From waste to resource: Cleaner production of geopolymer bricks from sewage sludge via production parameter optimization

The study investigates the use of sewage sludge in geopolymerisation for sustainable solidification, specifically as an aluminosilicate source in geopolymer brick production. The study encompasses a comprehensive experimental program aimed at examining the effects of activator concentration, curing temperature, curing time, and liquid-to-solid ratio (L/S) on the mechanical and microstructural properties of geopolymer brick samples derived from sewage sludge. Results indicate that the characterization results reveal that a 12 M NaOH concentration led to full geopolymerisation, ensuring uniform interaction between sewage sludge and the alkaline activator, resulting in increased UCS(UCS). Results indicate that the highest UCS ranging from 17.23 to 18.7 MPa were attained at NaOH concentration, temperature, curing time, Na₂SiO₃/NaOH and L/S ratios of 12 M, 100 °C, 28days, 1.9 and 0.6 respectively. At 0.8, the highest workability of 295 mm which was optimal to produce SS geopolymers. Calcination at 750 °C led to the increase in amorphous content from 2.21 % to 7 %, which is satisfactorily suitable for geopolymerization. XRD reveals that zeolite and Na-(Ca)-S-H are the main phases that leads to an improved mechanical performance of SS geopolymers. Additionally, Fourier transform infrared spectroscopy (FTIR) analysis further confirms that the presence of an intense peak band associated to Si-O-Al bonds in the 900–1000 cm⁻¹ range indicate the formation of hydration products like Na(C)-A-S-H gel, contributing significantly to concrete strength. The investigation has demonstrated that sewage sludge can be improved to generate construction materials with increased value and address the issue of environmental pollution.