Alkali activated materials vs geopolymers: Role of boron as an eco-friendly replacement

In this paper, the influence of boron ions on the strength development and microstructure variations of fly ash-based geopolymers and alkali-activated slag (AAS) was investigated through utilizing borax-based alkali-activators. In addition to cost-effective aspects, the substitution of boron ions in geopolymer and AAS matrixes meant to minimize the downside of the silicate compounds on the environment. The binders were produced by introducing borax to alkali-activator solutions. Granulated ground blast furnace slag (GGBS) and Class F fly ash were used to produce AAS and geopolymer specimens respectively. Results showed a decline in the strength of geopolymer binder as a result of the increase in the amount of borax. However, the formation of B-O bonds in geopolymer matrices prevented an unexpected behaviour of the strength derived from an imbalance in the alkali activator solution. Substitutions of more than 60% of sodium silicate in the fly ash-based geopolymer was not reasonable. The compensatory role of boron in the strength of AAS was undeniable where even introducing more than 30% of borax led to a slight growth in the AAS strength. It was concluded that borax-based binders could be potential candidates for high volume substitution of sodium silicate in both geopolymers and AAS.