Valorisation of spent diatomaceous earth as alternative sodium silicate source for the activation of copper slag

The cement industry faces mounting pressure to reduce its environmental footprint, as Portland cement production accounts for approximately 8% of global CO₂ emissions. Alkali-activated cements offer a sustainable alternative by utilizing industrial byproducts. This study investigates the valorisation of spent diatomaceous earth (SDE) as an alternative source of sodium silicate to activate copper slag-based alkali activated cements. Sodium silicate solutions with an activation modulus ranging from 0.51 to 0.83 were synthesised by dissolving SDE in a 10 M NaOH solution at 80 C. These alternative activators were then used to produce copper slag-based alkali-activated cements with a liquid/solid ratio of 0.4. The specimens were then cured at room temperature and evaluated after 7 and 28 days for their mechanical properties, physical characteristics, thermal conductivity and microstructural development. Their performance was then compared with that of conventional NaOH and commercial sodium silicate activators. Despite having lower activation modulus, SDE-based systems achieved superior compressive strength (38.4 MPa at Ms = 0.83) compared to commercial activators (27.8 MPa at Ms = 1.0). They exhibited enhanced densification (bulk density: 2474–2518 kg/m3), reduced porosity (20.01%) and excellent thermal insulation (0.45–0.49 W/m·K). Microstructural analyses confirmed the formation of dense N-(Fe)-S-H gel networks, with increased SDE content correlating to progressive polymerisation. The exceptional performance of SDE-derived activators demonstrates successful valorisation of industrial waste streams while reducing dependence on energy intensive commercial sodium silicate. This approach addresses dual sustainability challenges by simultaneously recycling spent diatomaceous earth and copper slag, contributing to circular economy principles in the cement industry. • Spent diatomaceous earth successfully synthesized sodium silicate activators. • SDE-activated copper slag achieved 38.4 MPa, surpassing commercial activators. • Enhanced densification and reduced porosity with SDE-based activation. • Excellent thermal insulation for energy-efficient buildings. • Dual waste valorisation promotes circular economy in cement industry.