This paper reports for the first time the experimentally tested elastic modulus values of the fiber-reinforced geopolymer composites (FRGCs). Further, the deflection hardening behavior and sand addition effect on the ambient-cured one-part steel-polyethylene FRGCs are also reported wherein the total fiber volume fraction used was 2%. It was found that although the cylinders’ compressive strength of the blended-based FRGCs was relatively higher than that of the slag-based FRGCs, the latter showed a relatively larger elastic modulus in comparison with the former. The addition of fiber increased the elastic modulus of the geopolymer composites relative to the none-fibrous composites at which the elastic modulus improvement was related to the steel (ST) volume. Further, all the FRGCs exhibited a deflection hardening behavior which was directly related to the polyethylene (PE) volume included in the composite. Generally, the slag-based FRGCs showed better flexural behavior (ie modulus of rupture, deflection capacity, and multiple-cracking behavior) in reference to the blended-based FRGCs. The sand addition (ie small size like 212 µm with low content such as 30% by mass) was found to improve both flexural cracking and ultimate strengths without affecting the deflection capacity nor the cracking behavior. This could be due to the better volume stability and lower shrinkage of the geopolymer matrix after sand addition.