Influence of ground granulated blast furnace slag on recycled concrete powder-based geopolymer cured at ambient temperature: Rheology, mechanical properties, reaction kinetics and air-void characteristics

In this paper, ground granulated blast-furnace slag was used to enhance the properties of recycled concrete powder based geopolymer cured at ambient conditions. The rheology, workability, strength, hydration reaction kinetics, microstructure, and air-void characteristics of the geopolymer were investigated. The results showed that adding the slag accelerated the setting process and reduced the water absorption and porosity of the geopolymer mixtures. Excellent mechanical properties were achieved when the slag content exceeds 30 %. The 28d compressive and flexural strength of the geopolymer with 50 % slag addition reached 61.2 and 5.17 MPa, respectively. The yield stress and plastic viscosity of the geopolymer increased as the slag content increased. A quantitative model between the slag content, resting time, and rheological parameters was established, with high accuracy, realistically reflecting the rheological properties of the geopolymer. The addition of the slag provided nucleation sites for the C-S-H gel, which promoted the hydration reaction, increased the hydration products and the total heat release, and formed a denser and more uniform microstructure, thereby increasing the strength of the geopolymer. The slag addition was also found to reduce the air content, spacing factor, and average chord length of the geopolymer matrix. As the slag content increased, the air bubbles became smaller and more stable, and exhibited a more uniform distribution.