Alkali Activation of Clay and Water Potabilization Sludge Binary Blends: Influence of Composition and Curing Conditions

Abstract

This work aims to evaluate the compatibility and features of alkali-activated blends obtained by replacing carbonate-rich illitic clay with either untreated or heat-treated water potabilization sludge (WPS). The experimental setting was created looking towards producing environmentally friendly solutions such as precursors that are sourced from the same territory, room-temperature curing in realistic environmental conditions, and activation exclusively with sodium hydroxide (NaOH) solutions. A multi-analytical characterization of the blends using X-ray powder diffraction (XRPD), an optical microscope (OM), a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDX), and a mechanical test demonstrated that up 75% of calcinated sludge and 25% of uncalcinated sludge could be successfully incorporated into the clay-based blends, offering a valuable alternative to landfill disposal of WPS. The matrices’ features were affected both by the amount of sludge in the blends and by the environmental conditions curing. since mineralogical investigations, OM and SEM observations showed the formation of secondary crystalline phases, mainly zeolitesin addition to amorphous gel. The mechanical strength results reached values between ~3 and 9 MPa, suggesting the possible use of the investigated alkali-activated blends for the formulation of precast building materials. Furthermore, to assure the replication of these alkali-activated blends, uncontrolled (T °C and RH%) curing does not appear to be the most appropriate solution. The study demonstrated that WPS, traditionally destined for landfill, could be a resource for the production of alkaline-activated materials by partially replacing unrenewable raw materials. thus resulting in the creation of eco-sustainable and economic processes as WPSare a widely and locally available industrial byproduct. However, a better control of mix designs and curing conditions is necessary for the upscaling of the here investigated blends.