Physical and mechanical properties of cement-based products containing incineration bottom ash

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Abstract

This paper presents the results of a wider experimental programme conducted in the framework of the NNAPICS (“Neural Network Analysis for Prediction of Interactions in Cement/Waste Systems”) project funded by the European Commission and a number of industrial partners under Brite-EuRamIII. Based on the fact that bottom ashes from waste incineration are classified as non-hazardous wastes according to the European Waste Catalogue, the aim of the present work was to investigate the feasibility of addressing the potential use of such residues in cement-based mixtures. This issue was suggested by the analysis of the properties of different bottom ashes coming from Italian municipal and hospital solid waste incinerators, which showed a chemical composition potentially suitable for such applications. Different mixes were prepared by blending bottom ash with ordinary Portland cement in different proportions and at different water dosages. The solidified products were tested for setting time and bulk density, unconfined compressive strength and evaporable water content at different curing times. The results of the experimental campaign were analysed through a statistical procedure (analysis of variance), in order to investigate the effect of mixture composition (waste replacement level and water dosage) on the product properties.

Introduction

The amount of bottom ash, including grate siftings, produced by waste incineration, typically ranges from 30 to 35% of burnt waste by weight.

While great concern has been addressed so far in order to assess the feasibility of treating air pollution control (APC) residues in view of either reuse or disposal, much less information concerning bottom ash is available in the literature.

Nevertheless, the amount of bottom ash generated represents the major portion of the solid residues resulting from waste combustion. Consequently, bottom ashes, although classified as non-hazardous wastes according to the European Waste Catalogue, actually require high expenditures for their disposal.

In light of the above considerations, technical approaches aimed at reusing incineration bottom ashes as secondary materials must be strongly encouraged. This may result in two main beneficial effects: (1) reduction of the amount of residues to be landfilled, (2) partial substitution of raw materials in industrial applications. Among the various applications proposed for bottom ash, a number of studies are available on use of bottom ash either in road construction (Alkemade et al., 1994) or as an aggregate in concrete (Pera et al., 1997). In some cases, it was recognised that the high chemical reactivity exhibited by bottom ash could cause some long-term detrimental effects on the properties of the final product (Crignon et al., 1999). On the other hand, advantage may be taken from the presence of reactive compounds in bottom ash, such as oxides and alumino-silicates, with a view to the formulation of blended cement. It should also be stressed that in some cases, depending on the composition of the original waste, the combustion technology adopted as well as the combustion conditions, bottom ash may also exhibit pozzolanic activity as a consequence of high contents of amorphous, highly reactive silica.

Section snippets

Incinerator bottom ash composition

Studies on bottom ash mineralogy (Clozel-Leloup et al., 1999) revealed that, as expected, incineration bottom ash is predominantly composed of high-temperature solids (primary phases). Most of these phases are metastable under natural conditions and can be chemically transformed into thermodynamically stable assemblages or minerals, often called secondary phases (Zevenbergen and Comans, 1994).

In addition to unburned material, fresh incinerator bottom ash may contain a low-density,

Materials and methods

The experimental campaign was carried out on three municipal solid waste incineration bottom ashes (MBA) and one hospital waste incineration bottom ash (HBA) coming from different Italian incineration facilities.

Bottom ashes were characterised for their physical (including bulk density, water content, loss on ignition) as well as chemical properties (including phase oxide composition, elemental composition, anion content). The laboratory testing programme followed a wider protocol adopted by

Statistical analysis

A two-way replicated factorial design (Box et al., 1978, Montgomery, 1991) for prediction of each product property, namely unconfined compressive strength, evaporable water content and setting time (the response variables) was analysed by means of the analysis of variance (ANOVA). The independence between the response variables was assumed throughout the analysis, so that univariate linear models were fitted.

The analysis was aimed at inspecting the effect of bottom ash dosage and water content

Results and discussion

Table 2 shows the physical properties of bottom ashes and their phase oxide composition along with that of OPC, as determined through total X-ray fluorescence (TXRF). It can be seen that the bottom ashes are mainly composed of Si, Al and Ca oxides, accounting for 79–82% of the material on a dry weight basis. In particular, the Si and Al oxides content ranges from 52 to 70%. The ternary composition plot for the bottom ashes based on the content of the three earlier mentioned oxides is depicted

Conclusions

The influence of waste dosage and water content on the properties of solidified incinerator bottom ash–Portland cement mixtures was investigated through a two-way replicated factorial design. Different response variables including mechanical strength, evaporable water content and setting times were analysed.

The results showed that for unconfined compressive strength no interactions between the factors were recognisable. Moreover, linear dependence of strength on both waste dosage and water

Acknowledgements

The experimental work was conducted within the framework of the Industrial and Materials Technologies Programme (Brite-EuRamIII) research project “Neural Network Analysis for Prediction of Interactions in Cement/Waste Systems”, involving Imperial College (UK), Trinity College (I), Universidad de Cantabria (E), University of Rome “La Sapienza” (IT), University of Surrey (UK), British Nuclear Fuels (UK), Euroresiduos S.A. (E) and GE.SE.NU. S.p.A. (IT) as partners. Financial support of the

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