An Experimental Study on GGBS-Fly Ash Geopolymer Concrete: Mechanical Properties and Structural Performance of Reinforced Deep Beams
The shift from Ordinary Portland Cement (OPC) to geopolymer concrete offers significant environmental advantages. However, its application to shear-critical structural elements, such as reinforced deep beams, remains insufficiently researched. This experimental study investigates the mechanical properties and structural performance of Ground Granulated Blast-furnace Slag (GGBS)-fly ash geopolymer concrete with alkaline liquid-to-geopolymer (AL/GS) ratios of 0.7, 0.8, and 0.9. The compressive strength, indirect tensile strength, and behavior of reinforced deep beams were evaluated under ambient and heat-cured conditions. The results showed that an AL/GS ratio of 0.7 yielded the highest compressive strength, 65.7 MPa at 90 days, under ambient curing. The deep beam tests revealed that a reduction in the 28-day compressive strength from 59 MPa (AL/GS = 0.7) to 42 MPa (AL/GS = 0.9) caused a dramatic 68% drop in the ultimate load-carrying capacity (from 325 to 108 kN). This pronounced sensitivity is attributed to the primary strut-and-tie mechanism, where higher compressive strength significantly enhances the efficiency of the concrete strut. The geopolymer concrete also exhibited more brittle tensile behavior compared to conventional predictive models. These findings highlight the importance of optimizing the AL/GS ratio to achieve high and uniform compressive strength for the safe structural application of GGBS-fly ash geopolymer concrete in shear-critical structural members.