Approach to design sustainable alkali-activated slag recycled aggregate concrete: Mechanical and microstructural characterization

There is a steep increase in demand for concrete as the need for infrastructure is increasing with a rapid increase in urbanization. Consequently, there has been a surging demand for cement across the globe. By using alkaline concrete activated with ground granulated blast furnace slag (GGBS), cement usage can be eliminated in concrete, which addresses the issue of CO2 emissions concerned with the cement manufacturing process and promotes the use of industrial by-products as sustainable building materials. The current study focuses on proposing a methodology to design sustainable GGBS-based alkali-activated concrete incorporated with 100 % coarse recycled coarse aggregates (RCA) recovered from construction and demolition (C&D) waste, for various strengths by optimizing the mix proportions and verifying the same through experiments. From the current study, it is evident that this mix design procedure can be adopted to design alkali-activated slag recycled aggregate concrete (AASRAC) mixes for strengths ranging from 44 MPa to 85 MPa. The optimum values of alkalinity factor (lambda) and NaOH concentration (M) i.e., 1.5 and 12, have a significant role in the development of high strengths which is attributed to the formation of hydrotalcite. Micrographs captured using scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) studies reveal that the alkaline binders promote the development of a denser and stronger interfacial transition zone (ITZ) that enhances the strength of lower AA/B concretes at the microstructural level as Ca/Si ratio is higher for lower AA/B concretes. Global warming potential (GWP) analysis reveals that the alkali-activated concretes have significantly lower GWP compared to conventional OPC-based concrete by approximately 2.5 times. The mix-design chart proposed in this study may pave the way to designing structural grade AASRAC with enhanced performance for the desired strength, which may find its application in fast-track construction and manufacturing of prefabricated elements.