Influence of BFS Content in the Mechanical Properties and Acid Attack Resistance of Fly Ash Based Geopolymers

This paper analyzes the influence of an extra calcium source (added as blast furnace slag) on certain properties of fly ash based geopolymers using sodium silicate as activating solution. Geopolymers were manufactured with different fly ash/blast furnace slag (FA/BFS) ratios: 100/0, 80/20, 60/40, 40/60, and 20/80. The properties studied in the geopolymers were their compressive strength and acid attack resistance. The addition of BFS improves geopolymer compressive strength at early stages. Compressive strength increases as the proportion of BFS in the mixture increases. However, maximum compressive strength was found for a determined BFS content value in the geopolymers, so that a clear reduction of this parameter was appreciated when the BFS content increased above this value. The resistance to acid attack improves in the presence of BFS possibly due to the formation of a calcium sulphate precipitate layer that increases the compressive strength and also operates as a protective coating that hinders geopolymer deterioration.