Effect of recycled concrete powder and GGBS/FA ratio on the performance and microstructure of geopolymer mortar

Abstract

Reusing construction waste and reducing the carbon emissions of construction materials are critical for achieving sustainable development in the construction industry. This study developed a geopolymer mortar (RSFGM) by incorporating recycled concrete powder (RCP) into a binary precursor system composed of granulated blast furnace slag (GGBS) and fly ash (FA) to enhance both mechanical performance and environmental sustainability. The effects of RCP replacement ratios and GGBS/FA ratios on the flowability, setting time, mechanical strength, drying shrinkage, microstructure, and environmental impact were systematically evaluated. The results showed that the flowability, flexural strength, compressive strength, and elastic modulus of RSFGM decreased as the RCP replacement ratio increased, while the setting time was significantly extended. For instance, a 40% RCP replacement caused decreases of 33.21% in flowability, 34.7% in compressive strength, and 54.72% in elastic modulus compared to the control mixture without RCP. Increasing the GGBS/FA ratio resulted in microstructural densification, enhanced mechanical strength, and a shortened setting time, primarily due to the higher calcium content and greater reactivity of GGBS. An optimal balance between mechanical properties and environmental benefits was achieved with a GGBS-to-FA ratio of 7:3 and an RCP replacement ratio of 20%, yielding flexural and compressive strengths of 9.2 MPa and 76.9 MPa, respectively. Additionally, a novel drying shrinkage prediction model of RSFGM was proposed to capture the effect of physical parameters. This study highlights the potential of RCP as a sustainable precursor, offering new insights into the design of low-carbon RSFGM.