Abstract
Geopolymer concrete lowers the carbon burden of construction
but remains brittle in tension, and the question of which fibre combination
corrects that brittleness at least cost to compressive strength is still open.
This study examines thirteen mixes of fly ash and GGBS based geopolymer
concrete under a two factor design: four fibre systems (steel alone, steel with
polypropylene, steel with glass, and a ternary blend of all three) at three
total volume fractions (0.5, 1.0 and 1.5 per cent), together with an
unreinforced control. Specimens were tested for compressive strength at 7, 28
and 56 days, split tensile and flexural strength at 28 days, impact resistance
by drop weight, water absorption and mass loss after 90 days in 5 per cent
sulphuric acid. Two way analysis of variance shows that fibre system had no
significant effect on compressive strength (F(3, 24) = 2.144, p = .121) while
dosage did (F(2, 24) = 20.330, p < .001), and that both factors mattered for
flexural strength (system F(3, 24) = 10.002, p < .001; dosage F(2, 24) =
89.831, p < .001) and for impact resistance. No interaction reached
significance for any response. Tukey comparisons place the ternary blend
significantly above steel alone in flexure (p < .001) but not above the
steel and polypropylene pair (p = .347). Compressive strength peaked at 1.0 per
cent and fell at 1.5 per cent, and a quadratic regression locates the
stationary point at 0.936 per cent steel. Flexural strength correlated almost
perfectly with split tensile strength (r = .985) and strongly with impact
resistance (r = .937). The ternary blend at 1.5 per cent gave the largest gains
in flexure (68.9 per cent) and impact (228.0 per cent) but sacrificed
compressive strength relative to the 1.0 per cent mixes. On the evidence, a
ternary hybrid near 1.0 per cent total volume represents the practical optimum,
since the additional flexural gain from 1.0 to 1.5 per cent was not
statistically significant (p = .123) while the compressive penalty was real.