Characteristics of boroaluminosilicate geopolymers

• Fly ash particles were alkali activated by a new class of alkali activator.
• Anhydrous borax was used as a part of alkali activator of high strength geopolymer pastes.
• Formation of B–O bond helped geopolymers to gain high strengths.
• Boroaluminosilicate geopolymers showed different types of microstructures.

In this work, a method for incorporating fly ash in constructional technology was developed by production of boroaluminosilicate geopolymers. These new class of geopolymers were appropriately synthesized from mixtures of fly ash and anhydrous borax and compressive strengths as high as 64 MPa were achieved by using suitable amount of borax. Different types of microstructures were observed by changing mixture proportion of the specimens. All of specimens revealed a brittle fracture with no crack branching in the mixture. Additionally, less unreacted fly ash particle was observed in the considered pastes indicating a totally different fracture mechanism in boroaluminosilicate geopolymers than aluminosilicate ones. FT-IR analyses of the mixtures revealed an additional B–O bond in comparison with aluminosilicate geopolymers. It was shown that this bond is a key factor to determine compressive strength and specimens with no B–O bond have lower strengths.