The microstructure of concrete made with municipal waste incinerator bottom ash as an aggregate component

Abstract

The interaction of municipal solid waste incinerator bottom ash (MSWI bottom ash), when utilized as an aggregate in concrete, with the cement matrix was investigated. The most prominent reaction observed in lab and field concrete was the formation of aluminium hydroxide and the release of hydrogen gas from aluminium grains reacting in the alkaline environment. The expansive aluminium reaction was identified as a main cause of extensive spalling on the concrete surface. Due to the higher content of bottle glass as part of the ash, in all samples, reaction products of an alkali-silica reaction (ASR) could be observed as well. However, damage due to ASR were less severe than those caused by the aluminium reaction. The expansion rates were low and only a few of the lab samples showed cracking. Microstructural analysis of the samples indicated clearly that a large quantity of the alkali-silica gel which was formed was accommodated in the pores and voids without exerting any strain on the material.

Introduction

The incineration of municipal solid waste is becoming increasingly important for waste management because of the new European regulations which prohibit storing of untreated waste in landfills. Research concerning the use of residues from waste incinerators in concrete production is therefore a positive advance in sustainable development, by saving natural resources and decreasing waste volume stored in landfills. Today, modern reprocessing techniques lead to incinerator slag or ashes, which have the potential to be used as aggregates or mineral additions in cementitious building materials. Because of a highly sophisticated reprocessing technique residues with relatively stable composition, relatively well defined properties, and with contents of harmful components, (such as heavy metals and hydrocarbons) below legal limits, can be produced. Attempts are being made to custom design material properties according to their application by varying reprocessing methods.

In recent years many studies were performed in order to find possibilities for incorporating municipal solid waste incinerator bottom ashes (MSWI bottom ashes) into building materials or structures. MSWI bottom ash is now widely used in road construction as a gravel material in compacted base layers [1], [2], [3], [4]. Appendino et al. [5] demonstrated that bottom ashes can also be sintered together with other waste materials, to form glass-ceramic bodies, with possible applications as tile or brick material. For cement based materials MSWI bottom ash was studied as a possible mineral addition [6], [7], [8], [9] or as a constituent raw material for cement production [10], [11], [12], [13], [14].

MSWI bottom ash was considered as an aggregate component of concrete for some time. The first attempts at dealing with technological and physical properties of fresh and hardened concrete, together with chemical analyses, generally show the potential to use bottom ash as an aggregate material [15], [16], [17], [18]. The present paper is part of an ongoing study which investigates the effects and performance of MSWI bottom ash aggregates in concrete, and focuses on the microstructural aspects of this material in interaction with the cement paste. The main goal was therefore to analyze reactions occurring between the ash constituents and the cementitious binder on a microlevel and, its possible impact on the performance and durability of the concrete.