Coupling effects of sewage sludge and recycled fine aggregate on the properties of geopolymer recycled mortars

The utilization of sludge from sewage treatment plants, ground granulated blast furnace slag (GGBFS) and recycled fine aggregate (RFA) from demolition of abandoned buildings in the production of geopolymer recycled aggregate mortar (GRM) offers a sustainable solution to reduce Ordinary Portland cement (OPC) application, and to address the growing volume of municipal sludge and construction waste. This work aims to investigate the workability, mechanical performance and microstructure of GGBFS and sewage sludge ash (SSA) based alkali activated GRM. GGBFS and SSA based geopolymer was used to replace 100% of OPC and RFA was employed to substitute various proportions (0%, 15%, 30% and 50%) of natural fine aggregates (NFA). Influences of SSA and RFA content on the consistency, setting time, compressive strength and flexural strength of GRM were investigated. The geopolymerization products were characterized through scanning electron microscopy (SEM), x-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) tests. Results show that the consistency value and setting time of GRM decrease when the SSA and RFA content increases. When SSA content is 10% and RFA content is 15% (S10R15 sample), sample's microstructure becomes more compact, displaying robust mechanical properties. And S10R15 sample attained an average 28-day compressive strength of 51.7 MPa and flexural strength of 7.2 MPa. This demonstrates that GRM exhibits satisfactory working performance, mechanical properties. This study is expected to offer more insights into the application of SSA and RFA.