2CO3, K2CO3, NaOH, and KOH through a facile preparation technique. The results demonstrate that the activator of Na2SiO3·9H2O + KOH presents the highest mechanical strength, due to the synergy activation between the inherent ≡Si-O-Si≡ silicon-chain precursor derived from the Na2SiO3 and K+’s catalysis. It reveals that the K+ plays a crucial role in the Na2SiO3-activated fly ash geopolymer, which is the rate-determining step of the enhanced crosslinking and propagation of N-(C)-A-S-H chains, leading to an increase in weight loss temperatures of specimens from TG/DTG results. Furthermore, the adding silica fume facilitates as-formed amorphous silicates, which also could fill into the pores of N-(C)-A-S-H amorphous gels and present a uniform and compact morphology, leading to an increase in the pore volume of the pore diameter less than 100 nm. It explores an efficient and cost-effective preparation of fly-ash-based geopolymer for developing solid waste recycling techniques.