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 mm 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 mm, 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.

Introduction

The continuous production of waste from construction and demolition activities is a major environmental concern in European countries. EU Commission estimates reveal that construction and demolition waste (CDW) accounts for approximately 25e30% of all waste generated in Europe (European Commission, 2011). Several researches have demonstrated the potential for the reuse of this waste as an alternative material in civil engineering (Cardoso et al., 2016; Contreras et al., 2016; Vieira and Pereira, 2015). This is due to the presence of solid particles from a variety of construction materials which can form new aggregate grains of variable toughness and hardness, depending on composition, which in turn is conditioned by the type of construction and/or demolition work. In fact, an unselected CDW usually includes constituent materials in different proportions including (recycled) concrete fragments (RC), ceramic products such as crushed bricks and tiles (BT), reclaimed asphalt pavement grains (RA), natural aggregates and excavated soils (NA), together with occasional small particles of glass, wood, metals, and plastic (Jimenez, 2013
). Member states are being encouraged by the European Directive 2008/98/EC (European Parliament, 2008) to increase the use of recycled non-hazardous waste resources in place of natural materials to a minimum of 70% (by weight) by 2020. Consequently, construction activity has witnessed the increased use of alternative materials and recycled aggregate from CDW in numerous civil works (Vandecasteele et al., 2013).