Alkali-activation of aggregate fines from construction and demolition waste: valorisation in view of road pavement subbase applications

This study investigates the potential of fine particles of recycled construction and demolition waste (CDW) aggregate to undergo alkali-activation when mixed with an appropriate alkaline solution. The fine is a natural by-product of the milling process and includes particles from four main material sources (i.e., recycled concrete, recycled asphalt, crushed bricks and tiles, and natural aggregate and excavated soil) and other occasional elements which are too small for identification. The fine was obtained by sifting the material through a 125 μm sieve. Since the reactivity of unselected material depends on its constituents, these were also individually investigated. Firstly, the four constituents of CDW recycled aggregates were separated, then milled to a size smaller than 125 μm, before being tested to measure their reactivity to an alkaline solution. A preliminary chemical and mineralogical characterization of the five powders was carried out to identify the main crystalline phases and ascertain the presence of aluminosilicates needed for the alkali activation process. Particles of each powder were afterwards mixed with three concentrations of the same alkaline solution with a liquid/solid mass ratio of 0.4, cast in prismatic moulds, and cured at room temperature. Mechanical tests after 3, 7, and 28 days of curing demonstrated that powders react positively in a basic environment, showing an increase in strength without any thermal treatment. Hardened pastes of undivided fine aggregate and recycled asphalt exhibited the best results in terms of flexural and compressive strength with the more concentrated solution. A Field Emission Scanning Electron Microscopy analysis was also carried out to observe the microstructure and to support an interpretation of the mechanical strength data. Results demonstrated the feasibility of using a solution to activate unselected CDW fine particles to stabilize CDW aggregates. In full scale applications, CDW aggregates can be stabilized without the addition of any binder.