۱- Introduction

۲- Experimental program

۳- Investigated parameters

۴- Results and discussion

۵- Conclusions

۶- Conflict of interest

References

Abstract

The paper presents the assessment of metakaolin-slag-potassium-silicate geopolymer mixtures containing concrete and fired clay aggregates from Construction and Demolition Waste (CDW). An extensive characterization was carried out from a mechanical and physical standpoint, aimed at exploring their potential as a building material and their suitability for exploitation at industrial level. Based on the obtained experimental results, geopolymers with CDW showed promising properties for use in building elements even with 50% of aggregates and more, although further aspects need dedicated investigations.

Introduction

Construction and Demolition Wastes (CDWs) typically comprise inert mineral materials (concrete, bricks, tiles and ceramics, etc.), with smaller amounts of other components (e.g. wood, glass, plasterboard, bituminous mixtures and tar) [1]. CDWs are one of the main sources of waste in Europe, according to official data released by the European Union (EU) [2]. Although estimations may vary, due to illegal dumping and to different waste definitions and reporting mechanisms in force in various Countries [3], CDWs approximately represent one third of the total waste generated by economic activities and households, which in EU-28 are about 2.5– ۳ billion tonnes per year [2]. Thus, the European CDWs stream is about 0.8–۱ billion tonnes per year. As underlined by Peng et al. [4], the recycling of CDWs is of primary importance for several reasons: CDWs are heavy and bulky, thus undesirable for landfill disposal; many of them are potentially very relevant for recovery and reuse; their recycling is environmentally significant since it would reduce the consumption of energy and natural resources, the emission of CO2, and would promote the achievement of recycling goals (70% by weight in the European Union, according to the Waste Framework Directive [5]) and the preservation of valuable space in landfills. Incidentally, it is worth mentioning that a lack of harmonisation still exists in EU, with End-of-Waste (EoW) criteria not fully developed or consistent across different Countries [6]. Currently, two of the main destinations of recycled CDWs are: unbound aggregates for road sub-bases [7] and bound aggregates for concrete mixes [8–۱۰], the latter being a higher added value recycling pathway. According to Nixon [10], just after the Second World War the use as aggregate in fresh concrete of brick debris left by intensive bombardments was documented, and later on concrete rubble coming from demolished fortifications was included as well. Conversely, after that period of intensive rebuilding, there was little research interest until the Seventies, when the increasing availability of CDWs and the expected future scarcity of natural aggregates promoted more systematic investigations on recycled aggregates. Recent studies demonstrated that the production of structural Recycled Aggregates Concrete (RAC) with properties comparable to those of standard concretes is feasible through a careful optimization of CDWs typologies [11,12], grading [13] and mixing approach [14]. This is mainly due to an improved refinement of the Interfacial Transition Zone (ITZ) between old aggregates and the new cementitious matrix [15,16]. Nevertheless, the use of RAC is restrained by several drawbacks. It is worth mentioning, among them, the inferior mechanical properties and the greater drying shrinkage generally exhibited by RACs in comparison to concrete made with virgin aggregates, their lower resistance to carbonation and chloride penetration, and the still low cost of natural aggregates [9–۱۱].