Abstract
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Fly ash-based geopolymer (FAGP) and alkali-activated slag (AAS) concrete are produced by mixing alkaline
solutions with aluminosilicate materials. As the FAGP and AAS concrete are free of Portland cement,
they have a low carbon footprint and consume low energy during the production process. This paper
compares the engineering properties of normal strength and high strength FAGP and AAS concrete with
OPC concrete. The engineering properties considered in this study included workability, dry density,
ultrasonic pulse velocity (UPV), compressive strength, indirect tensile strength, flexural strength, direct
tensile strength, and stress-strain behaviour in compression and direct tension. Microstructural observations
using scanning electronic microscopy (SEM) are also presented. It was found that the dry density
and UPV of FAGP and AAS concrete were lower than those of OPC concrete of similar compressive
strength. The tensile strength of FAGP and AAS concrete was comparable to the tensile strength of OPC
concrete when the compressive strength of the concrete was about 35 MPa (normal strength concrete).
However, the tensile strength of FAGP and AAS concrete was higher than the tensile strength of OPC concrete
when the compressive strength of concrete was about 65 MPa (high strength concrete). The modulus
of elasticity of FAGP and AAS concrete in compression and direct tension was lower than the
modulus of elasticity of OPC concrete of similar compressive strength. The SEM results indicated that
the microstructures of FAGP and AAS concrete were more compact and homogeneous than the
microstructures of OPC concrete at 7 days, but less compact and homogeneous than the microstructures
of OPC concrete at 28 days for the concrete of similar compressive strength.