Flexural strength of plain and fibre-reinforced boroaluminosilicate geopolymer

• Anhydrous borax was used as a part of alkali activator of geopolymer pastes.
• Formation of BO bond helped geopolymers to gain high strengths.
• Reinforced and plain boroaluminosilicate geopolymers reveal high flexural strengths.

In the present work, flexural strength of plain and fibre-reinforced boroaluminosilicate geopolymers is studied. Traditional aluminosilicate geopolymers are produced by alkali activation of an aluminosilicate source. Alkali activator is normally made by mixing a high alkali solution (such as sodium hydroxide) and a silica-rich source (such as sodium silicate). Alkali activation of fly ash in this study, to fabricate boroaluminosilicate binders, was performed by mixtures of anhydrous borax and sodium hydroxide. Flexural strength of the specimens in unreinforced and reinforced conditions was measured by three-point bending. Reinforced specimens were prepared by using 2, 3 and 5 wt.% of steel fibres, with length and diameter of 30 and 0.5 mm respectively. The highest flexural strength of unreinforced specimens was 9.5 ± 0.4 MPa, with borax to NaOH solution weight ratio of 0.912 and alkali activator to fly ash weight ratio of 0.9. Reinforcing of this mixture by 5 wt.% of steel fibres resulted in the highest flexural strength, 11.8 ± 0.9 MPa. Maximum and minimum average increase of flexural strength of about 47% and 5% were achieved by adding 5 and 2 wt.% of steel fibres to some mixtures respectively. Results indicated the ability of these new classes of construction materials for using in flexural load-bearing sections in both unreinforced and reinforced situations.